[Milsurplus] Intersting item (BC-732, ILS)

[Mike Morrow]

Larry wrote:

>Never saw one of these before.  Not mine evidently.
>220175972613

Clete and others have pointed out that this is an USAAF SCS-51 ILS control box (BC-732).  More specifically, it is for the RC-103 ILS localizer VHF receiver, which provides output to the left-right pointer needle of the I-101 pilot's indicator.  Subsequently, a UHF glideslope receiver of the AN/ARN-5 series was added to the ILS.  The glideslope channel is selected by the same switch on the BC-732 that selects the localizer channel.  By end of WWII, there were six VHF localizer channels paired with three UHF glideslope channels.  The output of the glideslope receiver was sent to the up-down pointer needle of the I-101.  There is an antenna mast (AS-27/ARN-5) consisting of a straight UHF dipole and a swept-back VHF dipole that served the ILS.

This is in addition to the 75 MHz marker beacon receiver system.  It has always interested me that this very excellent late-WWII ILS system is in *every* significant detail the same that is still in worldwide use today, 65 years later.

I have been trying to complete a RC-103 and AN/ARN-5 ILS for some time now, but I've never been able to locate the mount for the BC-732 control box or the mount for the BC-733 localizer receiver.  The control box is common, and can be easily located NIB for about $15.  The BC-733 is still found at larger BA fleamarkets, often at haul-it-away-free prices.

The USN tried to implement a quite different ILS, based on the pre-war commercial Air-Track (TM) ILS.  That system used a MF localizer receiver (initially, an RU-series receiver) and a special VHF glideslope receiver that operated on frequencies that are now part of the FM broadcast band.  Receiver outputs were displayed on a pointer-indicator that was visually similar to, but not interchangable with, the USAAF I-101 ILS indicator.  This USN ILS was the ZA or ZAX system, which apparently developed to the AN/ARN-9.  This is the ILS ro which AN/ARC-5 manuals refer when they describe the purpose of the MX-19/ARC-5 ILS audio adapter and the associated special audio circuitry in the R-23, -23A, -24, and -148 receivers.  It appears that the USN abandoned the Air-Track system by 1944.  Given the superiority of the USAAF ILS, that was a very good decision. 

The only part of the AN/ARN-9 that I've ever seen is the ID-24/ARN-9 pointer-indicator.  I'd love to find a copy of the AN/ARN-9 manual someday.  I doubt that much AN/ARN-9 hardware or documentation survives.

Mike / KK5F