Let's see if this gets to the list !
100 percent modulated AM transmitter
will show 22 percent power output increase
during a sustained 100 percent modulation
condition !
Processing modulation to achieve up to
140 percent positive peaks ( strongly assymetric
modulation ) is standard practice, as well
as segmented band processing ( i.e., having
the audio frequency range between 50 Hz and
5000 Hz ( in Region II ) or between 50 Hz and
4500 Hz ( in Regions I and III ) processed
separately in three, four, six and even eight
sub-bands....
This can lead to a nightmare when trying
to adjust the system for a "more competitive
on the air signal "....
BUT, nothing in my opinion justifies DOWNWARD MODULATION..... amplitude
modulation applied
to a carrier wave properly, MUST ALWAYS result in an increase in antenna
current ( more properly current delivered by the feeder to the antenna
tuning network ) ....
PSM transmitters, using minimum economic
steps ( 48 steps ) will follow the audio
waveform, but never as well as 64 or 96 step
PSM modulators.... and PSM using fiber optic
coupling to the power supply IGBT modules
sometimes produce rather "funny"audio when
for some odd reasons an IGBT module falls
"" out of step "" !!! This may cause downward
modulation for obvious reasons.
AM modulation of present day broadcast
transmitters faces yet another problem....
and it is no other than the type of radio
receivers involved at the receiving end...
Mainly small portables with tinny speakers,
home table top radios ( a few of them ) in
plastic cases and with poor low quality
speakers, and automobile radios ( that
in most cases are normally tuned to the
FM broadcast band anyway )
The poor quality receivers then require a very
special form of audio processing.... that includes
the shaping of the audio frequency response
curve, the above mentioned segmented band
audio processing, and a lot of tweaking to
achieve the MAXIMUM PUNCH demanded
by station managers...
Commercial stations with mostly voice content like talk shows do totally
different processing than
stations running the ""oldies"" format , and
international shortwave broadcast stations,
many of which are running on shoestring budgets
sometimes don't even reach 100 percent
modulation when the modulator tubes or "valves"
are used until the filaments ( directly heated
cathodes )are almost exhausted.
Again, there is nothing like a well designed
Class B high power modulator with enough
reserve power to handle modulation peaks,
working with a well designed Class C RF
power amplifier, and both powered from a
well regulated and well filtered HT power supply.
The problem with that old technology is
simply COST... something that is hard to
forget...
A Pulse Step Modulation AM transmitter with
64 IGBT modules, will NEVER require
changing extremely expensive modulator
and driver tubes that have a rather limited
service life... Changing 5 or 10 IGBT modules
per year is nothing compared to the cost
of a pair of modulator tubes for even a
small by today standards 50 kW AM transmitter.
This has led even to "" conversions kits "" that
replace the whole AM Class B modulator
and three phase HT power supply with an
""adapter"" that turns a standard AM high level
plate modulated transmitter into a PSM rig !
By the way, as more and more 1kw and 5 kW
standard AM transmitters are seeing retirement
they should become available for LF conversions
that can be done easily and at low cost...
Only problem is you will need an extension
to the house to install them !!!
73 and DX
Arnie Coro
CO2KK
Soon, hopefully, to be on LF !!!
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