I have a 160M full length dipole. The center used to be at about
40 feet. About the time version 5.57 of the firmware came out,
the mast supporting the center feed point broke and I re-rigged
with the center at about 22 feet. (I hate debugging when there
were two changes.)
Measurement of the lowered antenna with a MFJ-259B antenna
analyzer show 1.2:1 SWR match at 1.87 KHz with 2,5:1 and 3.3:1
at the band edges from the shack. Because of the mismatch at the
band ends, I usually use the antenna using the KAT3A tuner
installed in my K3. (I don't have similar figures for the higher
antenna, but the SWR curve matches the computer model.)
I used the antenna with modest success including 10 DXCCs and 41
US states confirmed.
When I installed 5.57, I noticed significant power drops on
160M, stepping down from 100W to 25W and 50W. With the help of
Elecraft support, we have concluded that the antenna is
irritating the new firmware feature:
K3 MCU 5.57 / DSP 2.88 / FPF 1.26, 2-16-2017
* FASTER RESPONSE TO HIGH REFLECTED POWER. The transceiver
will now reduce
drive more quickly when transient conditions cause
excessive reflected
power. This roll-back response does not change the set
power level (PWR
knob); instead, it reduces the drive to get below the
reflected power
limit. This means that once a better match is restored
(such as when using
an antenna tuner), the set power level will automatically
be restored.
When I backed off to version 5.50, 160M performance seems to
have gone back to what it was.
Measuring the open-circuit voltage on the antenna feed line with
a oscilloscope shows about 600mV peak to peak. I estimate, based
on eyeballing the trace and measuring other antennas, that about
200mV of that is 60 cycle hum. Elecraft support thinks this is
enough voltage to affect the internal K3 SWR bridge.
The voltage is most likely from AM broadcast stations. The
strongest station, measured with the P3, is KLIV (5000W) about 8
miles from my QTH.
So now, the question is what to do about the problem. Several
things I have been able to think of are:
* Use a broadcast band filter. DXEngineering has one that will
handle 200W, but it wants 50 ohms impedance on both ends.
Therefore it is recommended to install it between the power amp
(internal to the K3) and the tuner (also internal to the K3). I
might be tempted to insert one in the K3, but the tuner and
power amp are connected through a header, not a simple cable I
could reroute to install the filter.
* Get a KAT500 and the filter. Expensive, but straight forward.
* Sometimes the power remains at 100W when the internal tuner is
in bypass mode, but not always. It might remain stable enough
for QSO, but flipping back and fourth seems to be a pain.
Does anyone have any suggestions?
Thanks - Bill AE6JV
-----------------------------------------------------------------------
Bill Frantz | Concurrency is hard. 12 out | Periwinkle
(408)356-8506 | 10 programmers get it wrong. | 16345
Englewood Ave
www.pwpconsult.com | - Jeff Frantz | Los Gatos,
CA 95032
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