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Rhombic Termination Resistors

Updated: Apr 5

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

Also phasing 2 or more antennas can be done, but it takes a lot of land for all the antennas.


Look for new old stock at hamfest and surplus stores

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 hard 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. The Longer the wavelength antenna, there is more RF power in the wire being radiated, so there is less power going to the load resistor. I have never burned up a resistor.

Also a Ladder line static bleeder of some type is needed see,

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.

Dissipation Lines can also be used:

It has long been the practice to use high dissipation stainless steel wire transmission lines for rhombic termination. Until recently, a four wire 300 ohm line of 16 guage stainless steel wire was used to terminate each 300 ohm stacked pair of rhombics. The line attenuation is about 2.5 db per 100 ft at 15 mcs and the line length of 800 ft is adequate. The wire used is a stainless steel which has good magnetic properties and has teh following composition: carbon 0.20%/nickel 1.7%/chromium 16%/iron (balance).

Delay Line Absorbers: By an unknown person, a good read

Recently in order to simplify the aerial terminations, dissipation lines of compact dimensions, in the form of a delay line, were developed. The delay line is a co-axial form and consists of a continuous helical winding of 16 guage stainless steel wire having a winding pitch of eight turns per inch wound on threaded ceramic formers assembled on an inner copper tube. This assembly is coaxially l ocated in an outer copper tube. The characteritic impedance of a delay line is given approximately by:

Z9o) = sqroot (L/C)

where L and C are the inductance and capacitance per unit length. Once the helix pitch and diameter which determine L and have been chosen, Z(o) may be adjusted to the required value by the choice of the diameter of the outer tube which controls C. In the final design, the helix length is 5 ft and the copper tubes are i in outer diameter and 2.25 in inner diameter. This provides an impedance of nominally 325 ohms with negligible reactance when correctly terminated. The assembly of such a delay and its impedance frequency characteristics are shown in figure 26.

The delay line attenuation varies between about 6 and 16 db from 5 to 25 mcs. The terminating resistances are rated at 300 watts and the load is capable of dissipating at least 1.2 kw continuously at any frequency. The dissipation is limited by the terminating resistance at low frequencies and by temperature rise in the delay line due to the rising attenuation of high frequencies. A pair of delay line loads provides a balanced 650 ohm load for each rhombic element. The loads are fitted on the masts at the element termination. From Table III the following continuous ratings are obtained assuming a maximum dissipation of 1.2 Kw per delay line element (table values: two element: 15-28 kw from 8-18 mcs and four element: 35-73 kw from 8-18 mcs).

Only at low frequencies is a two element array incapable of absorbing the maximum continuous power output of 20 kw of the high power transmitters. However on ISB telephony transmission where the average power is very low the two element array may be safely operated at peak powers of 40 kw. The introduction of the delay line terminations has resulted in considerable simplification of the construction of multielement rhombic arrays.

Also: Look for, Collins numbers are: (1) 754-9057-001 and described as "1KW Termination Kit" 2) 774-6261-001 " 3) DAA805-68-C-0020 " "Transformer, Radio Frequency /TF506/TRC-136" 4) 758-5322-001 " "50-600 ohm Balun", 7649858-001 also labeled - 764-9604-001-D with "REV B" one - 764 9058 001 with "REV C"

For one of these: NATO Stock Number 5985-21-882-1782 or VTR-600 termination resistor is comprised of passive components used to terminate a 600-ohm antenna while dissipating the power applied to the system.

The VTR-600 will terminate a 600-ohm antenna through the H.F. band of 2MHz to 30MHz.

The VTR-600 will dissipate 1KW of power at CW continuously for one hour and 500W continuously for extended operations.

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