Rob,
I would suggest that on 40 meters and 80 meters, your loop is presenting
a very low impedance to the shack end of the feedline.
Tuners of all types do not work well into very low impedance loads.
Add or subtract somewhere between 1/8 and 1/4 wavelength of feedline at
the problem frequencies and see if that helps your problem. Of course,
that solution possibly could introduce problems on other bands, but it
is worth a try.
73,
Don W3FPR
On 7/3/2015 2:29 PM, Rob KK4R wrote:
I have been having problems with my K3 auto tuner using an antenna that has
worked well and been tuning easily for nearly all bands. My first reaction
was to assume the antenna, an 80m horizontal loop, was the problem, but I am
now convinced otherwise. To prove that, I did the following:
- took the antenna down, refurbished it, and replaced the feed line (not
wasted energy);
- tried other baluns;
- confirmed that my LDG tuner has no problem tuning; and
- tuned it manually with the K3 tuner.
The problem behavior is this. On 40m and 80m, the K3 will not find a decent
SWR (<1.7:1) when auto-tuning. On 80m it has done fine tuning one direction
across the band and failed going the other direction over a significant
portion of the band. This is repeatable. On 40m, the SWR for the antenna
system at the rig end is only about 1.7:1 at the low end and 1.4 at the high
end. At the low end, the K3 tuner will not find a match under when the ATU
TUNE button is used, except in particular circumstances. Using the menu
system to check tuner values, it ends up with the capacitor connected at the
antenna side at a low value and with a low value of series inductance.
Effectively this is near the bypass mode. Hitting the ATU TUNE button to
cause a long tune cycle makes no difference. Manually, the capacitor can be
switched to the transmitter side, and a match near 1:1 is easily set by
adjusting L and C.
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