[Elecraft] [KX3] Why more power-efficient at or below 3 W?

[Wayne Burdick]

There are two windings on the PA output transformer: 1:1 and 1:4. We select one of these based on power level, battery voltage, and in some cases, operating mode.

In the 1:1 setting, the PA drain impedance is 50 ohms, making it more efficient for at power levels, reducing current drain.

In the 1:4 setting, the PA drain impedance is 12.5 ohms; the transformer steps it back up to 50 ohms for the low-pass filters. In this case it's more efficient at higher power levels, at the expense of higher current drain.

Wayne
N6KR



> On Jan 26, 2021, at 11:07 AM, Julia Tuttle <julia at juliatuttle.net> wrote:
> 
> The manual states in a couple spots that the KX3 can transmit more
> power-efficiently when output power is at or below 3 Watts.
> 
> Why is this? Is there an (additional?) amplifier stage that is bypassed at
> these lower power levels?
> 
> Thanks,
> 
> Julie
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to n6kr at elecraft.com