Rhombic Termination Resistors
Updated: Nov 12, 2022
Large 600 to 800 ohm non-inductive resistors are used to terminate the antennas, making them highly directional with FB of more than 30 db on these long antennas. The resistor must be sized to at least 1/3 of the input power (watts) going in to the feed side of the antenna for SSB and up to 50% on other modes.
This remote antenna direction and termination control box at K0UO is 1500 feet from the station and uses UHF link radios to switch antenna direction.
The green box has both non-inductive resistors and spark gap lightning protection in it
It is possible to improve the low efficiency and gain of unidirectional rhombics by replacing the termination resistor by a low-loss balanced resonant stub transmission line. The behavior of stubs is due to standing waves along their length, their reactive properties are determined by their physical length in relation to the wavelength of the radio waves. This reflects the power that would have been wasted in the termination resistor back into the antenna with the correct phase to reinforce the excitation from the transmitter. This circuit can increase the radiation efficiency of transmitting antennas to the 70-80% range, at the cost of increased complexity.
The Rhombic is a very high-gain antenna however it require a lot of Acres and the efficiency when terminated is only about 50%. An alternate impedance-termination system, which was only used for a few large broadcast stations where input powers were above 50 kw, is called the re-entrant line termination. Clyde Haehnlen SK, developed the specifications for the Voice of America antenna system at the Bethany, OH Relay Station. That re-entrant Rhombic was 90% efficient by re-phasing the power instead of heating up termination resistors, in this system, the Rhombic is terminated in a transmission line, which in turn is coupled back to the input through the proper voltage-matching and phasing networks. Thus, the energy in the dissipation line is fed back to the antenna, so that considerably less than 50 percent of the energy is wasted. The old VOA Bethany site in Ohio had efficiency up to over 90%.
Clyde provided K0UO with design information for re-phasing a few years ago before his passing.
The normally displaced terminated power is returned to the input line by properly phasing and adjusted to the voltage magnitude through the use of stub line of proper values and space a long the return line. Impedance of the line is corrected in a like manner in some cases combined with one of the re-entrant stub lines, all stubs are shortened and grounded at the midpoint for lightning protection. This feeds-backs the wasted RF energy "In-Phase", back into the feeder end of the antenna. For any variation from the stubs frequency, the stub must be returned.
K0UO is now using re-entrant line termination equipment which is re-phasing the power instead of heating up termination resistors. (See Info below for high-power non-inductive power resistors.)
UPDATE: As of June 2022 the station is using a re-entrant line termination (Finally got it right in 2022).
So are there other ways to get higher efficiency from a Rhombic? A parasitic or active reflector could be used or couple an out of phased reflector depending on the situation. Recirculating the power from the termination resistor in these phased coupled antennas could add about 3db power for the antenna to radiate while increasing the efficiency.
One way might be by the terminating with a dipole antenna, using a feed line and balun matching to a 50 ohm dipole. Also crossing the two feeder might work to clean up the pattern, modeling is difficult. See FIG 1, 2, & 3 below and Patents
See US2517238A - Radiating termination for a rhombic antenna - Google Patents
Also phasing 2 or more antennas can be done, but it takes a lot of land for all the antennas.
Termination Resistors (palomar-engineers.com)
A GREAT TIP on non-inductive power resistors:
You could terminate the antenna, to a 50 ohm high power dummy load (1000 watt cantina), since large high-power non-inductive power resistors are so had to find. 16:1 or 12:1 balun to a 50 ohm power resistor (rated a 33 to 50% of the input power), or you could use an exponential taper feed with a1:4 balun, to transform the 600 to 800 ohm unbalanced to 50 Ohms balanced termination.
Also a Ladder line static bleeder of some type is needed see,
Ladder Line Static Bleeder with Ground Lug (palomar-engineers.com)
For day-to-day use of the antenna in amateur radio service, remember amateurs are not point-to-point shortwave broadcasters, military or wire services, Amateurs just want to make QSOs! Also most amateur radio operators don't have tens of thousands of dollars to spend on tall towers and stacked monoband beams, or the ability to climb and maintain such structures. Rhombic antennas were the ultimate antenna design back in the Golden Age of Wireless. However, building one required a large tract of land and a lot of tall telephone poles, because they have dimensions several times the wavelength. To most amateurs the positive thing is there are no large mono-band antennas to maintain, or rotators to fix, and rhombics allows for instantaneous direction and band switching. They normally can be intalled at very low cost, if you have trees to hang them from, all that is needed is a lot of wire and time! Also the key concept with traveling-wave antennas is that there are no standing waves, which means that the current and voltage levels are the same everywhere along the antenna conductors. So the rhombic antenna does have the very distinct advantage of working over very wide frequency ranges with flat SWR and high gain.