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Thread: TSO Com?

  1. #161

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    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?

  2. #162

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    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?

  3. #163

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    Quote Originally Posted by bob turner View Post
    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.

  4. #164
    BC12D-4-85's Avatar
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    https://www.youtube.com/watch?v=bbnllGycQws

    If interested skip the early ad (cut back to the small YT format) and forward to about 6 minutes to watch an analyzer and stub in action. There's some math but don't worry the demo is worth the effort.

    Gary

  5. #165

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    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?

  6. #166
    BC12D-4-85's Avatar
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    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

  7. #167

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    Quote Originally Posted by bob turner View Post
    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.

  8. #168

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    Quote Originally Posted by wireweinie View Post
    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

  9. #169

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    Quote Originally Posted by astjp2 View Post
    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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  10. #170

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    Quote Originally Posted by frequent_flyer View Post
    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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    Last edited by frequent_flyer; 09-04-2021 at 04:32 PM.
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  11. #171
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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
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  12. #172

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    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.
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  13. #173
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    Quote Originally Posted by bob turner View Post
    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.

  14. #174
    wireweinie's Avatar
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    Just a wee bit less electrical interference in Soldotna than San Diego.

    Web
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  15. #175

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    This one ok?
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  16. #176

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    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.

  17. #177

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    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?

  18. #178
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    Be prepared for the swat team and the news media to show up at 3am!
    N1PA
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  19. #179

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    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."

  20. #180

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    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.

  21. #181

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    Quote Originally Posted by bob turner View Post
    The filter still lets the difference frequency in.
    The stub was intended to attenuate the UHF ATC frequency and bench tests showed it did that quite well. It is not possible for the stub to attenuate the difference frequency. If the difference frequency is actually being radiated from the ATC antenna site it can only be fixed there.
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  22. #182
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    Quote Originally Posted by bob turner View Post
    ....Another topic - I am setting my personal Cub up for a GTR-200.....
    I love the SL-40 with the PM-501, but am simplifying in preparation for adding the GTR, which needs no external intercom.
    Isn't the GTR200 the same model radio you've had all the trouble with, resulting in 5 pages of discussion here?
    Hmmm....isn't there an expression about that--
    something along the lines of doing the same thing over but expecting different results?
    Are you switching radios just because of the GTR's built-in intercom?
    I would probably think about just sticking with the SL40 & separate intercom if it worked well.
    You know, the old "if it aint broke, don't fix it" thing.
    Last edited by hotrod180; 10-15-2021 at 09:23 AM.
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  23. #183
    wireweinie's Avatar
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    Even if you go with the '200, my offer to fab the harness still stands.

    Web
    Life's tough . . . wear a cup.

  24. #184

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    Thanks.
    The SL-40 seems to be a better radio, but the features of the GTR-200 outweigh it by a big bunch.

    The SL-40 uses less battery power, and has none of the interference problems I describe. But in the J3, which is flown from either seat, its display is inadequate and its memory access is the worst I have yet encountered.

    The GTR can be seen and operated from either seat. It has stunning display and memory setups. And its intercom is the best for open cockpit that I have ever tried.

    Balance that out against this interference problem, and the verdict is GTR-200. I have used a lot of comm radios, and for Cubs and Stearmans, this one is the best. If they changed the squelch range, it would be even better. Sure, it sucks batteries down, but have you seen what is happening with batteries?

    I don't make a living installing avionics, but I have done King, Narco, Edo-Aire, and these two, along with simple intercoms. I can do the GTR harness while the Greek cooks dinner and the News Hour plays. It is that simple.

  25. #185
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    Just for others information; if you need to operate one radio from two seats, look at the Trig TRY91 as it can be installed with two separate control heads. This means front seat/back seat or even two separate cockpits can control a single radio.

    Web
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  26. #186

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    Bob---what battery are u using with GTR-200 ?---- I want to install it in my J-3 C-90-8 (NO generator)---Odessey PC-680 is ????---maybe a 10 AMP lithium ???Capt Cub---


    "You cannot teach experience, you must acquire it."
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  27. #187

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    I have for decades used 7 Ah gel cells, and before that motorcycle batteries. One gel cell is good for five hours with the SL-40, but only one hour feeding the GTR. We graduated to 24 volts using two in series and get 12 tach hours in the busy pattern.

    Right now the Cubs with GTRs are using the EarthX ETX-16C, and we are watching the timing - but I hooked both up so I can check volts on the preflight (or at any time) and can select an alternate battery in flight.

    We are using the Odyssey PC-545 for the starter, and as a switchable backup for the GTR. Garmin engineers insist that the GTR is current-driven, but our experience is that it goes into “low power” mode around 12.4 Volts, when we still have enough oomph for maybe six more starts.

    A brand new 545 gives us 75 starts; after several cycles it drops to 60. That’s usually 3 months!
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  28. #188

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    Today I got SoCal interference in a KX-155 King radio. Since I was the only one in the pattern, I mentioned it to our friendly local controller. He said "yeah, we get SoCal interference, but it hasn't been as bad lately."

    The Garmin is more susceptible, but now I realize that others have had this problem, and are just not as ornery as am I.

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