[Elecraft] KAT500 investigation results (WAS KPA-1500 issues)

[Randy Farmer]

First things first: My key line connections are correct. A little bit of 
trial and error with the amp on on 160 meters shows that the 
intermittent high VSWR faults I was encountering were due to a "feature" 
of my antenna system. Apparently there is water in the FCP transformer 
box or a bad coax cable that was causing an intermittent arc or other 
breakdown at high power. The problem is much worse at the high end of 
the band, where the antenna VSWR is higher (imagine that!).

All of this led me to spend a couple of hours investigating what the 
KAT500 was actually doing on 160 meters. It turns out that, despite what 
the manual says, the tuning bins on 160, and presumably everywhere below 
3 MHz, are actually 8 kHz wide, not 10 kHz. This was verified by slowly 
tuning the radio and listening to the presets changing. My 160 antenna 
has a VSWR that changes rapidly enough as a function of frequency that 
most every bin had a slightly different tuning solution. This made it 
possible to see the bin boundaries across most of the band. The bins 
start at 1800 kHz and are spaced exactly every 8 kHz. Tuning slowly with 
fine tuning engaged it can be seen that, for example, when the radio is 
tuned from 1807.999 to 1808.000 kHz the KAT will change to the next 
preset. I re-trained the tuner by tuning to the center of each bin: 
1804, 1812, 1816... all across the band and executing a "Tune" operation 
at each frequency.

I had always wondered why the KAT utility reported frequency values that 
were frequently quite different from the actual transmit frequency. It 
turns out that the "Observed Frequency" value displayed in the utility 
Operate window is the low end of the particular tuning bin that has been 
selected by the selection algorithm. 1836 reports as 1832, for example. 
Since my antennas for 80 meters and higher are not extremely high Q, it 
would be difficult to do a similar experiment to determine what the 
actual tuning bin widths are on the higher frequency ranges.

I realize this behavior may be of academic interest only, since the 
firmware does seem to do a good job of selecting the appropriate tuning 
solution, but for a very high Q antenna such as is frequently found on 
160 and/or highly compromised shortened antennas for other HF bands it 
could be important. It would be nice if the Elecraft folks would correct 
the manual to show what the actual tuning bin widths are for each range.

73...
Randy, W8FN

> On 11/29/2018 8:48 PM, Don Wilhelm wrote:
>> Randy,
>>
>> That indicates that you do not have your KAT500 and amplifier 
>> properly connected.
>>
>> The KEYOUT line from the transceiver should go first to the KAT500, 
>> and then from the KAT500 to the amplifier.
>> When the KAT500 needs to tune, the amplifier will not be keyed and 
>> tuning will take place at the power output of the transceiver.
>> After tuning, the amplifier will be engaged.
>>
>> 73,
>> Don W3FPR