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Amp feed back and high swr at radio

J

Jeremy Hull

New Member
Aug 17, 2020
32
6
8
45
This is a “600Hd base unit”...using HG2879c pills.
Driven by a Stryker 955

problem...
I get feed back to the radio receive when I unkey and swr is in the 2.8-3 range on the radio face...verified with a meter as well. This is only on a.m.
*brand new amp...no reflect rf when it left the builder.
*antenna grounded with #10 copper to a ground rod at the base.

Things I’ve tried...
1. New lmr400 from the radio to the antenna (thought the amp might be to much for the rg8x I had in line.
2. Retuned the antenna with an analyzer to near perfection...swr between flat and 1.3 under all conditions even with the amp on.
3. Tried a different antenna...A99
No change with any of these.

Things I’m thinking about trying...
1.A fresh ground on all equipment to a second rod.
2. Pull the 955 from my truck and see if it does the same.
3. Try a small driver between the 955 and 6 pill...the builder tested it this way with about 250 pep drive.
4. Possibly run a dedicated 110 circuit just for the amp...I know it would be stretching it otherwise.

Any other ideas??
 

As a side note...I tried this set up into my 1500w dummy load...no issues. Maybe that piece of info will lend a clue? I know I’m missing something simple.
 
Oscillation problems tend to go away when a dummy load is used. It could be harmonics or spurs from the radio or an issue with the amplifier. If you have a cobra 25 or 29 laying around give it a try. If you still have issues you can try different jumpers between the radio and amp. 6 or 12 feet might make it play.
 
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If the amp was oscillating in tx with the antenna but ok on the dummyload id expect higher vswr with the amp on & oscillating, it could be oscillating when you unkey,

what was the first antenna you tried before the a99 ?
has the a99 got radials ?

cmc can cause the amp to go crazy when you unkey & you see strange things like big signal on the rig & no rx, if that's what happening

have you tried a choke at the antenna ?

changing the jumper length as 543 said changes common mode impedance of the coax braid & that can cure issues,

try winding a choke in the coax that can work too.
 
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Ah, coax jumper with deteriorated connection to the shield?

Have you put a second SWR meter between the radio and the amplifier? If the amplifier's input is too far from 50 ohms this can lead to unstable behavior.

Sounds like an amplifier designed to push the wattmeter as far as possible, not one meant to have clean output.

A higher drive-power level is NOT the solution.

73
 
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543_Dallas said:
Oscillation problems tend to go away when a dummy load is used. It could be harmonics or spurs from the radio or an issue with the amplifier. If you have a cobra 25 or 29 laying around give it a try. If you still have issues you can try different jumpers between the radio and amp. 6 or 12 feet might make it play.
Click to expand...
I’ll try a different radio and see how that works. Also, all the jumpers are 9’ so I will try a 6’ and 12’. Thanks so much!
 
bob85 said:
If the amp was oscillating in tx with the antenna but ok on the dummyload id expect higher vswr with the amp on & oscillating, it could be oscillating when you unkey,

what was the first antenna you tried before the a99 ?
has the a99 got radials ?

cmc can cause the amp to go crazy when you unkey & you see strange things like big signal on the rig & no rx, if that's what happening

have you tried a choke at the antenna ?

changing the jumper length as 543 said changes common mode impedance of the coax braid & that can cure issues,

try winding a choke in the coax that can work too.
Click to expand...

First antenna was a Hustler Ground plane. A99 doesn’t have ground plane radials. And I do not have a choke at the antenna but could certainly give that a try. Thanks!!
 
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nomadradio said:
Ah, coax jumper with deteriorated connection to the shield?

Have you put a second SWR meter between the radio and the amplifier? If the amplifier's input is too far from 50 ohms this can lead to unstable behavior.

Sounds like an amplifier designed to push the wattmeter as far as possible, not one meant to have clean output.

A higher drive-power level is NOT the solution.

73
Click to expand...
Swr between the amp and radio shows high swr as well.
 
good luck jeremy hull,
a dummyload can hide evils that only show up when you try using an amp on a real antenna, let's hope that's not the case for you.
 
By the way, would a coil of coax at the antenna provide a good choke or do I need to order an actual Balun choke? I’d coax works about how many coil should I start with and what diameter? Lmr400 ok or something else? Thanks!
 
Jeremy Hull said:
By the way, would a coil of coax at the antenna provide a good choke or do I need to order an actual Balun choke? I’d coax works about how many coil should I start with and what diameter? Lmr400 ok or something else? Thanks!
Click to expand...

Be careful with the lmr400. The center insulator is foam. A good choke would be about 5 turns on a 4.5 inch form.

choke_impedances.png
 
Jeremy Hull said:
I


I have some lmr400 flex on the way for another project...would that be a better choice for the coil?
Click to expand...
If it has a solid center insulator like rg213 then it would be better. IMO

You can use the lmr400 just don't bend it sharply or wrap it around a small form.
 

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