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Question for amp gurus

C

CherokeeSoccer

Member
Sep 1, 2020
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I’m having high swr issues when I put amp inline and power is on. I’m just getting back into cb radio after many years so I’m far from an expert. I have learned just enough to probably be dangerous. lol I’ve read through numerous threads here and I’m thinking it’s a harmonics issue. The amp is a no name 250 that I’m guessing is a knockoff. On high it’s showing around 160 watts output. My setup is with a Cobra 142 gtl with a 3.5w dk and a new Maco V58. Without the amp, I have it tuned to close to 1.2 across all channels. With the amp inline and on, swr jumps up to around 3.

I have read in various places that a low pass filter between the amp and antenna can help with the dirty harmonics of the amp. First question is, is this true? Second question is, if they do indeed help with harmonics, do I want a band pass filter or a low pass filter? ICE makes both kinds of filters for 11 meters and they have a cut off of anything above 30 MHz. Any input will be greatly appreciated!

Link to ICE filter page:
https://www.iceradioproducts.com/filtersrf.html
 

I tested all jumpers as well as tried various lengths. From my understanding of the research I’ve done, coax length doesn’t really do anything but fool the meter into thinking the swr is good. I tried it anyway with no changes.
 
Well, you're on the right track.

Best way to see if that amp needs tuning, is to throw the SWR meter in-line - BETWEEN AMP AND RADIO.

Then I'd look into TURNING DOWN the wattage going to that amp....

Each time you turn down the watts going into the amp - then recheck the SWR (FWD to REF each time)- between both the Radio and the AMP.

IF that SWR is not changing - then the problem may be with the amp.

Ok, let's do the next approach to verify that your radio isn't sending that amp this stuff causing your SWR...

You checked the radio to amp already - now lets check SWR from AMP to ANTENNA

Again, you really should keep the watts of the radio down low, to help keep your amp alive while you check...

If you have your Low -pass filter...INSTALL IT between RADIO AND AMP

Using your low watts OUT of the RADIO - now check your SWR from AMP to ANTENNA - is it still high or is it going down when you use less watts?

IF the answer is yes, your amp needs the help - it may be tuned to pick up everything from here to Neptune in signal out of that radio to "swing it" - you need to rework the input or output or both sets - of winds in that amp to equalize the SWR condition that is causing this. The issue may not be the amp itself, but it's output network is converting it's output impedance to something more like 75 ohms or higher. Read this as it's not 50 ohm output, which the antenna wants to use to give you a low SWR - you must check and rework the amp to rewind those networks to make it closer to 50 ohms and reset the wattage inputs to using less watts to help keep that amp from going thru the roof.
 
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Handy Andy said:
Well, you're on the right track.

Best way to see if that amp needs tuning, is to throw the SWR meter in-line - BETWEEN AMP AND RADIO.

Then I'd look into TURNING DOWN the wattage going to that amp....

Each time you turn down the watts going into the amp - then recheck the SWR (FWD to REF each time)- between both the Radio and the AMP.

IF that SWR is not changing - then the problem may be with the amp.

Ok, let's do the next approach to verify that your radio isn't sending that amp this stuff causing your SWR...

You checked the radio to amp already - now lets check SWR from AMP to ANTENNA

Again, you really should keep the watts of the radio down low, to help keep your amp alive while you check...

If you have your Low -pass filter...INSTALL IT between RADIO AND AMP

Using your low watts OUT of the RADIO - now check your SWR from AMP to ANTENNA - is it still high or is it going down when you use less watts?

IF the answer is yes, your amp needs the help - it may be tuned to pick up everything from here to Neptune in signal out of that radio to "swing it" - you need to rework the input or output or both sets - of winds in that amp to equalize the SWR condition that is causing this. The issue may not be the amp itself, but it's output network is converting it's output impedance to something more like 75 ohms or higher. Read this as it's not 50 ohm output, which the antenna wants to use to give you a low SWR - you must check and rework the amp to rewind those networks to make it closer to 50 ohms and reset the wattage inputs to using less watts to help keep that amp from going thru the roof.
Click to expand...
That mostly makes sense to me. I didn’t think 3.5 watts would be too high a drive for the amp but that may be it. I think I can figure out how to turn the 142 down to whatever it may need to be to make a difference. I am confused about putting a filter between the radio and amp though. I thought the idea was to get rid of stray RF from bad harmonics coming OUT of the amp. Are you saying that the radio could be creating an out of frequency signal into the amp? Without the amp I’ve been getting very good reports on how it sounds. Not even going to try and transmit other than short bursts to check swr with the amp on until I get this figured out. As far as “tuning” the amp as you described, that’s probably above what I have the knowledge to do. I have recapped a couple of old radios but this is a whole different ballgame. I really appreciate your input! I don’t have the filter yet because I’m really not sure which one I mentioned in the op will be the correct one. I will definitely follow the procedures you outlined to narrow things down.
 
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On a Cobra 142GTL, you can turn down the deadkey by adjusting VR6, it's located near the Audio IC, which is along the right side of the board. Adjust it so it's down to about 1.5-2W deadkey, no need for anything higher for that amp. Check to see if you have high SWR. If the problem persists, there's a mismatch issue inside of the amp that will need to be addressed. A lot of those amps aren't tuned real well from factory, and do require some extra tuning to lower the input or output SWR.


~Cheers~
 
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The most-common cause of seeing the SWR jump when the amplifier is active will be harmonics. Odds are the builder of this amplifier didn't spend a dime to include a low-pass filter inside the thing. All legitimate ham linears have harmonic filtering.

Since you're running at 27 MHz, the typical TVI filter meant for ham radios below 30 MHz should do the job.

Only one way to tell.

73
 
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nomadradio said:
The most-common cause of seeing the SWR jump when the amplifier is active will be harmonics. Odds are the builder of this amplifier didn't spend a dime to include a low-pass filter inside the thing. All legitimate ham linears have harmonic filtering.

Since you're running at 27 MHz, the typical TVI filter meant for ham radios below 30 MHz should do the job.

Only one way to tell.

73
Click to expand...

That is what I was thinking from the research I’ve done on these types of amps. My question is, do I want a band pass filter or a low pass filter? ICE makes both kinds of filters for 11 meters and they have a cut off of anything above 30 MHz. Any input will be greatly appreciated!

Link to ICE filter page:
https://www.iceradioproducts.com/filtersrf.html
 
Last edited:
CherokeeSoccer said:
I tested all jumpers as well as tried various lengths. From my understanding of the research I’ve done, coax length doesn’t really do anything but fool the meter into thinking the swr is good. I tried it anyway with no changes.
Click to expand...

Coax length shouldn't be an issue - sometimes a jumper cable that is too short between a radio and a amp can cause problems but that doesn't sound like it's the case for you.

The reason I suggested checking and swapping coax cable is because cheap coax can often fail or there can be bad connections with the connectors. People throw an old cable between the radio and amp and or amp and meter and get problems and think it's the amp when it's actually the coax.

Did you try turning down the deadkey and seeing if that had any effect?

I'd pick up a lowpass filter - more useful in the long run.
 
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