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Commercial AM getting into a 2m repeater ?

Discussion in 'RFinderPi - Open Source radio interface based on R' started by KD0CAC, Feb 1, 2017.

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  1. KD0CAC

    KD0CAC Ham Member QRZ Page

    One of the guys that look after a club repeater had to shut it down a few times as the weather changed -10* F , to 48* F and rain , now back to 17* .
    This is on top of a commercial building about 4 stories high , the short tower [ 2 sections 25G ] with mast vertical on top , then tied to the lighting ground cables .
    Figuring a bad connection somewhere , but haven't found yet .
    So a few of us were talking the other day about it , and I mentioned that would we be able to use a low power AM transmitter to induce RF into the system to try to pin down the loose / bad connection ?
    It's intermittent , happens a couple times a yr. or so , maybe .
     
  2. KD0CAC

    KD0CAC Ham Member QRZ Page

    Wanted to bump , I can build stuff and do some repair , but can not design stuff so hoping to get some ideas about building , if it could work ?
    So building a low power AM transmitter [ handheld ] and maybe use a small beam / log to try to inject signal into objects , loose connections etc. around towers / feed lines , in order to correct , and keep out interference in - in this case repeater , but also ham shacks in general ?
     
  3. AG6QR

    AG6QR Subscriber QRZ Page

    Are you talking about the commercial AM broadcast band? The frequency of that band is about 540 kHz through 1700 kHz. Meaning the wavelength is around 180 to 550 meters. A beam for a quarter-mile wavelength isn't going to be portable. Besides, you should keep the power down to Part 15 limits when you're playing outside the ham bands (your ham license doesn't give you any privileges outside the ham bands). Instead of using a beam from miles away, why not try to get close to the repeater, and use a tiny, inefficient antenna.

    But if this is intermittent, it may be difficult to track down, regardless. Does it come and go with changes to the field strength of the broadcaster? I'd guess it's more likely to be coming and going with changes to some connection somewhere around the repeater, which sometimes acts as a diode when loose, and thereby demodulates AM. If my suspicion is correct, then when the connection is solid and working correctly, no AM transmitter, neither the broadcaster's nor your own, will trigger the issue, but when the connection is faulty, any AM transmitter, including the broadcaster, will cause the problem. If that's the case, then going around the repeater with an AM transmitter won't tell you much. The better thing to do is to go around the repeater site when the problem is happening, and tighten connections or at least tap on things to see if any change happens.

    This is posted on the Raspberry Pi forum. You might try posting this on one of the relevant technical forums to get better answers.
     
  4. KL7KN

    KL7KN Ham Member QRZ Page

    I have read this six times and still don't what the problem really is that you are asking about.

    Does the repeater suffer for intermittent TX issues?

    Does the repeater sometime start transmitting a local AM station's signal? The two are vastly different problems.

    Could you perhaps state it a bit differently?
     
  5. KL7KN

    KL7KN Ham Member QRZ Page

    I have read this six times and still don't what the problem really is that you are asking about.

    A good model for describing an issue is SOAP - Symptoms, Observations and analysis....
     
  6. KD0CAC

    KD0CAC Ham Member QRZ Page

    This is on the roof of university with light rods & cabling , are tower is tied into there ground cables .
    I have not heard this , the guy that does the work on our club repeater told me about it , the AM broadcast station getting into the repeater - then the repeater keying up with some part of what ever is getting into it .
    He mentioned this to me and said that it has happened mostly when there is high humidity in warm weather , or like recently a thaw in the 40s and lot of snow melt , last year there was the same and someone else went & found a loose connection on one of the ground cables & tightened it and it went away , but tuff to find because it was probably a combination of weather conditions , how the AM station was operating / power & direction .
    Might have to bring a garden hose up to make happen ;)
     
    KL7KN likes this.
  7. KL7KN

    KL7KN Ham Member QRZ Page

    Ah so.

    You many have a problem with a second, unrelated frequency getting into the receiver front end and mixing - and thus now hear the AM Station.

    Had that happen to an Air Traffic Control system I maintained. Periodically the radio is the control tower would play music from a local AM station. Since the receivers used a multiport cavity filter to a single antenna, I pulled that out of the circuit - IOW , ran an ant straight to the RX.

    Still had the problem. That confirm the issue was in the front end. re-tuning the receiver didn't help and a replacement had the same issue. So that eliminated my equipment as the source of the problem

    So, I put a stub filter on the coax feed. That stopped the music. I was able to call in a specialist team that tracked the problem to a spur on a paging transmitter that was mixing in the receiver front end.

    The paging system owner received a nice visit from some Federal folks and the problem went away.

    May I suggest:
    Use a spectrum analyzer to retune the system cavity filters to ensure they are good.
    Consider a secondary filter in the RX feed.

    Ensure your receiver is up to spec. for the LOs and any other frequency setting circuits.

    Let us know how it turns out.

    More on a simple stub filter:

    [​IMG]

    [​IMG]

    and simple to tune
    [​IMG]
     
  8. KD0CAC

    KD0CAC Ham Member QRZ Page

    Thanks for the reply , have to get up there & do some checking .
    On the stub filter , your drawing shows on leg / tee , the next picture shows 2 , more info on 2 tee's , I would guess that the distance between the 2 tee's is also frequency related ?
     
  9. W6KCS

    W6KCS Ham Member QRZ Page

    Could you describe the system including cavities, preamp, type of coax, etc? We can only make wild guesses without knowing what's there exactly. Any chance you could record some of the audio? Unless you know what, if anything, is causing a mix, you don't need to make anything yet.
     
  10. KD0CAC

    KD0CAC Ham Member QRZ Page

    I do not have a lot of spec.s memorized , but the repeater is a 2m Motorola Micro , some large cavities but do not remember brand etc. , 7/8ths hardline , 2 sections of 25G , the antenna is not one of the large Station Masters , but ham type about 13' tall .
    On a commercial roof with the tower tied into the lightning ground system - this is where we suspect the issue , but unknown for sure .
    The weather is not the conditions that seem to make it happen , wet and in the 40's & higher , it was like that with a snow melt a few weeks ago .
    I have not heard it and do not have a recording .
    We had issues in the past , one was very erratic , turned out to be a computer 1 floor down and has been changed out .
    Then maybe the same thing a yr. or so ago , and a connection to the ground system was tightened up and may or may not be the same now - I did go on the roof a couple weeks ago and found nothing loose .
     

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