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The Hangtenna - Centre Fed Collinear For 70cm UHF.

By Stephen Stebbing 2013-12-14 22:43:46 1 comment

This antenna came about as a result of my requiring something quick and cheap for testing a new 70cm transceiver and having nowhere to mount it except of the side of an existing mast. It is a centre-fed two-element collinear made from a coat hanger.

I know that the theoretical gain of a two element collinear is 3dB, but in this case the elements are so close together that the interaction between them will reduce the gain. From measurements I’ve seen, it’s probably around 1.5 to 2dB which leaves a little left over after feed cable loss is taken into account.

1. Dimensions

Hangtenna dimensions diagram

I calculated the lengths of elements and the matching stub using the formula:

wavelength(m) = 300/frequency(MHz)

I know that in practice, the actual length is usually 95-98% of the calculated length so I cut them to 100% reasoning that I could measure the resonant frequencies and trim them down to size.

I was hoping to have the use of a UHF antenna analyser to check the resonances, but unfortunately it was not available so I just left the lengths at 100%. As it turned out, the antenna seems to have a wide enough bandwidth for this to be okay.

band element length (mm)
70cm A 344
B 172
C 20
Balun 227
UHF CB A 315
B 157
C 20
Balun 208

I haven’t tried this antenna on UHF CB, but expect that it should work reasonably well.

1.1. Balun

Four to one coaxial balun diagram

The balun is a standard 4:1 type made from rg58c coaxial cable. The length was calculated by multiplying the half wavelength by the velocity factor (0.66 for rg58c) giving 227mm for 70cm.

If both ends of the half wavelength section are shorted and the resonant frequency measured, it will be found to be half of the frequency measured with one end shorted and one end open. My dip meter only goes up to around 240MHz so I was able to confirm the length of the balun by dipping with both ends shorted. It measured 215-220MHz or so, which was nicely in range of the target frequency of 436MHz.

2. Construction

Prototype on the bench

Construction was straightforward; the elements were soldered to the matching section and a spacer made from hot-melt glue. I put the spacer in the centre of the matching section as I reasoned that having it at the end, where the impedance is very high, could adversly affect performance.

The balun wires were twisted tighly around the matching section and slid back and forth until minimum SWR was measured as 1:1 at about 4cm from the shorted end. At this point they were soldered in place and then sealed with hold-melt glue for waterproofing.

I avoided poking my eyes out by putting a small piece of black plastic tube on each end. These were removed when the antenna was mounted.

The completed antenna and mounting

The photo at the left shows the antenna after almost a year of being mounted on the mast. The two coats of paint seem to have protected it well enough as there were no signs of rust and the hot-melt glue had not turned particularly brittle.

The two supporting rods were pieces of plastic irrigation riser pipe. I figured that connecting them to the half way point of each section, where the voltage is minimum, would have the least effect on the antenna’s properties.

With the antenna mounted, the SWR increased to around 1.5 at the band edges and 1.2 at the centre which was probably due to its proximity to the mast and the paint and support rods. Next time I will wait until the antenna is mounted before finding the feed point and soldering the balun to the matching section.

3. Results

The results were pleasing, but having nothing to compare it against makes objective analysis impossible. I had good signal strength on all the local 70cm repeaters and reasonable strength into the Blue Mountains repeater some 60-70km away.

When I get my hands on a UHF antenna analyser, I will build another antenna and get the lengths correct for the band centres. Once this is done, it will be even easier to build this antenna by using a single piece of wire and a bending jig made from some nails in a piece of board.

All in all, I was happy with this antenna and can recommend it for when something quick and cheap is required for the UHF bands.

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