[TowerTalk] Tower and antenna decisions

[Jim Lux]

On 10/27/13 11:46 AM, Richard (Rick) Karlquist wrote:
>
>> That's not necessarily the case.. if the amplifier has variable bias and
>> drain voltages that adjust to what's needed for the match, you can
>> optimize the efficiency at all powers. A simple design with fixed
>> Vdd/Vcc, yes, you'll have a bigger power supply and efficiency will be
>> bad.
>
> That scheme works if the load is resistive and other than 50 ohms, like
> 100 ohms or 25 ohms.  Unfortunately, it doesn't work if load is
> partially reactive, like 50 + j50 ohms.  That is a power factor problem
> and there is simply no way to get good efficiency other than tuning out
> the reactance AFAIK.  One could imagine a hybrid
> system where there was a rudimentary tuner that only tuned out the
> reactive component, and then the scheme you suggested dealt with
> the resistive component not being 50 ohms.  By the time you jump
> through all those hoops, again, it appears to make more sense to
> have an amplifier that just amplifies and a power supply that just
> puts out 48V and a tuner that just tunes.
>
> If you have something better in mind, please educate us.
>
>

You're right.

Basically what you are doing is synthesizing a reactive component with 
an amplifier, much like people do for AF filters.

However, I'm not convinced that with sufficiently clever design you 
couldn't do better than a variable resistor in series with a fixed power 
supply (which is what traditional amplifiers really are, or at least can 
be modeled as such).

In the DC power area, with DC/DC power converters (e.g. motor 
controllers) they deal with reactive loads just fine, and have high 
efficiency.  I don't see that being particularly easy today at RF, but 
that doesn't mean that we shouldn't be looking in that direction.