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Ron Smith G17 heading for eBay: any assembly instruction?

Discussion in 'audio' started by andrewsutton, Jul 27, 2021.

  1. Baluns again a used and abused item…used for matching impedance of an aerial to the feed and/or preventing common mode interference being fed back to the receiver and so increasing noise floor…simple explanations of how they work are, in my Ham experience, often side of the mark; I’ve made and used many but the why and wherefore are a complex subject indeedo_O
     
  2. davidsrsb

    davidsrsb pfm Member

    Getting baluns and RF transformers to work without introducing a few db of extra loss can be hard. The simple ones you see in 75 Ohm to 300 Ohm transformers measure very badly.
     
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  4. davidsrsb

    davidsrsb pfm Member

    Amateur radio antennas are usually very narrow band, so optimizing is quite easy. Even 88-108 MHz is a bit wide for a Yagi, TV even worse.
     
  5. Mike Reed

    Mike Reed pfm Member

    .

    As I had a G23, I'd agree with that, and a rotator involves two masts and extra weight, in effect, so the chimney has to be up to it. The G17 is a different kettle of fish and I can't understand why a 12 foot mast (or 10, even?) would induce sufficiently more wind load over, say a 6' mast, which would be a minimum, I'd've thought.

    I had a 6 element on a 12' steel mast on my 1960s chimney, which isn't exactly sturdy, installed by local riggers. I do think your riggers were a bit precious, even though they were probably just covering themselves. I bet Ron wouldn't have hesitated !
     
  6. True - all these things are trade offs - there’s no such thing as a perfect antenna - you can optimise certain features such as gain, front to back ratio, side rejection and beam pattern but these are all a balance of which areas are of interest to you
     
    Eyebroughty likes this.
  7. Jim Audiomisc

    Jim Audiomisc pfm Member

    The problem in practice may be that the effect of not ensuring an ideal balun means the outer of the coax becomes a part of the antenna. Given that the lengths, shapes, etc, vary the consequences vary from case to case. I tend to suspect that in practice for FM radio the main result is the signal is more prone to interference as the most obvious user-experience.

    However, where these days can you buy decent baluns for 75 Ohm coax covering 88-108 MHz for Band II use?

    That said, where can you buy a decent bandpass filter for the same?
     
  8. Jim Audiomisc

    Jim Audiomisc pfm Member

    Torque, I suspect.
     
  9. Mike Reed

    Mike Reed pfm Member

    On the chimney structure, I suppose, but if an extra 6' was too much of a gamble, it sounds like it wasn't really suitable for a G17 anyway with its 50 lbs wind load @ 100m.p.h. (the G23 is 72 lbs !!)
     
  10. davidsrsb

    davidsrsb pfm Member

    Just wondering why you would want a bandpass on FM, the users below the band are military mobile radio. Away from army bases the frequencies above the band are the main issue
    A low pass would be simpler and lower insertion loss
     
  11. Only guesswork but….It wouldn’t necessarily be the frequencies above and below the fm broadcast band it could be proximity to other radio sources causing overload in the receiver as they are not really built for heavy rejection of out of band interference (ie like amateur radio tx/RX) so a band pass would be best to give the RX front end the best chance
     
  12. davidsrsb

    davidsrsb pfm Member

    Sources below 88 MHz are unusual, that band is tactical VHF radio in most of the world.
    Not many countries allow amateurs the 4 m band.
    The bands just above 108 MHz are cluttered with aviation and various mobile radio systems including taxi radios in the old days, do they still use them?
     
  13. Jim Audiomisc

    Jim Audiomisc pfm Member

    Yes. What I had in mind was bursts of RF interence that are big enough to hit the downconversion mixer because the tunable RF filters aren't enough to avoid this. We get this on occasion from one of the VHF antennas we use. So came to mind when reading this thread.
     
    seagullsnacker likes this.
  14. davidsrsb

    davidsrsb pfm Member

    Most of this can be avoided by using decent coax and a ferrite sleeve or bazooka balun at the antenna end so that the coax shield is not acting as an antenna indoors.
    Most coax installed in the old days is rubbish with very meager screening
     
  15. Yes most consumer coax is just shite with virtually no screening…it’s prob better to get decent 50ohm stuff than worrying about using 75ohm for the matching to the tuner (negligible difference); whack as many turns around some ferrite close to the tuner input and at the antenna feed point if you can as well
     
  16. davidsrsb

    davidsrsb pfm Member

    At 100 MHz simply passing through a close fitting ferrite sleeve is enough, the multiple turns trick is for HF.
    You want the balun at the antenna end so that the braid running through the house is earth

    For coax, the satellite cable is cheap and technically superb, low loss and excellent shielding
     
  17. It won’t harm to have multiple turns on a ferrite;to get properly retentive about though you need to know which ferrite type and applicable attenuation at required frequencies - some satellite cable is superb I agree but equally some I’ve seen is atrocious!
     

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