Once again the History of Science Museum in the heart of Oxford has allowed us to take over their historic grade 1 listed building to run a radio station for International Marconi Day. The station comprised an Elecraft K3 transceiver driving a loaded dipole strung across the front of the building and supported by telescopic poles. In spite of the local noise and short time period we had available, we made over 30 contacts and the best DX was Japan. Several other IMD stations were worked in various modes.
In addition we also had Brian G2KQ’s unique collection of vintage equipment, most of which could be demonstrated. Upstairs, as well as the traditional Morse Code activity put on by the museum, Ron G0XGM, brought in his aircraft direction finding receiver and challenged visitors to decode the Morse from the local non-directional beacons (NDBs). This proved extremely popular, in spite of QRM from the station in the basement.
We had over 500 visitors and the event was judged a success. The History of Science Museum is a department of the University of Oxford and is home to the Marconi Collection.
On Saturday 24th April 2021 we participated in International Marconi Day, celebrating Marconi’s birthday. Many of the other special event stations on the air transmitted from locations around the world which had some significance to Marconi, so what has Oxford got to do with him?
Well, Marconi did receive an honorary degree from the University in 1904, but it was 100 years later that the large Marconi collection was presented to the Museum of the History of Science, as it was called then, and to the Bodleian Library. A selection of his most significant early apparatus is on display in the museum. ODARS was invited to the original opening of the collection in 2005 and have put on a station at the museum most years since.
This year, of course, we had the pandemic and so we asked OFCOM to use the special event callsign from our home stations, and this was approved. So we did Working DX from Home, or at least tried to. The museum asked me to look into ways the public could be involved, as they could when we operated from the physical museum, and here is what we came up with.
Anyone may visit a web site called dxsummit.dx and put the special call sign GB4MHS into the search box. The most recent entry will have our current operating frequency. If you happen to have a shortwave receiver that can handle single sideband you should then be able to find us.
However, if you don’t have a receiver you can use an Internet one. Just visit http://websdr.ewi.utwente.nl:8901/ and enter the frequency. You may have to select USB or LSB depending on the wave band in use. Good luck and let us know if you hear us!
In this design, the radiating elements are cut from a steel tape measure. Very thin and light. I have chosen three elements as a reasonable portability/performance balance.
The overall length which is 500 mm is about half that of a conventional design.
This design is intended for occasional outdoor field trips. It was not built as an “erect outdoors and leave for ever” antenna. If you don’t want the added complications of this short beam, you can still make a conventional design with steel tape, just for light weight and ease of construction. Lots of designs on the internet.
The significant feature of this design is the short length.
If the element spacing is reduced, the feed point impedance reduces.
The idea is to close up the spacing until the impedance is 12.5 ohms and then introduce a 4:1 Guanella transmission line transformer between the dipole and the feeder.
The antenna as shown in the second picture below weighs about 280gm/10oz
The starting point
The elements can be folded to take up less space. Trussed up ready to go out. Springs open when the croc. Clips are removed!
3-Element Portable Beam Performance
Measured with a 2-meter feeder on the antenna to a Daiwa reflectometer and then a 200mm lead to a Baofeng GT-5TP running 5 to 6 Watts
I have used the materials I had to hand. A fiberglass tent pole 530 mm long for the main spar and bits from an old fishing pole for the element supports. Others will necessarily have to find different answers.
The steel tape measure was a ‘Jumbo’ type, about 25 mm wide edge-to-edge across the curl.
The steel tape, although very springy, is surprisingly easy to drill with a normal HSS drill bit. It can be cut to length with ordinary scissors.
The tuning of the dipole is sensitive to even a few mm error. Start with it 10-15 mm oversize on the two elements and reduce each very carefully 2 or 3 mm at a time to tune.
Ready to go out into the fields!
Does it work?
On my first walk, one mile North of my home village of Cassington, near Oxford, with open fields around me and about 90m a.s.l., I was able to get into Reading (RD) and Swindon (WH) repeaters both about twenty miles away. Contacts commented on my very good signals.
Next walk, I shall try direct calling on 144.5.
It outperforms my MFJ1714 which is IMHO the best telescopic for a hand-held.
ODARS celebrated Marconi day at Oxford History of Science Museum.
Our special event station made over 20 contacts. A vertical antenna, strapped to the museum railings, was used. This was fed with a Furuno automatic ATU. ODARS members provided visitors with opportunities for Morse sending and receiving experience, and Brian demonstrated Marconi era radio equipment.
Many thanks to all members who took part,. We were enthusiastically supported by the Museum staff!
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.”
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.
The introduction in the volume on magnetrons is a fascinating (for me)
summary of their development.
pp 207 – 210 and 284 – 286 of vol1 give some details of the SCR-584, a search and track radar so good that many are still in use today.
pp 136 – 141 of vol2 give more details about the APN-1 altimeter.
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.