A Tale of Two Antennas: Brian Hawes G2KQ on the World War Two APN-1 Radio Altimeter

Reading the report on Dayton Hamvention 2018 in the July issue of RadCom my attention was caught by the comment “One ham was delighted to find an antenna for a WW2 radio altimeter and so complete his set.” Now there’s a coincidence. I was at Hamvention this year and one of the items on my flea-market search list was a second AT-4/ARN-1 antenna to complete my RT-7/APN-1 radio altimeter kit. Could there have been two people looking for such an obscure item to complete a set? Could there have been more than one on sale? It seems unlikely. How KC0G, who wrote the item, got this news I do not (yet) know: maybe he sat near me at lunch. Here is a short note to explain the details.

The story started at the Newbury Rally, about six years ago, when I saw an RT-7/APN-1 transmitter-receiver unit for sale in near mint condition. This is the heart of a 440MHz frequency-modulated radio altimeter. It is designed to both measure and control the height of an aircraft in the range 0 – 4000 feet and over 10,000 sets were manufactured for the American Air Force during WW2. The unit is neat in its use of ‘acorn’ valves in the r.f. sections, and also an electro-mechanical variable capacitor to frequency modulate the transmitter. The latter was to an extent the unit’s downfall when they appeared on the surplus market. During the late 1950s or thereabouts one of the popular magazines ran an article on an f.m. 10.7Mc/s (we had cycles in those days) i.f. alignment generator using this capacitor and as a result many units were stripped. Some attempts were also made by amateurs to use them on the 70-cm band but the low power output, about 100-mW, limited their usefulness. The principles of operation of the altimeter are described in several articles on the internet and a couple of references are appended.

My appetite was further whetted when a later Newbury rally yielded the altitude switch and an altitude indicator meter was found on e-bay. Fair Radio in Ohio U.S.A. supplied a copy of the original manual for the set. The antennas, however, were a little harder to find.

I found one antenna at Hamvention in 2013, but these parts are uncommon and I was therefore more than delighted to find a second one at Hamvention 2018.

I was fortunate enough to visit the Udvar-Hazy National Air and Space Museum near Washington Dulles airport in 2013, while visiting my friend Frank Gentges (K0BRA), now sadly S.K. a great loss. Udvar Hazy has the B-29 “Enola Gay” used to drop the first atomic bomb and I was fascinated to see those two ARN-1 antennas, one under each wing. The aircraft is fenced off from the public, but I persuaded one of the museum staff to take my camera underneath and photograph the antennas.

As to what passed between myself and Phil Miller Tate (M1GWZ) who travelled to Hamvention with me, my actual comment was “I’m so pleased to have found that second antenna, now all I need are a few cables and a B-29 and I shall have a working system.”


1.The two dipole antennas, each 290mm long
The two dipole antennas, each 290mm long
Top view of the RT-7/APN-1 unit, r.f. covers removed
Top view of the RT-7/APN-1 unit, r.f. covers removed
The ‘Enola gay’ in the Udvar-Hazy Museum near Washington D.C.
The ‘Enola gay’ in the Udvar-Hazy Museum near Washington D.C.
One of the ARN-1 antennas under Enola Gay’s wing
One of the ARN-1 antennas under Enola Gay’s wing


I mentioned the M.I.T. radiation Lab series on Tuesday. These are a twenty-seven volume, plus index, write-up of the work of U.S.A on radar systems during WW2. They are of course dated now but are an important historical summary. The first two volumes are more general in their approach but the introduction chapters of all are generally quite readable.