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

[Jack Brindle]

Randy;

The resolution of the frequency being reported from the K3 to the KAT500 is 8 KHz. This probably does a better job explaining why the relays switch every 8 KHz than bin size. I would bet that once in a while you will find two adjacent frequency segments that fall in the same bin, and no change occurs.
I also suspect that the RF frequency count resolution is 8 KHz (as it is in the KPA500), which will show similar frequencies when transmitting. When operating the KAT500 with the K3/K3S, we usually have the K3 tell the KAT its current frequency, which is why it switches during receive and on 8 KHz boundaries.

I sympathize with you on the water issues. We see a lot of that kind of problem. My “favorite” is the right-angle coax adapter (this one bit me). The cheaper ones do a poor job of constraining the inner conductor, and at high power will arc to ground. I changed out all my adapters for good ones (read: more expensive), and no longer have the issue. Figuring out the problem is usually rather difficult since you can’t actually see the arc results because they are inside an enclosure. You are lucky to find your issue so easily, but then water intrusion is a bit more obvious.

As far as calculating bin size, I suspect that K3WYC’s solution of sending FA; response frequencies into the KAT500’s serial port might actually yield a better solution since those frequencies can have 1KHz (or better) resolution. In this case the relays should switch very close to the actual boundary instead of  at 8KHz boundaries. What I don’t know is if the KAT500 uses RF counted frequency for its bin determination or if it can use the externally reported frequency. I suspect it is the former.

Glad you found the issue. Perhaps you now might find a way to lower the antenna Q to require fewer bins (and perhaps wider bandwidth), but it is good to know the KAT is providing a solution for your antenna the way it is (well, without the water).

73!
Jack, W6FB



> On Nov 30, 2018, at 11:01 AM, Randy Farmer <w8fn at tx.rr.com> wrote:
> 
> 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
> 
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