[ARC5] VHF Guard (121.5 MHz), and Other Guard Channels

[Mike Morrow]

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