Regarding the thread I started a couple of weeks
ago...
I found some strong intermod products from AM (medium
wave) broadcast stations on and around 80m when using
a large inverted L and low-pass L-network antenna
tuner. The strongest was the combination of 2 x 1310
kHz + 580 kHz, two local 50 kW (I think) stations in
Ottawa, which was about S9 on 3770 kHz using my K2/100
and not there at all on a Drake TR-7. The intermod
was affected by the preamp and attenuator settings
only to the same degree as normal signals.
After some investigation I have found that this
intermodulation is occurring in the basic K2,
apparently either in the T-R switch or the final (10
watt) amplifier transistors. The KPA100 80m
pi-network actually was acting as an antenna tuner at
1310 kHz which made the intermod slightly worse when
connected through the PA than when in QRP
configuration. VE3TG's (QRP-only) K2 was also tested
on the same antenna and gave nearly identical intermod
levels compared to mine (going by S-meter readings).
So it seems highly unlikely to be a build problem.
The total input voltage at the K2/100 antenna input
from the various local broadcasters was measured on an
oscilloscope at about 1.5V peak-to-peak.
I didn't find an obvious spot to add a high pass
filter to the K2, so I will probably consider building
a 100W-rated filter to add externally between the rig
and the antenna. In most cases a high-pass antenna
tuner is likely to be sufficient, I think.
It would be interesting if someone with the right
skills and equipment was able to make some 3rd order
intercept point measurements on a K2 for out-of-band
signals to see if the expected intermod levels agree
with my observations.
73,
Steve VE3SMA
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