Dorking & District Radio Society

Portable Antenna Construction

Colin Berry, M0GXV

This activity was inspired by an article in Practical Wireless April 2015, "A Practical 40m Beam for the Small Garden" by Steve Telenius-Lowe, PJ4DX. The club decided to build a half-sized version for 20m to support our various portable activities, and also as an opportunity for members to learn about the theory and practice of designing an antenna.

The design consists of two identical modules, consisting of a quarter wave vertical radiator and two radials, with a connection box including a simple balun. These are positioned a quarter wavelength apart and supplied via different lengths of feeder to give a 90 degree phase difference. In practice because the elements pick up each other's near field, they are fed via 84 and 155 degree phasing cables. The result should be a directional antenna with a broad lobe at the front of +3dB compared to a single element, and a 20dB null at the back. By swapping over the phasing cables, the direction can be reversed.

After a couple of planning and design meetings, and obtaining the components, two evening meetings were scheduled to involve the rest of the membership in the actual construction. At each session, there were two teams performing complementary activities, led by Tom G4DFA and Colin M0GXV. The first session started with a presentation on the project and a demonstration of how our MFJ antenna analyser could be used to measure the velocity factor of the coaxial cable we were using. The first activity then followed, to determine this factor and calculate the lengths of the two phasing cables to give the antenna its directionality.

The two phasing lines then needed to be cut accurately to length and terminated with a PL259 at each end. These will be fed from the transceiver via a "T-shaped" splitter.

The resonant frequency of the pair may be higher than for each element in isolation, so the vertical elements were prudently cut oversized, with the expectation of shortening them when the antenna was set up. These each had a 4mm banana plug attached. The radials are a generous quarter wave, and are attached to strings to enable them to be pegged out off the ground. The driven end was formed into a loop and soldered, ready to be attached to a bolt and wing-nut on each side of the connection box.

The connections were made via sockets fitted to a plastic box, which also contained a short length of feeder threaded through ferrite beads to act as a balun to prevent any radiation from the phasing lines. Extensible fibre glass poles were used to hold up the elements, with the connection box mounted about 2 metres off the ground and the radials pulled out at either side.

At our first field test, conducted in a local park, we checked the resonant frequency of each module separately, and as expected it was lower than needed. Rather than cut the verticals, we decided to try folding a length back down the pole and securing it with reusable cable ties. However we found we needed to fold about twice as much as predicted. This kept our options open to retune the antenna later if needed. When both modules were connected, the resonant frequency rose further, and even more needed to be folded down. Eventually we obtained a resonance within the 20m band and an impedance close enough to 50 ohms to enable an ATU to get the SWR down to 1:1.

Without delay, members queued up to try for a QSO. Sure enough, someone from Europe in the right direction came back and reported a good clear signal, which was very encouraging. We had access to a field strength meter and walked round the park with it to verify that there was indeed some directionality. This proved inconclusive as we couldn't easily keep a constant distance away, the meter went out of tune as we moved it around, and communicating to the operator was tricky. And of course by now the operators weren't really interested in conducting a scientific experiment with a pure carrier, there was some real DX to enjoy. The day was finally concluded with a shared lunch and a great sense of achievement.

Back home, a theoretical NEC antenna model was created, which showed a cardioid pattern and front null at the horizon. However we had been in a grassy park and when we tried simulating these real earth conditions the front null rose above the horizon and the single rear null moved to the sides, leaving some gain at the back. There was some forward gain compared to a single vertical under the same realistic earth conditions. The antenna would obviously not compare well with a 3 element beam at roof height; however the antenna we had made is very portable, can be used in a rural low noise environment, and the setting up is part of the fun. Additionally, it is vertically rather than horizontally polarised.

Later in the year, it was used effectively at Polesden Lacey with the callsign GB0NT, where trees were in unsuitable places for a G5RV to be strung up. Even on a rainy day, it was found that the radials needed to be clear of the ground to work properly, rather than on the grass. It will prove to be a useful addition to the DDRS tradition of club /P operation at Devil's Dyke and Leith Hill alongside the traditional VHF.

Photography by Walter Blanchard