high SWR at radio only when amplifier turned on

[towerdog]

I just finished my mobile installation, TRC-465, Midland 500w NMO mount ant, and a Gray 300 bi-linear.

I had used the amp before with my base setup, no issues other than I never really checked it other than the built in SWR function on my kenwood.

Tested jumper from amp to radio with a 50ohm load, no issue.

Problem is, when I turn the amp on, VSWR jumps up to like 4 or 5 to 1 on high power, less on low, like 2 or 3 to 1.
When amp is cut off I get a 1.2:1 same as before I put the amp in.

I read somewhere on here of a similar problem with a Texas Star but I cant find it. It uses similar "sandbar" type resistors like the TS.

The worst part is, I think a schematic is going to be very difficult to find, the "300 bi-linear" came in many variations. Hope this is a common thing and easy to fix.

 

[MudDuck442]

Assuming I am reading the post correctly..these amplifiers have a few issues. The input swr on them is pretty high on a lot of the "editions" of these amps. The input section was changed several times and so was the swamping part of that circuit. It can be fixed but requires a rework. Also these amps on some of the series lacked negative feedback as well. On some of them the output matching was done horrible as well. I know these are not the things you want to hear but the builder at one point knew of a high 3rd harmonic issue that was created by misdesign and also a spur..and did nothing because it helped wattmeter readings.

 

[psycho]

Assuming I am reading the post correctly..these amplifiers have a few issues. The input swr on them is pretty high on a lot of the "editions" of these amps. The input section was changed several times and so was the swamping part of that circuit. It can be fixed but requires a rework. Also these amps on some of the series lacked negative feedback as well. On some of them the output matching was done horrible as well. I know these are not the things you want to hear but the builder at one point knew of a high 3rd harmonic issue that was created by misdesign and also a spur..and did nothing because it helped wattmeter readings.

The input tuning on Gray 300s was never an issue and there were only 2 versions. 1 version had 4 x MRF492s and the other had 4 x MRF455s. Obviously the amp had been screwed around with.
The output tuning was another story and these amps were notorious for high reflect on the low power stages and low reflect on the high power stages.

 

[towerdog]

something burned out. that much ref power would burn a radio up quick, I am talking something like 5:1 with it on high power. This amp gave years of reliable service to the previous owner, a distant relative who passed away and his son gave it to me. Back in the day, when I was a young boy he had the most powerful setup in the county, I recall riding with grandaddy and hearing "rabbit" 2 counties over, 30 minutes away from him being able to hear you. Only person of anyone I know to get popped by the FCC, and when they raided his stuff they missed this one.

Before I got it it was dead, it appeared as though the power wires had ripped out the back and shorted out the PCB burning up one of the traces of the PCB. I redid everything with solid copper to bypass the bad trace and wiring around the switches and it was back in action.

I used it with about 4 different radios at my base, about 90% on SSB for about 3 months, talked to stations all over the world and local at night, stations I could barely hear could hear me loud and clear.
It worked, it is what it is, a cheap dirty "export only" amp, but it got the job done and did it well.
I am pretty sure its class AB biased as well, read something about if there is an inductor in line with the collector its probably class AB. Not really sure but I seen class-c biased amps on SSB and when turned up high. sound like ass. Over the air, the Grey did not sound, like ass.

Point being, it worked before with no issue until it went in my car last weekend.

The last radio I hooked it up to was my kenwood ts-440, perhaps it drove it too hard and burned out something, as the kenwood can put out enough power to fry it 10 times over, and the built in meter is useless at less than 20 watts RMS so I had to use a cheapo power meter to set my power low enough not to cook something.

When I get home next weekend I am going to check the obvious first, and hook it up to a load. Perhaps my new NMO midland antenna, identical in every way to a Maxrad, is crap. Maybe somewhere in the system its arcing out, supposed to be good to 500 watts but maybe defective coax or coil.

So next weekend I am going to check,
(1)line and antenna
(2)sandbar resistors
(3)???? anything else y'all can think off

thanks

 

[9C1Driver]

"high SWR at radio only when amplifier turned"

With an amplifier in-line you check SWR after the amplifier with an external meter. Do not use the radio's SWR meter. Problem solved.

 

[W5LZ]

It's pretty easy to say that if the antenna system (feed line and antenna) are 50 ohms impedance then the output of the amplifier isn't 50 ohms impedance if the SWR between amplifier and antenna system is higher than 'normal'. That's a fairly common occurrence when using 'no tune' circuits in amplifiers.
It's a very good idea to check the amplifier into a dummy load rather than the antenna. That's because a dummy load is 50 ohms impedance and any antenna system is seldom 50 ohms impedance. You eliminate one 'variable' that way. If when checking into a 50 ohm dummy load the SWR is reasonable then it's a safe bet that the antenna isn't 50 ohms input impedance.
- 'Doc

(It's also a good idea to not expect to see 'perfect' readings using an SWR meter. That's because there are always some variations in the characteristic impedance of things, nothing's 'perfect'. So, a little bit 'off', is actually about as good as you can realistically expect. As long as that 'little bit' really is a little bit, it's not a biggy.)

 

[MudDuck442]

Sorry psycho, I have to disagree, not going to get into a pissing contest, but I have seen at least 4 different versions, duplicates of duplicates maybe... who knows, but they all looked like they came out of the same garage.

 

[towerdog]

Found the problem, harmonics. Checked with another radio for crap in/crap out, and my radio is clean. TX into load and no problems. TX into my base antenna, with about 2mhz bandwidth and i got 1.8-2.4:1, low setting to high setting. Mobile is a very narrow bandwidth antenna, so that explain the really high readings in the car. I got the SWR below 2:1 by putting ground straps on the radio and amp. Still goes through the roof on high power setting though.
No sandbar resistors on this thing either, i was thinking of another amp.
I was reading some old Secret CB articles. Seems high VSWR is almost the norm on amplifiers from those days. I dont think power reflected at the harmonic frequencies will burn out finals as fast as power on the actual frequency. I also got some ideas about replacing the caps on the input and output with trimmers, or run a TVI filter backwards between radio and amp....or sell the thing and buy a RM.

 

[bob85]

It's pretty easy to say that if the antenna system (feed line and antenna) are 50 ohms impedance then the output of the amplifier isn't 50 ohms impedance if the SWR between amplifier and antenna system is higher than 'normal'. That's a fairly common occurrence when using 'no tune' circuits in amplifiers.

no tune amps never have never will be the common cause of high vswr,

there is absolutely no need for an amplifiers output impedance to be 50ohm in order to have a low vswr, amplifier output impedance changes with drive level,

even mistuning an amp that does have output tuning does not cause high vswr unless the amplifier is inherently unstable under certain load conditions,

vswr is determined only by the characteristic impedance of the coax compared to the load impedance,

if turning an amplifier on causes high vswr on an otherwise sound antenna system the amp is unstable/oscillating/spewing harmonics on a frequency where the antenna is mismatched to the 50ohm coax,

testing the same unstable amp into a dummyload rather than the antenna won't eliminate a variable,
it only eliminates your ability to see you have an issue with harmonics as the dummyload will be a good match to the coax at the harmonic frequencies causing the high vswr on the antenna system.

 

[W5LZ]

An amplifier that's 'spewing' harmonics, or is unstable is certainly one reason for a high SWR, but I'm afraid you're mistaken about the mis-tuned output circuits not being a possible problem too. Those 'no-tune- circuits are never going to be exactly 'right' in all circumstances, they can't be by their very nature. An amplifier's output impedance -IS- one of the things that does determine the system's SWR. That's the only reason the output impedance is made to be 50 ohms which is the typical impedance of a common antenna system. 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).
The use of a dummy load is one way of providing a 50 ohm, ->NON-REACTIVE<-, resistive load. 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 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.

I'm afraid that almost every statement you made in post #9 is incorrect.
"no tune amps never have never will be the common cause of high vswr..."
You may not have ever seen it, but I certainly have and I think a lot of other people wouldn't agree with that.
"there is absolutely no need for an amplifiers output impedance to be 50ohm in order to have a low vswr..."
Then why even bother using a 'no-tune' circuit? Of course the output impedance makes a difference.
"vswr is determined only by the characteristic impedance of the coax compared to the load impedance..."
But only if you are doing that SWR measuring between coax and the load. 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.)
"if turning an amplifier on causes high vswr on an otherwise sound antenna system the amp is unstable/oscillating/spewing harmonics on a frequency where the antenna is mismatched to the 50ohm coax..."
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.
"testing the same unstable amp into a dummyload rather than the antenna won't eliminate a variable..."
Of course it does! It eliminates the possibility of reactance in the antenna system since that dummy load doesn't have any reactance.
- 'Doc

 

[bob85]

lmao,

a mistuned output circuit only causes high vswr on an unstable amp that takes off under certain load conditions, its nothing to do with the amp output not been 50ohms,

if the 50ohm coax is terminated with a good 50ohm dummyload you will always have a low vswr, amp output impedance has nothing to do with it,
high vswr is caused by a mismatch between coax and load,

im sure i have had many times the number of no tune amps on my bench than you ever will, i never saw one that caused high vswr on a sound antenna system unless it was spewing harmonics like a palomar/boomer

point us to the respected source of info that says an amplifiers output impedance must match the coax characteristic impedance in order to have a low vswr doc,

point us to the info that says any amplifiers output impedance is the same at different drive levels,

 

[Shockwave]

Twenty five years of field experience and five years of paid education tells me Bob is right on track here. I don't like to pick sides but I certainly will if it involves providing accurate information to the forum. The output impedance of any RF amplifier can only be set at 50 ohms for any ONE level of output power. If the mode of operation causes output power to fluctuate (AM or SSB for example) or you change the drive level to the amp, the output impedance will certainly deviate from 50 ohms. This will NOT cause a high SWR problem.

Here is another way to look at this. Most people understand that maximum power transfer only occurs when the impedance through the system matches. Therefore common sense should tell us that any output level below maximum will result in a source impedance higher than 50 ohms.

Additionally, a solid state amp that causes a severe increase in SWR is almost always the result of a defective amplifier. This can even be the case when the high SWR is observed on the input to the amp because when a solid state amp oscillates, this can be reflected back into the input stages as well. Be careful who you take advice from online because inaccurate info is prevelent online.

 

[bob85]

Here is another way to look at this. Most people understand that maximum power transfer only occurs when the impedance through the system matches. Therefore common sense should tell us that any output level below maximum will result in a source impedance higher than 50 ohms.

only if you have half an idea of how amps work, have built them and had to calculate optimum load conditions from tube or transistor datasheets,

i know some of the guys on here like yourself know which way is up, thanks.

 

[Shockwave]

Some of this I recall from my years working at Motorola. Right when I was getting out of Two-Way radio work, Motorola was coming out with all their new programable no tune radios. Just plug the computer into the mic jack and program the new info including power output with a PA that was not designed to be retuned.

I remember specific information about setting the power level on these radios that relates very well to this topic. The 45 watt radios could only be turned down to 25 watts. If you set the output below 25 watts, the impedance of the power amp would rise too high, dropping more of the RF power in the PA amp and cause it to run hotter than it would at full rated power.

 

[Shockwave]

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).

Doc, we've covered this at least three times in the past and you still promote this incorrect idea. I beg you to connect an older tube output HF rig to your antenna and check the SWR. Now rock the load control trough it's range recalibrating the SWR meter and checking the reading. At no time will you be able to cause an increase in SWR even though you changed the rigs output impedance all the way from about 25 ohms to 100. Do it and check back with us so you can get on the same page.