Transmitting low power through the PA

[Julian, G4ILO]

I believe that the K3 and K2 are unique in that they switch the PA out of circuit when low power (below 12W in the case of the K3) is used. Why do they do this? Would it not generate a more linear signal and result in less stress on the low power PA components if the high power PA was used, where fitted, even for power levels of 10W or less?

I have recently become interested in the WSPR beacon mode (http://physics.princeton.edu/pulsar/K1JT), which involves transmitting a continuous signal for periods of 2 minutes. At a level of 5W the underside of the K3 gets uncomfortably hot. Since this is the sort of thing you want to leave running all day, I'd prefer the 100W PA with its big heatsink and pair of fans to take most of the strain. Is this possible?

[Dave, G4AON]

Julian the problem with running low power through a high power PA is
that you get poor efficiency. A typical HF transceiver running 5 Watts
output from a 100W PA may be drawing 8 Amps to do so. The Elecraft
solution nicely improves efficiency and works well for most situations.

73 Dave, G4AON
----------------------------------------------------------------------------
I believe that the K3 and K2 are unique in that they switch the PA out of
circuit when low power (below 12W in the case of the K3) is used. Why do
they do this? Would it not generate a more linear signal and result in less
stress on the low power PA components if the high power PA was used, where
fitted, even for power levels of 10W or less?

I have recently become interested in the WSPR beacon mode
(http://physics.princeton.edu/pulsar/K1JT), which involves transmitting a
continuous signal for periods of 2 minutes. At a level of 5W the underside
of the K3 gets uncomfortably hot. Since this is the sort of thing you want
to leave running all day, I'd prefer the 100W PA with its big heatsink and
pair of fans to take most of the strain. Is this possible?

-----
Julian, G4ILO. K2 #392 K3 #222.
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[Julian, G4ILO]

Dave G4AON wrote

Julian the problem with running low power through a high power PA is
that you get poor efficiency. A typical HF transceiver running 5 Watts
output from a 100W PA may be drawing 8 Amps to do so. The Elecraft
solution nicely improves efficiency and works well for most situations.

Okay, but if you are more concerned with reliability than the cost of the electricity bill, wouldn't it be better to run all the components conservatively well within their ratings, than endure the constant heating up and cooling down of the low power PA components?

[AC7AC]

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[hb9brj]

Julian, G4ILO wrote

I have recently become interested in the WSPR beacon mode (http://physics.princeton.edu/pulsar/K1JT), which involves transmitting a continuous signal for periods of 2 minutes. At a level of 5W the underside of the K3 gets uncomfortably hot. Since this is the sort of thing you want to leave running all day, I'd prefer the 100W PA with its big heatsink and pair of fans to take most of the strain. Is this possible?

Julian, I'm quite active in WSPR and can confirm that the LPA module is getting hot. When I contacted Elecraft, they sent me a procedure to align several bias currents on the LPA board. To get access, you have to remove the KPA3. I realigned the bias currents but did not notice any significant difference in the temperature of the bottom cover.

As a workaround, you can set PWR to let's say 15W and reduce the LINE IN (MIC) setting.

Playing with PWR and MIC I can reduce my power all the way down to 100mW (measured with an OHR WM-2 QRP wattmeter). And to my surprise my signal was heard 2500km away...

73, Markus