I have not seen an answer to your request for information about the
subject radios, so I will give you what I have.
I have a copy of U.S. Army Technical manual (TM) 11-487, dated 2 October
1944, titled "Electrical Communications Systems Equipment". This is a
directory of all, or nearly all, of the ground communications equipment
used by the U.S. Army in WW-II. It contains brief descriptions of each
item, frequency range, a list of other major items used with each piece
of equipment, a picture of the equipment in most cases, etc.
TM 11-487 lists TM 11-859 as being the manual Radio Set SCR-593. The
BC-728 is shown as being the one and only major item comprising Radio
Set SCR-593. As you know, it is a receiver for AM only, covering 2.0 -
6.0 MHz. The receiver is supposed to have a seven foot long,
telescoping whip antenna, AN-75-A, and was powered by a 2.0 volt
lead-acid wet cell, BB-54, which could be recharged by proper connection
of the receiver to a 6 or 12 volt vehicle battery. The vehicular mount
was an FT-338. TM 11-487 states that the purpose of the receiver was
for "...reception of alert or warning messages. Used by CAC/AAA (Coast
Artillery Corps/Anti-aircraft Artillery) and ENG (Engineers)." The
photo of the set shows it being carried by a soldier at his waist,
supported by a shoulder strap. The photo does not show a canvas cover.
Now, about the SCR-510 and 610. First, they are FM sets, not AM as your
message says. Also, the SCR-510 operates in the range of 21.0 MHz to
27.9 MHz, while the SCR-610 operates in the range 27.0 to 38.9 MHz. The
BC-620 is the Receiver-Transmitter unit for the SCR-509 and 510, while
the Receiver-Transmitter unit for the SCR-609 and 610 is the BC-659.
The SCR-509 and 609 were equipped to be "pack sets" or field sets. They
had a Case CS-79 which attached to the bottom of the
receiver-transmitter unit and contained dry batteries for operation of
the set independently of any other power source. The SCR-510 and 610
were equipped for vehicular use. They had either a PE-97 power supply
for 6 or 12 volt operation, or a PE-120 for 6, 12, or 24 volt
operation. The power supply attached to the bottom of the
receiver-transmitter. The vehicular mount was the FT-250. The antenna
for BC-620 was an eight foot, telescoping whip, AN-45, which fastened to
the rear of the receiver-transmitter. For vehicular use, there were a
number of optional antennas using various mast sections and bases. The
important point to keep in mind is that the total length of the antenna
AND the connecting wire from the antenna base to the radio are CRITICAL
to proper operation and prevention of damage to the radio! The antenna
for the BC-659 was a 13 foot, telescoping whip, AN-29, which fastened to
the rear of the radio. The other remarks about the BC-620 antenna also
apply to this one. The BC-620 did not have a loudspeaker, so either a
TS-13 telephone-type handset or an HS-30 headset and a T-17 microphone
were required to operate the set. The BC-659 does have a loudspeaker,
so either a TS-13 or a T-17 mic are sufficient to operate. If you need
further info about the SCR-609/610, I can probably provide it as I have
TM 11-615 which covers that set. I do not have TM 11-605, which covers
the SCR-509/510.
History. Hmmm... All that was a little before my time in the
business. I was a Signal Corps instructor during the Korean War, so the
following is based upon what I know, plus what I have have read, plus
what I learned from talking with people older than I who were in the
business "way back then": late '30s, early '40s.
As for the frequency selection. Just prior to WW-II, the Army realized
that better communications were needed. More frequency channels were
needed, and more reliability of communication was needed. At the time,
most of the Army's tactical radio equipment operated in the low end of
the so-called HF spectrum, i.e.: about 2 MHz to 12 MHz. This range of
frequencies is subject to all kinds of interference such as reception of
sky-wave propagation from transmitters half-way around the world,
day-night fading, atmospheric noise ("static"), etc. Moving tactical
radio operations above 20 MHz reduced those problems because all those
problems diminish as frequency is increased. More channels were
provided for by a (then) new frequency allocation scheme. The range 20
MHz to 27.9 MHz was designated as the "Armored" (tanks, etc.) band, and
27 MHz to 38.9 MHz was designated as the "Artillery" band. Notice the
overlap between bands to allow communication between armor and arty.
This was accomplished by introducing a new family of tactical radios
including the SCR-509/510, 609/610, 508/528, 608/628, and others. Much
later, about the time the Korean War started, another whole new family
of radio sets was introduced which included equipment for a third band,
38 MHz to 54.9 MHz for the Infantry. Channel allocations were 100 KHz
apart, on even .1 MHz frequencies, which provided (then) LOTS of
operational channels, simplified stocking of crystals for
crystal-controlled sets, and made "netting" of groups of radios easy.
FM was chosen as the type of modulation for several reasons, all of
which come down to reliable communication. The clarity of FM was, and
is, far superior to AM. Also, FM receivers exhibit what is known as
"capture effect." What that means is that an FM receiver will "lock on
to" the strongest signal on the frequency to which it is tuned, and
reject weaker signals on that frequency as well as signals on adjacent
channels. This characteristic of FM receivers greatly minimizes
interference problems. I have read that, during WW-II, Germans who
captured American FM equipment in operating condition were amazed at how
much better our FM stuff worked than the AM equipment which they were
still using. A side issue to our use of FM is that Edwin F. Armstrong,
the American who invented it and held the patents, gave the U.S.
Government an unlimited license at no cost to use the technology. This
included manufacture of equipment by many different companies, for
military use, without having to pay license royalties to Armstrong.
I hope this is of some use to you, Laurent, and if I can be of further
assistance don't hesitate to ask. I MAY even answer more quickly the
next time (but no promises!).
Regards,
Ed Greeley
Mobile, Alabama, USA