No subject

initial demise) in the USAF, when it became necessary because of the vast
number of ANG units that were co-located at civilian airports.  Many
aircraft  were fitted with VHF, such as the Wilcox 807 and later the
ARC-186, when they were transferred to the ANG.

Scott W7SVJ

-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of Mike Morrow
Sent: Friday, March 29, 2013 2:05 PM
To: arc5 at mailman.qth.net
Subject: [ARC5] VHF Guard (121.5 MHz), and Other Guard Channels

I wrote:

> The Collins AN/ARC-27 and the Bendix AN/ARC-33 can *not* receive
> 121.5 MHz.  They are UHF-AM only, with a 243 MHz guard channel.

Bob wrote:

> ...I thought they received both.

I'd bet that the survival radios AN/URC-4 (243/121.5 MHz) or the mainly
civilian-use AN/URC-14* (121.5 MHz) when transmitting on
121.5 MHz probably put out a detectable second harmonic on 243 MHz.
(The AN/URC-14* is the 121.5 MHz version of the military 243 MHz AN/URC-11*.
Most (not all) have no military order or contract info on them, and were
used in survival kits for some US commercial
airlines.)

The use of the 121.5 MHz guard frequency by US armed forces is rather rare
after UHF-AM became universal.

The first set that had a VHF guard channel was the USN RT-19/ARC-4, but it
was on 140.58 MHz until 121.5 MHz became standard after WWII.

The second set that had a VHF guard channel was the USN RT-18/ARC-1, but it
was on 140.58 MHz until 121.5 MHz became standard after WWII.

The USAAF SCR-522-A and the AN/ARC-3 (and 16-channel AN/ARC-36 and
48-channel AN/ARC-49 versions) have no guard channel, although the RF
connector marked "IF" on most AN/ARC-3, -36, and -49 receivers was present
to support the addition of an external guard receiver that was never
deployed.  The USN AN/ARC-5 had no guard channel.
But all of these would have one channel that was set to 140.58 or
121.5 MHz.

After WWII, the USN pretty much dropped interest in VHF-AM.  One of the
first successful UHF-AM sets is the RT-58/ARC-12 10-channel unit that was a
direct UHF replacement for the RT-18/ARC-1 VHF unit in existing AN/ARC-1
installations.  The RT-58/ARC-12 has a
243 MHz guard channel.

The USAAF and US Army retained extensive VHF-AM capability in many aircraft,
even after UHF-AM sets like the AN/ARC-27, -33, -34, etc.
became available in the 1950s.  But other than sporadic use of VHF Type 12
gear in smaller aircraft, the AN/ARC-3, -36, -49 served the VHF needs of the
USAF until the AN/ARC-73* digital-tuned (vacuum tube with some
semiconductors) VHF sets appeared in the very late 1950s.  But the AN/ARC-73
had no guard channel.  The AN/ARC-134 (Wilcox 807B) solid state VHF set came
into service in the late 1960s.  The AN/ARC-134 has no guard channel.  There
were also the small panel-mounted transceivers like the VHF-AM AN/ARC-115*
by 1970, and those finally had a dedicated guard channel.

There is a VHF-FM tactical panel-mount transceiver called the
AN/ARC-114* that has an FM guard channel of 40.5 MHz, which is one-third of
the VHF guard frequency of 121.5 MHz, and one-sixth of the UHF guard
frequency of 243 MHz.  The Army's little 1968
AN/URC-68 four-channel portable radio has a channel "G" that covers
243 MHz AM when in UHF mode, or 40.5 MHz FM when in FM mode.  It can be set
to alternately send a beacon tone signal on 40.5 MHz FM, then 243 MHz AM,
back and forth.  Even though this channel is marked G for "guard", the
AN/URC-68 doesn't have a guard channel that is in operation even though some
other channel is selected.

By 1970, a US Army pilot on a UH-1D/H in Vietnam would typically have
control on their C-1611*/AIC panel of

radio 1 (VHF-FM tactical, AN/ARC-131),
radio 2 (UHF-AM command, AN/ARC-51BX),
radio 3 (VHF-AM command, AN/ARC-134), and maybe radio 4 (HF-USB liaison,
AN/ARC-102).

Only the AN/ARC-51BX (radio 2) had a guard channel, on UHF.

Pilots I know say that the VHF capability was used most often for
communication with AH-1 gunship support.  There wasn't much need for a VHF
guard on 121.5 MHz, but 243 MHz UHF guard on the AN/ARC-51BX was vital,
including broadcast of ARC LIGHT operations coming into the region.

Thus, it seems that the USN wasn't interested in VHF at all, and the USAF
and Army weren't much interested in having a guard channel in most of their
VHF sets until the mid- to late-1960s.

Mike / KK5F
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