Top post...

Well, friends, rather than speculate as we most certainly love to do, I just
got off the phone with John, the KFI AM Engineer. (11:15am CDT Oct 21)

He said that althought that meter is a pretty standard toroid type power
sampler (very like the common ham units, but it has no reflected output), it
is not designed to read PEP, but couldn't say what they did in the design.
He went on to speculate that the meter ballistics(how fast the meter
responds to changes) may be a factor. He also said that due to the level of
compression used, you won't see much dynamic range (variation of power with
modulation level changes) He said that "it *does* vary if you look
closely."
He advised that you *should have* looked at the final current meter. It
*does* vary widely.

He also said that Harris is doing "some pretty interesting things in
that transmitter." He described the transmitter as a "50kw D to A
converter" with hot redundancy (extra amps on line) and if one of those
smaller amps dies, you can see a "notch" in the output waveform, but that it
doesn't cause any high frequency sidebands--due in part to the filtering
effect of the antenna bandwidth, but he couldn't elaborate if this was the
only bandwidth limiting factor preventing splatter from said notches.
Thinking about it I wonder if "notch" was not the best word, but that "flat
spot" might be mbtter...but then, I didn't think to ask...Oh well.

So there ya go...

One comment I feel compelled to make here. It is all too common for us to
ASSUME that EVERY measurement made is absolutely 100% correct. I see
Engineers do this all the time (and though 99.99% of the time this probably
is right, there are those times when some corrupting factor gives a strange
reading and the experienced Engineer is the one who figures out that
something is amiss IN THE SHORTEST TIME-- cuz' we all get confused by
strange observations). It is sort of a "It says so on the HP digital meter,
so it has to be 100%" assumption. Easy trap to fall into. Meter says
"power, therefore it must be reading MY interpretation of power."

Heep the faith AND keep asking...
--
Steve N, K,9;d, c. i My email has no u's.



"Roy Lewallen" wrote in message
...
I have some ideas, but that's all. I hope someone with more recent
direct experience with AM broadcasting than mine who really knows the
answer will comment.

I will go out on a limb and speculate that the carrier isn't being
reduced during modulation. If it were, simple envelope detectors would
produce serious distortion. And if the carrier isn't being reduced, then
the power has to be greater when modulation is present. But let's see if
an expert will comment -- if I'm wrong I'll gladly eat my words.

Roy Lewallen, W7EL

Bill Turner wrote:


Last year I took a tour of the KFI transmitter site in Southern
California and was fascinated by the 50kW transmitter. On the
transmitter's front panel was a meter calibrated in output power. It
read *steady* at 50kW, except when the operator dropped the power
momentarily to 5kW, just to show he could.

Ever since then, I've kicked myself for not asking why the power didn't
rise with modulation. The transmitter was a Harris model DX50 (IIRC)
which uses dozens of low power solid state modules which are switched on
and off digitally to produce the RF output. Could it be that as they
are switched on and off, they also are driven in such a way as to
maintain constant power? In other words, when modulation is added the
carrier power is reduced? It's the only thing that comes to mind, but
there may be another reason.

Ideas?

--
Bill W6WRT