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The 70cm Townsman
Aerial The Townsman aerial was designed by Bernard Howlett, G3JAM, in the late 1970s and published in the UK journal "Wireless World" in February 1980. It was an interesting dual-band design, but directions were given for a single-band operation (including dimensions for up into the UK TV Band (470-860 MHz). The need arose at our local Air Training Corps for an aerial which would perform well at 435-440MHz. (The Air Training Corps is an organisation for youngsters between 13-18 years. It is closely associated with the Royal Air Force.) Gain was not an important issue, but the lack of a ground plane definitely is. Tests with the usual J-pole arrangement (and the Slim Jim) proved that physical constraints and area coverage of the target area were not quite what we needed, so the attempt was made to use the Townsman. Tests so far have shown that it is better than the Slim Jim, albeit at a cost of increased patience and construction. The Townsman is probably unique in the method by which the end impedance of a half-wave radiator is matched to the 50 ohm coax feeder. Put simply, it is a transformer which equates to a "round wire over a flat plane" transmission line. By adjusting the height of the wire over the flat plane, the impedance rises. Insulation between the radiating
element and the transformer is by use of a small piece of Perspex or
similar material. In our case, the transformer is a 1cm
wide strip of brass about 2mm, (1/16 inches), thick. The radiating
element can be made of the same material or, as in our case, 10mm copper
tube (small-bore central heating pipe). The usual feed is UR43 (rg58, but
it is possible to use rg213
(urm67).
"Constructors in the U.S.A. are advised to
shorten the radiator to about 28.5cm, (about 11.25 inches). Fine
adjustments in length can be made with some THICK copper wire
soldered to the top of the pipe. Other materials for the radiator
may require different techniques." Radiating element 30cm Transformer 13cm Space 8mm (between transformer and radiating element) Hairpin 7.24cm total
length. (I used 18swg enamelled copper, but the original
spec called for "pvc insulated hook-up
wire")
Photos:
The picture above
shows that the Townsman 70cm antenna can go to the top of the U.S.A
440 band! Notes: U.S.A. builders refer to new updated lengths and tuning above and below! If you are using RG8-type cable, I used a length of 1.2mm enamelled copper wire to connect the cable to the radiating element. If you use UR43 or RG58, simply extend the inner conductor. A 6BA solder tag makes for an ideal connection. The hairpin match loop is connected between the inner conductor of the feed coax and the transformer strip. Again, it's total length is 7.24cm total length. Tuning for best SWR is done by adjusting the spacing between the transformer sections very carefully....a tiny bit at a time. This can be a very time consuming procedure but you may hit the "sweet spot" the first try! Adjust for the lowest swr at your transmit frequency. With the dimensions as shown, a good match can be obtained over quite a wide range of frequencies. Further tests are planned to see what influence adjustment of the length of the radiating element does with relative field strength. I can promise you, though, that
with the dimensions and materials given, it WILL work in the 440MHz range,
although I suspect a little trimming of the radiator might be in order. 73
DAVE, M0UXB The VHF Townsman Aerial: further notes. Bandwidth A broader bandwidth may be achieved by use of an Aluminium tube. A second prototype using 15mm thin-walled lightweight tube gave a good account of itself; 6MHz, about 1.5:1 at each end of the (149-155MHz) band. Transformer A 4" length of 1.5mm insulated (mains) wire forms the hairpin, and another length forms the feed to the base of the radiator. A model made for covering 149-155 MHz has a shorter transformer (by about an inch). Note: In the
UK, single core, 1.5mm Twin & Earth wire is used on 230v lighting
circuits. Spacing The spacing between the end of the transformer and the radiator is kept at 20mm. Radiator An improvement in conductivity between the bolt (which holds the radiator to the Perspex insulator) and the aluminium can be made by use of a little copper or graphite loaded grease. Don’t over-do it.
The dipole radiating element was shortened to 37 inches. If you plan on using higher powers, a little extra insulation over the transformer securing bolt will help. Outside covering I had an old fibre-glass aerial tube (taken from a broken aerial), and used that. Provided that tuning is done with the radiator in the tube, no de-tuning should occur. Other scaling Rough
calculations indicate the following: F = 145
MHz F =
151MHz
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