SWR issues with more power..

I don't see an SWR issue at all. Don't be a slave to the meter.

2.7:1 SWR is like 21.1% loss, 78.9% effective radiated power, 3.16 Watts available.

Or 65 Watts - 21.1 % = 51.3 Watts.

Could be a bad connection. Might need a low pass filter.

My guess it the amplifier is amplifying a spur from the radio or the amp is oscillating and spewing harmonics. Do you have another radio that you can try at the same power level? Preferably an unmodified type accepted cb radio.

Do you have an oscilloscope? If you can look at the waveform you can see if the amplifier is oscillating. Are you 100% sure the coax is good and the connectors are properly soldered? You may try removing the choke, swapping jumper lengths or grounding stuff. It may or may not help...you're taking shots in the dark and might get lucky.

Running a ground wire up the pole to a ground rod shouldn't make a difference but it could if you have a common mode issue. A 25 foot wire will not make an rf ground connection. You will basically have a radial hanging straight down that happens to be connected to a ground rod.

w9cll said:

I don't see an SWR issue at all. Don't be a slave to the meter.

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Maybe not an swr issue but possibly a higher swr reading caused by harmonics from the amp. The ratio should not change with the power level. That antenna should have a very low swr on 12 thru 10. Something is wrong if it doesn't

harmonics making meter give false reading

CB590 said:

I'd almost do a standard 102" while with radials.. .

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IMO that would be a better antenna. Use 3 pieces of wire for the radials and they can be used as guy wires.

My thoughts you are having spurious emissions being rejected by the input tuning / matching coils on the A99 giving you a false SWR reading also. Since the antenna has feed through capacitors internally you should provide a good ground to bleed off static directly from the antenna and not your radio. The ground if done correct will provide a decent counterpoise. If all else fails barrow a antenna tuner and sweep it to see if it’s good.

Are you recalibrating the SWR meter when you modulate to the higher power reading? If not, that is why you are seeing the jump.
What meter are you using?

I 2nd the notion of a harmonic spur (or two) when the amp is on. It would explain the skyrocketing SWR (2.7+) for if the Amp is clean that's one thing - but you say this is in AM mode that you get these readings.

May be a retune of the radio is a necessity, or restore the AMC and ALC of the radio - but that won't fix a noisy carrier AM problem nor on the FM side of things.

Now if FM is clean, then the AM side shows the SWR? Then it refers back to the radio for the spur is what is causing the SWR reflection at the SWR meter AFTER the amp. (Bandwidth or drive level) - FM? Not as much especially if not excessively deviating for the carrier is fixed - not like AM. AM envelope drive - can clip if something like Bias or as mentioned earlier AMC and ALC are not doing the job and riding the Mic gain isn't enough.

Worry about the SWR from the Amp output. The one after the amp is going to be the tell-tale sign of; not of if a neighbor will ever knock - more like, how soon.

AFTERTHOUGHT:
This may have already come up, but as you seem to be concerned about the AM yet the Radio seems to tolerate it, may mean a mod or two - especially an NPC mod, can make the radio exhibit a highly wildly - swinging carrier. This swing in carrier can add a lot more power than the amp can cleanly produce. Now if this is the case, just bring the AM carrier up a bit more and use less emphasis on the swing and re-set the AMC so you don't hurt the amp.

The Balun/Choke you've got is prolly' signaling you have a common mode current - or radiating coax caused by the "grounding problem" you speak of. Something is not equal - but again, speculation persists here - so do the simple stuff like the radio before you have to do some heavy hauling, lifting and otherwise dirty work with fixing a poor grounding loop problem.

The "ground" - unless it's tied off all to the same point - aka the ground at the mast instead of both - or just back at the station. Try grounding at just one location first, the station, then mast - if you get a radiating current - SWR can indicate this ground loop because the Balun or choke is not the matching network - it's to prevent the antenna from re-coupling back to the coax - then you may need to investigate the grounding points and work accordingly. The Balun Choke is telling you something is wrong - so you have nothing to lose if Grounds are poor because unless the radio is really dirty and a simple bias and retune can't seem to tame the SWR - Grounds you're speaking of aren't on the same level...

Hope this helps!.

I wouldn't be so quick to blame the class C thing. Yes it will cause IMD issues, but when checking swr with an unmodulated carrier it's no different than any other amp.

If the amp is just unstable and breaking into oscillation the lack of a bias circuit might actually be making things better.

A quick and dirty method to check for oscillation without a scope is a pep watt meter. Turn the mic gain all the way down, key up in am mode at a setting that gives the high swr and switch between pep and average. The readings should be equal. If they aren't you have a problem.