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

Fri Mar 29 17:36:56 EDT 2013

I THINK I was confusing GUARD CHANNEL with the EMERGECY frequency.

I remember the earlier VHF sets being crystaled for 121.5 but there was no 
guard channel. The pilot just changed to that channel if necessary.

I have owned URC-4 transceivers. The frequency is selected by how you extend 
the antenna.

But the ARC-27 and ARC-33 did have guard channels.

When I returned to the States in 1954 the Fighter Wings had UHF radios in 
the fighters. But support aircraft like our C-45s still had ARC-5 VHF sets. 
It was not considered necessary to upgrade the support aircraft to UHF.

I did see the ARC-5 VHF sets in th radio shop until at least 1959. But I 
never saw an ARC-3 here in the states.

Did civil airports have UHF capability in the 50's. I thought that was all 
VHF.

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"
----- Original Message ----- 
From: "Mike Morrow" <kk5f at earthlink.net>
To: <arc5 at mailman.qth.net>
Sent: Friday, March 29, 2013 2:05 PM
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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