[ARC5] Pre-WW2 technology

[Mike Morrow]

James Cole wrote:

> I am curious about something I recently read: that the ARC-5 (and immediate
> predecessor) was designed in 1938 -  that the basic design of the ARC-5
> series was designed well before WW2. What surprises me is that I was under
> the impression that in the late 30's superhets were either rare or unknown
> (?) and the concept of "miniaturization" i.e., compact design, was
> unheard-of. How was it that the ARC-5 (ARA) emerged from this? Was it some
> particular engineer? In my view the ARC-5 receivers are models of
> ingenuity, and yet they seem to have evolved from...where?

Mike Hanz wrote:

> Oh my...now you've done it...you've released the Tauson monster.  My 
> download bandwidth was bad enough as it wuz... :-)

Mike is too modest to point out the interesting pictures of the A.R.C.
Type K receivers, and the system diagram dated December 1938, that are
found at his site.  See http://aafradio.org/NASM/Hmmmm.html .

While waiting for Michael Tauson to join in, here's my outlook below.

The subject of superhet vs. TRF receiver design that one hears can be
misleading.  The U.S. military sets derived from the Type K (1938) are
far from being the earliest U.S. military aircraft superhets.  The BC-189
(1935) and BC-224-A (1936) liaison receivers, along with the BC-310 (1936)
RDF receiver are all examples of superhets pre-dating the *original* Type
K by at least a year or two.  Then there's commercial stuff like the Radio
Frequency Labs B-3 beacon band receiver and the Bendix RA-1 liaison
aircraft receiver that existed well before the Type K.  I suppose the Type
K derived sets were the first U.S. *command* sets with superhet receivers.

Here's my quick and dirty outline of Type K derived command set timelines:

The original Type K development as a MF/HF command set took place for the
USN in 1937 and 1938.

The first military implementation of the Type K design was the RAT/RAT-1
(1939) two-receiver set (13500 to 27000 kHz) that appears to have been 
*designed* to extend the coverage of existing A.R.C.-made RU-* liaison
receivers (224 to 13575 kHz) to match the coverage of the General Electric
GO-4, -5, -6 (1938/39) liaison transmitters (with 26500 kHz high end).
Subsequently A.R.C. produced the RAV (1940) liaison receiver set (190 to
27000 kHz) that incorporated the RAT-1 and eliminated the RU-* by using
eight local-control-only Type K receivers in two racks of four.  That
appears to have been an unsuccessful approach, compared to the much more
successful competitor from General Electric.  That G.E. RAX/RAX-1 (1940)
three-receiver liaison set (200 to 27000 kHz) weighed less, took up less
space, used less power, had far fewer parts, and had better performing
receivers with more RF and IF amplification and much better bandspread.
The RAX-1 was made in the many thousands throughout WWII, while the RAV
was made in only 50 sets in 1940.

The Type K design didn't achieve much until the liaison receiver designs
were abandoned in favor of the original Type K design as a **command** set.
Three of the original RAV receivers (1500 to 9100 kHz) were incorporated
into the ARA (1940) command receiver set, while two other RAV receivers
(190 to 1500 kHz) were slightly modified to eliminate loop antenna 
connections to join the ARA.  Five transmitters (2100 to 9100 kHz) were
part of the associated ATA (1940).

It appears from studying A.R.C. part number assignments that the ARA
design originally did not include a receiver for 1500 to 3000 kHz, nor
the ATA a transmitter for 2100 to 3000 kHz.  That is somewhat similar
to the design evolution of the USAAF's SCR-274-N (1941) command set.
That set never included a 1500 to 3000 kHz receiver or a 2100 to 3000
kHz transmitter.  But when first issued the SCR-274-N also lacked a
3000 to 4000 kHz transmitter and a 520 to 1500 kHz receiver.  One
suspects that the need to cover the universal air-to-tower civil air
frequency of 3105 kHz motivated adding that transmitter, while potential
use of the AN/ARR-1 ZB-homing system definitely required adding that
receiver.  The only electrical difference between the USN ARA/ATA
and USAAF SCR-274-N systems is the use of low impedance AF by the
USN sets, versus the use of high impedance or configurable high/low
impedance AF by the USAAF sets.

Finally, there is the USN's product-improved, lessons-learned successor
to their ARA/ATA known as the AN/ARC-5 (late 1943).  The receivers all
now had real AGC, and the two LF/MF navigation receivers had a special
AF output delivered to the front adapter panel for ILS localizer service
use.  The loop antenna connections missing since the RAV return for the
navigation receivers.  The three MF/HF communications receivers were
stabilized for locked fixed-tune operation.  The AN/ARC-5 MF/HF
transmitters now numbered eight, covering 500 to 9100 kHz.  These used
shunt- vs. series-fed PA tanks and plate vs. screen modulation.

A four-channel VHF-AM receiver and transmitter were added, plus a
dedicated ZB-homing system receiver (AN/ARR-2) was an integral part
of the receiver system design.

So...in command set development the USN 1938 Type K led to the USN
1940 ARA/ATA which was adopted in simplified form by the USAAF as
the 1941 SCR-274-N.  All of this is before U.S. entry into WWII.
During WWII, the USN late-1943 (or early 1944) AN/ARC-5 superceded
the ARA/ATA as the USN's current command set.  The USAAF never
advanced their HF command set beyond the SCR-274-N.  Their AN/ARC-3
VHF command set took its place.

Comments and corrections or differences in opinion and interpretation
are eagerly solicited.

Mike / KK5F