[ARC5] ARC-5/SCR-274 Navagation Receivers?

Mike Morrow kk5f at earthlink.net
Fri Apr 29 20:26:23 EDT 2011


Bob asked:

> Can someone explain to me how the ARC-5/SCR-274 navigation receivers
> were used.

The use of A-N direction LF/MF beacons has already been explained.  The
existence of such an network from the 1930s until final phase out (in
the 1960s, I believe) across the country meant that even for post-WWII
civil aviation, if a plane had only one piece of radio gear, it would be
a 200 to 400 kHz beacon receiver.  Each directional beacon had four
paths to the beacon on which the A signal and the N signal would merge
and produce a steady 1040 Hz (IIRC) tone.  That was all that was necessary
to indicate to the pilot that the aircraft was travelling along that
path/bearing to or from the beacon.  And the four paths radiating from
the beacon's location did not have to be separated by 90 degrees either.
They were usually "tilted" slightly to align with the next airfield
in the network.  In the USA, most directional LF/MF beacons had been
shutdown by the early 1960s, leaving only non-directional beacons for
which a RDF or ADF must be used to home in.  

The other common military and civil use of the beacon band receiver
was to copy airport towers. Almost all airports with radio facilities
used the standard 278 kHz (or some other nearby frequency) for the
tower-to-aircraft link.  A low-powered aircraft transmitter on 3105 kHz
(later 3023.5 kHz in the early 1950s) served as the standard 
aircraft-to-tower link.  This system survived into the mid-1950s,
and was replaced by systems using a tower/beacon band receiver and a
VHF-AM transmitter (Example: A.R.C. Type 11).  Ultimately, LF/MF
was discarded for tower communications as two-way VHF AM took over
(Example: A.R.C. Type 12).

>There must have been a loop antenna and ADF display that were not
>an actual part of the ARC-5/SCR-274 components.

As others have explained, there were no such additions.  There was
a proposed USAAF set called the AN/ARA-1 that was intended to work
with a BC-453-B to provide directional indication, but I don't
believe it ever got fielded.

The use of a DU-* loop with the USN's RU-* receivers was fairly
common in aircraft with two-man crew in the 1930s, before the
USN ZB/YE-YG 246 MHz homing system was fielded.  The ZB-* adapter
requires a broadcast band receiver to process the ZB output.
The ZB-* became the AN/ARR-1, and it was acquired by both the
USN and the USAAF for use in the Pacific.  This function is the
only real reason for the existance of the ARA/CBY-46145,
SCR-274-N/BC-946-B, and R-24/ARC-5 broadcast band receivers.
However, by the time the R-24/ARC-5 was available, the WECo
AN/ARR-2 246 MHz homing receiver superceded the USN's need for
the AN/ARR-1 and R-24.  It fits into the AN/ARC-5 receiver racks
just like the R-24 would have.  This ZB/YE-YG homing system was
an extremely valuable navigation system for returning pilots
back to their carrier or island, and served on into the mid-1950s.

There was one other ultimately obscure use for the R-23/ARC-5.
The USN proposed to use an ILS system called the ZA-*.  That
was the Navy version of the pre-war commercial Air-Track ILS.
The localizer receiver for this ILS was an RU or ARA beacon
band receiver, while the glide slope receiver was a ZA VHF
receiver around 100 MHz.  You'll sometimes read a statement
in AN/ARC-5 tech manuals that the R-23 and R-24 have a special
audio feed to the front adapter panel for ILS.  The ILS
when AN/ARC-5 was designed was to be an improved ZA system
called the AN/ARN-9.  It would have been fed 90/120 Hz
audio from the R-23 through the MX-19/ARC-5 audio adapter panel.
Look at a R-23 or -24 schematic and you'll see how the 12A6
cathode circuit differs markedly from that of the earlier
navigation receivers, or even from the AN/ARC-5 communications
receivers.  Fortunately, the USN dropped this hard-to-use ILS
and adopted the much superior USAAF SCS-51 ILS (RC-103, AN/ARN-5).

>I'd also like to find documentation on the ARN-6 and ARN-7 radios.

The R-5/ARN-7 is an older design compared to the R-101/ARN-6.
The AN/ARN-7 is essentially nothing more than the excellent
SCR-269-G with four bands instead of three (a band was added for
100 to 200 kHz).  Also, the C-4/ARN-7 control has a MCW/CW switch.

Leslie asked:

> I'd like to add to Bob's question by noting that (at least) later ARC-5
> sets have a two-terminal attachment for the navigational loop, including
> a switching mechanism whereas the earlier ARA and BC-453 sets don't. 
> What were the circumstances that led to the change, and when was it
> made.

Actually, the loop connections on the R-23 and -24 are throwbacks to
the original A.R.C. navigation receivers that were part of the eight
receiver RAV liaison receiver system.  The CBY-46102 (beacon band) and
the CBY-46103 (broadcast band) had those loop connections and switch.
Only about 50 RAV sets were procured, but the CBY-46129 (beacon) and
the CBY-46145 (broadcast) receivers for the ARA were identical to the
RAV versions except for the loop connections and switch having been
eliminated.  For whatever reasons, when the improved R-23 and
R-24 versions were designed for the new AN/ARC-5, the old RAV loop
connections/switch were resurrected.  But there doesn't seem to be
any evidence that the R-23 was ever used with a loop, just as there
doesn't seem to be any evidence that the R-24 was ever used at all,
since the AN/ARR-2 homing receiver was available.

Mike / KK5F


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