[Elecraft] KPA500 - Full Power Keydown

[Jim Brown]

On Sat,4/25/2015 7:55 AM, Johnny Siu wrote:
> Can KPA500 be used for RTTY in a contest condition say at 500W?  If not, do we need to turn it down to 300W?
> Can anyone share with real life experience of using KPA500 in RTTY contest?

I have done that many times, and have posted that IN THIS THREAD. When 
the KPA500 was in beta test, I was loaned a KPA500 for the weekend of a 
major RTTY contest and told to "run it with all the lights lit." I did, 
and it worked great. I bought one as soon as I could, and regularly run 
it in RTTY contests with all the lights lit. According to my calibrated 
LP-100A wattmeter, I get more than 600W out into a matched load on all 
bands except 6M, where I get at least 500W.

The KPA500 is VERY well protected. The fan will speed up as the output 
devices warm up, and if the SWR is too high, drive will be reduced by a 
voltage divider and output power will be reduced. In an RTTY contest, if 
you're in Run mode (calling CQ) and getting answers to most calls, the 
fan will run at a medium speed. If you're not getting answers, it will 
get loud, but the amp will keep on going at full power.

I run JT65 and other WSJT modes on 6M for weak signal work, mostly at 
full power. Likewise, I run JT65 on 160M, at power levels anywhere 
between 10W and 600W, depending on conditions. Again, the KPA500 gives 
me full power. The fan will usually speed up a bit near the end of a 
JT65 transmission.

To correct a perceived misunderstanding about JT65 and operating power. 
JT65 is a WEAK SIGNAL mode, NOT a QRP mode. On the HF bands, it is 
considered good practice to run fairly low power on JT65, JT9, and PSK31 
so that stations with lousy receivers are not overloaded, and also 
because these modes have good enough noise rejection that relatively 
little power is needed IF you have decent antennas, and IF both stations 
have relatively little RX noise. A good USB sound card can easily 
improve JT65 decoding by more than 10 dB -- that's 10X the power.

An example -- a year or so ago, I ran a sked with a guy in ND on 160M. I 
have very good TX and RX antennas on 160M with a Tascam US100 USB sound 
card, and I heard him just fine, even though he was running only about 
50W. As I recall, his signal was about -12 or -14. But he didn't hear me 
at 40W, 100W, 200W. I had to go to 400W to get over his RX noise! This 
is not uncommon -- I can often decode 8-10 signals per pass on 160M, 
many of them from east of Chicago, with signals in the range of -14 to 
-20, but I've got to run 400-500W to get them to hear me. By contrast, 
NO3M, whose 160M station in western PA has very good RX antennas, 
consistently hears me running 5W on CW.

73, Jim K9YC (near Santa Cruz, CA)