Rhombic antenna experiments by Jelmer PA5R

Introduction

Some years ago I became interested in a special type of antenna: the Rhombic antenna. I wanted to know if it really was as good as you hear in all the stories about them, and, if we could use them in our contest applications. As the name already indicates, the antenna has a rhomboid shape in the horizontal plane that is formed by 4 wires. It's an interesting antenna because it has some very nice characteristics:

  • Very broadband
  • Easy to construct
  • Cheap
  • High gain
  • Low noise


There are some drawbacks like the size of the antenna and that it is in a fixed direction (e.g. none rotatable). So using it as your only multi band antenna for all bands will not work!

 

How does it work?
The rhombic is made of 4 wires of length L in a rhomboid shape in the horizontal plane (see fig 1). It is symmetrical around the long side of the antenna. Good symmetry is of vital importance for the performance of this antenna. The width of the antenna's main lobe is determined by the angles q and a, often referred to as tilt and apex angle. In general, the wider the rhombic (greater a and smaller q) the wider the beam and vice versa. Of coarse q and a are linked since the sum of half the tilt angle and apex angles is always 90 degrees: q/2 + a/2 = 90

image002

In principle, it's what's called a traveling wave or non resonant antenna, in stead of a standing wave or resonant antenna like a dipole. A balun turns the unbalanced RF signal into a balanced signal, to be connected to the antenna feeder. When the signal enters the rhombic, the transmission line is effectively widening. While traveling along the widening wires, the energy is gradually radiated. Essentially, the rhombic is just a tapered feed line very much like a horn antenna or Vivaldi radiator both commonly used as broadband antennas in microwave applications. This also explains why the antenna only works when L is sufficiently long, generally a few wavelengths at least, because the longer L, the more the radiated energy adds up in one direction.

image00402

OK, but why does it fold back again? That is because when the wave reaches the widest point and the wire would just stop there, energy will be reflected back towards the transmitter and will interfere with the forward traveling wave, disturbing the antenna pattern. So to avoid this problem, one should terminate both wires using a matched shunt resistor to ground. When you do this at both ends at the widest point of the antenna, you have made yourself a so called V-beam, another type of traveling wave antenna. This antenna is widely used in the HAM world. One problem is that a good ground connection (for RF.) is very hard to make causing a significant loss in efficiency of the antenna. The rhombic just folds back again and then uses one resistor that is connected between the two ends of the antenna wires. With this configuration, there is no need for any ground at all since the voltage across the resistor is equal in amplitude and 180 degrees out of phase. The middle of the resistor hereby becomes a virtual ground for the RF signal, and can therefore even be used to connect to DC ground for avoiding static build up in the antenna. The resistor value is double that of a resistor for a V-beam, and is around 800 Ohms. Because the antenna is all symmetrical, this is also the impedance presented at the feeding point. In case you want to transmit on your rhombic, the terminating resistor should be able to handle half the RF power you put in, and be true resistor (non inductive). So any high power wire wound resistor is just not an option!

So basically, the following aspects are important when constructing a rhomic antenna:

  • L should be long enough (2 to 4l at the lowest frequency does the job)
  • The values of q and a determining the shape of the main lobe
  • Symmetry of the total antenna system, including balun, feed lines and resistor.
    This seems simple to do, but really is not! First you need a very good balun that works properly over the entire frequency range of the rhombic. Then you have to set up this
    huge antenna with pinpoint accuracy. This for me has proven the most difficult!
    And last you need a high power, non inductive load that is electrically symmetrical
    around a central ground tab 

Construction of the rhombic:

For the size of the rhombic, I chose 80m per side, so 320m in total (1000 ft). Because the wire is unsupported along each side, normal copper wire cannot be used because it stretches out too much. I have tried 3 different types of wire: galvanized steel, military telephone wire (steel with 3 copper wires inside) and phosphor bronze wire (available at surplus shops). The latter has the advantage of being as strong as steel wire, but without the high resistance. In theory, the phosphor bronze wire should yield the best results, but in practice I didn't notice any significant difference. This may be because the whole antenna is by definition high impedant, so no large currents will flow in the wires. Whatever type of wire you choose, make it as thin and light as possible otherwise the necessary force to stretch the full antenna when it's in the air may become too much for the supporting structures!

For determining the angles
q and a, MMANA (by JE3HHT) was used to model the antenna. The height was set to 18m (54 ft) over average ground. Using MMANA, the antenna was simulated for 40m and 20m The plots below show the basic model and some simulation results.

fig302

Fig. 3: Basic model 320m rhombic.

fig4

Fig. 4: 20m pattern 320m rhombic @ 18m

fig5

Fig. 5: 40m pattern 320m rhombic @ 18m

For the power load I took 3 non inductive power resistors of 220 Ohm in series. The resistors are flange mounted 100W types and were bolted on an aluminum heat sink. To further increase power handling, the resistors and heat sink were placed in a can with oil.

Matching of the antenna to the 50 Ohm TRX was done in two steps: first a 1:4 balun, to transform the 50 Ohms unbalanced to 200 Ohms balanced. Next is a 20m long tapered open wire feed line, starting with a spacing of 10 mm and tapering out to 300 mm at the end. The taper is made logarithmic, but I suspect a linear taper would do just as fine. I just wanted it to be perfect HI. The main thing is that it should at least be ╝ lambda at your lowest frequency. The tapered line was made of insulated copper litze and using PVC electrical tubing for spacers. The pictures below show the matching line.
 

fig6
fig7

Fig. 6 & 7: Log tapered impedance matching line

Setting up and testing:

Setting up such a large antenna is almost a project by itself. The rhombic is 135m (400 ft) long and 60m (180 ft) wide. The four supporting masts have to be placed very accurate to ensure a pure symmetrical romboid shape of the antenna. We used a 50m measuring tape and worked out from the center of the antenna. Starting with the front and back masts, the distance from the center of the supporting masts were marked by a rod. We then would optically align it over the center. Next, for the side masts the same principle, but you have to make sure this line is at an exact 90 degrees on the first one. All masts were fitted with an insulator and a small pull. This way you can control each individual fixing point of the antenna for fine tuning. When the antenna is up, you will immediately see if it is symmetrical, only then the insulators and pulleys  all run straight towards the center of the antenna.

The forces the masts will  experience are quite large if you stretch the antenna so it is horizontal. Actually, we could never get it all straight, there still was a slight slag in the four sides. The antenna wants to pull the supporting masts inside, so extra guying in opposite direction (away from the center of the antenna) is help full. The wind load of all that wire is huge!
 

fig8

Fig. 8: One of the side masts holding the rhombic up.

From the first tests on I liked the performance of the antenna. The VSWR was below 1.5 : 1 from 3 to 30 MHz. Using the termination, the antenna was extremely low noise and had a constant low VSWR for every frequency: a true non-resonant antenna! When leaving out the termination and shorting the two legs together, the noise level went up a little. This is because the antenna is now receiving from two directions, so in theory the noise level should increase by 3dB. By removing the termination, the antenna becomes a resonant antenna, so the VSWR will not be low for the entire frequency range. I found that whenever one side of the rhombic is a multiple of ╝ lambda (odd or even!), the antenna resonates and shows a good VSWR. But outside these frequencies the VSWR increases. When you keep the coaxial cable short or use a balanced tuner, this actually is not a real problem. But I wonder what happens to the radiation pattern for frequencies were the antenna is not resonant.

During the times we set up the antenna for testing, I was able to work nicely on it from 80m trough 10m. Best performance was on 20m and 15m, set up towards USA the signals were truly booming. On 10m the antenna simulations showed that the beam width becomes very narrow, so we were "illuminating" just a small part of the US. I wanted to make a comparison with a well known antenna, like a small triband beam or so. But unfortunately, this never happened so I cannot really say how good the rhombic is. On 40m and 80m the beam width becomes much wider and because of it's limited height most of the energy goes way to high for real DX work. But what does help is low noise floor because of the antennas good directivity. This enables you to hear very weak signals.

One other thing we experienced during testing is the enormous build up of static electricity if the antenna is not grounded. Just before a big summer rainstorm, the noise level slowly went up to 9+10dB and I disconnected the antenna to save my transceiver. During the storm, the antenna plug was literally sparking, certainly destroying your rig if you leave it in!

Contest use:

After setting up the antennas a couple of times in the field, it was time to use them in a contest. By now, we had two 320m rhombics, but only one termination. So we decided to set up one antenna to USA using the termination and one to Japan with the ends tight together. This would allow us to work into South America on the back of the antenna. The USA rhombic was at 18m high (54 ft) and the JA/SA rhombic was at 12m (36 ft). The antennas were to be used for DX on 40m, together with a vertical and a dipole. We could switch between the four antennas. During the night, the rombics did a great job. Many signals were not even heard on the vertical, while perfectly readable on the rhombic. Even when DX stations were not in the direction of the beam, they could often be worked better on the rhombics that on the vertical because the noise level was so much lower. Because we were in a contest, there was no time for comparison of the rhombics with our 20m, 15m and 10m beams, something I really wanted to try. Maybe some other time thanů

Conclusions:

All in all, the rhombic is a very nice antenna to work with. It's a very low noise antenna with high directivity also on the lower bands. A comparable yagi or quad beam antenna (at least 3 to 10 elements from 80 trough 10m!) would be much more expensive to buy or much more difficult to construct. The two only serious drawbacks of the rhombic I think are it's size and the fixed beam heading. By using relays and extra open feeder, you can use it in two directions, but still this is not very practical when used for contesting. Another thing that does not help in a contest is the rhombics inherent bandwidth, making your station susceptible to interference caused by the transmitters working on the other bands. When you have enough square miles of land for 4 or more rhombics and work single band, I would go for it!

For our contest group the rhombic antennas were used twice as 40m beam antennas. While performing great, we decided not to use them anymore because of their enormous consumption of square meters on the contest field and the amount of work needed for setting it up compared to the improvement in station performance.

More interesting information on "Rhombics" can be found on the following links:

http://www.mindspring.com/~cummings7/rhombic.html
http://www.angelfire.com/tx5/wieser/rhombic.htm
http://www.tpub.com/neets/book10/42o.htm
http://www.fullwave.com/fsj/rhom1.htm