high SWR at radio only when amplifier turned on

[W5LZ]

You are right, there are still some misconceptions, but I'm not the one making them. What exactly is the definition of SWR? Now, how did I mis-state what I said? I would suggest you take another shot at it, that one just doesn't 'fly' at all. You are welcome to 'think' of it in any way that makes you happy, but don't try to re-write physics (as I'm accused of doing), it just doesn't work.
- 'Doc

 

[MudDuck442]

Yep..mistuning the amp is just that...mistuning the amp..may reduce power output but the vswr stays the same.

My guess is that the amplifier is producing a strong 3rd harmonic but at the same time the fact that he said that one antenna system shows a better vswr than another tends me to think it is some weird by-product due to an oscillation. If it was only harmonics it would be pretty uniform that this would happen all the time using the same meter and different antenna systems unless by some strange chance the antenna system is just very lossy and makes everything look OK vs. The other antenna system that does not look good with the amp on.

Could be close in back oscillations that the one system is broad banded enough to "pass through" and radiate (oh my) on one but not on the other.

Also and last..I have seen these CB solid state amps oscillate on one antenna system but not another depending on the load, type of load (reactive, resistive, etc) VCC power and other factors..they are just so unstable and touchy a leaf could fall and send it into self oscillation.

Anyways...take a snapshot of the thing. Maybe by just adding some negative feedback of the proper sorts, checking for missing components or "mods" or improper design we can get this thing at least usable. Best to get it on a bench and try different drive levels add some reacance and try different load types reactive and resistive between the amp and dummy load (used this method before to check for stability and vswr ruggedness before) but at the same time turn on the spectrum analyzer and see what we will see. In less than 2 mins we would know what we are dealing with. Fixed, cleaned up, and made run better a lot of cb amps this way especially the "supa bowl boxes" where some guy convinces people that a wattmeter's maul watts are good and blowing out transistors every week is normal because "dats how we roll on da bowl" or because it's "comp class"....actually it was poor tuning of stages, poor design and a lot of oscillation problems self destructing these things. You would be surprised what goes on....or maybe not for that matter.

Didn't want to hijack here..just wanted to add some ideas and at the same time share some experiences.

 

[Shockwave]

You are right, there are still some misconceptions, but I'm not the one making them. What exactly is the definition of SWR? Now, how did I mis-state what I said? I would suggest you take another shot at it, that one just doesn't 'fly' at all. You are welcome to 'think' of it in any way that makes you happy, but don't try to re-write physics (as I'm accused of doing), it just doesn't work.
- 'Doc

Doc, stop debating and filling the forum with incorrect technical nonsense and go tune a tube rig just like I said. I'm dieing for you to come back after doing this and realize there is nothing you can do to an amplifiers output impedance that will cause a change in measured SWR once the meter is recalibrated.

There is no arguing that adjusting the load control on a tube PA will alter the output impedance. This is very basic RF theory here. In fact the specs on lots of tube equipment states that the output can be loaded into antennas ranging from 25 to 100 ohms. Hopefully we can agree on this because there is no chance of bringing you into the light if not.

You've been presented with several ways to prove yourself wrong but you refuse to do this in favor of teaching others your misconceptions. That's offensive to those who spent time trying to share accurate technical info with others. Especially when you post your views that cannot be reproduced in the field. Until you have put your ideas to the test and realize they won't hold a drop of water on this topic, you're wasting everyone's time with misinformation.

I'm not trying to be disrespectful here but you have wasted more time misinforming people than it would have taken you to load up a tube rig and see how far off these ideas are. What is the reason for this? Do you not have any access to a tube rig to run this 60 second test yourself or is it too hard to admit a mistake. Either way the forum can easily learn this lesson on their own by running the test I suggested to you many times before.

 

[bob85]

DOC "An amplifier that's 'spewing' harmonics, or is unstable is certainly one reason for a high SWR,"

bingo

DOC "but I'm afraid you're mistaken about the mis-tuned output circuits not being a possible problem too"

only if the amp is unstable, it is not because the amp output impedance is not 50ohm

DOC "Those 'no-tune- circuits are never going to be exactly 'right' in all circumstances they can't be by their very nature"

if you understood amplifiers you would know any amp is only exactly right at one drive level

DOC "An amplifier's output impedance -IS- one of the things that does determine the system's SWR"

no it is not

DOC "That's the only reason the output impedance is made to be 50 ohms which is the typical impedance of a common antenna system"

no that's not true, they are made close to 50ohms for best power transfere

DOC "If that amplifier's output impedance isn't the same as the antenna system's impedance there will be an impedance mismatch, or an SWR of greater than 1:1 (definition of SWR)."

that's not true, amplifier output impedance does not cause high vswr unless you also incorrectly still believe vswr meters compare whats in front of them to whats behind them which they do not

DOC "The use of a dummy load is one way of providing a 50 ohm, ->NON-REACTIVE<-, resistive load."

that's true

DOC "If by operating an amplifier into a good dummy load you see a very high SWR and if you are using 50 ohm coax, you can certainly assume that the output impedance of the amplifier isn't 50 ohms"

that's technobull, you won't see a very high vswr if the coax is terminated with a none reactive dummyload of its own characteristic impedance

DOC "That's just one of the common ways of determining if the output impedance of things is within a usable range, run it into a good dummy load and see what the SWR is"

common to who? you and the people you hypnotized? show us an example of this been used as you claim, its technobull

DOC "I'm afraid that almost every statement you made in post #9 is incorrect"

don't be afraid, i agree your statements are incorrect but you can learn the truth if you drop the dogmatic attitude, do some simple tests, do some reading from respected sources

DOC "You may not have ever seen it, but I certainly have and I think a lot of other people wouldn't agree with that"

you may have seen it but you misunderstood what was causing it,
you won't find anybody that has half a clue will agree with you,

DOC "Then why even bother using a 'no-tune' circuit?"

in order to improve power transfere

DOC "Of course the output impedance makes a difference"

only to power transfere, it does not cause high vswr like you keep telling people who come to the forum for advice

DOC "but only if you are doing that SWR measuring between coax and the load"

vswr ignoring line loss measures the same anywhere along the line,
if it does not you have cm currents or a faulty meter,
you can find that info in the arrl and many alternative publications,
are you saying they are all wrong?,

DOC "If the output impedance at the end of the coax opposite the load isn't the same as the coax and load then there will be SWR there
(That's also the most common point where SWR is measured.)"

that's technobull that can be tested with a simple test or learned from respected sources, you refuse to do either

DOC "Nope, it means that the impedance of the output of that amplifier isn't the same as the antenna system, which should be 50 ohms"

amplifier output impedance does not cause vswr, show us your reference material that argues with every respected source that disproves your technobull

DOC "Of course it does! It eliminates the possibility of reactance in the antenna system since that dummy load doesn't have any reactance.
- 'Doc"

all using the dummyload does is mask the issue by creating a good match over a very wide frequency range.

 

[W5LZ]

Thank you. You've just proved me right.
- 'Doc

 

[Shockwave]

Thank you. You've just proved me right.
- 'Doc

Doc, it would appear you're just being silly now. Sad that you would use the forum to entertain yourself at the expense of those trying to learn here.

 

[W5LZ]

Shockwave,
And perhaps it would be to your advantage to take a look at what was said in 'bob85's last post, the 'rebuttals'. Then you tell me who's entertaining themselves at the expense of those trying to learn here.
- 'Doc

 

[Shockwave]

Shockwave,
And perhaps it would be to your advantage to take a look at what was said in 'bob85's last post, the 'rebuttals'. Then you tell me who's entertaining themselves at the expense of those trying to learn here.
- 'Doc

I don't think you considered the frustration involved with trying to explain to you 50 different reasons why the SWR meter only reads what is placed after it and is in no way influenced by the source impedance. Bob's rebuttals may have contained some sarcasm however, they also contained accurate information that you ignore as you promote this false idea regarding SWR and any relation to source impedance. If the source impedance does not match the load, the result is less than maximum power transfer. Not an increase in SWR.

I feel his frustration also because I have to tell you, it's very aggravating to spend a good deal of time explaining things here only to have you try and tell everyone it's not so. Of course this could have been over two years ago with you learning too if you only tried to adjust your SWR by turning the load control on any tube power amp and found the same results as the rest of the world does. NO CHANGE IN SWR REGARDLESS OF SOURCE IMPEDANCE. Maybe yelling will help

 

[psycho]

To answer the question.....something has gone wrong in the input tuning of the amp.
Could be a trimmer, capacitor, power stage switch, or resistors on the power stage switch. Also could be the possibility that some super tech was screwing with the amp and put different component values than what was in there to begin with.
Impossible to diagnose from a lazy boy.

 

[W5LZ]

You can yell that the earth is flat but it doesn't make it so. Neither is that BS about the input impedance not determining what an SWR meter reads, only what comes after it.
- 'Doc

 

[vkrules]

To answer the question.....something has gone wrong in the input tuning of the amp.
Could be a trimmer, capacitor, power stage switch, or resistors on the power stage switch. Also could be the possibility that some super tech was screwing with the amp and put different component values than what was in there to begin with.
Impossible to diagnose from a lazy boy.

I agree with you. if the SWR is high, between rig and amp, with the amp running. It's most likely input tuning problem. Or it could be rf getting into the transciever.

 

[bob85]

doc you should be on tv

http://http://www.youtube.com/watch?v=bQmhTW4xqb0

 

[Mudfoot]

doc you should be on tv

http://http://www.youtube.com/watch?v=bQmhTW4xqb0

Link no worky.

 

[bob85]

works ok here

 

[Mudfoot]

Strange...

It works from main page, but will not load from Tapatalk.