[KX3] JT65 and JT9 possible TX issues

[Bob Towers-2]

I was interested to read contributions on this topic.

I recently had my first few JT65 QSOs on 20m using my K2/100. I set the
power level to 5 watts and was astonished to hear the fan at the back of
the K2 come on in the middle of a TX session. The heatsink coped pretty
well and no one complained of drift, so JT65 QSOs were entirely
possible. Had I increased the power to 10 watts, I suspect there may
have been problems.

As far as I'm concerned, JT65 is a weak signal mode and in my book even
5 watts is probably too much!

73

Bob Towers
MM0RKT
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[Jim Brown-10]

On Sat,5/9/2015 2:14 PM, Bob Towers wrote:
> As far as I'm concerned, JT65 is a weak signal mode and in my book
> even 5 watts is probably too much!

Don't confuse "weak signal mode" with QRP. It is considered good
operating practice on the HF bands to run low power -- I'd guess that
3-30 W is pretty much the range. This is because the noise rejection of
JT65 and JT9 makes low power and wet strings possible on the HF bands,
AND because running higher power can drive cheaper receivers into overload.

All of the WSJT modes are often used at high power for weak signal
communication -- like meteor scatter, moonbounce, and long range tropo
at VHF, and for long hauls on 160M. A few years ago, I heard a ZS
calling CQ on 160M. That's a 7,000 mile path for me, so I called him
with the highest power I had available. I didn't work him. :)

The K2/100 and K3/100 will switch in the 100W amp at some power level
determined by software. I don't recall what those levels are, but I'm
certain that going to 18W will get you there. That's less than one
S-unit, so you're not going to make a mess of the JT65 "window" at that
power level. A bit of experimentation with the power control will get
you the answer. And if 5W is too much, go to 3W. That's only 2 dB less.

73, Jim K9YC
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[k6dgw]

On 5/9/2015 2:14 PM, Bob Towers wrote:

> I recently had my first few JT65 QSOs on 20m using my K2/100. I set the
> power level to 5 watts and was astonished to hear the fan at the back of
> the K2 come on in the middle of a TX session. The heatsink coped pretty
> well and no one complained of drift, so JT65 QSOs were entirely
> possible. Had I increased the power to 10 watts, I suspect there may
> have been problems.

For RTTY, I stuck a computer fan to the top of the amp-in-the-lid with a
little tab of velcro, blowing down.  Heat sink stayed totally cool even
after calling CQ with 3 sec breaks almost continuously.  I finally sold
the amp, I began using the K2 as a QRP field rig after I got the K3 so I
can't send you a photo, but it was pretty easy.  When I wasn't on RTTY,
I just pulled the fan off.

If you lower the power below the amp's threshold, I believe it shuts
down and you're using the QRP PA's in the basic K2.  I don't know how
the thermodynamics work in that situation ... that is, if the amp's heat
sink functions with the base PA transistors or not.  It may be that
15-20W [using the amp] would work better than 5W using the basic PA's.
>
> As far as I'm concerned, JT65 is a weak signal mode and in my book even
> 5 watts is probably too much!

Sort of.  JT65 *is* a weak signal mode, and if you're working a QRP
station with perhaps a compromised antenna, he will likely be weak.
Ditto for an EME station, however there is nothing inherently "QRP"
about JT65, and "weak signal" does not automatically imply low power.
In EME usage, some pretty hefty powers and antennas are employed ... and
you'll still be weak coming back down. :-)

JT65 will permit QSO's at and below the noise level regardless of what
the transmit power is ... the trade-off is it requires a lot of patience
and you don't get to be very creative in what you say.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 50th Running of the Cal QSO Party 3-4 Oct 2015
- www.cqp.org



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