On Mon, 22 May 2006 19:47:14 GMT, biascomms
wrote:

Gary Schafer wrote:

Cathode modulation is very similar to grid modulation in performance.
After all audio is placed between the grid and cathode just the same
as it is with conventional grid modulation. The amount of cathode to
plate modulation is minimal.

Correct, and the distortion can be pretty nasty, too.

Screen modulation is similar in performance to grid modulation also.
Efficiency is around 35% carrier efficiency. Very tricky to tune up
properly.

It was the /only/ type of AM I used at first, as I was unable to get big mod
transformers. It worked well enough, though it was difficult to get close
to 100% mod. Lots of envelope feedback cured the non-linearity
distortions.

Low level modulation with a linear amplifier behind the modulated
driver also has an efficiency of around 35%. Excellent AM can be
generated this way as is done with some SSB transmitters in the AM
mode. If properly set up one can not tell the difference between it
and a high level plate modulated transmitter except that the plate
modulated transmitter will probably have higher distortion.

Low-level mod with a linear is very inefficient (with respect to DC input),
but is sometimes the only really practical way to proceed. Again, lots of
envelope feedback can sort out the distortions caused by less than perfect
PAs!

Bob

Plate modulated transmitters are not all that low distortion in
general. It is much easier to get a very clean low distortion AM
signal by doing it at low levels and amplifying with a linear amp,
provided the amplifier is run properly.
At low levels modulation can be done with class A modulation stages as
opposed to typical class B high level modulators and non linear
modulation transformers.

Even with 33% carrier efficiency of the linear amp the total amount of
AC power required from the power company isn't much different. With
the linear amp there are no large modulator tube filaments to light,
screen, bias and plate supplies to run the modulator.

With the new FCC rules limiting the peak envelope power to 1500 watts
it makes more sense to use low level modulation and an amplifier than
it did with the old rules. Now plate efficiency does not matter, you
can still put the same amount of power into the antenna no matter what
the efficiency of the transmitter is. In the past you wanted the
highest plate efficiency that you could muster to get the most power
into the antenna.

I believe that the thinking of the past that "plate modulation is the
only way to go" has more to do with not coming to grips with the fact
that it is about how much power we can put into the antenna and not
how much plate input we run as in the past.

73
Gary K4FMX