It really sounds like something is breaking down when you transmit on
12m, not sure where RFI fits into this. The impedance at the balun-end
of your ladder line can be just about anything depending on frequency
[and length of the line]. The 4:1 balun will divide the magnitude of
that impedance by 4. "Anything" divided by 4 is pretty much still
"anything."
If the SWR went up slowly, it would suggest you're warming up something
in the system that doesn't really want to be that warm. You say it goes
up suddenly? This would suggest something has arced over. Your comment
about reducing the power "stops it" is really good evidence of this.
In the days of NTSC TV, saturating the ferrite in your balun would
generally get some aggravation from your neighbors, waveforms get very
square at saturation. It also would create SWR problems. Perhaps the Z
is very low on 12M, currents very high. I don't know what balun you have.
If you have or can borrow one, put an impedance bridge on the coax side
of your balun and see what the impedance is on all the ham bands. I
really doubt this has anything to do with an RFI problem.
73,
Fred K6DGW
- Northern California Contest Club
- CU in the 2013 Cal QSO Party 5-6 Oct 2013
- www.cqp.org
On 1/8/2013 5:06 PM, Ron W3ZV wrote:
At least I'm pretty sure its an RFI problem. I have a K3, KAT500 and
the KPA500 connected as recommended with Elecraft DB15HD cables. The
antenna is a 120ft. doublet fed with ladder line to a 4:1 balun and
10ft of coax to the KAT500. I have a comment point ground to a ground
rod six foot from the shack. I have operated the amp at 400+ watts
from 80m to 6m. The only band that gives me a problem is 12m. When I
am operating at 400 watts ssb, the amp will fault with a high swr
indication within 5 to 20 seconds. Immediately prior to the fault the
swr on the amp display was 1.2:1. Reducing power to below 300 watts
seems to avoid the problem.
Assuming the consensus is that this is RFI, what would be the most
likely suspect? Coax cables in/out? DB15HD cable? Power cable? What's
the easiest way to diagnose and correct the problem? Thanks
Ron W3ZV
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