WCAP Update

[Garrett Wollman]

<<On Mon, 02 Sep 2013 11:09:01 -0400, John Francini <francini@mac.com> said:

> So then, out of curiosity, for a 5kW transmitter, how much input
> power is needed for a typical vacuum-tube unit versus its
> solid-state equivalent? Would a 5kW transmitter require 10kW from
> the mains (thus dissipating 5kW in other ways (mainly heat))?  Would
> a solid-state unit draw significantly less (like, say, around 6kW
> for the same output power? 

If I recall correctly, the last new generation of small tube
transmitters was something like 55-60% efficient (mostly, I think,
because they only had one tube stage left, the modulator and power
supplies having been replaced with solid-state devices).  Tube finals
can actually be fairly efficient -- the rules assume 80% efficiency
for a plate-modulated final, based on DC power input -- but the power
supply and other ancillary electronics will drag the overall
efficiency down substantially.  (See 47 CFR 73.51(e)-(f).)  For
directional stations, there's also loss in the phasor.

But note that AM station power is stated for an unmodulated carrier.
Once you start superimposing audio on it, the power consumption will
increase.  At 100% modulation, the power output is four times the
carrier power, so a 5-kW station sending a pure tone at 100%
modulation is actually generating 20 kW of power (and thus consuming
as much as 45 kW of electricity, for an inefficient transmitter, or as
little as 22 kW for a very good one).

Of course, AM stations rarely broadcast pure tones for any length of
time, so the average power consumption will be less than this, which
is the key to various efficiency improvements that have been proposed
that allow stations to reduce carrier power during periods of low
modulation such as silences.  Such techniques are commonly used in
(what's left of) shortwave international broadcasting, and the FCC has
been studying them for domestic AM broadcasting as well -- much to the
delight of AM transmitter manufacturers, who have been in a bit of a
slump lately, with so many of their customers being unable or
unwilling to invest in new equipment without a prospect of substantial
returns.

-GAWollman