HF RDF Antenna

 

The Adcock HF RDF Antenna

RDF Antenna - Combiner

The Adcock antenna has been used for a long time for RDF. It is basically an interferometer.

When both of the two dipoles see exactly the same signal strength and RF phase, no signal is delivered to the receiver. This condition happens only when the signal source is equal distance from the two dipoles.

As the antenna is rotated through 360 degrees, there will be two directions where the signal will drop to a very low strength. This can be a very deep and narrow null. The direction readout is then read, and the direction of the signal is then known, with an ambiguity; we don't know which of the two null directions is the correct direction.
The directional ambiguity will be resolved when another station takes a bearing from some significant distance away. The two correct headings will cross, while the two incorrect ones will not cross.

Since this antenna only responds to vertically polarized components of signals, poor performance can occur when the signal is horizontal or nearly horizontal. Since purely horizontal signals are rare, it should function on most signals.

Construction

Construction is not critical. The only really important factors are electrical balance, and a lot of clearance distance to other objects. These factors may require the antenna be placed out in the open, away from the radio operating position.

The simplest arrangement would be a free-standing support with the vertical mast pivoted on a compass rose on the ground. This can be wood. Metal can be used for the support and for the cross-boom, as long as the two dipoles both always see the same capacitance from the structure. Symmetry is the name of the game here.

A remote speaker and remote S meter can be mounted at the antenna in a weatherproof box. The operator can tune in the suspect signal, and then go out to rotate the antenna and take the heading.

Detail of the Combiner circuit

The combiner circuit mounted on the cross boom is not critical for the antenna to DF. If a tiny toroid is used, use small diameter wire. If a large toroid is available, larger wire can be used. #30 wire would probably work for a 1/2 to 1 inch diameter toroid.The number of turns is not critical, but slightly greater sensitivity for weak signal

s may be achieved through experimentation.The feed line lengths on each side must be identical.The length of the dipoles is not at all important, so long as they are not resonant at some frequency of major interest. Phase characteristics of a resonant antenna are strongly affected by very slight detunings. If the two dipoles are not exactly the same (they won't be!) then the antenna will exhibit maximum error at the resonant frequency.

Because this is a broadband low-cost antenna, it is not possible to have good impedance match between the antenna and the feed line at all frequencies. Experimentation with turns ratio (bifiler turns versus coax feed turns) may provide an optimum match at some desired frequency, such as 20 meters. Mismatch on a receiving antenna is usually not important. Even with a poor antenna one can hear most of the signals on the band, because receivers today are so very sensitive.