Rhombic Termination Resistors
- skylarkcolo
- May 23, 2021
- 9 min read
Updated: Oct 27
Large non-inductive resistors, ranging from 600 to 1000 ohms, are employed to terminate the antennas, enhancing their directionality with a front-to-back ratio of over 30 dB on these lengthy antennas. For SSB, the resistor should be rated for at least one-third of the input power (in watts) supplied to the antenna's feed side, and up to 50% for other modes. My antennas typically use resistors in the 700 to 900-ohm range; use what you can find, but ensure they meet the requirements.
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 non-inductive resistors.
I currently utilize a Re-entrant that achieves 90% efficiency by re-phasing the power back into the antenna, rather than dissipating it as heat in termination resistors. Learn more about it below in this blog. The Rhombic arrays are now highly efficiency (90%) using, re-entrant line termination system
The low efficiency and gain of unidirectional rhombics can be enhanced by substituting the termination resistor with a low-loss balanced resonant stub transmission line. Stubs function due to standing waves along their length, and their reactive properties are influenced by their physical length in relation to the wavelength of the radio waves. This setup reflects the power that would have been lost in the termination resistor back into the antenna with the appropriate phase, thereby strengthening the signal from the transmitter. This approach can boost the radiation efficiency of transmitting antennas to the 70-80% range, although it adds complexity.
The Rhombic is a very high-gain antenna however it requires 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%.
The Rhombic antenna is known for its very high gain, but it requires a significant amount of land and has an efficiency of only about 50% when terminated. An alternative impedance-termination system, used only in a few large broadcast stations with input powers exceeding 50 kW, is known as the re-entrant line termination. Clyde Haehnlen SK developed the specifications for the Voice of America antenna system at the Bethany, OH Relay Station. This re-entrant Rhombic achieved 90% efficiency by re-phasing the power instead of dissipating it in termination resistors. In this system, the Rhombic is terminated in a transmission line, which is then coupled back to the input through appropriate voltage-matching and phasing networks. As a result, the energy in the dissipation line is fed back to the antenna, reducing the energy waste to well below 50%. The old VOA Bethany site in Ohio achieved efficiency levels of 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.
The Re-entrant Rhombic array is one of the highest forward gain HF antennas with its 90% efficiency
My station is now using re-entrant line termination equipment which is re-phasing the power instead of heating up termination resistors.
UPDATE: As of June 2022 the station is using a re-entrant line termination (Finally got it right in 2022). The high efficiency re-entrant rhombic antennas lines were developed for each ham band to eliminate the problem of dissipating up to 50% of transmitter power in the antenna termination. This is now accomplished at my site by proper control of the phasing and impedance matching of the return power and recirculating it back into the antenna input. By using the Re-entrant line power recirculatating system, which significantly increases efficiency to around 90%, by proper impedance matching and phasing to recombine the power, and replacing the termination resistor.
Are there alternative methods to enhance the efficiency of a Rhombic antenna? One option is to use a parasitic or active reflector, or to couple an out-of-phase reflector, depending on the specific circumstances. By recirculating the power from the termination resistor in these phased-coupled antennas, it is possible to add approximately 3dB of power for the antenna to radiate, thereby boosting efficiency.
Above, an exponential taper feed line extends down to the ground for incorporating terminating, phasing, or fed-systems at ground level. This particular line transitions from 900 ohms to 200 ohms. By adding a 4:1 balun, you can easily connect to 50-ohm loads near the ground.
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
The rhombic antennas can be connected either serially or parallelly as shown below:
See
Look for new old stock at hamfest and surplus stores
Above, $49 for a 470 ohm 500 watt non inductive terminating resistor RPS500DH CC28 just right for Vee=Beams, or put two together for 940 ohm which works great for most Rhombic setups.
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 a 4:1 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 for high power systems, 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 the 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 located 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 MHz. 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 MHz and four element: 35-73 kw from 8-18 MHz).
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.
Above: Using a spark gap for static and lightning protection which also protects your terminating resistors. In the above, the there are two 150 ohm resistors in series on both sides of the ground, you did not have to mid ground however if using two or more resistors it is a good safety precaution.
The K0UO antenna test range site makes use of the 4KS Walz airport and its surrounding area as a practical learning environment for STEM (Scientific, Technical, Engineering, & Mathematics) antenna projects in a real-world outdoor setting. If your group has a university aerospace or antenna research STEM program, please let me know.
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