This article is for tubes but tuned inputs are tuned inputs. Some circuits are more finicky about coax length than others.
https://www.w8ji.com/tuned_input_circuit.htm
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Amp feed back and high swr at radio
- Thread starter Jeremy Hull
- Start date
This article is for tubes but tuned inputs are tuned inputs. Some circuits are more finicky about coax length than others.
https://www.w8ji.com/tuned_input_circuit.htm
Ok, then the amp needs the help - theres' not much that can be done if swapping radios doesn't eliminate the problem.
But IT Helps! It PROVES the issue is squarely on the amp - there may be more that one issue going on here.
One, input tuning of the Amp - 50 ohms - that depends on how the SWR between the RADIO and The AMP appear, if there is a SWR mis-match - that may require a known good load but not at 50 ohms, you'll need to use a 75 ohm load to offset, upset - the loading on that amp.
This may sound confusing, but the concept here is to work into a known load that is not perfect, but allows the system to work this without excessive losses and dissipation issues - it's survivable for both radio and amp - at least supposing things are working. You don't do this full power, just enough to get a measurements of SWR at Amp to Load, and Radio to AMP - using the load - either way it is a short test to determine the SWR problems.
Its a lot like the "DUT" on Datasheets - your test jig is your amp - your input is your CB your known reference - you just have to figure out the proper conjugate for both the admittance and transfer characteristics.
Are they reflected back to the radio or is the amp capable of absorbing the mis-match and not pass it back to the radio - or is the amp so mis-tuned - the only recourse is to take it apart and retune the amp to work right.? (Sorry for the long question but it's conditional on many levels.)
Nowadays, though, the good techs use an antenna analyzer, and others simply tune into a 50 ohm perfect load and obtain their highest wattage transfer - you pick...
But IT Helps! It PROVES the issue is squarely on the amp - there may be more that one issue going on here.
One, input tuning of the Amp - 50 ohms - that depends on how the SWR between the RADIO and The AMP appear, if there is a SWR mis-match - that may require a known good load but not at 50 ohms, you'll need to use a 75 ohm load to offset, upset - the loading on that amp.
This may sound confusing, but the concept here is to work into a known load that is not perfect, but allows the system to work this without excessive losses and dissipation issues - it's survivable for both radio and amp - at least supposing things are working. You don't do this full power, just enough to get a measurements of SWR at Amp to Load, and Radio to AMP - using the load - either way it is a short test to determine the SWR problems.
Its a lot like the "DUT" on Datasheets - your test jig is your amp - your input is your CB your known reference - you just have to figure out the proper conjugate for both the admittance and transfer characteristics.
Are they reflected back to the radio or is the amp capable of absorbing the mis-match and not pass it back to the radio - or is the amp so mis-tuned - the only recourse is to take it apart and retune the amp to work right.? (Sorry for the long question but it's conditional on many levels.)
Nowadays, though, the good techs use an antenna analyzer, and others simply tune into a 50 ohm perfect load and obtain their highest wattage transfer - you pick...
So if I’m thinking correctly, any mismatch between the amp and radio (for instance anything that’s not exactly 50 ohms) will then only amplify a mismatch beyond the amp such as an antenna that isn’t exactly 50 ohms.
Correct.
If you know what you should be off, then the measurement is simply the amount reflected back.
This goes for the device you are testing to antenna load (your dummy load).
The SWR will be different, it could be higher or lower - the key here is comparison to known good references of one or more radios
If that reflected power shows SWR to be the same as all other radio equipment you plan to use with it - which in rare instances it may be true, but most likely it isn't - the upset SWR trying to return to the amp - the amps own output network with attempt to change it showing a DIFFERENT SWR conditions than any other equipment using this same setup.
In some "rare" instances, the load is the SAME Resistive value (or closer to) as the output networks' IMPEDANCE of the amp (or other tuned stage) then that means the coax connecting the two - is causing the condition - because it's NOT the same in impedance as the load or that output network.
To determine this, you'll need two lengths of known good 50 ohm coax, one at 18' and the other at 9 foot - test them both in this condition or to test FOR this condition - if the SWR's at your 27MHz band are different between the two - your load and amp are tuned more for each other, than the coax is.
- This experiment shows how coax can be used as a balun, and in some antennas, they are - like the PDL II or a co-phased array.
This is what makes the "urban myth" of coax lengths seem to ring true, but the condition the coax is in versus the expected condition the equipment wants - generates these moments of reflection - SIC.
Alright, so I finally got everything working properly. Well, maybe a better way to say it is I got an acceptable swr at the radio and at the antenna with the amp operating. By using the very helpful info here I made several adjustments in coax jumpers, air coils and some ferrite here and there. Swr at the radio is 1.5 or less (unusually 1.2-1.3) on all used frequencies under all situations. Swr at the antenna system is near flat. I still plan to put the amp on its own circuit, hopefully tomorrow if time allows. And at some point, I may try to adjust the input tune on the amp to aim for perfection but I feel like everything is plenty safe for now. Just wanted to say thanks for all the suggestions and help! Not only did it fix my problem but I was able to learn more about radio equipment for future projects! Thank y’all so much...God bless each of you!!
The coax length myth is a long and winding road. Some of the same folks that say coax length doesn't matter will tell you to use specific lengths of balanced feedline aka "ladder line" Any feedline not terminated into it's surge impedance will act as an impedance transformer.
Realize that, if you use the "jumper" method, you have not solved the problem - only that using jumpers as a temporary fix for other hidden issues, can cause other problems.
Like;
- - if your amp fails,
- - causes erratic operation,
- - gets you jailed
- - we cannot be held responsible to you for your own efforts at trying to tune this Fish.
- Remember there's always a bigger Fish...
If you value those pills do not run that amp with any swr over 1.3. HG pills are known to lose a lot of hfe gain when driven hard. Im sure the 200 watt drive was a test but still can drain hfe. Be nice and it should last a while.