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High SWR's, trouble with carrier on amp
- Thread starter bassman21
- Start date
The choke might help correct my issue, i don't know yet.
I am using a (meant for mobile use) 1/4 wave base load vertical, perhaps D.C. grounding the counter-poise/ground plane will solve the issue I have. More reading/testing required on my part.
Probably common mode current on the shield. A few ferrites would likely have done the same thing. The RF isolator is just a choke, not a balun at all.
I think I'm going to upgrade the coax for better shielding too. That might fix the issue. I already had some beeds on there and it was still doing it.
Is this a mobile install? Usually you have issues with common mode when the antenna is mount is less than desirable or there isn't enough counterpoise under the antenna.
Did you try the choke at the antenna end of the coax? That will force more return current to flow where it should and keep it off the outside of the coax shield. The choke right after the amplifier may stop the symptoms but is isn't the best solution.
Actually, those results do seem to indicate that you do have a faulty feedline.
(unless the feedline is open) all the power delivered to the antenna is actually radiated, it has nothing to do with a "target frequency".
stories about finals being burnt up due to high VSWR are myths.
Yes, finals can be destroyed, but not due to VSWR
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can you install a dummy load at the antenna location as some have mentioned to see how the coax is working. if the swr lowers the coax is good.
have you verified the grounds at each pl295 connector is soldered and good. a loose ground can add to this also.
I had a ground come loose on a pl259 end on my work bench years ago and was only testing a new connex 3300 and it would not even read as if the radio was putting out any power. I could put a cobra 29 on the bench coax and it would work just fine. thought I had a bad new radio and was about to tear into it for a problem and a buddy of mine asked about my coax jumper and changing it out I found the radio was fine. just the extra power of the radio was causing it to show as if it was bad.
so be sure your coax is good and then go from there. so many things the guy's have mentioned can cause your problem. you will just have to eliminate each item to confirm what is wrong.
have you verified the grounds at each pl295 connector is soldered and good. a loose ground can add to this also.
I had a ground come loose on a pl259 end on my work bench years ago and was only testing a new connex 3300 and it would not even read as if the radio was putting out any power. I could put a cobra 29 on the bench coax and it would work just fine. thought I had a bad new radio and was about to tear into it for a problem and a buddy of mine asked about my coax jumper and changing it out I found the radio was fine. just the extra power of the radio was causing it to show as if it was bad.
so be sure your coax is good and then go from there. so many things the guy's have mentioned can cause your problem. you will just have to eliminate each item to confirm what is wrong.
A coax jumper that does not have the shield braid SOLDERED to the body of the plug is a popular way to cause this symptom.
If all your coax cables are okay, a sudden "jump" in power as you slowly increase the radio's carrier is oscillation. RF is feeding back from the final transistors into the driver. The added power you see when it "jumps" is mostly NOT on your channel frequency, but at a different one generated by the feedback.
Usual cure is to insert a choke or two, with disc caps to ground on the power where it feeds into the driver transistor.
A close look at ground screws, especially on the coax sockets may reveal a problem caused by a bad ground connection. A ground lug under one screw on each coax connector should have as short a connection as possible to the circuit-board ground.
73
If all your coax cables are okay, a sudden "jump" in power as you slowly increase the radio's carrier is oscillation. RF is feeding back from the final transistors into the driver. The added power you see when it "jumps" is mostly NOT on your channel frequency, but at a different one generated by the feedback.
Usual cure is to insert a choke or two, with disc caps to ground on the power where it feeds into the driver transistor.
A close look at ground screws, especially on the coax sockets may reveal a problem caused by a bad ground connection. A ground lug under one screw on each coax connector should have as short a connection as possible to the circuit-board ground.
73
Amplifiers without driver stages can oscillate. Could be tuning as in capacitors out of tolerance, bad build practice, mismatched (possibly damaged) transistors.
Everyone wants to swamp the input or add caps vs fixing the issue...I used to. If all the components are found to be in tolerance then you might need to stray from the original design that usually works. A little more negative feedback on the final section can work wonders.
His problem may be all in the antenna system. Hopefully he will go about this systematically and not start replacing random things.
What type of coax are you using between radio and amp and amp and meter? How long are the lengths?
The problem ended up being not one, but two RG8x jumpers of the same brand. I changed to different jumpers and the issues was resolved. I also upgraded from the Wilson 1000 magnet mount to the Sirio magnet mount and changed out the coax on the mount from the cheap aluminum RG58 to copper RG8x. I checked all the resistors on the amp and they are within range. I think the box is slightly out of tune which is why this box had the issue and my other didn't.
Glad you got it figured out, with those types of amps I've seen a lot of people have sweet or keying issues that were resolved with changing out to better coax or longer jumper lengths.
Glad to hear the fix was simple.
Does lead me to suspect that a close look at the 'problem' jumpers will reveal that the shield braid is either not soldered, or at best poorly soldered.
73
Does lead me to suspect that a close look at the 'problem' jumpers will reveal that the shield braid is either not soldered, or at best poorly soldered.
73
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