Rhombic Array Termination Resistors, or Not
Updated: Aug 15
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 in the past typically used resistors in the 700 to 900-ohm range; use what you can find, but ensure they meet the requirements.
I 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. Discover more about it below in this blog. My Rhombic arrays now operate with high efficiency (90%) using the re-entrant line termination system.
Resistors play a vital role in sustaining a stable traveling wave within antennas by effectively absorbing the energy that reaches their ends. This absorption prevents any reflected energy from traveling back and forming standing waves, which can significantly degrade the antenna's performance. By ensuring that the current maintains a progressive phase along the length of the antenna, these resistors support unidirectional wave travel and contribute to stable radiation patterns. Matching the resistor value to the characteristic impedance of the antenna is essential for minimizing reflections, which leads to consistent traveling wave behavior. This consistency is crucial for enhancing both directionality and bandwidth, allowing the antenna to perform effectively in various applications and under different conditions.
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 90% 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 renowned for its high gain; however, it necessitates a substantial amount of land and achieves only about 50% efficiency when terminated. An alternative impedance-termination system, implemented in a limited number of 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 (VOA) antenna system at the Bethany, OH Relay Station. This re-entrant Rhombic attained 90% efficiency by re-phasing the power rather than dissipating it in termination resistors. In this system, the Rhombic is terminated in a transmission line, which is subsequently coupled back to the input through appropriate voltage-matching and phasing networks. Consequently, the energy in the dissipation line is redirected back to the antenna, reducing energy waste to significantly below 50%. The former VOA Bethany site in Ohio achieved efficiency levels exceeding 90%.
Prior to his passing, Clyde provided K0UO with design information for re-phasing.
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. Drawing of what K0UO, now use s a Re-entrant system which is 90% efficient by re-phasing the power back in the antenna, instead of heating up termination resistors.
I'm using the array on many HF ham bands 160 meters to 6 meters, which involved considerable engineering time to get it right, this was not needed by the VOA in the 1940s, they only used a few frequencies.
The Re-entrant Rhombic array is one of the highest forward gain HF antennas with its 90% efficiency
My station now confidently employs re-entrant line termination equipment, re-phasing the power instead of heating up termination resistors.
UPDATE: As of June 2022, the station utilizes re-entrant line termination. The high-efficiency re-entrant rhombic antenna lines, developed for each ham band, effectively eliminate the issue of dissipating up to 50% of transmitter power in the antenna termination. At my site, this is achieved by expertly controlling the phasing and impedance matching of the return power, recirculating it back into the antenna input. By implementing the Re-entrant line power recirculating system, efficiency is significantly increased to around 90% through precise impedance matching and phasing to recombine the power, 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.
Elimination of Internal SWR "Reflections"
When RF energy travels down a line with a sudden impedance jump, a standing wave is created due to reflections. By changing the impedance continuously and exponentially over a distance that is long relative to the operating wavelength (typically at least 1/2 wave at the lowest operating frequency), RF energy transitions smoothly without creating major standing waves (SWR 1:1).
Ultra-Wideband Impedance Matching
Unlike traditional discrete matching units or quarter-wave matching stubs—which only provide a perfect impedance match at a single frequency and its odd harmonics—a properly designed exponential taper provides an impedance match across several octaves (e.g., from 160 meters through 6 meters) without requiring physical readjustments or retuning.
Practical Ground-Level Integration used at K0UO
High-impedance wire arrays often present feedpoint impedances between 600 to 900 ohms high up in the air. The exponential taper line drops down to ground level while gradually transforming that high impedance down to a lower, manageable level (such as 200 ohm). From there, a standard 4:1 current balun can easily step the system down to standard 50 ohm coaxial cable or low-impedance switching hardware.
Summary of Advantages for Large HF Arrays
Continuous Multi-Band Operation: Eliminates the need for band-specific tuners or physical stub adjustments.
High Power Handling: Since it uses air-dielectric open-wire conductors, it handles multi-kilowatt power levels without dielectric breakdown or thermal loss.
Low Loss: Offers significantly lower attenuation across long physical runs compared to coaxial feedlines.
Additionally, fed into an antenna instead of the non-inductive resistor
One approach could involve terminating with a dipole antenna, or preferably a directional Log LPDA or Yagi beam. This would use a feed line from the far end of the antenna to a 12:1 balun, matching it to a 50-ohm beam. Crossing the two feeders might help refine the pattern, though modeling this can be challenging.
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 ham-fest 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 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 regular use of antennas in amateur radio service, it is important to remember that amateurs are not point-to-point shortwave broadcasters, military, or wire services. Amateurs primarily aim to make QSOs. Additionally, most amateur radio operators do not have the financial resources to invest in expensive tall towers and stacked monoband beams, nor do they have the capacity to climb and maintain such structures. Rhombic antennas were considered the pinnacle of antenna design during the Golden Age of Wireless. However, constructing one required a large area and numerous tall telephone poles, as their dimensions are several times the wavelength. For most amateurs, a significant advantage is that there are no large monoband antennas to maintain or rotators to repair, and rhombics allow for instant direction and band switching. They can typically be installed at a very low cost if trees are available to support them, requiring only a substantial amount of wire and time. A key concept with traveling-wave antennas is the absence of standing waves, meaning that the current and voltage levels remain consistent along the antenna conductors. Thus, the rhombic antenna offers the distinct advantage of operating over a wide range of frequencies 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 mHz 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.
Ideal for use as a Beverage wire antenna terminator, DX Engineering Beverage Termination Resistors withstand nearby lightning strikes significantly better than hard-to-find carbon composition resistors. They are superior to carbon composition resistors in real life, retaining resistance value despite exposure to heat and power surges. These special non-inductive resistors can absorb extreme amounts of power for short periods without damage. They feature the highest surge immunity of any 2 watt leaded (with leads) carbon or metal resistors. Metal-film and carbon-film resistors easily fail from even minor electrical disturbances. https://www.dxengineering.com/parts/dxe-ecm-r470-2
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.
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.
Historic Preservation: The K0UO station actively preserves and utilizes parts and insulators from legendary, historic radio arrays, including those from W6AM (Don Wallace), W7YRV Roy, BBC, Voice of America (VOA) and many others.
"I hope others will carry on the tradition, and art of building the large Rhombic Arrays in the future". It is truly the "PHD" of wire antennas.
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Good read!