On Mon, 08 Dec 2008 17:13:48 -0800, Jim Lux
wrote:

Jeff Liebermann wrote:

Good point, but that's not what I had in mind. I was aware of the
problem and suggested that the power control be run by a micro. The
algorithm needs to be worked out, but basically, the weakest signal
sets the power level. The range of power adjustment also will not be
huge or large enough to have much of an effect on distant stations.
Our current 60 watt amplifier might have the power output reduced to
about 20 watts or 4.5dB power reduction. That's less than one S-unit
and probably wouldn't be noticeable even by distant stations. However,
it will make a huge difference in power consumption and battery
operation run time.

Would it? A lot of amplifiers don't change their power consumption all
that much, especially if they're designed for saturated operation (i.e.
if you reduce the input drive, they either distort, or drop into linear
mode and dissipation goes up).

For FM repeater use, the amplifiers are usually Class C or overdriven
Class AB. The output devices are alternately driven into saturation
and cutoff. I have some measurements somewhere for our various
repeaters buried somewhere. As I vaguely recall, the overall system
efficiency was about 50% for VHF and about 35% for the UHF repeater.
Of course, everything is properly logged and documented:
http://www.LearnByDestroying.com/k6bj/K6BJ%20Repeater/slides/Documentation.html

There are a variety of methods of reducing power, but for Class C, the
easiest is to reduce the collector or drain voltage on the RF output
stages. This works well down to about 20% of rated output power, but
often causes nasty oscillations and parasitics. It also burns quite a
bit of power in the linear voltage regulator. Not ideal, but I know
it works from experience designing marine VHF radios, which were
adjustable and stable from 10-25 watts RF out.

My guess(tm) is that the VHF rack burns about 10A in transmit at
13.6VDC. The UHF box burns about 15A in transmit. The idea is to
reduce this to the absolute minimum, and still have a usable machine.
Ideally, the power control should be automatic.

You're correct that the efficiency drops badly at lower power, but
reducing the overall current consumption is the goal here. The
efficiency may be lousy at low power, but if the overall current
consumption drops proportionately, it's good enough. The idea is to
be able to run the repeater for several days, in an emergency, on
battery power:
http://www.LearnByDestroying.com/k6bj/K6BJ%20Repeater/slides/Batteries.html
I might setup a switching regulator to reduce the power dissipated in
the voltage regulator circuit. I've never tried this but it seems
easy enough. Power amp distortion is not an issue here as the FM
repeater doesn't have any linear components in the RF signal.

The K6BJ repeater:
http://www.k6bj.org
http://www.LearnByDestroying.com/k6b...ter/index.html
http://www.LearnByDestroying.com/k6bj/IRLP/index.html

if you have an amplifier designed for variable power output, then what
you say might be a good idea.

Nope. None of the amps are designed to be operated that way. Few
commerical FM transmitters and power amps are designed for adjustable
power. Such details have never stopped me in the past. However, club
politics, other priorities, and my health issues have slowed me down,
thus nothing has been done.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558