Adjusting SWR with amp

[stevejrfan]

It a d&a pdx400 ive tryed everything i know to do even what oldtech03 said seems like its all working fine fan works tubes glow bright relays all work when keyed only for a very quick key and release could be something internal i guess

 

[stevejrfan]

Ok guys heres my problem my swr jumped up out the roof when i hook up my amp no power on same thing with it on out the roof i changed jumpers still the same i can take amp out of line just radio and meter under 1.5 ive tryed just amp and radio no meter swr out the roof new antenna and new coax plus 2 more new jumpers 6 ft from the 3 ft cant figure out why my swrs are high only when i put amp in line took amp to a freind of mine hooked amp up to his set up and amp works fine ive tryed everything i know other than moving it to another room any ideas thanks

 

[GrandNational]

SWR doesn't change with power levels. The accuracy of the meter reading the SWR can and does change with different power levels, that's why you calibrate them.
If the SWR changes when the amplifier is in/out of line, when using a correctly calibrated meter, then it means that the output impedances of the amplifier and radio are not the same. Which one's right? Good question, beats me! Odds say that the radio's output impedance is 'right'er than the typical amplifier's. There are 'rules' about making radios, not about "CB" amplifiers.
So, which should you adjust for? Depends on how much you use that amplifier. If you use it all the time, then make the proper adjustments. If the difference isn't all that much, and if you don't use the amplifier that much, leave it alone.
Where you make those 'adjustments' means a lot. If the amplifier's output isn't at least close to the radio's (hopefully around 50 ohms), then the problem is in the amplifier. Fix the problem -where- the problem exists.
All that assumes you're taking the SWR measurements in the proper place. If you aren't, then they don't mean anything anyway. Right?
- 'Doc

Technically it does, with an amp you’re magnifying a mismatch that may not be seen with low power scenarios. Even with an analyzer you can only go so far then you tune from there to get it closer.

 

[338_MtRushmore]

Technically it does, with an amp you’re magnifying a mismatch that may not be seen with low power scenarios. Even with an analyzer you can only go so far then you tune from there to get it closer.

Yes, it can change if your amp is producing trash that the antenna can't get rid of.

 

[Tallman]

Technically it does, with an amp you’re magnifying a mismatch that may not be seen with low power scenarios. Even with an analyzer you can only go so far then you tune from there to get it closer.

Nope! The antenna tuning does not change with power levels. If you have the antenna tuned to the primary frequency it is good from 1 milliwatt to 10 Kilowatts.
The only reason it changes is because of dirty drive signals ie: a dirty cb radio that has been hacked and tuned up for ghost watts.
When the Harmonic Suppression filters are damaged in the "Tuning" process the dirty radio puts out a trashy signal and your problems begin.

 

[GrandNational]

Nope! The antenna tuning does not change with power levels. If you have the antenna tuned to the primary frequency it is good from 1 mill watt to 10 Kilowatts.
The only reason it changes is because of dirty drive signals ie: a dirty cb radio that has been hacked and tuned up for ghost watts.
When the Harmonic Suppression filters are damaged in the "Tuning" process the dirty radio puts out a trashy signal and your problems begin.

So you’re saying if I take a clean radio and clean amp and use only a carrier and no audio for testing purposes that the swr will not change?

 

[GrandNational]

Nope! The antenna tuning does not change with power levels. If you have the antenna tuned to the primary frequency it is good from 1 mill watt to 10 Kilowatts.
The only reason it changes is because of dirty drive signals ie: a dirty cb radio that has been hacked and tuned up for ghost watts.
When the Harmonic Suppression filters are damaged in the "Tuning" process the dirty radio puts out a trashy signal and your problems begin.

So let’s use a mobile setup for instance. So when I setup a pair of antennas and I’m tuning the hot with an analyzer and do a test key and I have say 10w reflect with carrier only and I go back and tune for lowest reflect and I get around 2w are you saying that I tuned the antenna for trash instead of getting a better match? Sure some people turn every screw and knock traps out of whack and yes an amp will reproduce what’s been stuck in the input but with the right mod levels you can get pretty clean signals no matter the amp.

 

[338_MtRushmore]

Is this a mobile setup, or a keydown setup?

 

[Tallman]

So you’re saying if I take a clean radio and clean amp and use only a carrier and no audio for testing purposes that the swr will not change?

That is correct.

 

[Tallman]

So let’s use a mobile setup for instance. So when I setup a pair of antennas and I’m tuning the hot with an analyzer and do a test key and I have say 10w reflect with carrier only and I go back and tune for lowest reflect and I get around 2w are you saying that I tuned the antenna for trash instead of getting a better match? Sure some people turn every screw and knock traps out of whack and yes an amp will reproduce what’s been stuck in the input but with the right mod levels you can get pretty clean signals no matter the amp.

You should have less reflected power than ten watts. If your system is clean and you put a 100 watts in you should have less than 1 watt reflected.
If your radio has been "Tuned" you are throwing a bunch of harmonics at the antenna and your SWR will fluctuate with power levels. The antenna will reflect all of the harmonics back regardless of the power levels.

I always recommend tuning with the amp out of the circuit altogether. I use just the antenna analyzer.

http://www.mfjenterprises.com/Product.php?productid=MFJ-259C

 

[Dmans]

stevejrfan,

Ok guys heres my problem my swr jumped up out the roof when i hook up my amp no power on same thing with it on out the roof

The above statement sounds to me like an internal (amplifier) connection problem.

took amp to a freind of mine hooked amp up to his set up and amp works fine

The next statement (above) sounds like an internal (amplifier) connection that got moved, jostled or pure dumb luck with moving the unit.

A very careful inspection of all solder joints from the input SO-239 of the amplifier to the output SO-239 is in order. Don't forget relay contact points and the Hi/Low switch or any connection with coax cable attached.

Good Luck!

73's
David

 

[338_MtRushmore]

So let’s use a mobile setup for instance. So when I setup a pair of antennas and I’m tuning the hot with an analyzer and do a test key and I have say 10w reflect with carrier only and I go back and tune for lowest reflect and I get around 2w are you saying that I tuned the antenna for trash instead of getting a better match? Sure some people turn every screw and knock traps out of whack and yes an amp will reproduce what’s been stuck in the input but with the right mod levels you can get pretty clean signals no matter the amp.

I could see this being reasonable. Matching the antenna with an analyser for 1.1, then dead keying "tuned" cobra 29 into a davemade at 500 watts for a higher vswr.

Without details, it is hard to say

 

[OLJEFF]

different power readings from different radios will change it very slightly (the SWR), if the antenna is grounded well the match should stay fairly the same with other radios of the basic same caliber . When you put your amp in line and check the SWR with it on high it should read slightly higher , no more then 1.5 and you should be OK. Lets say you have a 1.1 match and it goes up to 1.3 to 5 , I'd say you were OK , if it goes up past 2 , I'd say you had a problem , as I said , could be ground concerning the antenna ? or it could even be circuitry in the amplifier that has lost it's values in parts that help keep the SWR down in the first place. They have antenna analizers at shops that can be put on your system that help to ween out any problem you might have. IM sure others will have ideas for you on this. Good luck to you.

Mine is three or higher. The match with the imput impedence of the amplifier must be off. How is that adjusted?

 

[OLJEFF]

Oh. Come on. Is there is an adjustment to match the impedence of the radio to the amplifier. Surely all should be considered and the match should be optimal. The match between the amplifier and the antenna should also be optimal. Is there an answer and not a rationalization? Truely we are looking for a knowledgable response. Or is that too much to expect from this forum?

 

[Shadetree Mechanic]

Oh. Come on. Is there is an adjustment to match the impedence of the radio to the amplifier. Surely all should be considered and the match should be optimal. The match between the amplifier and the antenna should also be optimal. Is there an answer and not a rationalization? Truely we are looking for a knowledgable response. Or is that too much to expect from this forum?

Yes