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

Posted by wayne burdick on Jan 26, 2021; 7:32pm
URL: http://elecraft.85.s1.nabble.com/KX3-Why-more-power-efficient-at-or-below-3-W-tp7667847p7667849.html

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 <[hidden email]> 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
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