Rhombic Antenna Circuitry

Author: Edmund A. Laport

From a circuital standpoint the rhombic antenna, when terminated at the far end in its characteristic impedance Z₀, has an input impedance which is predominantly resistive and equal approximately to Z₀. For the three-wire type, this value is of the order of 600 ohms, which matches well with ordinary two-wire balanced feeders. After construction the input impedance should be measured and the feeder impedance matched to the measured value. In practice, true traveling waves never exist on the system. There are always some reflections from the side corners, though it is possible to adjust the termination so that the reflections are minimized on either the forward half or the rear half of the antenna. If a system is constructed carefully so as not to have sharp corners where the feeder and the terminating line attach to the antenna, and if large metallic fittings near the supports are avoided, a satisfactorily uniform input impedance of high power factor is realized, over a large range of frequencies.

When used for receiving, the terminal resistance for a unidirectional system is usually installed directly at the far end in the form of a non-inductive resistor. This resistor should have a high impulse rating to maintain stable resistance and to withstand induction from lightning. Some designs include discharge circuits to ground at the termination by dividing the terminal resistance into two sections and connecting the center to one side of a small spark gap, the other side of which is connected to ground with a wire (if wood poles are used), broken by a series of gaps, or by using a steel support as the ground connection.

For medium- and high-power transmitting purposes, the terminal resistance is almost always a balanced lossy line of high dissipation capacity. These are discussed in the following chapter.

Static-charge drains are a necessity in most regions, where the antenna may become highly charged from rain, dry snow or flying sand, and electric storms. Draining is most easily accomplished at the end of a dissipation line but can also be done in the feeder, using resistors or drain inductors having impedances high enough at all working frequencies not to introduce reflections. Drain resistors or coils are connected from each side of the system to ground.

The directivity of a rhombic antenna can be reversed by interchanging the feeder and the terminal resistance. Figure 3.80 shows an arrangement for this purpose, with switches located on the ground near the center of the system where feeders run to each end of the antenna. The main feeder and the dissipation line are also centered at this point and connected into the switching circuits.

For simultaneous reception of unidirectional signals from both directions, lines can be brought from each end to separate receivers. Each line must be correctly terminated, either by the receiver or by a resistive network, so that there is no reflection from the receiver inputs. To use this system successfully, there must be virtually zero radiation from the receivers, or they may mutually interfere.

In large receiving stations using many antennas and receivers, concentric lines have been used to simplify antenna-receiver switching. The use of concentric lines only to reduce feeder pickup is seldom justified because the spurious directive responses of a rhombic antenna are greater than the direct pickup on ordinary open-wire lines. When concentric lines are used, the balanced antenna impedance is matched to the unbalanced line impedance with a wide-band transformer.

If a balanced four-wire cross-connected open-wire line is used, the two balanced impedances can be matched by using a tapered intermediate matching line designed for the lowest working frequency. Either exponential or linear taper may be used if properly designed.

Here, again, there is no justification for using a four-wire line because of its low pickup quality when it is used with a rhombic antenna having comparatively greater omnidirectional pickup.