Last time my dad flew the plane the tower told him they got a call from Center and were instructed not to let us fly until we fixed our radio. Apparently when we transmit on the local ground frequency (121.7) they are hear us on 121.5.

I figured it was our antenna and the radio manufacture agreed that it was likely the cause but they said this radio has a very powerful transmitter so there will be some emissions on adjacent frequency’s but it is well within FCC limits. They even tested using the frequencies in question and said ‘emissions at 121.5 when tuned to 121.7Mhz are about -55dbm down relative to carrier.’ They also said if there is a receiver near by it will pick up the signal on an adjacent frequency. So I guess if Center has a receiver set up at my home field there isn’t much I can do?

I replaced the antenna but still have the issue. I’m going to call the local avionics shop on Monday but doubt they even have the equipment to diagnose the issue.

If your radio is legal the Center(or any ATC facility) cannot tell you not to fly.

Bonanza Man wrote:If your radio is legal the Center(or any ATC facility) cannot tell you not to fly.

Can you suggest a path forward? Have an avionics shop verify emissions are within FCC specs then tell ATC the radio is legal and meets spec? What if they are still picking me up on 121.5? I can see that being an issue since this radio puts out a true 12W and Center has a guard receiver on the field.

No advice only sympathy. I freaking detest radio issues, and we’ve had plenty of them with our Rans and Dynon comm. We’ve been through 3 different radios, I believe 4-5 antennas. The latest radio revision is better, they had some sort of internal grounding issue, but we still get garbled transmission at full engine power on the 118.XX tower frequency. It’s fine on the 122.XX and above. Oh yeah and other planes in the air can hear us fine on tower but not the tower!!!! Of course their shit is perfect if you ask them and not their fault (works fine on ground though[emoji849]). I now carry my portable ICOM ready to plug in the headset in a second when I go to town. Of course no radio repairman around so it’s all hunt and peck.

Can you reduce your microphone gain so you don’t overmodulate?

Thanks for the empathy GB. I’ve Brent trying to learn this radio stuff but I’m at the point I just want this thing working.

Chris, I can adjust the mic gain and was just wondering it that would make a difference. My DC headset works great but my dads Zulus have a touch of feedback so I guess we’ll have to adjust the gain depending on which headphones are being used.

Is it possible to explain in simple terms what magic gain has to do with modulation and sideband splatter?

The carrier can be clearly heard on 121.5 and we didn’t have any headsets plugged in when working with it yesterday. Idk if that means anything.

True splatter is a case of over modulation of your radio. During cycles where the modulation amplitude exceeds 100%, the carrier frequency is effectively dithered (splattered) across a much larger bandwidth. The reasons are easy to understand if you read about "am overmodulation". Sidebands are an inescapable side effect of AM, but the channel spacing is such that they are very weak by design. There are ways to suppress sidebands virtually entirely, but they almost require special receiver gear in addition to the obvious transmitter differences. Splatter happens in a fashion that is highly distorted and the sidebands are often unintelligible and akin to fingernails on the chalkboard.

The solution to the problem of excess sideband splatter is simple and straightforward: reduce the percent modulation for the transmitter.

In each of the six radio models I have ever worked on, it has been a single screwdriver adjustment inside the radio. In fact, I did not have a manual for all but two of them...I hooked up a balun to the output of the radio, attached an old scope (at least 60 MHz, but 100+ MHz is way, way better, and I now use an inexpensive 500mbps USB scope), trigger to the audio (108+ MHz is too fast to trigger with a cheap scope), and find the adjustment that adjusts the percent modulation. A quarter turn and back on each until you find the winner. Then you can either eyeball the modulation (70% to 80% sounds fine and won't generally saturate the modulation). If it is low (less than 50%) you can move it higher as well and it will sound way less "tinny". The adjustments often have "mod" printed nearby.

Use beeswax on the ones you messed with and test out.

If you are interfering with other equipment the FCC can and will tell you to stop using it. If you ignore them and continue using it anyway it will become very expensive. As others have mentioned, it is probably a matter of too much modulation causing the PA to splatter, any radio shop should be able to fix this quick and easy. Rather than randomly playing with settings and hope for the best, have the radio shop set it up with a known audio signal and set the modulation correctly, then go thru all of the headsets you use and set the mic gain on them (if they have a mic gain) to match the radio. If you are using an intercom in the plane, have the headsets checked in the plane with the intercom on, there may be a difference, or the intercom may have a gain adjustment. This really isn't a major operation, but it must be done by the book, especially if the FCC/FAA is already looking at you. When it is all done the radio will sound better to everyone else, and you will hear a difference in the headset sidetone when you talk. The fact that it is a 12 watt radio really isn't a factor.

Whee, you can try to fix this yourself and you might eventually succeed, but it sounds like the tower has already had enough of your radio, and without diagnostic equipment you'll really just be guessing whether you fixed it or not... you'll just be waiting for the next nastygram from the tower and possible involvement from the FCC, as well as violating the radio warranty, if it has one.

Radio shops cost what they cost for exactly one reason: they're worth it. Just bite the bullet and take it to a radio shop and get a quote for a guaranteed fix. It'll be a LOT cheaper than the cheapest fine that the FCC will issue, and it's really the responsible thing to do. Wonky radio's are a pain in the ass to EVERYONE, not just the tower.

Bummer of a problem, but like most things in aviation it's nothing money won't fix, so consider it a lucky problem.

good luck

Whee, which radio are you using?

I made a couple of small adjustments on the potentiometers on my radio (Icom A200) - with a noisy cockpit my audio was clipping and distorting and tower would complain. I turned my radio’s mic gain down and then adjusted the radio side tone back to a level that was comfortable in my ears. Did it on the ground then went for a test flight and it was good to go. Easy peasy.

Not a radio expert but could the side band spillage be due to too high of a mic gain with all those Watts you’re pushing?

Thanks for the thoughts/ideas guys. I find this stuff fascinating but I know so very little about it. Following a install manual is easy but when radios don’t work like they should it’s hunt and peck for me which is horribly inefficient.

This particular radio has digital modulation control so I don’t think there is a screw to adjust it and the local avionics shop can’t do anything to change it. From the manual: “Modulation is digitally controlled to achieve a modulation index of 70%, hard limited to a maximum of 80%. Modulation depth is regulated independently for positive and negative waves.

Undesired our of channel TX products : <-60db referred to unmodulated carrier at maximum power”

Mic gain is adjustable via the setup menu and there is a procedure for doing so in the most recent version of the installation manual.

I don’t think the local avionics shop can fix the issue but they might be able to diagnose if there is actually anything wrong. I’ll call them tomorrow.

I’m not sure what radio you’re using but maybe have them send you another one. If you call, at least in my experience, they talk to you to like you know what you’re doing. Might as well be talking about thermodynamics or differential equations with a non-engineer. Makes just about as much sense to me.

CamTom12 wrote:Whee, which radio are you using?

...

Not a radio expert but could the side band spillage be due to too high of a mic gain with all those Watts you’re pushing?

It’s a MGL V16. I really like the radio and it works great. Tower says I’m crystal clear. I’ve talked to people a over a hundred miles away with some big hills in between. It really seems to be a good radio I just need to figure out if there is actually an issue here and if there is how to fix it.

I haven’t adjusted the mic gain and have no idea if it will make any difference. I’ll try it tomorrow.

It's not likely, but quite possible the FAA technician misconfigured the RCO receive frequency range. Also possible they have a bad radio. If so, it would receive too wide of a range and repeat on 121.500

Reach out to Rainier and I'm sure he can help. He browses the vaf forum so might be worth creeping over that way. I had nothing but issues with my MGL stuff and switched everything to garmin

Rainier is great and always really helpful.

I think we are back in business. We did a firmware update and adjusted the mic gain. I tuned a handheld radio .2mhz off frequency and could pickup a transmission from the airplane if I was within 50ft. MGL said this was possible but well within the specs defined by FCC. I’m the first case of such an issue but they are currently testing a firmware update that should completely remove this energy. To be extra cautious I’ve turned my output down to 5W and will leave it there till the new firmware is ready and loaded into my radio.

We put some MGL gauges in the S20. We found their tech support folks very helpful. Glad you’re getting a bead on it.

Or call Matt down in Torrance CA. He’s the US contact for MGL.

I would play with mic gain and test with a handheld near by.

If I get time I can try the same with my MGL and handheld. But I don’t remember having any issues.

What plane is this in?

Dumb question: Why do you need more than 5 watts x mit power, anyway?

NASA talked to the moon on less power.

MTV

Actually you don't need more than 5 watts, I've talked just fine over 200 miles on 1 watt plenty of times. The original idea of 5 watts for a transmitter is in an AM radio is your modulation has to be 1/2 the power of your transmitter, so if you had a 2 1/2 watt speaker and a 5 watt transmitter, you could use the same audio circuit for your transmit modulator and your receive audio driver, it saves money and in the time of tubes it meant less heat. 10 watt, 12 watt, and 15 watt transmitters are a simple matter of marketing. If someone will pay more for it, the companies will make it. Don't mean to ruffle anyone's feathers, that's just how it is.