[Elecraft] K2/100 and RTTY / PSK31 duty cycles

[John, KI6WX]

Bill;
The KPA100 is less efficient at lower power levels.  Most amplifiers are 
designed to have their best efficiency at their maximum power output.

Some digital modes do not transmit 100% power even when the transmitter is 
sending a continuous character stream.  PSK31 is an example; it is both 
phase and amplitude modulated.  For other modes, such as any FSK mode 
including RTTY, the transmitter is running full power output continuously. 
These modes should only be run at 25 watts unless you can supply better 
cooling to the KPA100 heatsink.
-John
 KI6WX

----- Original Message ----- 
From: "Bill Coleman" <aa4lr at arrl.net>


>
> I'm just starting to get into RTTY with my K2 and I'm wondering how much 
> power I should use.
>
> The KPA100 manual recommends 40 watts, with 25 watts for "higher-duty 
> cycle modes such as MFSK".
>
> Now, this statement confuses me for two reasons. First, where does 40 
> watts come from? The KPA100 is rated for 100 watts of CW. Since CW has a 
> 50% duty cycle, it would seem that the raw heat dissipation of the KPA100 
> should handle 50 watts of the 100% duty cycle of RTTY.
>
> So, how do we arrive at 40 watts? Is the KPA100 less efficient when 
> producing 40-50 watts than it is producing 100 watts?
>
> Second question -- how can one have a "higher" duty cycle than 100%? Can 
> someone explain how MFSK can produce more of a strain on the amplifier 
> than being directly keyed down on a single tone continously?
>