[ARC5] Interesting documents - Notes on AN/ARC-5 Improvements

[Mike Morrow]

> I still wonder why the receivers were tuneable but the
> transmitters were set at a fied frequency...

Before the AN/ARC-5, command set receiver LO thermal and mechanical stability was not good.  The pilot needed to be able to remotely tune his broad bandwidth ARA or SCR-274-N receivers to the associated transmitter.  Pilot-tuneable command sets were always a curse, especially since the command sets lacked any way to net the receiver to the transmitter in-flight.

That is why by 1943 the new USN AN/ARC-5 command set used stabilized communications receivers R-25, R-26, and R-27/ARC-5 that were specifically designed to be locked pre-flight on a specific fixed frequency the same as the associated transmitter.  No remote tuning control was typically provided.

Only the R-23/ARC-5 beacon band navigation receiver (not typically installed in carrier aircraft, where the R-4/ARR-2 VHF homing receiver was substituted) was used with a C-26/ARC-5 remote tuning box.

A common single-seat carrier aircraft installation would likely have:
Three-Receiver Rack (MT-65/ARC-5) with
1.  R-4A/ARR-2 246 MHz Homing
2.  R-26/ARC-5 3.0 to 6.0 MHz Stabilized Locked-Tuned Communications
3.  R-28/ARC-5 100 to 156 MHz Four-Channel Communications

Two-Transmitter Rack (MT-71/ARC-5) with
1.  T-23/ARC-5 100 to 156 MHz Four-Channel Communications*
2.  T-20/ARC-5 4.0 to 5.3 MHz Communications
* Or T-126/ARC-5 100 to 146 MHz

Controls
C-38/ARC-5 for R-4A, R-26, R-28 (no channel select)
C-30A/ARC-5 for T-20, T-23, and R-28 (channel select)

The receivers except R-4A were connected for A3 voice only, and the transmitter control box emission switch was set for A3 voice and covered.

So...in this ultimate command and homing set, there's no tuning or mode controls for the pilot to worry with (except for the R-4A MF modulation frequency channel selector and VOICE/NAV mode switch).  He's got four channels of VHF plus one channel of HF plus one channel of VHF homing with selection of six pre-set MF modulation frequencies.

That's a tremendous increase in capability in a much easier to use system, compared to the two MF/HF channels, plus one VHF homing or beacon band receiver of the earlier ARA/ATA and SCR-274-N command sets.  And...no tuning dials anywhere.

Pilots universally found the channel-select VHF sets far less troublesome to use, compared to the old coffee-grinder HF sets.  But the locked fixed-tuned receivers and transmitters that the AN/ARC-5 used provided similar capabilty to select a fixed MF or HF channel without any pilot tuning needed...or possible.

Mike / KK5F