K2/100 and RTTY / PSK31 duty cycles

[Bill Coleman-2]

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?


Bill Coleman, AA4LR, PP-ASEL        Mail: [hidden email]
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

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[Don Wilhelm-3]

Bill,

I myself have been wondering about the recommendation for 25 watts vs. the
40 watts - BUT,
The real fact is that the amount of heat is the real determining factor.
One must give consideration for the transmit/receive cycles as well as the
duty cycle of the actual transmitted signal.  If the transmit time is
reduced, the ratings can be increased.

I know that does not provide a really concise answer - so you will have to
create your own based on your typical transmit vs. receive times for
whatever mode you are planning ot use.

You may want to add an auxilary fan to the KPA100 - a 12 volt muffin fan rom
a defunct computer power supply works great - just lay it flat on the KPA100
heat sink positioned so it draws air away from the heat sink.  You can even
add a series resistor to slow the fan a bit and reduce the noise.  Monitor
the temperature rise and experiment with the series resistor so you end up
with a decent operating temperature for whatever mode you are using.

73,
Don W3FPR

> -----Original Message-----

> 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?
>
>
> Bill Coleman, AA4LR, PP-ASEL        Mail: [hidden email]
> Quote: "Not within a thousand years will man ever fly!"
>              -- Wilbur Wright, 1901
>

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[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" <[hidden email]>

>
> 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?
>

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