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.




ODARS Scouts Day, July 2018

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On 12 July the 43rd Scout Group was visited by 4 of ODARS members who demonstrated CW and SSB modes. The scouts had some hands-on experience of Morse code and phonetic alphabetic as part of their Communication badge. The scouts were split into two groups and each scout practised Morse code and was shown the workings of a radio. ODARS set up two radio stations for the event, one to demonstrate and listen to CW QSOs and one for SSB listening. All the scouts showed lots of interest in the event and they did very well, in particular when practising with Morse code.

Marconi Day at the Museum of the History of Science, 21 April 2018

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Many thanks to all of you who helped to make it go well on Saturday. I think everything went to plan, the FT-950 generated and decoded FT-8 signals, Morse code was as popular as ever, Brian’s historical apparatus worked impressively too. I know Colin had some issues with the aerial, but then, don’t we always! We had a steady steam of visitors and many engaged with us. I spoke to an amateur from the USA, and another visitor was going to text a friend in Finland to try and contact us. I even spoke to someone who said he was Marconi’s cousin! It was great to see David Walker, G3BLS there too.

Liz’s talk on women in radio telegraphy was interesting and well received, and people looked like they were listening to me on Marconi as well.

I believe we made around 25 contacts on FT-8 and 2 on CW (thanks Colin). I do have the log in ADIF format and I’ll translate it into human readable and distribute shortly. Robyn, the MHS Public Amazement Officier, took many photos which she will share with us. Anyone else who has good shots of just the ODARS team (no public) please sent to Mark for the web site.

Thanks again to everyone who helped make it happen.


Ken, M1SLH

Ken Taylor, M1SLH, on FT-8 digital mode, 21 November 2017

On 21 November 2017, Ken Taylor, M1SLH, gave a talk about FT-8 Digital Mode.

FT-8 digital mode
FT-8 digital mode

In July of this year a new digital mode was added to the well known WSJT suite of software for weak signal contacts. It is very sensitive and particularly fast, each exchange taking less than 15 seconds, and has become extremely popular.

Using forward error correction, advanced signal processing techniques and a very narrow bandwidth it had proved an easy and reliable way of making DX contacts on HF under less than favourable conditions as we slide down the sunspot cycle.

This talk centres on setting up and operating using FT8, without delving too much into the technicalities of how it does it. It is also planned to give a live demonstration. As several members have already been using the mode for a while, there will, no doubt, be an exchange of tips and tricks afterwards.

John Graham “Surplus Test Equipment” talk, 7 November 2017

On Tuesday 7 November 2017, John Graham delivered his exceptionally learned talk on Surplus Test Equipment for the Amateur Laboratory.  Below are a list of relevant links:

Yahoo groups:






Sources of pdf manuals










PDFs for sale: Artek, Mauritron, etc

Other links


Farson, AB4OJ: Noise Power Ratio (NPR) Testing of HF Receivers:  www.ab4oj.com/test/docs/npr_test.pdf

Verbana, I2VGO: Measurement of all products intermodulation on HF receivers: www.ab4oj.com/test/docs/test_npr.pdf

KE5FX GPIB Toolkit: http://www.ke5fx.com/gpib/readme.htm

The Gliwice Radio Tower by Phil Miller Tate M1GWZ, 19 September 2017

Gliwice Radio Tower
Image available by Andrzej Jarczewski at Polish Wikipedia

There were nine members present, one visitor who later joined, and our guest speaker Phil Miller Tate M1GWZ from Walton on Thames. Several apologies for absence were received.

Phil’s topic was the Gliwice radio station, now in Poland. Originally constructed as a medium wave broadcast station in the 1930’s, it is famous for it’s wooden tower, tallest in the world, probably, and
a botched attack on it by the German SS pretending to be Polish insurgents, giving Hitler an excuse for invading in 1939. It was a thoroughly interesting talk and delivered in an entertaining manner.

Today the station is a museum, and the larch wood tower still survives, held together with brass bolts replacing the original stone hardware. The tower now supports mobile phone antennae, and a low power FM station on 93.4MHz, for any FMDX fans.

We also welcomed James, M0JHF (also K0FOZ) from Cumnor, who has joined ODARS.

The next club meeting will be on Tuesday 3rd October.

ODARS Field Day, 2-3 September 2017

By Chris G7IVF

Many thanks to all those who helped make last weekend’s VHF contest 
event possible. I have submitted the log and for those who are 
interested, here is a link to a map of the contacts we made: http://www.rsgbcc.org/cgi-bin/vhfresults.pl?kml=/2017/wyfWgIfHOx4MEm34JqFCwnDIcyYwu24

The bulk of the interesting DX was obtained on Sunday morning, which was a pity, because just as it was starting to get interesting, the weather 
came and chased us away!

The list of claimed scores for comparison is at http://www.rsgbcc.org/cgi-bin/claim.pl?Contest=144MHz%20Trophy&year=2017

Demonstration of making contacts from the club meeting, 5 September 2017

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There were seven members present at the meeting on Tuesday 5 September 2017, plus an unexpected but very interesting guest.

After a bit of general discussion, Chris G7IVF gave us a report on the weekend activities in the 2m Trophy Contest. The set-up went well using a guyed 11m mast made of scaffolding pole sections for easy transport. This was mounted at the bottom on G4AZN’s rotator, supported by bearings made by Ted G3LPU, and using a new clamp specially purchased for this type of activity. The 2m aerial was Colin M0DDT’s 16 element yagi. This was attached to the K3 transceiver and Elecraft 2m transverter with LMR400 cable and powered from the club generator. However, on Sunday morning the weather worsened and with the prospect of rain, it was decided to abandon the attempt.

Our guest was Jim 7J1AJH. Originally from the USA he now lives in Tokyo Japan and is in Oxford attending a software conference, so looked us up and dropped in. Jim gave us some interesting insights into ham radio operating in Japan.

For the first time, I think, we then set up a live station and made some QSO’s from the hall! Chris brought in his portable apparatus he normally uses for 2m activity contests and was originally planning to set up in the garden outside. However, once again the rain came down so he set up inside to see what would happen. The equipment was the Yaesu FT-857D running 25w battery powered to a portable 5 element yagi which could be broken down for easy transportation. As the evening coincided with the 2m activity night a few local stations were heard at good strength. Chris then tried calling some of them and made 3 or 4 QSOs. To support the antenna, one or two members initially held it aloft, but it was eventually connected to a fibreglass pole, supported valiantly by Jim, our guest.