[TowerTalk] amplifiers for phased arrays

[Tom Rauch]

> 4 SDR-1000 digital radios  (w/100 Watt output) - 4@ $1300
> 4 SGC500 500W amplifiers - 4@$1200
> 4 high power computer controlled tuners  4 @ $500
> 4 radiating elements (one nifty approach would be a pair
of switch
> selectable horizontal dipoles at the top of each of 4
middling high posts)
> PC with necessary multichannel sound card interfaces $1000

Sounds like a great system if you want less significantly
than 1500 watts output, pee-poor IMD, and have time and
money to burn.

> Probably $15K by the time you get it all put together.
Compare that to a
> big tower and a SteppIR or two and a 2kW linear.  It's in
the same general
> ballpark.

Except the single amp and antenna would have much better
IMD, the power would be much higher, and it would be a less
complex and less expensive system. It also would not cost
near 15K to do this with a conventional setup.

> 10-15 years down the road, the potential is for small
modules, costing
> perhaps $500-600 each incorporating the digital stuff and
a suitable 100W
> or so PA, integrated with some reasonably compact
radiating element.  Put
> 10 or 15 of those modules on your roof, along a fenceline,
on a pair or
> three towers, etc.

You'd still never have 1500W power. Not unless the only
phasing you intended to use was a bi-directional pattern
with high sidelobe levels and low gain. And any compact
antennas would still be inefficient.

A simple four square with near optimum phasing has a power
division of 34%, 31%, 31%, and 4%. What does that do to the
idea of using 4 375w modules? It kills it.

You'd need three 500 watt modules and one 50 watt module.
They would also have to tolerate RF impedances varying at a
cyclic rate on the output ports without undue distortion.

Also if this fantasy array were ever built, you could NEVER
use an array with negative resistance element without
intentionally dumping (wasting) power. You also face the
severe technical problem of the amps being subjected to
power returning on the feedline due to mutual coupling from
other elements. I can easily imagine the headaches that
would cause, especially when SS PA's by themselves are awful
IMD producers!

I can't think of any advantage in using an overcomplicated
system like this in an HF application. Not one.

73 Tom