High SWR With Linear Amp

[brharrison]

Station setup from tower down to table top.

100' Tower
Maco M104 Beam
Yaesu G800 rotor
All coax = LMR 400 coax
Xforce 20012HD Base Amp
Dosy TC-4002-SW Inline Watt Meter
Browning Golden Eagle Mark III Receiver 23 channels
Browning Golden Eagle Mark III Transmitter 23 channels
W2IHY Technologies EQ-Plus
Astatic D-104 Microphone.

Problem I am experiencing:

SWR on channel 1 = 1.2
channel 23 = 1.3

These SWR's are without the linear amplifier running.
Just as soon as I turn the linear amplifier online my SWR's jump above 2+.
Any ideas as to what may be causing this and how may it be resolved?

Thanks for any help,

Ray

 

[Beetle]

Is the meter between the radio and amplifier, or between the amplifier and antenna?

 

[airplane1]

Check your coax jumpers, I do not know if it is yourproblem or not but I was using Rad shack jumpers and had the same happen.

I replaced all my jumpers with 9913F7 jumpers and every thing is great now.

AP

 

[RatsoW8]

If the OP does a forum search he'll find the other 100 posts on this same topic.

 

[W5LZ]

'Where' you do that measuring makes a big difference in determining what a problem is. So, as already asked, where are you doing that SWR measuring?
- 'Doc

 

[Shockwave]

Since the operators watt meter is also his SWR meter, I think we can assume the meter is after the amp. If you want to save yourself lots of time, stop checking your jumpers, do not attempt to adjust the gamma with the amp. Just pack the amp in an appropriate box and ship it back to X Force with the following note: Please stop checking your amps on the bench with a dummy load and start checking them into a resonant load before shipping them out. Your dummy load is consistently hiding oscillations within the amp that are not occurring on the fundamental frequency. Areas that could improve this unstable amplifier include proper negative feedback, insuring low inductance ground paths for the emitter connections and installing a tuned input circuit to the amp. There may be other issues with RF bypassing on the DC line especially if it has an internal driver. To make a long story short, it's the amp not the antenna.

 

[Mudfoot]

Amen.

You'd think they'd learn and build 'em right.

 

[psycho]

Since the operators watt meter is also his SWR meter, I think we can assume the meter is after the amp. If you want to save yourself lots of time, stop checking your jumpers, do not attempt to adjust the gamma with the amp. Just pack the amp in an appropriate box and ship it back to X Force with the following note: Please stop checking your amps on the bench with a dummy load and start checking them into a resonant load before shipping them out. Your dummy load is consistently hiding oscillations within the amp that are not occurring on the fundamental frequency. Areas that could improve this unstable amplifier include proper negative feedback, insuring low inductance ground paths for the emitter connections and installing a tuned input circuit to the amp. There may be other issues with RF bypassing on the DC line especially if it has an internal driver. To make a long story short, it's the amp not the antenna.

The only X Force, Worldwide, Fat Boy, and Davemade amps that acted that way were the ones with built in drivers......and certainly not every one. All the straight 2,4,6, and 8 pills I have ever used had very low reflect. Very unusual.

 

[psycho]

If the OP does a forum search he'll find the other 100 posts on this same topic.

Now on CBRT there are thousands of threads like this.

 

[Oatmeal]

Where is the swr/watt meter at ?......

 

[Shockwave]

The only X Force, Worldwide, Fat Boy, and Davemade amps that acted that way were the ones with built in drivers......and certainly not every one. All the straight 2,4,6, and 8 pills I have ever used had very low reflect. Very unusual.

This is not surprising since most builders do not know how to make the driver play nice in the same box as the final amp. The most important thing here is to RF bypass the DC line feeding the driver. We have to remove any RF on the DC before it can feed the driver stage. It's also a good idea to have a tuned circuit installed between the driver and final. Good builds will shield the driver from the final compartment. One easy way to do this is to cut copper flashing to the appropriate size and solder it down to the ground of the board between the two stages. Notch the bottom edge of the copper so any traces on the circuit board can pass under it without shorting.

 

[Oatmeal]

Hmmm, didnt no this stuff about the amps that has a driver in them....

Im running a XF amp, but mine is a HD amp, and I run my 2995dx with it...
Sweet combo...

 

[brharrison]

If the OP does a forum search he'll find the other 100 posts on this same topic.

Somehow I am not surprised to see this response.
Your response was very helpful btw..I truly appreciate the time you took to offer some assistance with that most helpful repsonse.
FYI - I did a forum search and I read numerous posts for hrs "before I even considered posting my question and my setup"... on this forum and many others and did not find a solution to the problem that I was experiencing which BTW is now taken care. If you had taken the time to see in my setup that I am running a set of vintage Browning Golden Eagle Mark III's and you do a search for yourself and see just how many solutions you find with that setup.

As for the rest of you kind folks, I appreciate you guys and gals for taking the time to offer a solution to the problem I was experiencing. I checked everything from the crown down to the table top and found no issues. I have in the past experienced high SWR with those Brownings and that was leading me back to them so I spoke to some kind folks on another forum and they suggested I place a balun air choke at the feed point to my beam at the top of my tower and this resolved my SWR issues with the linear AMP running. Again I really appreciate the help that was offered.

Have a Merry Christmas.

Respectfully,

Ray

 

[Shockwave]

I checked everything from the crown down to the table top and found no issues. I have in the past experienced high SWR with those Brownings and that was leading me back to them so I spoke to some kind folks on another forum and they suggested I place a balun air choke at the feed point to my beam at the top of my tower and this resolved my SWR issues with the linear AMP running. Again I really appreciate the help that was offered.

Have a Merry Christmas.

Respectfully,

Ray

This cure deserves a closer look. If there was a significant issue with coax radiation before placing the coax balun at the feedpoint, we would expect to see a high SWR when the amp was off also. The degree of mismatch would not change from great without the amp to terrible with it. SWR is a ratio between forward and reflected power and should remain fairly constant until you exceed the power limitations of the antennas matching network.

The sad thing is so many transistorized amplifier problems are misdiagnosed as other issues because an unstable amp can so easily drift between a condition of stability and one where it's kicking out tons of spurious emissions as a result of self oscillations occurring within the amplifier circuit. All types of myths have spawned from this problem. For example, changing coax lengths, adding a TVI filter, changing jumper lengths, or adding a choke.

This all stems from the fact the slightest change in reactance or load impedance can start or stop the offending oscillation. Even if you find a combination that provides a good SWR, the alarm bells should still be ringing because the amp is still very close to breaking into an oscillation again. Maybe next time it rains, a change in frequency, slight reduction in drive power, or using SSB can all bring this problem right back to the surface because the root of the problem has not been addressed.

 

[Captain Kilowatt]

This cure deserves a closer look. If there was a significant issue with coax radiation before placing the coax balun at the feedpoint, we would expect to see a high SWR when the amp was off also. The degree of mismatch would not change from great without the amp to terrible with it. SWR is a ratio between forward and reflected power and should remain fairly constant until you exceed the power limitations of the antennas matching network.

The sad thing is so many transistorized amplifier problems are misdiagnosed as other issues because an unstable amp can so easily drift between a condition of stability and one where it's kicking out tons of spurious emissions as a result of self oscillations occurring within the amplifier circuit. All types of myths have spawned from this problem. For example, changing coax lengths, adding a TVI filter, changing jumper lengths, or adding a choke.

This all stems from the fact the slightest change in reactance or load impedance can start or stop the offending oscillation. Even if you find a combination that provides a good SWR, the alarm bells should still be ringing because the amp is still very close to breaking into an oscillation again. Maybe next time it rains, a change in frequency, slight reduction in drive power, or using SSB can all bring this problem right back to the surface because the root of the problem has not been addressed.

Agreed. It sounds to me like the issue has been simply masked and not solved. There is a considerable difference between the two but both have the same result. Unless you fully understand what is actually causing the problem you cannot truly be sure you solved it versus simply making it disappear. Sometimes two wrongs actually make a right...........or at least make things appear to be right.