Building an Inexpensive HF Directive Gain Array
The antennas described here are neither new nor revolutionary, but are frequently overlooked by those searching for an economical high performance design. These 3-element wire beams cost less than $50 in materials, and also provide several other benefits. They can survive high winds, can be located close to the ground so as not to attract lightning, and yet can still provide high gain and good front-to-back and front-to-side ratios. They can be fed directly with 52Ω coax, using no special matching devices or antenna tuners, while maintaining an extremely low SWR over a very wide frequency range. In addition, a simple means of reversing the direction of fire is described.
The key to this design is the focus on the intrinsic impedance of each of the components involved. By matching the impedances of each component, the efficiency of the antenna system approaches 100% and does not require the use of tuners or other matching devices.
The Familiar Dipole
The driven element of the beam is a simple half-wavelength dipole that most of us are quite familiar. As the ARRL Handbook states, the height above ground of a half-wave dipole determines its radiation angle and its radiation resistance. Setting the issue of the radiation angle aside for the moment, note that a radiation resistance of about 50 ohms occurs at two-tenths of a wavelength above ground. Note, also, that the radiation resistance induced by “average earth” coincides with that of “ideal earth” at this same two-tenths of a wavelength height. Therefore, we should place our driven element two-tenths of a wavelength above ground.
Another consideration in a half-wave dipole is the effect of the ratio of the conductor diameter to its length in determining the center feedpoint impedance. For 52Ω, a ratio of about 20 to 1 is required, however since this is obviously impractical we may note that the ratio of #16 awg wire, which is about 5000 to 1, yields a center feedpoint impedance of about 65Ω—not too far from the desired result. Since there is little to be gained by using thicker wire, I used #16 awg for reasons of wind resistance, weight, and availability.
To conclude our discussion of the half-wave dipole, don’t forget we should have a balun at the feedpoint in order to prevent unwanted RF from coming back down the outer shield of the transmission line and spoiling the desired radiation pattern. A simple current-type choke was made by coiling up the required number of turns of my coax feedline, as described in the ARRL Handbook.
Now, let us visualize this dipole as the driven element of a two-element beam. Here we come to the issue of the spacing between the elements. Assuming the second element is to be a parasitically coupled reflector, then the spacing should be about two-tenths of a wavelength to yield best gain while also maintaining a radiation resistance in the driven element of about 50 ohms. As far as the length of the reflector is concerned, best gain occurs if it is between 5-8% longer than the driven element. Since it is best to err on the high side, a length of 8% longer is recommended.
Now, visualize a third element, a parasitically coupled director. The spacing which produces best gain, as well as maintaining our 50-ohm driven element impedance, is once again two-tenths of a wavelength. The length of the director should be about 5% shorter than the driven element. In addition, since the spacing is the same as for the reflector, we can later exchange the reflector and director to reverse the direction of fire.
Finally, we come to the matter of the mechanical construction of the array. You’ll need 6 support poles. I used some 2″ steel poles, which are usually available as scrap pretty inexpensively. I measured out the distances and dug 6 holes about 4′ deep. Next, I found some PVC tubing just large enough for the steel poles to slip into. I capped them off at the bottom in order to keep the poles isolated from ground. (Grounded metal objects become resonant at all quarter wavelength multiples, but ungrounded metal objects only become resonant at full wavelengths. We don’t want the poles to become resonant and de-tune the array.)
I set the PVC tubing into the holes with concrete and slipped the steel poles into the tubes. Next, I attached the dipole to one of the center support poles using a fixed length of rope at one end and then secured it to the opposite pole with a random length of rope. The rope went through a small pulley and had a 2-pound rock hanging from it as a weight to keep it taught. I checked the SWR and pruned it down to 1 to 1 at the center of the band (in this case 20 meters).
I installed the reflector in a similar fashion, parallel to the driven element, using the proper length of rope at the fixed end, so that its 8% greater length would be equally divided relative to either end of the driven element. As before, I then attached the other end to the opposite pole with a random length of rope which went through another pulley and also had a 2-pound rock for a weight. I checked the SWR and, as expected, there was no change. (In some cases, there may be a slight rise, however, it should drop back down when the third element is installed.)
Now its a 2 Element Beam
At this point, the dipole has now become a 2-element beam. The radiation angle of a dipole located two-tenths of a wavelength above RF ground is not good—approximately 78 degrees. However, the addition of the reflector dramatically lowers the radiation angle to approximately 45 degrees. This, combined with a foward gain of about 3.5 dB, should make a noticeable difference in signals now being received from the direction the antenna is pointed.
Then, I added the director, parallel to the others, on the opposite side of the driven element from the reflector. I attached it to the last pole, using the required length of rope at the fixed end so that its 5% shorter length would be equally centered relative to the other elements. Again, the other end was tied to the opposite pole with a random length of rope going through another pulley with a 2-pound rock as a weight. A check of the SWR showed it to be under 1.2 to 1 across the band. Gain had now increased to about 6 dB relative to the dipole. Front-to-back and front-to-side ratios were as expected—about 15 dB. Most importantly, the radiation angle had now dropped to about 37 degrees, which is low enough to work DX effectively.
Can Work DX
Factors such as the type and moisture content of the soil will cause the RF ground to actually occur somewhere below the surface. This may further lower the radiation angle. In general, angles such as 15 or 20 degrees are more desirable for DX work. But, 37 degrees can still provide good DX performance, especially if the DX station is located across a large body of water where RF reflection is more efficient.
Due to the low height of this antenna system above ground, the SWR may be slightly higher over better RF grounds because some of the power is being reflected back up and striking the antenna. Correct interpretation of the cause of this SWR reading tells us the antenna system is still working fine, and there’s nothing to worry about.
Originally posted on the AntennaX Online Magazine by Jean-Philippe Lestrale, W2FGV
Last Updated : 19th March 2024