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Mute it, Kirk: Radio Transmitter Muting

Posted by Kirk Harnack [TWiRT] on Mar 17, 2014 10:25:00 AM

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TWiRT 204Kirk has a transmitter problem related to operation during antenna icing conditions. Between Chris Tobin’s help and ideas from the TWiRT chat room, I think we’ve got a solution! Plus, Chris Tobin explains how transmitter muting and safety lockouts are USED in multi-transmitter master antenna sites.





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Announcer: This week in Radio Tech Episode 204 is brought to you by the Omnia A/XE and Omnia F/XE software audio processors for Windows. For live stream processing and encoding and encoding choose Omnia A/XE. For file based processing and encoding it's Omnia F/XE. Both give you precise Omnia audio processing and real Fraunhofer encoding. On the web at omniaaudio.com.

Kirk has a transmitter problem related to operation during FM antenna icing conditions, between Chris Tobin's help and ideas from the TWiRT chat room I think we've got a solution. Plus Chris Tobin explains how transmitter muting and safety lock outs are used in multi-transmitter master antenna sites.

Kirk: Hey, welcome in to This Week in Radio Tech. I'm Kirk Harnack, your host, and so glad to be here. This is our 204th episode, so we've been going strong with this for about four years, Holy Moly.

This is the show where we talk about broadcast engineering, radio engineering more specifically, audio, RF, all the techniques, the coded audio, istreaming, and all that from the microphone to the beacon at the top of the tower. Someday I'll get my beacon back in the shot here, it's kind of bright, but I'll get it back in the shop.

On this show we've got a couple of technical problems to solve. One of them is mine and we'll see if we can get Chris Tobin to think of a technical problem that he has to solve. He should give me the problem and I'll try to solve it, and we'll also check in the chat room.

If you are watching live, you ought to sign into the chat room. You can be anonymous, just go with a name they make for you, or you can put your real name, or some alias in there if you want to and participate in the chat room. I always get some great ideas from the chat room about what to talk about or solutions to problems that we're not thinking up.

Our show is brought to you by my friends at Omnia Audio and the Omnia A/XE software processer on stream encoder, also by Omnia F/XE the file based audio processer and re-encoder. It's cool, just check it out.

Our co-host, as usual, Chris Tobin is with us. Hey, Chris, welcome in.

Chris: Hello Kirk. I'm doing well. It's going to be a fun time.

I'm running off a MacBook today, something last minute. My Windows laptop that I normally use died. I'm not sure exactly what went wrong, but something in the mechanical portions of the hard drive just started screeching and squealing, and I lost it. So if I suddenly disappear or something happens, because I had to at the last minute just throw everything in and make it work.

Kirk: So what did you do to make your MacBook work? Was Skype already on it.

Chris: Skype was already on it. My Logitech webcam, I did not need to do anything, I just plugged it in and it went without a hitch. I'm using a USB audio interface; I had to download the drivers for that. It's an Edirol UA-25EX, which I use for everything and it's seems to be the most stable little I used for the last four years now. It's real nice.

Kirk: People seem to like it when we show other stuff on the show. I'm moving my camera right now and we'll see if we can get the... Oh, that's it? So that goes between your Lav mic and your USB?

Chris: That is correct. That is correct. So the Lav mic plugs in, it has phantom power. I have my earpiece and it's a nice little box.

Kirk: So if we can put my camera on, there is my audio interface that I use week-in and week-out, not all the time, but almost all the time. That is a Shure X2U. It's XLR on this end, mic level. It can be phantom or not. I think the phantom power is only 12 volts, but that seem to work. Then it has a mic preamp level control. It has a volume control for the built-in headphone.

Then it has a mix control to mix the mic preamp audio, it steers off to the headphone output, and the return audio from the computer coming back... as if this is an external sound card... is also mixed and you can adjust the balance of that mix right here, between those two thing.

It's held up for, I guess, about three years I've had it. It's gone all around the world with me. It works quite well.

I'm sorry about all the camera movement there, we'll put that back on. There we go. So there you go, a little inside baseball as to what we're using.

Well, so I mentioned that on this show we're going to solve a couple of technical problems. We may even go in a bit of depth here.

You folks know that I'm the engineer for a small group of radio stations. My full-time job is I work for the folks at the Telos Alliance. So I'm the head of the Telos division, so phone systems, and codecs with ISDN and IP audio codecs. But I also take care of some radio stations in Mississippi and in American Samoa.

So we have one station that... I don't really have time to do all the engineering, so we do hire in people from time to time. One engineer that we do hire in from time to time is Mike Patton. Mike's been a guest on this show. Mike is a fantastic contract engineer. He lives in Baton Rouge, Louisiana and he services stations all over The South. He does NRSC measurements and major rebuilds.

We had him rebuild an old FM transmitter for us. "Old" I say, it's old, it's 14 or 15 years old. It's branded Armstrong, but it's really made by the folks at R.V.R. in Italy. It's a single-tube FM transmitter. It's capable of doing 3.5 kilowatts. It's not very tall. It's this thing... it's about, I guess, chest height, so maybe 3.5 or 4 feet tall. I guess it's about 4 feet tall.

It has a single tube in it. That tube is a, I want to say a, 3CX5000. It's definitely a triode, so it needs a lot of drive. So if you're familiar at all with the, I guess we've got an hour to talk here and at least a half an hour on my problem.

So this transmitter is really an amplifier. It had and exciter in it, like a 30 watt exciter. Then it had a solid state I.P.A., that doesn't mean beer, it means intermediate power amplifier. And that solid state I.P.A. would take about 19 or 20 watts coming in, maybe 9 or 10 watts, anyway it would take a small amount of power coming in and it would shove out up to 300 watts going back out.

Then it takes about, depending on how much power you want to put on the transmitter, anywhere from 200 to 270 watts to drive the final tube in this transmitter. Again, it's an Armstrong 3.5 kilowatt FM transmitter, and being a grounded grid tube it only has about 10dB of gain. So it's not like a tetrode tube where we have more gain, upwards to almost 20 dB of gain with some of the 4CX tubes in a tube design transmitter.

The bottom line is, you've got to feed it a lot of power, but it's also very stable. It doesn't need to be neutralized like other tube amplifiers do, and it doesn't need that extra screen voltage power supply. It doesn't even need a biased power supply, it simply a filament inside the tube and a grid, which is grounded; hence the grounded grid name of this kind of transmitter, and an anode or the plate, which is sitting at a very high voltage, 5,000 or 6,000 volts or so.

So you feed RF into the cathode, the filament and the cathode in this case are the same thing, in some tubes the filament heats up the cathode and you apply power to the cathode, or supply a signal and inputs it into the cathode or the grid. In this case, though, the cathode and the filament are the same thing. You feed it 200, 250 watts are so and it gets amplified up to 3,000 to 3.500 watts of RF power coming it.

It's a Class-C amplifier, which means the output of it is kind of dirty. So it has to go through a low pass filter before it goes to the antenna, because it contains harmonics of your FM signal, those have to be suppressed. So you have to have a harmonic filter.

So the transmitter was designed, I say all that to get to this point. We no longer used the original FM exciter, and we no longer used the original intermediate power amplifier, so we just used the transmitter an amplifier. We'd feed it a couple hundred watts of RF from a new exciter from Nautel that amplifies that and ships it out to the antenna. So we're just using it as an RF amplifier at the final stage. This exciter from Nautel is their VS300, so it's capable of putting out 300 watts. I think it's loafing along at 200 watts, or so, to feed this tube.

So, here's the problem, the original design of the transmitter is like this. You should know that because the exciter feeds the intermediate power amplifier in the original design, and the intermediate power amplifier feeds the tube, they simplified things by not having different circuits that turn different things on.

Basically, when you start the transmitter, you'd turn on the filaments, and then if you want to you can turn on the plate voltage, the high voltage on the tube, but it won't come on yet until the filaments have warmed. So there's this timer that runs for about 90 seconds to three minutes, somewhere in there. When the filament is hot enough then it applies the plates.

Now in a grounded grid design it's perfectly okay to apply the plates even if there's no signal coming in to the cathode of the tube. You can apply full plate voltage, the tube will not run away. It'll draw a little bit of power, a little bit of current. But it won't get hot, it won't run away, you won't have a meltdown.

Then when the plates come on another relay is closed that applies AC power to the intermediate power amplifier and to the exciter. That's the original design of the transmitter.

So, the intermediate power amplifier it comes up, in a second or two it's ready to go, but the exciter takes time. The exciter has an oscillator circuit that has to come up, it has to lock into the right frequency and start modulating.

So the exciter typically would take 15 to 20 seconds to start putting out RF power, and the original transmitter design and the Nautel one as well, doesn't just wham put on its output power, it ramps up. Once it's ready to go, once the oscillator's working and everything, the power output of the exciter goes to what you preset it for. The original exciter has a digit control, of course, the Nautel has a digital control, and it ramps up.

So the net result is, you turn the transmitter on, turn the plates on, 90 seconds later the plates come on, and at that moment power is applied to the exciter and the I.P.A. The exciter a few seconds later, it ramps up and comes on, so the power going into the I.P.A. ramps up. The power going from the I.P.A., of course, into the P.A., the tube, it ramps up too. So the transmitter looks beautiful. I mean, the output power just goes, like this, and there you go. You're on the air and it ramps up all nicely.

Well, when we pulled out the original exciter, we pulled out the original I.P.A. We put in the Nautel exciter/transmitter, because it will do 300 watts, we put blank panels were the old stuff used to be, the Nautel sits separately. All this works, except we're following the same logic as the original transmitter design did. The Nautel exciter is not brought on until the plates are brought on and, via the original design, that ought to be okay. So the Nautel comes on, it's power comes up, the transmitter all comes up, everything's cool.

Everything's fine as long as you don't have one of those unusual problems like ice on the antenna. If there's ice on the antenna, the transmitter tries to come up to power, it gets too much reflected power from the FM antenna on top of the AM tower. And the transmitter, not the exciter, but the transmitter says, "Un ah, can't do this," and shuts off. It shuts the plates of the transmitter off, the filaments stay going, the exciter goes and we're off the air.

Now, we don't have any real good expertise at the station. We've got a great general manager, but he doesn't know how to go into the menus of the Nautel exciter and reduce the power to which it could come up to. So there's no feedback from the transmitter, and it's reflected power circuit, back to the Nautel exciter to tell it to reduce its power. All there is AC power coming into it, or no AC power coming into it. If the transmitter's not on, the only place the Nautel gets it's AC power to come up with is from the transmitter.

So you can kind of figure out the problem here. The Nautel exciter is not on long enough to ramp its power down, even if you know how to get to the menu, because by now the transmitter's already figured out that it's got too much reflected power and it shuts off. So there's never an opportunity with this design, with this logic layout, to reduce the power of the Nautel exciter, which is the controlling factor for the whole thing.

There is no power control on the transmitter. Yeah, you could unload the plate circuit, but that would cause more problems. So there is no screen control. There's no way to ramp the power up and down on the P.A. itself, there's only adjustment of the exciter, and the exciter's only on long enough to realize, "Oh, there's a problem," the transmitter shuts it back off.

So that's my problem. How do I make it possible, I mean, I can't even set the Nautel exciter to a different preset remotely, because it's not on. If there's ice on the antenna it's no on long enough, it shuts right back off in however long it takes the power to ramp off, in 2, 3, 4 seconds it's shut right back off again.

So how do I solve this problem. Now you could say, "Well, Kirk..." I'll tell you what, I'll shut up. Chris, do you have any ideas about this? Do you understand what my dilemma is here?

Chris: Yeah, I had many a Harris transmitter that used to fold back on crazy weather days, and sometimes they did their job and sometimes that didn't do it well. Well, they did it too well, I should say, they pretty much turned themselves off.

On your transmitter, the Armstrong labeled one, is there a circuit that you can adjust for the fold back status or the fold back threshold? Or, is it just literally, the antenna goes out of tune and then the amplifier just says, "That's it. I'm out of tune. I'm shutting down."

Kirk: There is a threshold, it's settable. It's a little bit hard to find, I believe, it's one of several multi-turn trim pots in the transmitter's controller circuitry. It's got a fairly sophisticated controller circuitry. Sophisticated doesn't make it easy to adjust, but it's preset from the factory. Mike Patton, I'm pretty sure, checked its setting. There isn't a fold back, because there's nothing to control. This transmitter had absolutely no feedback from the VSWR detection circuit to the exciter control.

I don't know how the original design, R.V.R. then relabeled Armstrong, I don't think their design had any particular fold back. If there was reflected power, the transmitter shuts off, you're welcome to take care of it some other way if you want to.

It seems like when that happened, I may have in the past when it was at a previous transmitter site. I may have gone out to the transmitter site, plugged a separate power cord into the exciter, brought the exciter up, and real quickly went to the right menu and lowered its power set point, realizing that I was putting RF power into the unpowered I.P.A. at that moment. So I didn't want to burn up the first transistor in the I.P.A.

So real quickly, I knew where to go. So in 2 or 3 seconds I'm applying about 10 or 20 watts and I'd run it down to 5 or 6 watts or so. Then turn the transmitter back on and it was either happy or closed to happy at that point, even with the reflected power.

So here's one idea that came to mind. Now this is a Rube Goldberg sort of solution, but it may be the most convenient way. Build up an A/B switch in the rack. You put it in the offline position, and it supplies power to the Nautel exciter, and at the same time it powers a coaxial switch in the exciters RJ-8 co-ax and switches it to a dummy load.

So you throw this switch, the exciter comes on independent of the transmitter, and puts the exciter into a dummy load. You can do all the checking and setting you want at that point. You could even have such a switch remote controllable with the dial-up remote control, or with an IP based remote control. So you throw the switch, go to another preset... I mean, I made different presets in the Nautel exciter for lower power levels.

But after I left the site it occurred to me, "Wait, our onsite guy won't have a chance to go to those lower power levels, because the exciter has to be on to do that, and it won't be on unless the transmitter's on supplying AC power to the exciter, so there's precious little window of opportunity to do anything in terms of time.

There is one other possibility that I just thought of. Right on the display of the Nautel exciter it tells you how to turn the exciter off. The lawyers at Nautel really wanted to make sure that the exciter could be turned off easily, because there isn't an on/off switch. It's a key combination. It's like X box and the checkmark, or maybe it's X and Down.

I think it's X and Navigate down, you hold those together and bam... the exciter RF goes off, but the exciter stays on. So one could turn the transmitter on, as soon as it comes and the exciter is beginning to ramp up, hit that key combination to turn the exciter off. I think you can do that in two or three seconds.

Now the transmitter's on, the plate voltage is on, but there's no RF coming out of the exciter. Then you go to the menu and you go to Preset Number 2, or Preset Number 3, which are lower power levels, and then you turn the RF back on, on exciter. Then it could be on at a lower power level, hopefully not tripping the VSWR alarm.

Chris: Does the exciter that you, the Nautel, have any kind of remote access its inner workings to do a fold back circuit? The VS300, is that part of a larger frame amplifier? Do it have any feedback loop capability that you could create a voltage, say, from a [inaudible 00:19:43] through line, you know, reflected and scaling voltage for what it's [inaudible 00:19:47]

Kirk: Yeah, It's got some GPIO remote control, so it's got some pin remote control.

Chris: Right

Kirk: And I think it's pretty definable. Some of them are fixed definition and I think some you can assign them. So probably I could take a VSWR signal from the transmitter and hit a button, but the problem is that as soon as the VSWR trips on the transmitter the power's off of the exciter anyway, and hitting a GPI on the exciter, it's too late at that point.

Chris: Well, you can take your thinking of the coax relay, rather than the coax relay being in the off position have the coax relay energized in normal operation. When it losses power from the P.A. the energized position goes to de-energized, which automatically puts the dummy mode across the output of the exciter. But the exciter still has power coming from a different source. At that point your GPIO, or whatever other device, can be used to alter the power RF, and then you can use an IP controlled power strip of some sort to turn back on the P.A. after you've determined that your exciter is at the level you need because of the weather conditions.

Kirk: By the way, in the chat room Arnie Karl has made few things,
"Climb the tower with a hair dryer, easy."

Chris: A very popular technique that's used a lot by antenna manufacturers for installations in the northeast has always been, the antenna's slightly detuned during normal weather conditions, nice sunny day. Then as the icing condition occurs the tuning of the antenna moves more toward center, so the P.A. stays within the window of tuning. Or, you can broaden the tuning on the P.A. a little bit to accept that change in the antenna, the noise might go up a little bit.

That I remember from years and years ago working with a couple of Continental sites and ERI antennas were designed that way. You did some measurements and said, "Wow. Our antenna is mistuned." I called up Tom at ERI and he's like, "Yeah, that's intentional and here's why." And it worked. I have to say we had icing conditions where you could see a half inch of ice on those rotatillers and the transmitter didn't like it, but it stayed up because it sort of went into the tuned frequency rather than way out of tune like it normally does.

Kirk: Yeah, it had farther to move before it caused a problem, because it was starting out at the other end of being detuned.

Chris: No.

Kirk: Not badly tuned, but I mean, like you said, under normal sunny days, no ice on the tower, it's actually slightly detuned in the opposite direction, that ice tends to detune it.

Chris: Right, right.

Kirk: If you start from the middle of being perfectly tuned, let's say a quarter inch of ice or a half inch of ice will detune it enough to cause problem, but if you start off slightly detuned the other direction, and the specs are still fine, they're as good as any other manufacturer, then as the ice builds up you go through perfect tuning in the middle. Then you come out the other end and up to a certain amount of ice, it's okay.

Chris: [inaudible 00:22:57] really. Well then I guess the issue is your power amplifier if very narrow banded, I guess. It doesn't like that.

Kirk: Well actually it's the antenna that's pretty narrow banded. If it is and ERI antenna it's a ring stub design, or it may be one of those helical designs. It's not a rotatiller. I wish it was, I like those, but it's an ERI [inaudible 00:23:20] design.

Chris: Well Shively makes some nice ones also. The 6800 Series is a very nice antenna. These also, I believe, do the tuning slightly off.

Jampro I'm not sure of. I've used their antennas, but I've never had a situation in ice conditions to see how they operate.

Yeah, not knowing the antenna design and a few other things, I'm not sure what else to offer up. But, a feedback loop through, say, a through line detector would probably be a good start, and use that voltage to try and tell the VS300, "Hey, I'm experiencing something, start throttling back."
If the GPIOs are programmable that's even better, but I don't know about the VS300 without reading them manual to give you a better answer.

Kirk: There's plenty of ideas here in the chat room and most of them involve some money. This is in Indianola, Mississippi, if we can trade some catfish for the solution then we'll have a solution. But, money cannot be used to solve the problem or at least not very much of it.

Wayne Co says, "Install a radome." For those of you who may not know, a radome is a fiberglass lid, tent, covering over the antenna, so that when ice does gather on the radome, instead of the antenna, that ice is several inches away from the antenna elements themselves and therefore they don't couple as much, so ice doesn't cause as much of a problem.

Also, ice may have more trouble sticking to the radome than it does sticking to the antenna itself. So that's what a radome does and that's certainly a possibility, but we'd have to buy radomes and get a tower crew. It's on a wimpy little AM tower, so radome would certainly increase the wind loading.

Somebody says, "Fake out the VSWR detection." That's a possibility. If not fake it out, at least loosen it up a little bit. I wasn't there, I don't know at what VSWR, how many watts reflected does it trip. I just know that they some ice a few weeks ago and it was enough to make it trip, so the transmitter was essentially off the air, because I didn't have a way to tell the general manager how to reduce the power, because there wasn't a convenient way to do it.

Chris: Well, you know what you could also consider. Give Armstrong a call, and talk to the guys and say, "Look, here's my situation. I'm a financially burdened operation, we're trying to maintain our week to week stuff. What's the tolerance of that transmitter's output that can handle the reflective power?"

They should be able to tell you, "Well, we normally set them for X; however, their design is such that you could probably go a couple of more watts in reflected and still operate without chaos ensuing." May be that's an approach to take. Then you just readjust the VSWR trip setting.

I'm just thinking out loud, given the other situation you have to worry about regarding the cost.

Kirk: This is Mississippi, they get ice every few years. Sure, they had an ice storm back 20 years that was just devastating, but usually we get some ice, it's not a lot, it may be enough to detune this small antenna. And you're exactly right, if Mike Patton followed the manufacturers advice about setting the VSWR trip point, it may be set at a fairly conservative setting, something that maybe even the lawyers just said to do, or some other spec said to do.

Maybe it trips at 50 watts reflected, and look, it'll operate all day long with 200 watts reflected. Maybe that's the case. So that may be the best thing to do, just loosen up that. But if I do that I'm going to talk to Armstrong first and see what they think would be a [inaudible 00:26:55].

Chris: Well, yeah, because what they're going to tell you most likely is, "Yeah, you can get away with X number of watts reflected; however, that means more heat out of the transmitter, so if you can make sure you can exhaust it better, or you do that. You're voltages, the power supply might not be able to supply enough current." Maybe the current rises when it starts to get in the mode.

These are things you have to watch out for that a lot of times we take for granted, because a lot of system designs we do we overdo it, so it doesn't happen. But, because you're in a part of the country that typically doesn't have this type of extreme weather, odds are when the antenna was designed and put together, they looked at the part of the country it was going to and said, "Oh yeah, you'll be fine." If this antenna was located in Vermont and with the transmitter set up you probably wouldn't be even discussing it right now.

Kirk: Just to answer a question in the chat room, I think it's a four element antenna, so it has a gain of approximately 2.2 minus the power that's lost in the 1 5/8 inch coax going up the tower. It's not that tall, it's just under 200 feet tall on the tower, there's not a lot of coax. It's kind of a run out there, so I'm going to guess there's probably 280 feet of coax involved, 1 5/8 inch, so there's a bit of loss.

Anyway, the TPO from the, I want to say this is a 3000 watt Class A, not a 6000 watt Class A, I think. So anyway, the transmitter's TPO, I don't know, maybe it's 6000 watt, the transmitter's TPO is right about 2000 watts. So it may be a 3000 watt Class A.`

Here's an idea, so Super Sat in the chat room says, "Why not power the exciter differently, separately, from the way it's getting it now, because that kind of is the problem, and somehow rig up the AC power output of the transmitter to activate the exciter through a closure?"

Chris: Yeah.

Kirk: That may be the best idea right there. In other words, allow a switch to turn the exciter on and off separately, but normally have a relay, a switch, or something so that the transmitter still controls the AC power like it does not. That works just fine and I kind of like that. It's fool proof. Nobody has to be there to hit any combination of buttons in the right sequence or something, but if I want to turn it on separately I can.

The only problem is that, and I would want to try to make sure that the exciter could not be powered up unless at least the filaments are on, so that the exciter itself doesn't get too much reflected power from the transmitter. Remember, the output of the exciter directly feeds the circuit that feeds the cathode filament of the tube.

Chris: You want the filaments warmed up before you do anything.

Kirk: Okay, otherwise the impedance there is going to be screwed.

Chris: Oh yeah.

Kirk: So the exciter itself may fold back very nicely. I'm sure the Nautel has a very sweet proportional fold back. It's not an on or off situation it's, "Okay, that's reflected power, I'm going to back off until I get 20 watts back or less, or whatever it may be."

So, yeah, if I could just power the exciter separately. Remember, there's nobody technical there at all. They may accidentally, I've I allow them to power the exciter without powering the transmitter at all, they're likely to turn exciter on and say, "Oh, it keeps folding back. It's not right, there's something wrong. There's red lights all over it." That's what I have to avoid.

Chris: Yeah, you're essentially bypassing the interlock.

Kirk: Yeah, bypassing the interlock and feeding it into a load that, it may not like it, or the exciter may not like the load, and the load itself may burn off over time. I don't think I'm going to burn up the cathode of the tube, per se, with 300 watts, probably normally it has several hundred watts of AC voltage going through it. There is no proportional analog voltage.

Somebody mentioned, "Well, put a directional coupler in there and then use the signal from the directional coupler. You can amplify it and then use that as a fold back." But, then I have to buy a directional coupler and that's hundreds and hundreds of dollars to buy a directional coupler.

Chris: Yeah, and what you need to do is come with a solution with the tools at hand, basically like they did with Apollo 13, "Here's what you've got."

Kirk: "Here's what you've got, solve the problem." Yes.

Chris: Here's a rectangular box and it needs to go into a circular cylinder, okay? But the people is, the people only supplies for a rectangular box and they have a circular cylinder. What do we do? So that's basically your situation.

Kirk: Yeah. Wayne Co suggests again, "Make a larger VSWR window on the transmitter." That would probably make the problem never happen again.

Chris: Yeah, I think that's the approach you should consider. That's a simple one.

Kirk: Yeah, and that approach involves no money, just a matter of turning a screw a few times to increase the VSWR trip point, but I would only do so with advice.

Chris: You know what? Not knowing the manufacture's history, some manufactures of transmitters were very notorious for being conservative in their trip points. Over time everybody learned to say, "Oh yeah, you've got a BEX, or you've got a Harris this, or a Gates so and so. You know what?
Here's what you've got to do to adjust it, because it's finicky these impedances. Yes, it's typical. It doesn't mean it's a problem, they just were very, very conservative."

It's very possible to call up Armstrong, talk with Mike or whomever and say, "Hey, look, this is the situation." He may go, "Well, I'll tell you what, try this, this, and this, because these guys tend to be a little bit on the conservative side." Then problem solved, you may be in great shape.

Kirk: Someone asked if heaters are available for the FM array. No, they're not. The antenna was not purchased with heaters. In fact, Chris, let's talk about heaters for a moment.

I know we're just getting out of the winter season. You guys got some snow there in New York, didn't you.

Chris: Oh, yeah.

Kirk: Did you?

Chris: We had icing. Next time, and if I can remember next week and if it doesn't take too much time, I have pictures from a rooftop of a skyscraper in Manhattan from a month ago when ice was forming on antenna elements, and I kid you not when I say this was about half an inch of ice and icicles hanging from these antennas. So, yes, we did get snow. We do have conditions that are not favorable for properly tuned radiators.

Kirk: So you're saying that in New York there are probably some stations that have been folded back on reduced power level with all this ice?

Chris: Absolutely. Absolutely yes. Antenna heaters work well, but they can be very annoying over the life of their use.

Kirk: My experience is they are not low maintenance.

Chris: No.

Kirk: And whenever you have to do maintenance several hundred feet in the air, it gets real expensive, real fast.

Chris: Yeah. The biggest problem is it's a heating element. The best analogy I can use, those of you have looked inside a fish tank, that require fish to have a certain temperature of the water, it's that glass test tube looking thing with a wire coil with a porcelain center, that's basically a heater. That's essentially what's inside your antenna for an FM station.

If you can picture that being on 24/7 in an environment that's constantly changing, so the antenna sits on top of a tower. Depending on the time of day the sun's beating down on it, quickly cooling off in the evening, the expansion and contraction of the elements. Over time those heating components start to fail.

The trick used by a lot of folks, rather than run them at regular full power, if it's a, say, 120 volt heater or a 208 heater, you ran them at half voltage. So that they were just sort of lukewarm 365 days a year. It sort of extended their life.

But, again, depending on where you are in the country, if you have extreme weather conditions there's nothing that saves you from those things failing. It's just, they are high-maintenance.

Kirk: Okay, well we'll try the easy way first. The next time I'm there we'll try the easy way first.

Of course, I've never been comfortable with the notion that I can't turn the exciter on without having the transmitter on. I have a dummy load that I bring with me, it's part of my standard fare when I drive down there. I can manually take a new jumper and go from the exciter's output into a dummy load. Then I can go plug the exciter up somewhere else, and turn it on to my heart's content.

But that's me there. The general manager that there has no idea what to do with the dummy load, and doesn't know what to do with patching a different piece of co-ax in.

Chris: Here's a question for you, does the VS300 have an external RF mute capability with a simple closer across the...

Kirk: That's a great idea, yeah.

Chris: ...and when it's in the mute mode, I guess you can still change the presets.

Kirk: Yeah, I think so.

Chris: So what if you have the exciter on a different power source and then when the transmitter trips off you have a relay that's always energized, so it's a positive on relay. It's like a failsafe, like an interlock. Basically, the transmitter is energized, everything's happy, hunky dory, everything is going along, suddenly the VSWR trip occurs, the transmitter shuts itself off.

That relay which was energized is now de-energized, breaks the circuit that is the RF mute, and now the exciter is muted, but stays on and you can walk up to it and go, okay, Present 12, or 3, here we go. Then have a reset on the transmitter, you can just power it back up.

Kirk: This station, it's an FM, is 24/7. It never goes off the air that we want it to. It would be smarter to power the exciter from a normal rack strip of power, not get its power from the transmitter, only when the plates are on. But use the plate interlock, use that same relay a connection that right now is providing that presence or absence of AC power to simply provide the presence or absence of a closer and hit the mute with GPI, with that on the VS300.

That way the VS300, it's electronics are on all the time. It's unmated whenever the plates are on on the transmitter. So when it's on, but muted, yeah, you can change different presets, that's the way to go.

Chris: Yeah, it was a design that Continental used on a couple of their early 10,000 watt transmitters, and stuff. People used to use it when there was an issue with the P.A. You'd mute the exciter, you still have control of the exciter, so you can go into a relay box and put the RF somewhere else. But, yeah, that would be the way to do it.

Also consider, if you go that approach, if you can, somewhere in the transmitter room or wherever you may have spare parts, you might want to come up with a bypass panel for the rack, so you don't have the transmitter cycling on and off the exciter because the VSWR is just straddling that trip point. You don't want to have up and down, up.

Kirk: Yeah, okay.

Chris: You might maybe even have instructions for the operators, hey, if these are the conditions that exist, hit this bypass, select Preset Number 2, reset bypass. Then you should come up and be okay at lower power. How's that.

Kirk: Good deal. I think we've got this solved. I'm going to use the muting. I'm going check to make sure there's muting, there should be.

Before we run to a commercial we should this notion of muting. I first became familiar with it in the context of AM radio stations, an AM station that has a directional array that is several towers, and it switches patterns.

So you're on the air with maybe one tower for daytime and 10,000 watts, but at night time you go to a three-tower pattern at maybe 5000 watts, so when you switch all these big relays from one pattern to another, you don't really want to do that hot. You don't want that with 10,000 watts coming out of the transmitter. You'd get all this arcing going on when the relays switch.

So what do you do? When you push the button to change the pattern the very first thing that happens is, a relay closes and its mutes the transmitter, assuming you're using the same transmitter for both. Sometimes if you shut a transmitter off, it takes it a second to quit putting out power, because it's got lots of juice still in the capacitors in the power supply. So you want to mute the transmitter, which instantly cuts off the RF. It may still have power in the power amplifier circuit, but it doesn't have any throughput power.

So you mute that, so now there's no energy on the big RF relays. You switch them and then you let go the mute, and the transmitter comes back on, perhaps at a different power level, perhaps even a different transmitter if you're switching transmitters. But the point there is, I learned about that with working with AM directionals.

Did you find you find that muting was used with pattern switching in your area?

Chris: Oh yeah. Yeah, my first job in broadcasting was an AM station, three-tower dogleg, the shape of an L, basically. Yep, we had a five kilowatt. Once in a while the RF relays would not properly mute and switch like they should, and boy, let me tell you, the silver plated contacts would splatter all over the insides of the cabinet. But, yeah, the muting was key.

Tube transmitters were great for muting, because they would be, like you say, slow discharge, so you would have to mute the transmitter, time it for, I think, about five seconds before you did a switch, that was enough time for everything to de-energize, switch, and then come back up. Solid state transmitters are great, because they can just mute, like an on/off switch. It's just boop. You know, watching a 50,000 watt solid state transmitter mute while switching patterns is like, oh boy, here we go, here we go. Oh, that went very nicely.

But then again, I have seen the 50,000 watt transmitter relay switches go and then one of them not properly seat and explode, so that wasn't funny.

Kirk: It is amazing that a muting control on a 50,000 kilowatt AM transmitter, you just close this little closure, it's just boop, and 50,000 watts in just a couple of cycles it's gone. Then you un-mute it and boop it comes right back. 50,000 watts, just like that.

Chris: Yeah, you're talking 25 amps of RF at the base of the tower, that's a lot. That's arc welding material.

Kirk: With FM transmitters I'm guessing, you tell me if I'm wrong, but I'm guessing that muting was put in as a feature on FM exciters, hence the rest of the transmitter, for the purpose of, let's say you had two FM transmitters that were combined. They're running exactly synchronously.

Let's say you need a 40 kilowatt transmitter power output to the antenna and you have two 20 kilowatt transmitters feeding through a 3dB hybrid combiner and you want to switch. Let's say one transmitter fails, you want to switch and put the other transmitter straight on the air. Before you do that RF coaxial switch with FM you want to mute that transmitter, make the switch, and then un-mute the transmitter.

Is that why they would put that in an FM exciter?

Chris: The FM exciter's had that, because you want to turn off the transmitter when you're switching between, say, your A and B transmitter's on a single antenna. Or, if you're in combined output, yes, you can do that.

I think it was put in there more for a safety and design, because if you have a traditional FM site where you have two transmitters and a single antenna you had to switch between the two. Now, you didn't want to switch between the two transmitters hot, so you have to mute one to make sure that one stayed muted while the other one came on.

They also used a mute switch, I believe, for, you know, you have a dummy load, so you make sure the fans that cool the dummy load are operating before the transmitter turns on and feeds the dummy load. So that's another failsafe.

Kirk: Ah, so you use external interlocks for things like the dummy load. You want to make sure the fan, the blower is running through that dummy load before you apply 10,000 watts to it

Chris: Right. The muting function is a byproduct of the interlock system of the transmitters.

Kirk: I'm so glad we had this conversation, because I wouldn't have thought of the muting function. Very few of the sites that I've ever taken care of used the muting function. There may have been a couple where we had a main and a backup transmitter, but most of my engineering has been at small and medium stations where switching to a backup was rarely an automatic or a remote control function.

It was, you go to the transmitter site, you patch this manually, because nobody would pay for a $7,000 RF switch, so you go do it very manually, and doing it manually you shut things off anyway.

Chris: Well, that's it, exactly yeah. I for worked at both sites. The one you described where there is no co-ax switch and sites where there is co-ax switch, and then sites that have a co-ax switch, but didn't bother using the muting function at all. That made for interesting occasions.

Kirk: It burns up [inaudible 00:43:37].

Chris: Yeah, that was a fun phone call, "Who are you?", "We don't have an engineer, but we have a problem. We're not sure what it is, but something's buzzing and burning in the transmitter building." I'm like, "I hope you vacated the building." They're like, "Yeah, yeah, yeah, everything's turned off, but we're not sure what to do."

It was a co-ax switch that didn't have a muting relay connected and they switched between their 10,000 watt FM, from the main to the backup, and never turned off the mains. So the switch turned, it just spins, breaks the circuit to the antenna, a 10,000 watt arc inside the co-ax switch, and now it was just a pile of melted metal between all the contacts.

It was an interesting smell. When we popped the cover on that switch there was some metal that oozed out of the shaft where it spins, where you have the handle where you can do a manual control.

Kirk: Oh, yeah.

Chris: Yeah.

RF is a funny thing. The laws of physics you can play with and twist, and if you try to break the rules they do sometimes come back at you in full force.

Kirk: You're watching This Week in Radio Tech, or listening to it. This is Episode Number 204. I'm Kirk Harnack along with Chris Tobin. We've morphed this show right into a good conversation about RF muting. I never thought that would be the subject of a show, but it turns out it's pretty important. We think we've solved the problem that I have at one of my radio stations and now we're talking about the whole concept.

Chris, after the break I want you to describe for us the muting and safety procedures, and VSWR protection procedures, when you have a multiple transmitter site with a combiner. So maybe at the top of the Empire State you can tell us about how that works; somebody in the chat room brought that question up.

We've been talking primarily about single stations, one transmitter into one antenna. That's what my stations are. But you've worked at places where things can go wrong in various places when you have 12 transmitters, or 8 transmitters, FMs I'm talking about here, being combined into one master antenna system. Where do you put the protections in order to protect what has to be protected, but not taking people off the air unnecessarily? So we'll get to that in just a minute.

Our show is brought to you by my friends at Omnia Audio. I want to mention to you a really popular product, Omnia A/XE. What is it? It is Windows software, it runs on XP or Windows 7. I believe it runs on Windows 8, as well. This is probably the least expensive way to do high quality streaming on the Internet.

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This is Episode 204 of This Week in Radio Tech. Chris Tobin, our usual co-
host, is here with me and we're talking about, we got on the subject of solving my problem in Mississippi with a little transmitter, when there's ice on the antenna. We think we figured out a way to allow it to get fixed.

Now we're going to move on to the subject of multi-transmitter sites, like they have at Empire State Building in New York or many sites, in big cities especially, across the country where you have 2, 3, 4, even 12 FM transmitters operating at reasonably high powers through a combiner, which is a big, honking device, it takes up a room, and it combines all this power from the different transmitters.

Each transmitter, by the way, thinks it's all alone. That's the key here. A combiner lets a transmitter operate power into it without reflecting any back, of its own or of the other stations. You can't just twist tie the outputs of transmitters together and send them to the antenna. That doesn't work, so you use a very specially designed combiner. Then you send all that power up to the antenna.

Now, Chris, tell us about how people protect themselves in those situations.

Chris: Okay. Well, at the Empire State Building, for an example, because it's a combined system of about 16 FM stations. You can image the amount of power that is generated by 16 50-Kilowatt E.R.P. stations, so that's about over 100,000 watts of RF, actually a little more than that.

In order to prevent trouble: first things first, at the Empire State Building you're required to understand something about RF safety, so this is one of things you'll be reading about, RF frequency site safety awareness training, and whatnot. This is one of many companies that offers these things. As a tenant you have to be aware of the safety involved.

In the case of master antenna, master FM, what you would do is, the procedures would be, all the stations agree to a shutdown. Say I have to do work on my FM transmitter. My combiner, one of the filters in the combined system is in trouble. Maybe it's detuned, maybe it's suffered an arc. Who knows?

However, whatever the case is, that needs to be pulled out of the system. Well, there are ways to bypass that filter and let everybody else continue on, but you don't do it live.

So what you do is you would schedule a shutdown. You can schedule a shutdown and switch to a backup antenna and it's a very simple, straight forward procedure. It's not done all the time, but when it's done it's done on the overnights.

But when you do such a procedure as a shutdown of a multi-combined system you have what's called the RF safety officer, or someone who's been designated the person who will do the following: verify that there is a person at each of the transmitter room themselves to shut off the transmitter. If a person is not available to be at the transmitter because the radio station may not have the personnel for that reason or the cost is involved, then the safety person has the ability to remotely shut off their transmitter using the interlock system. Then the safety person has to notify all of the operators.

Kirk: So the safety person may be at a central location, maybe in the combiner room itself, or right adjacent to it.

Chris: In the combiner room the RF safety person is watching.

Kirk: He can shut off people's transmitters from there for safety purposes, right?

Chris: Yes. Yes, the interlocking system goes through the combiner room, and then there's a monitoring point. You have RF watt meter panels that indicate power from all of the transmitters coming into the combiner system. There's also what they call the "RF plunger."

So if you can think of a coffee can cylinder sized device with a handle on it, and a rod that goes into the center. What that does is it shorts out your feed from the transmitter into the combiner system in the event that the interlock system fails. So you have all these little plungers...

Kirk: Oh, I saw those. That's what they do? They actually short out somebody's feed.

Chris: Right.

Kirk: And hopefully the transmitter shuts off at this point and doesn't become an arc welder.

Chris: Well yeah, it's designed to basically failsafe. You keep everything safe.

Kirk: [inaudible 00:53:46]

Chris: Then there's the procedure that every station calls up to the combiner room safety person and says, "I'm here. I'm powered off." The safety person confirms the meter readings and says, "Yes, you are." It stays off, lock them out, they turn off the interlock feature on that transmission line all the way through.

Once everything has been deemed powered down, RF safety goes through one more check and says, "Okay, fine." Then tells the antenna manufacturer or the vendor, whoever's on site to do the repairs, "You now may proceed with the maintenance that you're going to do."

Then you just reverse the order. That's in the simplest of nut shells. It basically comes down to, a combined site requires cooperation amongst all the tenants on the antenna system.

Kirk: I have a question here. You seem to be speaking kind of specifically about Empire State, because I take it at Empire State, New York these transmitter rooms are scattered about a couple or three different floors.

Chris: That's correct.

Kirk: And the people can't see each other.

Chris: That's correct.

Kirk: I have been to combined sites, like in Miami, where you might have several transmitters, and they may be in wire cages from each other, so people's property is secure. But you can physically see all the transmitters that are going into the combiner.

Chris: That's correct. So, you still use the same procedure, you may modify it. Say you have a room full of transmitters. Okay, you're a combined site, one big flat building, it's a single level. You've got 16 transmitters in a room and they're all sectioned off, by say, fences, or cages, whatever you want to call it.

You still should have someone physically verify, visually verify, that that transmitter that's part of the system is powered off, because you remember, the local transmitter settings could have been altered, somebody could have bypassed something. You don't know, so you can look at the transmitter and say, "Oh yeah, everything is off." Meanwhile there's RF coming out the spigot and going into the combined hybrid panel.

So you still use the same procedure, whether you're on top of The Empire State Building, where you can't see everybody, or if you're in a room that has everyone in there. You still do the safety.

It's like, have you ever watched movies with submarines or like an aircraft carrier. If you ever notice when a command is given to, say, change the direction of the boat and they say, "Ten degrees down bubble." For a submarine to go down, how many times do you hear that repeated?

Kirk: A lot.

Chris: And why is that? That's because you have to make sure that once you make that command the person who's going to execute it is confirming,
"Yes, this is what we want to do." Then they respond back with, "Ten degrees down bubble." This person who said, "Do it," now gets confirmation back that it's been done. He knows what to expect. He moves on the next thing he's executing.

The same is true at RF safety. You climb a tower. You send somebody up to the top of tower and you confirm they're there. They've confirmed that the power's been turned off already, so they ascend to the level that they need to work at. When they're ready to come down, they contact down below and say, "We're now descending." Then the guy at the ground level waits until he hears, "All clear," then tells the transmitter people, "Okay, you can power back up."

That's the same principal. So whether you're on top of a building, you're on a single level house at a transmitter tower, the same procedure just modify it a little bit. The idea is not to assume anything. When you assume we all know what happens, and in RF it can hurt, so it's a little different.

Kirk: [inaudible 00:57:03] yeah, okay.

Chris: But that's really what it comes down to, it's just, understand procedures. Say, "Hey, we're going to power down these 16 transmitters, let's confirm."

It's like, well here's another example again for confirmation, authentication, if you will. Have you ever watched a space shuttle launch?
Space shuttle operations, did you ever notice that the one guy in charge, what they call CapCom, Capsule Communications, he's says, "Flight," they say, "Flight, go, Health or Bio, go." What they're saying is, "We followed procedures, everything is in order, you now can proceed to the next level for what you need to execute."

So I'm the RF safety guy, I'm going to call up on the intercom and say,
"Transmitter Room 2?" "We're off." "Okay, Transmitter Room 3, 4, 5." Then I go, I look at my panel, I've got all zero indicators, "Great," I proceed to the next level. What is that? I activate the interlock bypass and boom, we go to work.

But the steps along the way can also make sure that, say, Transmitter Room 6, or Transmitter Cage 7 suddenly the guy says, "Oh my gosh, my transmitter just came back on." Because we're taking these executed steps, he can quickly jump on the intercom, or whatever method you have of communicating, and say, "Stop." Or, "Something is hot." And right away I, as the person orchestrating this knows to stop everything I'm doing and immediately move to the next procedure for emergency shutdown or whatever may be the case.

That's in a nutshell. It's gets more complicated, but that's in a nutshell.

Kirk: Sure. I've got two questions and then we're going to have to close out the show, we're getting tight on time.

Question number one, I take it that, at a place like Empire State Building or other sites where you've got combined transmitters, each transmitter, of course, has its own through line watt meter to see how much forward and how much reflected power. Is there another through line watt meter just as the RF power enters the combiner? Or, do we assume that that's good based on the transmitter's reading?

Chris: Oh, no. No, you have monitoring coming in and going out, so you have monitoring coming in from each of the transmitters, then you have monitoring going out of the combined system. So you can see the aggregate.

Kirk: See, now that's my next question, so out of the combined system you've got a huge amount of power, all of the combined power. If that reflected power there gets too high will that shut everybody off?

Chris: Yes.

Kirk: What happens if the combined power becomes too high?

Chris: It will shut down the system.

Kirk: Everybody?

Chris: Yeah.

Kirk: Okay. So the next question is, if there is some reflected power from the master antenna, but not enough to shut the system off, how much of that reflected power or that mistuning of the antenna, which causes reflected power, how much of that gets reflected through the combining system back to each individual's transmitter?

Chris: Probably very little, because the filtering process is the buffer between the devices, the impedance changes. So your transmitter sees the impedance of the filtering system and that's pretty much what it's tuning to, hopefully.

Kirk: Oh, okay.

Chris: It's like tuning a duplex for a two-way radio, or a HAM repeat, hopefully the antenna's reflected power is not enough to override that, then that creates a whole other series of issues. But, yes, there's always going to be a little bit of reflective.

Kirk: It sounds like there's a little bit of coupling there, but the combining system really does a pretty good job of isolating, not only transmitter s from each other, We know that, because one transmitter can't see the output of another transmitter. That would be bad. But it also buffers a transmitter, what it's seeing from the end of the circuit, the antenna.

Chris: Yeah, because by design the filtering technique, that's inherent in the way it's designed. You'll always have some type of reflection or something coming back, because there's just physics involved. Can you ever have zero VSWR? No, that's why you have a tolerance of 1.1, 1.3, 1.5. Sometimes a lot of people will tolerate 1.15 to 1.2 and that's it. But it's a balancing act and you just build accordingly and you maintain it.

The trick is, when you're doing combined systems you really do need to pay attention to proper preventive maintenance and understanding your limitations, because that's just a balancing act. I could tell you from experience over the years, it happened in New York many, many, many years ago, when one of the combiners didn't properly function, it blew out. It took everybody out and it was just a mess. It can happen, so if you don't stay on top of it you're in trouble.

Kirk: We're going to put a couple of websites in the show notes. It's been pointed out that Minneapolis Saint Paul has a big combiner system. We'll put a link from Scott Fibush who's a tower side of the week to that. I believe we also have some links to Empire State Building, the place we've been talking about in the last part of the show here. So we'll have a picture of that so you can visualize kind of what we're talking about.

We've got to go. Chris, this has been fun. I appreciate your participation and for helping me solve my problem.

Chris: You're welcome, any time.

Kirk: Our show's been brought to you by my friends at Omnia Audio, and the Omnia A/XE software-based, it runs on Windows, audio processor and stream encoder, and also Omnia F/XE the file-based audio processor.

Check those out on the web at Omniaaudio.com

Next week our guest it Rich Rarey from NPR labs. If you have ever listened to a show from National Public Radio there's a fair chance that you have heard Rich Rarey's name, especially on Fridays during the credits at the end of "All Things Considered." I used to hear Rich Rarey's name quite a bit there.

So Rich is a real gentleman, a great engineer. He's going to be with us next week on This Week in Radio Tech.

We're looking forward to the N.A.B. show coming up in a few weeks. Our show on Thursday during NAB, what is that date? April the third or tenth? Tenth, April the tenth, we're going to be live from the show floor at NAB for This Week in Radio Tech, so I'm looking forward to that, too. And more guests between then and now, so I hope you stay tuned.

Be sure you do check out the other shows on the GFQ Network. We're hosted on the GFQ by Andrew Zarian and his team, and there are plenty of other shows as well, like What the Tech, Mat Men, and just lots of other good stuff, so check it out on the GFQ Network. GFQlive.tv it's a good place to keep your browser tuned to.

All right, guys, that's it. We'll see you next week on This Week in Radio Tech.

Bye-bye, everybody

Topics: Radio Transmitters


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