Amp feed back and high swr at radio

[BJ radionut]

Jeremy: The issue is this. Think about the "Air Foam" center of the 400 or 400 Flex, as you turn the coil the foam will tend to collapse. This allows the center conductor to "migrate" towards the outside of the cable as the coil is made. This will decrease the amount of dielectric material between the outer shield and center conductor. This in mind reduces the power level of the cable, plus the length of the coil actually increases as the "migration" of center conductor occurs.
There are some whom will tell you the "solid" dielectric material tends to reduce this effect, and makes a better choice for winding a long lasting "ugly" balun or "Air Choke"
I tend to agree to a point, much depends on the cable manufactures specs on the minimum "Bending Radius" allowed before degradation of the cable properties begins.
All the Best
Gary

PS: LMR400 or Flex... Bend Radius: repeated inches 4.0
So as you can see the 5 turns on 4.5 inches coil required to correctly wind the "choke" is "barely" with-in specs before the cable begins to degrade> (?)

 

[Jeremy Hull]

Ok...so a quick update. I put up my G31 beams today...installed two air choke as recommended earlier in this thread and by those specs. Still seeing some feedback but not nearly as bad as before. I’m showing about 2:1 swr at the radio when the amp is active...barely anything with the amp off. Between the amp and beams I have nearly a perfect match with the amp inactive and about 1.2:1 active. I also noticed the the swr at the radio comes down to 1.3-1.5:1 after a few seconds when you modulate. I’m thinking now (in addition to providing good grounds to all equipment) that I may try an air choke or ferrite between the radio and amp. I’m also thinking I need to supply the amp with a dedicated circuit as I know the amp and all other equipment on one circuit is maxing that circuit out.

 

[nomadradio]

The higher SWR reading seen when the amplifier is active is really just a measurement of the amplifier's harmonic-frequency output. A low-pass filter on the amplifier's output side would bring the SWR reading back down when the amplifier is running.

73

 

[Jeremy Hull]

Low pass on the output side? That would cure issues between the radio and amp? I ask because I don’t have issues between the amp and antenna. (I’m still learning on all this.)

 

[nomadradio]

If there is a high SWR issue on the amplifier's input side, that's a whole separate issue. Only meant that cleaning up the amplifier's output would also clean up the SWR reading. That amplifier contains no harmonic suppression of any kind.

73

 

[Jeremy Hull]

Yes Sir...the problem is between the amp and radio. I’m running a Stryker 955 that shows swr on the radio...It showed 2.8-3 swr with the amp operating. I have three different swr meters that I’ve had inline after the amp and all read great between the amp and antenna. I’ve actually been making progress...installed and air choke between the amp and radio just a few minutes ago and seems to have cured the issue. Swr reads just slightly higher now with the amp operating.

 

[loosecannon]

lets start from the beginning.

how many transistors are in your amp?


has this radio been worked on by a shop or is it stock?
LC

 

[Jeremy Hull]

6 - hg2879/c transistors. Radio is tuned. But again, I think with the help of all the fine folks here...It’s performing properly as of late last night. Swr between the amp and radio is less than 1.5...swr between the amp and antenna is less than 1.3 with the amp operating. I still plan to ground all the equipment to a second separate rod from the antenna...add a dedicated circuit for the amp...and a few ferrite beads here and there just as a precaution. But everything is working great currently!!!

 

[Handy Andy]

Have you ruled out using a Lo-Pass filter between the Radio and the Amp?

The only reason I ask is to help the amp "not to produce" this SWR issue.

It may help you to know that if your amp is taking in everything that radio produces, even a small 2nd harmonic signal will be amplified and sent out to the antenna - thru that amp.

In turn, that antenna will see that harmonic, and that it's being a different frequency, means the antenna will see this power differently than the ones it gets around your 27MHz CB band - the antenna isn't going to be "tuned" to show 50 ohms impedance at that higher frequency.

As far as the power and its' frequency goes; to the antenna - it doesn't matter - but the reflection it causes from being at a different un-tuned frequency it's not meant to use - does show up as a bad SWR problem - or series of problems - having you think it's something it's not...

 

[Shadetree Mechanic]

If the coil of coax fixes the input swr, then the input tune on the amp needs some work. But if the coax coil works, then that will probably be ok too.

 

[Jeremy Hull]

Have you ruled out using a Lo-Pass filter between the Radio and the Amp?

The only reason I ask is to help the amp "not to produce" this SWR issue.

It may help you to know that if your amp is taking in everything that radio produces, even a small 2nd harmonic signal will be amplified and sent out to the antenna - thru that amp.

In turn, that antenna will see that harmonic, and that it's being a different frequency, means the antenna will see this power differently than the ones it gets around your 27MHz CB band - the antenna isn't going to be "tuned" to show 50 ohms impedance at that higher frequency.

As far as the power and its' frequency goes; to the antenna - it doesn't matter - but the reflection it causes from being at a different un-tuned frequency it's not meant to use - does show up as a bad SWR problem - or series of problems - having you think it's something it's not...

Very interesting...never thought of it that way. So I could/should put a low pass filter between the amp and radio?

 

[Jeremy Hull]

If the coil of coax fixes the input swr, then the input tune on the amp needs some work. But if the coax coil works, then that will probably be ok too.

Is it possible the input tune would/could be different directly from the builder to my setup? I ask because he was showing less than 1w reflect even when driving the amp with a 200w driver? Sorry for all the questions...trying to learn as I go here. I appreciate all the help!

 

[Handy Andy]

Very interesting...never thought of it that way. So I could/should put a low pass filter between the amp and radio?

The easiest way to know is to try...

It would not hurt, but do you have another mobile you can place in line instead, this may also show itself to be a contender for this. For if both radios produce this "SWR" problem, the issue then may be from the Amp and it's matching to the Radio - not the Radio to the Amp...

Put it in other terms - the Radio - your stuck with 50 ohms unbalanced - if the amp "appears" as an imbalance - this would show up as SWR issues too.

But if your radio is producing more output in other frequencies the amp can amplify BUT IT"S NOT DESIGNED FOR - can exhibit these same traits.

Best to try the filter to see if it removes the problem - else other methods will have to be looked at - including power delivery may be too much for the amps own good.

 

[Shadetree Mechanic]

Is it possible the input tune would/could be different directly from the builder to my setup? I ask because he was showing less than 1w reflect even when driving the amp with a 200w driver? Sorry for all the questions...trying to learn as I go here. I appreciate all the help!

It might depend on his setup on his bench. Different people might get different results with different setups. If the length of coax makes a difference with swr, then the impedance is not 50 ohms.

 

[Jeremy Hull]

I did try another radio (President McKinley), same result.