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high swr protection circuit idea

Discussion in 'General Ham Radio Discussion' started by Lil'Yeshua, Nov 30, 2012.

  1. Lil'Yeshua

    Lil'Yeshua .......

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    I just thought of something new to me. The Galaxy DX-959 has an auto swr checker. Is there such a thing as a high swr protection circuit that would prevent transmitting when a preset swr reading is detected?


     

  2. RatsoW8

    RatsoW8 Supporting Member, W9WDX ARC Member - WD8T

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    Most high power transceivers have "fold-back" circuitry build in to reduce output power and porotect the final output transistirs in the presence of high SWR. Not something you really need in a legal or well build CB radio. I'm sure it's not cost effective for manufacturers to include such circuitry. From what I've seen the export radio manufacturers are out to build as cheaply as possible. Minimum quality and maximum profit.

    I'm sure there are circuits out there you could find if you want to buid you own but it would be alot of work to add nothing of value on a CB radio..

    In other words your Galaxy isn't going to pop a transistor if you traamsmit into a mismatched load. Probably not even if you forget to connect an antenna for a few seconds. I've done that and never burned a final in a CB.
     
  3. fodendaf

    fodendaf Active Member

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    You could make a 100 ohm dummy load for the radio and hook it in parallel to the antenna coax. That way if the antenna swr rose above 2:1 the radios output would dissipate in the dummy load instead of the antenna. It would save the radio just the same.
     
  4. W5LZ

    W5LZ Crotchety Old Bastard

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    :)
    Something like 30 years ago there was an antenna matching device, solid-state (potted so you couldn't open it), that was guaranteed to provide a good SWR match to almost any antenna you could connect to it. It'd work on almost any frequency, didn't much matter what band the antenna was tuned for. It 'worked', not well, but it 'worked'. Wish I could remember the name of that thing! The manufacturer wouldn't say how it was made, trade secret you know. So, someone X-rayed the thing. Turns out it was a 50 ohm resistor across the feed point terminals. Ooo, the outrage over that was really, really big and it dropped off the market amazingly fast.
    Think about it.
    That thingy had the characteristics of an SWR protection circuit, it would always indicate a nice SWR. It was a 50 ohm resistor in parallel with an antenna. Use an old 'Cantenna' as that 50 ohm resistor and you'd have a no-tune 50 ohm antenna that'd work almost anywhere. Your trnasmitter wouldn't ever 'cut-back', always provide full power out. It'd have a REAL 1000 watt continuous duty power rating!
    - 'Doc

    (Sorry 'fodendaf', that one won't work.)
     
  5. fodendaf

    fodendaf Active Member

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    Really? I assumed that nested dipoles work on the same principle. The rf goes the path of lowest swr. And don't say that the impedance of the dummy load and the antenna are too close because I have heard of hams using nested dipoles all cut for the same band to make their station more broadbanded. And the non resonant dipoles don't damage the transmitter either. Can you explain why it won't work?
     
  6. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    The RF does not just "go" to the dipole with the lowest SWR but rather divides according to the actual impedance. If two or more dipoles have the same impedance the power will divide equally. In the case of multiple dipoles on the same band, like one cut for 3600 and another for 3900, the power may well be split equally when operating on 3750 and the expected SWR would be near 2:1 at most across the band.

    Now suppose we have a 50 ohm load in parallel with an antenna that presents a load of only 16 ohms. The antenna itself would present an SWR of 3:1 however since it is in parallel with an additional 50 ohms the actual SWR will be 4:1 because the impedance will be near 12 ohms. The old dummy load in parallel with the antenna trick works when the antenna impedance is higher than 50 ohms but not so good when it is less than 50 ohms. In any case it still wastes power dumping into a dummy load.
     
  7. fodendaf

    fodendaf Active Member

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    I see now. So you add the impedances using the resistors in parallel formula just the same. I never thought about it like that.
     
  8. Beetle

    Beetle Sr. Member

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    Hey 'Doc -- It was called the "MaxCom" or "MaxxCom". Invented by some guy named Sonny Irons. http://www.eham.net/ehamforum/smf/index.php?topic=61768.45;wap2
     
  9. fodendaf

    fodendaf Active Member

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    Does the 959 have a l.e.d that lights when the swr is high? You could use that to drive a transistor that opens a relay in the tx line from the microphone.
     
  10. Lil'Yeshua

    Lil'Yeshua .......

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    Yes it does. That's exactly what I was thinking. The swr warning led circuit is adjustable so that it could be set up to turn on and a predetermined level.
     
  11. W5LZ

    W5LZ Crotchety Old Bastard

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    Maxx-Com! That was it. It's also one of the first things I think about with several of the 'new' all band antennas being sold recently. They certainly 'work', but that never says how well they work, which is the 'out' for the advertising department. "Works better than anything I've ever had!", makes me wonder what in the world they have had in a few cases, you know?
    - 'Doc
     
  12. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Better make it a self latching relay. Once the PTT line is opened there is no RF to trigger the high SWR indicator so the relay will fall out only to be triggered again and then fall out again only to be trig..........Makes for a wicked case of relay chatter. IMHO if it has a high SWR indicator USE IT AND DON'T WORRY. I have yet to see a radio that blew the finals by accidentally transmitting into a near infinite SWR for a few seconds or even a minute or so. Low power radios like legal CB's are pretty hard to kill and high power gear usually has a fold back control.
     
  13. CHRISTOPHER PAUL RYAN

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    Have you seen the TWO VSWR DEFENDER MODELS by <TACTICALRADIOGEAR.COM>?
     

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