Eugen Berberich, DL8ZX
A Big Wheel Antenna for the 70cm Band
VHF Communications 2/1995
As a horizontally polarised circular beam antenna, the big wheel antenna has proved to be outstandingly suitable for the observation of activities in contests and as a transmit antenna for beacons, and it has other applications as well.
This article describes a format for the 70cm band. It is constructed as an "indoor" antenna, but can also be set up in the open air, if weather-resistant materials are used and if the connectors are potted. To keep the problems of materials procurement in the background, my construction proposal uses materials which can be found in any DIY supermarket, apart from the trimming capacitor.
1. OPERATING PRINCIPLE
The big wheel antenna consists of three horizontally mounted loops, each of which is one wavelength long, and which are wired up in parallel to the power splitter in the correct phase to give an impedance of approximately 50 Ohms. The inductive reactive component is compensated
for by means of a trimming capacitor. Only the external arc, with a length of l/2, acts as a radiator. The lambda/4 circuits (spokes) are there only for power splitting purposes, as in them the currents run in opposite directions and theoretically cancel each other out (1).
To give the antenna some stability, the spokes of the big wheel must be fixed to a base plate (Fig.1). The lid of a 70mm branch box, for example, is suitable for this purpose, with the holding clamps removed - or a disc made from epoxy material. Biscuit connectors are suitable for fastening the wires, with a hole in the middle for fastening using an M3 screw.
The "wheel" can be made from 2.8mm diameter bare Cu wire (e.g. 6mm square NYA or earth wire). The formation can be given additional stiffening through three additional biscuit connectors at the points where the spokes meet the wheel. A large shim or a piece of brass plate with a suitable hole acts as an earthing point directly on the BNC jack.
3. ASSEMBLY PROCEDURE
Measure and cut three pieces of wire, each 70cm long. Prepare the assembly plate (box lid) - i.e. drill a hole in the middle for the BNC jack and 2mm holes at 120° intervals to attach the biscuit connectors.
A 10mm shim (or a brass plate) with an external diameter as large as possible is attached at the same time as the BNC jack. The earth ends of the wheel are soldered on here later.
The three pieces of wire are now bent as in the diagram in Fig.2. Then push the biscuit connectors over the wires and prepare the wire ends to be soldered to the earthing disc and the internal conductor of the BNC jack (Fig.3). The spokes should run parallel as far as possible. The distance between
them is fixed by the biscuit connectors.
As the box lid is thermoplastic, the wires should be soldered to the earthing disc before assembly takes place. An assembly disc made of epoxy material would be more practical here, as it is less sensitive to heat, and would thus give the formation rather more stability.
After assembly, the formation is screwed together and calibrated. The three circuits can then be soldered to the internal conductor. In the final stage of the assembly, the 18pF foil trimmer is soldered directly to the BNC jack between the centre connector and earth, so that the rotor of the trimming capacitor is lying against earth.
4. SWR CALIBRATION
Set up the antenna at a sufficient distance from metallic components, and connect up a standing wave meter and a 70cm transmitter through a coaxial cable. Use the trimmer to set to the lowest SWR. Improvements can also be obtained by changing the "spoke distance" between the
two biscuit connectors. Any changes should be made on all three spokes.
5. EXPANSION OPTIONS
For my application (DF0ANN beacon), I stacked two of the antenna together to increase the gain by app. 3dB. The stacking distance is l . 0.8 and the two radiators are fed by
75W circuits with an electrical length of 0.75l giving a transformation to 100W. With two antenna wired in parallel, this gives 50 Ohms. The two planes must naturally be constructed to be equal and in phase. The stacking distance can be obtained using 3 hardwood rods or plastic
rods between the assembly plates. To stiffen the construction, and for the mounting of the BNC jack
for the two antenna, an assembly platform can be provided in the middle between the two antenna (Fig.4).