AN/APX-6

[Maury Markowitz]

I'm trying to understand the history of IFF and I've become confused...

I know that Mark III used ~176MHz as its primary frequency, and had only one "mode".
I also know that Mark X used ~1GHz and had three modes.

So here's the confusion:

http://www.iwm.org.uk/collections/item/object/30117414

This page at the IWM states that the AN/APX-6 is part of Mark IIIG. However:

http://members.home.nl/a.k.bouwknegt/index_bestanden/APX6.htm

This page has images that show it has three modes, and states it operates at ~1GHz.

Ugh.

Anyone have some clarity here?

 

[yellowaster]

I don't believe APX-6 was part of IFF Mk 3 or 3G. Take a look at references to SCR595 (ABK) and SCR695 (ABF) which were the US equivalents.

 

[Maury Markowitz]

So that seems to suggest that this is a Mark V unit, which British sources suggest was not used operationally. Perhaps they mean "... by the RAF.".

 

[yahya]

If this thread is still alive, let me correct some matters.

The early APX-6, which had a distinctive destructor switch on its control panel, adapted the 'security through obscurity.' It came out during the Korean War that this 'transpondor' as it was known in that time, was flawed, and its frequencies were picked up by the adversary. The basic APX-6 is elaborated on these websites:

https://www.vintageavionics.nl/index_bestanden/APX6.htm
https://www.jproc.ca/rrp/rrp3/avenger_apx6.html
Then came the APX-6A, a Mk X equivalent, which used known frequencies just like in the civilian aviation and supported military Mode 1 and Mode 2. It did not have the destructor switch on the control unit for it was not needed. The pre-flight setup had to be done on the ground from the outside of the airframe.

A manual for the APX-6A with sketches can be obtained from here:

https://nrc-digital-repository.canada.ca/eng/view/ft/?id=8180b33a-2345-4bf3-9004-aa6cda8ac19f
Finally, there was a peculiar version of the APX-6B, sometimes called the APX-25, which added the SIF capability, almost an equivalent to civilian 'squawk.' That variant used two control units: one for basic operation just like the APX-6A, and another one used to set the Mode 1 and Mode 3 SIF codes via the AN/APA-89 protrusion. Hence, the pre-flight setup on the ground from the outside of the airframe as in the earlier variants was not necessary. The pilot could change Mode 1, 2 and 3 codes manually from within the cockpit. Mode 2 was usually preset and was equal to the tail number.

That version is elaborated on the website as below:

https://www.vintageavionics.nl/index_bestanden/APX25.htm