[Elecraft] Antenna Problems with K3 firmware 5.57

[Bill Frantz]

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

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