[TowerTalk] amplifiers for phased arrays

[Jim Lux]

----- Original Message -----
From: "Rick Karlquist" <richard at karlquist.com>
To: "Jim Lux" <jimlux at earthlink.net>
Cc: <towertalk at contesting.com>
Sent: Saturday, March 05, 2005 9:41 PM
Subject: Re: [TowerTalk] amplifiers for phased arrays


> Jim Lux said:
>
> > that some ham, who IS an amplifier designer, would take that on.
There's
> > a
> > lot of new amplifier design approaches out there that are efficient,
have
> > better performance, etc., but the ham market (being relatively tiny)
seems
> > to be willing to buy the same old triode, bipolar, and fet designs that
> > have
> > been around for decades.
>
> Besides the DSP predistortion you mentioned, what are these "new amplifier
> design approaches?"  Do they work with an arbitrary load impedance?  What
> kind of efficiency are you thinking of?
>

I was thinking, in general, of things like envelope elimination/replacement,
or various modifications of outphasing, all of which can use high efficiency
stages (like, for instance, Class E/F or inverse). The real high efficiency
stuff is narrow band, but maybe it might be more cost effective to, for
instance, have a separate driver for each band with its own tuned tank. The
semiconductor technology isn't quite there for real cheap transistors at 30
MHz, but down at 7 or 14, you have things like Rutledge's kW amp using a
high power switching FET.  You might need some clever way to retune the
resonant circuits across the band.

Another intriguing approach, for signals with multiple carriers, each coded
(like COFDM), is an amplifier for each carrier (since the carrier is
constant envelope, even though the overall signal isn't, you can get higher
overall efficiency)

On the subject of impedance matching.. If you're closely coupled to the
antenna, there's no particular reason to ever go to 50 ohms.  Even more
intriguing is the possibility of supplying the reactive power with the
amplifier, rather than passive components (probably make your efficiency go
to heck, though).

I think a reasonable efficiency goal would be 70% or 80%, wall plug to RF.

For all I know, these techniques would be impractical, or have some
fundamental problem.  However, looking over the recent literature in MTT,
for instance, it seems that there might be something to tinker with.  You're
going up against decades of historical development on tube amps and fewer
decades, but still substantial time,  on solid state.  And, ham modulations
don't resemble modulations currently popular in the development world.

There HAVE been some hams poking at new amplifier topologies, but, as far as
I know, they've been looking at CW (or AM), and not the general SSB linear
amplifier.

> BTW, DSP predistortion is not, strictly, an amplifier design technique.
> It has to be applied to the whole transmitter.  It won't help with
> an amplifier module intended to amplify the output of an arbitrary
> exciter.

Certainly true, unless you do something crazy like digitize the output of
the exciter...but then, modulation is the  the easy part of a transmitter..
it's the amplifier that's the challenge.
>
>
> Rick N6RK
> (who IS an amplifier designer)
>