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TSO Com?

I love seeing people use FAA publications to support their positions. That AC even goes so far as to say it is acceptable for IFR!


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So it has been over a month since I have heard from the FAA. You may recall they read me my rights, told me I could be violated for each and every flight, and contacted me roughly twice a week for a month.

I asked for a written explanation as to why this was a major alteration, and shared the above feelings about "must meet TSO" along with their very own definition of "meets" - and have heard nothing since.

We did get a report of interference in a TSO Garmin 300, so it isn't the lack of a TSO that is causing the extra noise. But now I am afraid to poke them - they may want to start scrutinizing other things. And I admit - a serious sleuth could find things I have done wrong - if he looked hard enough. I think I blew through a frozen red light last week, after carefully checking for cars with racks on top.
 
So it has been over a month since I have heard from the FAA. You may recall they read me my rights, told me I could be violated for each and every flight, and contacted me roughly twice a week for a month..

Yes, a lot of time has gone by and you still have not answered - " I have asked several times for you to confirm that you hear the interfering signal when the GTR 200 is tuned to 125.675 but. so far, have seen no confirmation of that."

If the mixing is not at the TX site the signal will not be heard on 125.675 and perhaps may be fixable with a simple 1/4 wave stub at your antenna connector.
 
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His point is well taken, although I think I would have phrased it differently. I get to the airport and am usually so full of joy that I forget little easy tasks like this.

Today I took the J4 up - it has the newest and noisiest 200 - and with the cooperation of the tower folk, spent 15 minutes on 125.675. Nada. Not a peep. Silent.

While that may not be definitive (I could have picked the exact 15 minutes where no simultaneous transmissions occurred), I note that twice on the climbout (125.7) and once on the descent I had approach control interference.

Only thing I can figure is that there is some non-linear anomaly somewhere. Not sure what you mean about a quarter wave stub, since my antennae are all 1/4 wave. But willing to try . . .
 
His point is well taken, although I think I would have phrased it differently. I get to the airport and am usually so full of joy that I forget little easy tasks like this.

Today I took the J4 up - it has the newest and noisiest 200 - and with the cooperation of the tower folk, spent 15 minutes on 125.675. Nada. Not a peep. Silent.

While that may not be definitive (I could have picked the exact 15 minutes where no simultaneous transmissions occurred), I note that twice on the climbout (125.7) and once on the descent I had approach control interference.

Only thing I can figure is that there is some non-linear anomaly somewhere. Not sure what you mean about a quarter wave stub, since my antennae are all 1/4 wave. But willing to try . . .

The phrasing of my post was inappropriate and I have edited it.

Gary's later post shows he understands what I have in mind. If the mixing product is not heard on 125.675 then there is a good chance it is generated in the receiver itself. A possible way to prevent that is to notch out the 257.875 MHz signal before it gets to the radio. If you tell me what connector is used at the base of your antenna (probably BNC or TNC) I can try making a stub and tuning it (I have the tools to do that). You would put this stub between the antenna and the antenna coax with a T fitting and see if the problem is fixed.

So - let me know the antenna connector type, confirm 257.875 MHz is the UHF ATC frequency, and I'll see what I can come up with. No guarantee it will work but it's worth a try.

Here is a reference for those unfamiliar with using stubs as filters -
http://www.arcticpeak.com/antennapages/quaterwavestub.htm (there are many others).

Edited to add - a photo of the antenna base and coax would also be useful so I know what space I have to work with.
 
Plenty of space. BNC - I think most antennae are Conant, but the J 4 may be a coathanger with ceramic insulator. I know how to convert that to BNC. What an interesting idea.

I am going to redo my personal Cub to the Garmin. Its SL-40 is a good radio, but in the J3 it really is hard to work. That, and I can no longer really see the readout from the back seat, and it no longer likes to fly in the rain . . .
I will redo the entire wing root area, including lights, strobe, voltmeter, and battery source. Did all that on the J4 in April and we are delighted. So I can make room for almost anything. An actual quarter wave is around 14", right?
 
Oh - of course - closer to 7". Yes, 257.875. Your source says it also attenuates odd harmonics. I guess a transmitter could overcome that? Unless they mean all harmonics above the tuned frequency.

so do you cut the coax, then trim looking at a signal generator?
 
so do you cut the coax, then trim looking at a signal generator?

I'd rough cut it long then sweep it with a spectrum analyzer tracking generator. That will produce a plot of the notch which shows the center frequency and bandwidth. It should have no impact on VHF transmission or reception. If I were installing the stub on my aircraft I could measure the installed notch frequency. I have no control over your installation and the notch frequency may get shifted from my bench measurement. Still worth a try.
 
I'll go there in a minute. I can probably handle the math.

Let me review: we discovered the frequency pair by noting, during Covid, that when one controller was running both sectors we got crystal clear clearances, in their entirety, for approaches to Miramar and Palomar.

That is why I thought the difference frequency was sneaking through. We now get fragments, since each sector is run by a different controller. If just the UHF controller is talking we hear nothing.

So you are thinking that the mixing is going on inside the RF section of our receiver, and filtering out the UHF will prevent the mixing. Is it possible that the difference signal exists before it hits our antenna? If so, would it do any good to filter out the higher frequency?
 
I'm not the main sparky here but a spectrum analyzer inline with your antenna "might" detect an interfering product. Does the issue occur on the ground where such a device could be attached? Perhaps if so the FAA airways or whatever folks maintain the transmitters could help.

I've notched out amateur radio interference with stubs....but there are other options and means.

Gary
 
So you are thinking that the mixing is going on inside the RF section of our receiver, and filtering out the UHF will prevent the mixing. Is it possible that the difference signal exists before it hits our antenna? If so, would it do any good to filter out the higher frequency?

There were several possibilities but the two that seemed most likely were:

1. The mixing product is generated at the ATC transmitter site
2. The mixing product is generated in the COM radio

If the mixing product was generated at the transmitter site if would be heard on any radio tuned to the difference frequency. You could then have used this info in an attempt to get the TX site defect fixed. (The presence of the UHF and VHF signals at the transmitter site should not cause generation of a difference signal. Mixing requires there to be some defect at the transmitter site, most likely in the antenna feed system.)

You have confirmed that you don't hear the the signal when tuned to the difference frequency. This strongly suggests that the signal mixing is happening in your receiver. You can't stub out the VHF ATC signal but you should be able to stub out the UHF ATC signal. Hopefully that will stop the mixing. If it does it will be a simple inexpensive solution that can be applied to any com radio experiencing the problem in the vicinity of this ATC transmitter site.

No guarantees, but it's worth a try.
 
TSO's were not invented yet when CAR 3 was written. No requirement for it in any of the CAR's. I've never seen it mentioned in Part 135 either but guys have let stuff like that get written into their ops specs. Then you don't have any choice.

Web
Web, I have seen TSO'd wheels with 1936 dates on them with an airframe cert date of 1940...so I think that only some things were TSO's back then like wheels
 
Web, I have seen TSO'd wheels with 1936 dates on them with an airframe cert date of 1940...so I think that only some things were TSO's back then like wheels

I think if you look more closely, you will see they are TCd wheels, not TSO. Aircraft equipment like wheels, floats, skis, instruments were all Type Certificated under CAR 15 once the CARs were started back in the 1936-1938 time frame. Prior to that, the were Type Certificated under Aero Bulletin 7F. TSO wasn’t started until after the Federal Aviation Act of 1958.


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I'd rough cut it long then sweep it with a spectrum analyzer tracking generator. That will produce a plot of the notch which shows the center frequency and bandwidth.

Well here is a test stub. The frequency shifted slightly higher when I did the final cleanup and sleeving of the cut end but the ATC frequency is within the notch 3 dB bandwidth. You should have more than 30 dB attenuation of the ATC UHF frequency. If you want to try it send me your address by private message.
 

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Bob

Remember that you have a standing offer from me; Pick any radio other than that GTR-200 and I'll make a wire harness for free. Let me end the pain! lol

Web
 
It is not quite as painful as that. Other than the 125.7 interference, these are the best radios I have ever worked with, the best displays, the best memory circuits, easiest to install, and by an order of magnitude the best intercom.

Yes, I would love to try the stub.
 
It is not quite as painful as that. Other than the 125.7 interference, these are the best radios I have ever worked with, the best displays, the best memory circuits, easiest to install, and by an order of magnitude the best intercom.

Yes, I would love to try the stub.

I have had the GTR-200 in my bird for the last year or so and LOVE it. So far I have had zero issues with it. I don't use the built in intercom though.
 
Looks fine to me, it's certainly the type I would want you to use. I'll try to get the test stub in the mail to you tomorrow. I'll be very interested to hear if it has any impact on your ATC interference problem.

As I said in my private email I have tested it with 100 W at 50 Mhz and verified no (less than 1 dB) loss of receive sensitivity at 125.7 MHz.
 
First, a public thanks to Frequent. His stub arrived in the mail Friday. I installed it and tested it yesterday (Saturday).
It went in in about five minutes, thanks to an access panel i had previously installed. Test showed no interference with normal operation. The stub itself looks like it was done by a meticulous craftsman, with perfect shrink tube.

The Marine base shuts down on Saturday, so I assume SoCal inhibits the UHF transmitter. Monday will be the acid test.

In the spirit of keeping those interested in the TSO Com issue up to date, I have not heard a peep out of the FSDO for six weeks. You wll recall that they were contacting me twice a week for a month or so, including multiple personal visits and a notice that I could be violated for each and every flight. I understand that no contact is SOP when they decide not to prosecute - but maybe they are working diligently on my indictment?
 
Too early to really tell, but I think it worked!

Steve took the "stub-equipped" Cub up and I followed in the non-stub J4. Twice during our 20 minute session I heard Socal clearances, and each time I asked him "did you hear that?" (Yes, I explained to the controllers what we were doing - and we were basically alone in the north pattern.)

Both times he said "no."

This is not conclusive - the newer GTR-200s seem to get more interference than the older ones, and the J4 has a 2021 production radio. Still, I believe we may be on to something!

My spies tell me the feds are not happy with the SoCal antenna farm, and that there are high powered technical types looking at it. That is rumor at this point.

Still - things are looking "up."
 
Update - the interference is still coming through - although the frequency and duration of spurious receptions is down. Could be the Marines are not flying as much. We can easily handle the current noise. The filter still lets the difference frequency in.

I might add that it has been 2 1/2 months since I heard from the FSDO. Considering that I was getting urgent requests, Miranda warnings, etc., twice a week for over a month, I can only assume they finally figured out that non-TSO comms are legal as minor alterations, but just do not want to actually say so.

Another topic - I am setting my personal Cub up for a GTR-200, but for now am going to wire in the SL-40 as a "stand-alone" radio with the capability of using its internal intercom. I see only one input for PTT, so I am a bit puzzled - if two pilots have headsets on, and the intercom is disabled, are both mics hot? Wouldn't that be a very bad thing? I love the SL-40 with the PM-501, but am simplifying in preparation for adding the GTR, which needs no external intercom.
 
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