[ARC5] APX-6 IFF

[mstangelo at comcast.net]

The APX-6 and APR-4 brings back fond memories of my first and only foray into Microwave operation.

I read about the APX-6 in the surplus catalogs and found out there was an article in September 1960 and February 1961 issues of QST. I wrote to the VHF Editor of QST at the time, E.P. Tilton W1HDQ, about getting copies of the articles. He sent back a typewritten replay on ARRL stationary (I still have the letter) that he was the author and will send me copies of the articles.

I received the articles but was intimidated with the installation of the coax connectors on the anode and cathode transmitter cavities so I put the project on hold. In the February 1968 issue of QST I saw a letter describing another method providing the feedback by soldering inductance straps in the cathode cavity. I wrote and author Jim, W0PFP, asking details about the installation. I received a reply on a memo with an Iowa State University letterhead from Jim detailing the location of the straps. Apparently Jim worked in the Department of Electrical Engineering. (I also kept this memo).

I bought two APX-6 from BC Electronics in Chicago and did the IF strip and cavity mods. I built a dipole on an HN connector and checked out the transmitter by listening to the signal in a APR-4 receiver I picked up at a hamfest. Listening on a receiver I heard pulses which I found out years ago were from the ADS aircraft transponder. I tested the set's range in my parent's yard which was 200 feet long but did not to any further operation because by Ham friends weren't into experimentation. I forgot what I did with those sets.

Years later I mentioned the sets to an office worker who was in the Air Force stationed in Thailand during the Vietnam War. He remembers working on the KY encoders but had no memories the RF IFF unit. 

Mike N2MS