[BC] AM Loads

[RichardBJohnson at comcast.net]

The antenna bandwidth should be so large that normal audio
sidebands are not discriminated. It's the matching networks
that often have high Qs which cause problems with sidebands
even if the matching network is properly designed! As
an extreme example, suppose the antenna was so tall that
its resistive component was 5000 ohms. You need to match
this to 50 ohms. What is the minimum Q of such a network
if you did it in one fell swoop? No slide rules please. It's
Z1/Z2 = 100.  With a Q of 100, at 500 kHz, you are going to
affect modulation components at 500 / Q = 5kHz  

One of the reasons why vacuum tune PA output networks don't
make the impedance transition all at once is that the
Q will be horrible.

To answer the "rotation" question: Suppose the ATU
provided a perfect match at the carrier frequency, but
at 10 kHz below, the match was awful and at 1 kHz above
the match was okay. What would happen to the sidebands?

They would be unsymmetrical, resulting in distortion.

Suppose that everything was fine except that on one
side of the carrier the phase angle was positive and
at the other side it was not negative! What would this
do? Normally nothing unless that angle changed within
the sideband bandwidth. If the angle changed, then
the angle of the modulation sidebands will change
somewhat. If excessive, the result is distortion.

Note that if a tower has, say 30 ohms - j20, and
your network transforms this to 50 +/- j0, it will
do this only at a single frequency so there will be
some "rotation" no matter what you do. However, with
a minimum Q network (50/30 = 1.6), the bandwidth
should be very large so there shouldn't be a problem.

The idea of "rotation" comes from a Smith Chart
representation of the complex impedance. The Smith
Chart has a bunch of circles that rotate around
impedance coordinates.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Ron Youvan" <ka4inm at tampabay.rr.com>

   I'm not a radio guy, (but a HAM) but wouldn't the reflected power with
modulation be dependent on the high frequencies in the modulation and the
bandwidth of the antenna?
-- 
    Ron  KA4INM - around here the TAXing authorities have become quite skilled at taking
                  every opportunity to TAX and it shows.  (while giving very poor service)