[Elecraft] Duty Cycle of FT8 #Elecraft

[Bob McGraw]

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-request at mailman.qth.net wrote:
> Message: 1
> Date: Fri, 10 Nov 2023 09:27:18 -0500
> From: Pete Smith N4ZR<pete.n4zr at gmail.com>
> To:WSJTX at groups.io, Elecraft List<elecraft at mailman.qth.net>
> Subject: [Elecraft] Duty Cycle of FT8 #Elecraft
> Message-ID:<261a35c3-7996-433a-833a-b0a1e7cb22aa at 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