More about Rhombics – John Heys G3BDQ

I always look forward to reading the interesting and informative articles in VS that have been written by Eric Vast but his last offering that concerned rhombic antennas made and used more than fifty years ago prompts a few questions. The quoted gain figure of 45 dB is certainly ‘not on’. The huge UHF dishes, some more than a hundred metres across (using a natural ground hollow which has been ‘wired up’ as a reflector) can hardly achieve such a gain figure. On HF 20 dB is just something to strive for but is seldom realised. Remember you have to DOUBLE the antenna to gain 3 dB.

The practical size for a rhombic is to have no more than five or six wavelengths on each of its four sides. The angles can be calculated (or cribbed from tables) to show best gain and take off angle for frequency. The gain for a rhombic having each of its four sides five wavelengths long is just 12 dB. If you increase the number of wavelengths in each leg there is a diminishing return. This is because most of the RF has been radiated before the far ends of the antenna has been reached.

I once assisted the late G6QB to test a very long wire antenna strung around the parameter of a golf course not 100 miles from Bexhill. We had about 20 wavelengths on 14 MHz and fired a quarter of a KW into it. A small neon on a 20 ft pole was used to locate the voltage points and after about 700 feet the neon failed to respond. We actually dropped the far end (a high voltage point if ever there was one) and could hold it with impunity. All the RF had been radiated and nothing travelled so far down the wire.

An increase in a rhombic size (for a particular frequency) results in an extremely narrow radiation lobe, one which it would be difficult to align on the target. The non-inductive 700 to 800 ohm resistor which can handle 1/3 to 1/2 of the transmitter power is a difficult item to find. The commercial boys use several in series to make up the resistance and minimise the capacity between the resistor ends. The power dissipated will be less than the expected amount for much of the available power has been radiated before the end is reached. For SSB work the wattage of the resistor may be reduced further; down to perhaps 1/4 of the transmitter power.

If 100 watts to a bog-standard half wave dipole is taken as a reference, increasing the output power to 12.8 KW will result in a received signal strength 3 1/2 S. points greater than 100 watts (a gain (21 Db). The same 100 watts into a standard 12 dB gain rhombIc will give 2 S. Point gain (12 dB).

On an historical note the rhombic ‘King’ was the late Don C. Wallace W6AM of Rancho Palos Verdes, California. He had an antenna Ranch, not a Farm. On this plot 10 miles from his home at Long Beach he had 13 Rhombics of various sizes, and a Curtain antenna. He had numerous poles from 50 ft. to 140ft. in height. G6QB visited Don in 1964 and told me of the complicated cat’s-cradle of open wire feed lines. Finally, a modern technique to reduce the wattage of the terminating resistors is to feed them with a 700 ohm open wire feed line made with resistance wire. This helps a lot I understand.

John D Heys – G3BDQ

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