The complete FT-8 cycle is 30 seconds of which 12.63 seconds is the
actual transmitting period. Therefore (12.63/30)*100 = 42.1%.
As to backing down the power, the measured performance on my KPA500
indicates the highest dissipation does occur at lower power levels. On
20M at 500 watts output the dissipation is 368.0 watts for an efficiency
of 57.6%, while at 200 watts the dissipation if 388.7 watts for an
efficiency of 34.0%. I've concluded, with my KPA500, reducing the
output power is not advantageous with regard to amp performance.
Based on published data in the manual, the duty cycle at 500 watts is
100% for 10 minutes, and 5 minutes of standby. Applying the same
concept, then (10/15)*100 = 66.6%. Clearly FT-8 at 500 watts output is
well below the duty cycle rating of the KPA500.
I suggest one do the same measurements for the KPA1500 and find out what
the real numbers might be and publish the results. To be fair, one
needs an accurate power meter and a known good 50 ohm resistive load and
a little math.
73
Bob, K4TAX
On 11/13/2023 8:03 AM, elecraft-requ...@mailman.qth.net wrote:
Message: 1
Date: Fri, 10 Nov 2023 09:27:18 -0500
From: Pete Smith N4ZR<pete.n...@gmail.com>
To:ws...@groups.io, Elecraft List<elecraft@mailman.qth.net>
Subject: [Elecraft] Duty Cycle of FT8 #Elecraft
Message-ID:<261a35c3-7996-433a-833a-b0a1e7cb2...@gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
I started operating FT8 while my KPA-1500amp was out for repair.? Now
it's back and raising a question:
Operating with just my K-3, set at 100 watts, I would see 100 watts
indicated on its meter through the whole 15-second FT8 transmission..?
When I drive my amp, it only needs 30 watts for full CW power (on 24
MHz),? but the amp indicates 1200 watts input for the whole FT8 transmit
cycle.? I'm wondering if I should back off to protect the
amp,considering the duty cycle is so high - on the other hand, the
temperature reading profile of the amp seems to be pretty much the same
as on CW.
73, Pete N4ZR
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