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High SWR with amp

Discussion in 'CB Antennas' started by Supspt454, Jan 25, 2020.

  1. Supspt454

    Supspt454 Member

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    Running a Sirio P5000 magnet mount. SWR tunes to 1.25:1 on 1-40. It’s very broadbanded across 40 channels. This is running through an amp. When the amp is switched on the swr rises to about 2.5:1-3.0:1 and causes the radio to go into high swr shut off. I just replaced the mag mount base with a new Sirio magnet. It has RG58u coax which I think is not very good and my 18” jumper is tramflex rg8x which I think is much better. I’m thinking maybe I should just change everything out to a so239 stud mount with a good chassis ground and good coax. So I guess my question is, could the capacitor grounding of the magnet mount be causing the amp to see it as a poor ground? Any input is appreciated.


     

  2. w9cll

    w9cll W9WDX Amateur Radio Club Member

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    Mag mounts generally don't like amps. Hard mount the antenna and you will be okay.
     
  3. Tokin

    Tokin 42Zero Tampa

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    Puck it...
     
  4. dave457

    dave457 Sr. Member

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    Not always.... The amp may not be properly tuned
     
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  5. sp5it

    sp5it Master of puppets

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    How it behaves on dummy load?
     
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  6. w9cll

    w9cll W9WDX Amateur Radio Club Member

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    Hence I say GENERLLY..:-D
     
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  7. Supspt454

    Supspt454 Member

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    I don’t have a dummy load. Only dummy load I’ve got is myself.
     
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  8. Redbeard U812

    Redbeard U812 Sr. Member

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    Mag mounts don't ground as well. Insufficient ground plane. Hard mount, and do some bonding throughout the vehicle. Cab to frame, panels too. Mags are quick and easy, to deploy and take down but are notorious for no ground.
     
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  9. RT431

    RT431 In the swamp

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    If it's a Texas Star amp, that's normal, the input tuning on those suck. Sometimes changing the jumper length between the radio and amp will help, but not always. Sufficient grounding as mentioned and hard mounting the antenna should do the trick.
     
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  10. loosecannon

    loosecannon Sr. Member

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    Supspt454,

    remember that when your amp is off, and you take an SWR measurement using the built in meter on the radio, you are measuring the actual antenna SWR.

    HOWEVER!
    once you turn that amp on and do the exact same thing, you are not reading the antenna SWR anymore.
    you are reading the match between the output of the radio and the input tuning circuit of the amplifier.

    so your problem has nothing to do with your antenna, and everything to do with the input circuit on your amp.
    either it is not tuned properly, or you are not putting the right amount of wattage into the amp, or both.

    if you were to put an SWR meter between the output of the amp and the antenna, you would see that the antenna's SWR is still just fine.

    so what kind of radio is it?
    how many watts does it deadkey?

    what kind of amp is it?
    can you take a picture of the PC board inside the amp for us?
    once we can see what is going on inside the amp, we can recommend something to help fix the issue.
    LC
     
  11. Redbeard U812

    Redbeard U812 Sr. Member

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    Agree with LC....my palomar's both jump the swr when powered. I take swr with seperate meter, without linear in line or powered off. My linears always jump the swr up. But the mag mount sure doesn't make for a good ground, resulting in poor ground plane using roof or body panels.
     
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  12. nomadradio

    nomadradio Analog Retentive

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    Strictly speaking, the antenna's SWR didn't rise when the amplifier is keyed.

    The SWR reading went up, but the antenna didn't change.

    The reading is higher because the amplifier is delivering a "soup" of extra RF frequencies.

    The radio apparently has a clean output, mostly just the channel frequency, and that's when the antenna reads okay barefoot. If the radio had a 'dirty' output signal, the SWR would read higher barefoot.

    The antenna is tuned only for the 27 MHz band. The extra frequencies are not 27 MHz, and the antenna is not tuned for those. All amplifiers produce multiples of the input frequencies, called 'harmonics'. All legal ham amplifiers include filtering to reduce the unwanted frequencies coming out of the amplifier circuits.

    All not-legal amplifiers leave this out. Raises the cost to build, and the sale price. Doesn't make them sell better, so filtering is not included.

    Since the antenna is not tuned for the harmonic frequencies, the SWR for that part of your RF power is really, really high. The wattmeter can't tell them apart, and shows you the sum total of all reflected power at all those frequencies added together.

    Good chance that inserting an inline harmonic or "TVI" filter on the output of the amplifier would reduce the upward jump in SWR you see with the amplifier keyed.

    73
     
    #12 nomadradio, Jan 26, 2020
    Last edited: Jan 26, 2020
  13. Supspt454

    Supspt454 Member

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    Stryker 955, tuned for low DK and swings to factory rated spec, it’s not all cranked up, aligned, and amp is typical custom 2 pill HD. Let me say, thanks for all the advice. It’s good to see people responding to my problem and offering to help trouble shoot. Warms my heart that there is still good people in the world. Ok, with that out of the way, let me give a quick run down... radio is new and works awesome. Amp is new and I’m hoping it’s fine as well. I’m hoping it’s actually an antenna issue. I only mention that because of the age of this antenna. I’ve been using it since 2012. It’s had some issues on this vehicle for quite some time with several different radios. Mainly unwanted noise. It’s always had low swr readings on a inline meter, and usually the radios built in meters are somewhat close to the inline. So with all that said, I ordered a new Sirio magnet base the other day which came with much more coax length then the original even though it had the same part number. Anyhow, it is RG58U cable. I know it’s not the best but it’s what it came with. Now...I vaguely remember trying to use a Killer Bee 250HD in this same vehicle, with another radio, and same antenna years ago and I had the same issue. It threw the radio into swr warning Magnum S9 at the time. I also tried that same Tx Star 250HD with a CRE8900. Same antenna, same vehicle, same results. Evidently I’m a slow learner but maybe my antenna is failing other then the magnet? Everything that has been mentioned about coax length, and tuning in the previous post, I can understand that. Makes sense. I just can’t get over a few diff rigs and boxes in same truck with same antenna with same results. Call me dumb now thanks y’all. Keep em coming!
     
  14. Supspt454

    Supspt454 Member

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    Another thing. I did cut the coax from the antenna to a much shorter length today and soldered a good connector on. I had ran in a couple days with the long coax and after trimmed. Swr did not change one bit. And no change in amp results. Also tried Different lengths of coax from radio to amp. No change.
     
  15. loosecannon

    loosecannon Sr. Member

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    where are you measuring the SWR?
    before the amp or after the amp?

    also, a 955 into a 2 pill?
    why?

    that stryker alone should do a 25 watt deadkey and swing up to around 100 watts.
    even if you are getting 250 watts out of that amp, (which is pushing it), that's not even a 1 S-unit gain in someone's receiver.

    how many watts are you dead keying into the amp?
    most 2 pills like about a 2 watt deadkey.

    the chances of your antenna having a good SWR with amp off, and sky high with amp on are very slim, and it's much more probable that the amp is the issue.

    remember that an amplifier has a tuned circuit that changes the 50 ohm impedance that your radio puts out, to about a 3 ohm impedance that the power transistors want to see.

    then, after amplification, that 3 ohms has to be converted back up to 50 ohms by another tuned circuit.

    your SWR meter that is built into that stryker can't "see through" the amp when it's on, and only sees the match between the 50 ohms the radio is putting out, and whatever the input impedance on your amp is.

    if you have the ability, put a meter both before and after the amp and tell us the results.
    LC
     
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