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High SWR With Linear Amp

This problem is very common too. In fact it recently got me when I was hooking up a 2950 on my bench to monitor 10 meter activity while I'm working. I decided to chuck a typical 2 transistor amp behind it and boy was I surprised. Is was wiping out everything from the TV to the FM stereo. The SWR was poor after the amp and driving the amp. It has a 4 stage power switch and in one "gear" it worked OK while it oscillated in every other one.

It took 3 hours to correct this bad design. Adding a tuned input circuit, a 5 pole Chebyshev filter on the output and four more screws added to reinforce the ground path to the emitters of the transistors. I even found high inductance ferrite chokes placed inline with the existing RF feedback loop. The inductors were dropping nearly all of the feedback voltage across them before applying any of it back to the base of the transistors. One thing it had going for it was bias and today it runs super clean and unconditionally stable although it shows about 25% less on the Bird.

I see many folks taking a poor amp like this and using it to drive something bigger in the base like a SB-220. Now we have just made a bad situation ten times worse by adding 10 to 13 db of amplification to a signal that was filthy in the first place. These are important things to consider in the base since it can make or break your ability to use the station.
 
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hey have a 2x8 donkey stomper with a galaxy 959 radio a stryker a10 mag antenna with 18ft of rg8x tram flex coax and a 6ft jumper also rg8x tram coax without amp 1.2 swr with amp on it is in the red
 
Ok to resurrect an old thread ..... Was fine on the Imax testing VQ3 beam 1.1 almost across the board add amp power instant 2.3.
Same exact setup from radio to amp to include the same coax. The only thing changed was antenna. Also ordered a low pass filter just on balance but I didnt need it with the Imax....
Had no problems on the Imax with full power so what gives. Any constructive thoughts or criticisms welcome. I am new to the beam world so I wont pretend to know much as I havent even operated one yet as this vq3 still in test phase. I couldn't get the swr any lower in barefoot stage maybe 30 pep digital true peak reading MFJ meter. Go ahead have at me. Maybe I can learn something.
 
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This is why the RF source in an antenna analyzer must produce as little harmonic energy as possible. It just causes measurement errors.

Hard part to predict is which antenna will show how much error. Just goes to show that using a radio to check antennas is a second-best method.

And using the RF from an external amplifier for an antenna measurement is just asking for trouble. Some amplifiers are cleaner than others, but measuring that part of the picture takes one more piece of gear, just so you'll know why the meter lies to you. Harmonic energy in an antenna circuit just screws up measurement accuracy. The hard part to predict is how much any particular antenna will do this.

73
 

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