James H. Haynes haynes at CATS.UCSC.EDU
Fri Apr 18 13:34:00 EDT 1997

Just for those who may not know what Bill is talking about - in earlier days
there was an air navigation system that transmitted tone-modulated CW using
a tone frequency of 1020 Hz.  They also transmitted voice over these navaids.
So the airplane receiver would have a peak/notch filter for 1020 Hz with a
switch to select "range" (the tone only) or "voice" (suppress the tone) or
"both" (no filtering).

After WWII the surplus stores were full of these things - the FL-5 was a
filter that had a separate switch/jack box; the FL-8 was self-contained
filter and switch.

This so-called LF/MF Range or "four-arm beam" navaid was phased out in the
U.S. circa 1960; although they still use 1020 Hz MCW to send stations
identification on other navaids, and there are probably still some navaids
which also transmit voice.  (The idea there is that a plane with only one
radio needs to be able to receive voice instructions while receiving from
the navaid as well.)  So I don't know if filters like this are still used
in airplane radios.

Someone will have to refresh my memory on whether the operating impedance
for the FL-5 and FL-8 was 2000-ohm headphones or 400-ohm headphones.  I
wouldn't expect one of these filters to work at all well with an 8-ohm

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