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Thread: MYTHBUSTERS: Letís talk seriously about antennas, or 10 myths busted.

  1. #221
    Thanks, this makes more sense! By the way if some of you are interested here are some results:

    - Ezuhf receiver "sander" stock monopole - very bad VSWR > 3 far away from 433 mhz
    - Ezuhf transmitter rubber whip stock monopole - very bad VSWR > 4 far away wrom 433 mhz
    - Diamond SRH771 - perfect match for 433 mhz with VSWR<1.1 -/narrow band/
    - Several dipoles and one moxon from the IB Crazy tutorials - 310 mm is not perfect for 433 mhz -
    it seems a litle short, but seems close. I will try start cutting it from 330 mm next time. VSWR<1.5.

    I can make some videos these days. If you want me to test some particular antenna tell me. My metter can go up to 3 ghz.

  2. #222
    Instructor Pilot Channel 1's Avatar
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    Quote Originally Posted by IBCrazy View Post
    Consider your cable loss and the fact that few cables are truly 50 ohms at microwave frequencies and you will quickly find that VSWR varies more with cables than with the dipole attached to it.

    -Alex
    That all depends on the cable and who makes it, I have swept runs of RG-402, and it has no impedance problems at 5 GHz, the same applies to Commscope CMR-400 or Andrew LDF4.

    Now substitute any of those either with a Chinese knockoff of the same model number and all bets are off, just like it's impedance at considerably lower frequencies.

    Ya get what ya pay for when it comes to coax or connectors.

    Wayne
    Everybody loves a bunny.

  3. #223
    Thanks Alex, Wayne! So it seems the cable is the issue. I am using RG-402 for the 433 mhz dipoles and they are fine.
    Last edited by lkrustev; 10th November 2016 at 07:59 AM.

  4. #224
    Engineer for Jesus Christ IBCrazy's Avatar
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    Quote Originally Posted by Channel 1 View Post
    That all depends on the cable and who makes it, I have swept runs of RG-402, and it has no impedance problems at 5 GHz, the same applies to Commscope CMR-400 or Andrew LDF4.

    Now substitute any of those either with a Chinese knockoff of the same model number and all bets are off, just like it's impedance at considerably lower frequencies.

    Ya get what ya pay for when it comes to coax or connectors.

    Wayne
    Ever use a Belden cable? For being an authentic, those are the worst performing cables I have ever used. This is why i started using RG142. Freaking awesome cable up to 12GHz.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  5. #225
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    Quote Originally Posted by IBCrazy View Post
    Ever use a Belden cable? For being an authentic, those are the worst performing cables I have ever used.
    Belden has been living on their name for too many years now, overpriced and no longer the quality it used to be.

    I have found CommScope, Times Microwave and Honeywell to be some of the best cable on the market and fairly priced.

    Wayne
    Everybody loves a bunny.

  6. #226
    As some people say that the perfect match is not so important, because it decreases the transmitted power just slightly, I've performed some experiments and I had to disagree. It seems on practice that 0.5 difference in VSWR makes a big difference in the performance ... so maybe there are factors which are not considered - as example my theory is that the electronics of the transmitter is greatly disturbed by the returned power and the frequency changes. I have a powerful transmitter which with bad antenna shifted the channels so it is useless. With a good antenna it works just perfect.

    … so if 10% returned power does not affect the theoretical range too much to be worried, on a practice these 10% power could disturb the functioning of the transmitter, so we could get much significant cut in the range, then the predicted from the theory. Just a hypothesis, based on some practical observations.

    Last edited by lkrustev; 16th November 2016 at 03:58 AM.

  7. #227
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    Quote Originally Posted by lkrustev View Post
    As some people say that the perfect match is not so important, because it decreases the transmitted power just slightly, I've performed some experiments and I had to disagree.
    A perfect match is very important and is something to strive for.

    Wayne
    Everybody loves a bunny.

  8. #228
    Quote Originally Posted by Channel 1 View Post
    A perfect match is very important and is something to strive for.

    Wayne
    I think so. What are your arguments Wayne?

  9. #229
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    Quote Originally Posted by lkrustev View Post
    I think so. What are your arguments Wayne?
    Two constant problems with a mismatch, are loss of reflected power in the feed-line equaling lower ERP and feed-line radiation which can effect the desired pattern.

    And as the transmitter power increases, damage to the feed-line due to overvoltage caused by reflected waves combining with forward waves and transmitter output power fold-back, due to the detection of an abnormal SWR buy the output circuit (PA) in the transmitter.

    Wayne
    Everybody loves a bunny.

  10. #230
    Thanks Wayne!

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