Updated: Oct 30, 2020
K0UO is working on the "Lost Art of Rhombics" with three of these large wire antennas at the Kansas QTH beaming at the 6 main DX areas, and phasing for more steering.The "KING of Wire Antennas" is a W1AW type Rhombic. Each antenna was built for 40 meters and above, with over 600 to 700t foot per-leg and +1200' end to end, 6 to 7 Acres each on 80 to 100 foot wood poles, The Front to Back is like a brick wall, maybe +45dB or more. That makes them 4 to 6 Wave-Lengths long on each of the four sides or 16 to 24 wave-lengths total for 40 meters, these may be the largest 40 meter Rhombic antennas in current ham use. In the 1950s to 1970s W6AM had many 1000 footers and one was 1500ft, TF4M put up a larger one at one time, and Roy W7YRV/SK had nine at one time and he developed the X Rhombic, which he had one for every 20 degrees. That was a truly remarkable accomplishment for an amateur station (You must see his page, great info w7yrv.blogspot.com/2013/). It was an extreme pleasure for K0UO to meet Roy, even in his 90's, he was still an encyclopedia of knowledge when it comes to very high gain antennas. K0UO is very privileged that W7YRV has entrusted him with the schematics, drawing, and photos of these fabulous antennas. W1VDE Roger in OR has six and is on the air daily, alsoVK3MO, and in Texas N5APR and W5BY Jeff, have a few at their ranch's, KL7KK in AK has one along with a few VK's, and V55V (V55W) in Namibia had 2 but they are now down. The gain over the full size 40 meter wire 2 element beam at 100 feet high, is truly unbelievable, Gain before Amplification. The QTH also has two long 1200' Vee-beams, on 75-95 foot woodenpoles. The antennas are currently assisting a group with a project using TDoA (Time Difference of Arrival ) Direction Finding (DF) checking integrated statistical localization algorithm which allows the localization of HF transmitters based on AoA (Angle of Arrival). K0UO is best known as an antenna Experimenter, and Ragchewer 1st and DXer for fun! See the qrz.com website and lookup the K0UO call for more info.
A Rhombic Antenna = The highest development of a long-wire antenna.
The 100 foot wood poles are almost 36" wide at the base, on the main east-west continental US antenna four of the insulators are from the famous W6AM rhombic farm which was located in Southern California http://www.qsl.net/ne6i/w6am/antennas.html
How do the Rhombics preform?
A detailed modeling analysis of these rhombics has been done. Narda meters are used for testing both E and H fields in the far field, but the on the air tests and QSO's show the real power of these antennas. With more than 2200 feet of wire in the air each, “The RF Gotta-Go-Somewhere”. Note a full size 2 element 40 meter phased wire beam at 100 ft is used for a reference antenna. It can be truthfully be said, that "a Rhombic antenna occupies more space per db of gain than any other antenna"! The Rhombic is a very high-gain antenna however it require a lot of Acres and the efficiency when terminated is only about 50%.
Open-wire feed lines are connected to each end of the antennas ends, bringing these to a central point in the middle this allows switching in a termination resistor or a balun, which changes directions. The antennas also works very good in the "Bi-direction" mode."Most of the rhombic's performance limitations come from the high levels of spurious lobes and the poor efficiency (50%), especially over normal soil. The rhombic has one of the poorest gain-per-acre rankings of any high gain HF antenna array. On the other hand, a rhombic antenna does have the distinct advantage of working over very wide frequency ranges with flat SWR andhigh gain, something a basic monoband yagi can never do. The rhombic is also a simple antenna, requiring only four supports (three supports for the Vee beam, and one support for inverted Vee derivatives).
However some of those points can be very positive 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 alot of tall telephone poles, because they have dimensions several times the wavelength. To most amateurs the positive thing is there are no large monoband 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 alot 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.
Traveling wave antenna has no SWR: Due to ground resistance the electric field of the radio wave (E, big red arrows) is at an angle θ to the vertical, creating a horizontal component parallel to the antenna wire (small red arrows). The horizontal electric field creates a traveling wave of oscillating current (I, blue line) and voltage along the wire, which increases in amplitude with distance from the end. When it reaches the driven end (left), the current passes through the transmission line to the receiver. Radio waves in the other direction, toward the terminated end, create traveling waves which are absorbed by the terminating resistor R, so the antenna has a unidirectional pattern.
see traveling wave below
Think about this when building an antenna: The Maximum usable frequency (MUF) is important for determining the best HF frequency to use in communicating between two locations and most amateur radio operators understand it. But for general amateur radio work, maximum usable angle (MUA) is probably more important than maximum usable frequency. Maximum usable angle focuses on band availability, the types of antennas needed to take advantage of band openings, and the skip distances that can be expected. The maximum usable frequency equation for communicating is Fm = Fo / (sin Ae) . The main lobe elevation angle depends on the configuration of the transmitting antenna. When these antennas were designed, MUA to DX locations from K0UO's KS QTH had to be the main part of the that design. Using the ACE HF Pro software by Long Wave, allows K0UO to analyze the entire HF spectrum at any time on any band, from K0UO's QTH to any location.
A nice youtube overview of rhombic https://youtu.be/fmWlOisao-I
The Big project for 2020 is increasing the efficiency of the rhombic: 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 Haehnle n SK, developed the specifications for the Voice of America antenna system at the Bethany (Ohio) Relay Station that 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, some of 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. 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 reentrant stub lines, all stubs are shortened and grounded at the midpoint for lightning protection. This feedbacks 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 retuned.
So how do you get higher efficincy 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.
VOA antenna info, Clyde Haehnle, developed the specifications for the Voice of America antenna system at the Bethany (Ohio) Relay Station that Rhombic was 90% efficient by re-phasing the power instead of heating up termination resistors, https://www.youtube.com/watch?v=3w_ZXRJol_4