X-From_: [email protected] Wed Jul 31 14:28:14 2002
Envelope-to: [email protected]
From: "Prof. Arnaldo Coro Antich" <[email protected]>
To: <[email protected]>
Subject: RE: Downward modulation
Date: Wed, 31 Jul 2002 10:28:30 -0300
X-Mailer: Microsoft Outlook Express 6.00.2600.0000

Dear amigo Dick:
I was following your observations about the
high power AM broadcast transmitters in
Europe with great attention, and will pass
along some comments...
1. BBC needs to hire someone that knows
how to work with AM high power broadcast
transmitters urgently, as it seems the most
capable people have retired or changed jobs
after the Merlin Communications fiasco
2. DAM, Dynamic Amplitude Modulation
may perhaps save a little of the electricity
bill, but is not worth at all from the quality
point of view
3. PSM , pulse step modulation AM transmitters
are difficult to adjust, hard to understand for the
engineers at the site, and generate lots of
problems due to the switching transients that
happen with the SCRs or IGBT's whichever
the particular transmitter uses
4. PWM pulse width modulation AM transmitters
are a bit easier to tame... but they are also
troublesome
5. Downward modulation usually is an indication
of poor power supply regulation, something
that at some sites using several transmitters
carrying the same audio  ( on different frequencies to cover different
areas ) can be a real problem... BECAUSE dynamic regulation of the three
phase
AC power source then also becomes an
unsourmountable problem...
6. In my humble opinion after 45 years of
practical experience with AM transmitters from
1 watt up to 500 kW power level, there is no
match to a well designed high power transmitter using high level class B
plate modulation ( classic ) with lots of feedback, and a properly
designed and well matched Class C final
amplifier ( after 1965 or so, so called Class D
amplifiers enhancing the conduction angle of
the Class C stage by using a third and fifth
harmonic waveshaping circuits became popular,
reaching efficiencies that are pretty high indeed,
but also generating other problems )
I believe that downward modulation of ANY
AM A3 standard double sideband plus 100 percent carrier transmission is an
engineering
problem that demands immediate attention.
Modulation meters or monitors, usually have
a SET CARRIER meter, and any carrier shift
is easily seen on that meter, that is an essential
instrument to monitor transmitter performance all the time.
Notice that I have not said a word here about
AUDIO PROCESSING of any type whatsoever !
That's another story...
Among the interesting things I have came across
while doing field engineering work , are AM
transmitting antennas that have very narrow
bandwidth ( poorly designed and implemented
antenna tuning and matching systems  ) something that may prove particularly
interesting on the LF AM broadcast band, where I have
never worked, as here in ITU Region II it
is not used.
Please re-post to the LF list, as I can't do it
from my machine...
BTW, my good friend G3LDO while visiting
here in Cuba was the person that gave me
how to subscribe to the LF list.
I have a pair of 813's in the works for LF work,
but my QTH is a very poor location for
a LF transmitting antenna.
Anyway, we are talking to the PTT here
about the 136 kHz band, and have high hopes
of receiving soon a permit to operate with
some kind of experimental license, before
the band is officially approved and added to
our rules and regulations .
73 and DX
YOur friend in Havana
Arnie Coro
CO2KK
Host of Dxers Unlimited
Radio Havana Cuba