Why a Quad? Quads beams consist of 2 1 wavelength (approximately) loops, ordinarily arranged so that one is the driven element and the other is the reflector. Since the elements are double the size of the half-wavelength elements in a 2- element Yagi, we expect more gain-and we get it. In fact, a 2- element quad has almost as much gain as the 8'-boom Yagi or the wide-band Yagi we discussed in recent columns. However, the 2-element quad does not have as much gain as a fully optimized long-boom 3-element Yagi. Also, the front-to-back ratio of a quad fluctuates around the 20 dB figure, more like a 3-element Yagi than a 2-element Yagi.
Physically, a quad beam--when compared to a Yagi--trades volume for horizontal area. Horizontally, even the 8'-boom Yagi requires a rectangle about 8' long by 16.5' wide. A quad of similar performance has a footprint only 5' long by 9.5' wide, or about 1/3 the area. However, the Yagi is flat, while the quad occupies a 450 cubic foot volume. (That just means that it is as high as it is wide.)
If you decide that you want a quad for most of the HF bands, I recommend that you obtain one of the commercial models. These fairly complex structures are engineered for maximum strength and durability. However, if you need only a 10-meter model (and later want to tuck 6-meter or 2-meter quads within the same framework), then you can build one yourself with simple tools and materials.
The Quad Beam Structure. As shown in Figure 1, a good 10- meter quad can be built from two wire loops. If the wire you use is #14 bare copper (stranded or solid), the driven element loop is 105.3" per side or 421.2" overall. The reflector loop is placed 60" behind the driven element and is 110.3" per side or 441.2" overall. If you decrease the wire size of a loop antenna, the total length becomes smaller as well (and not longer, as with a linear wire antenna element). For #18 bare copper wire, the dimensions per side drop to 105" and 110" respectively--not a big drop. but a noticeable one.
These dimensions are for bare wire. Insulated wire has a 2-5% velocity factor, depending on the type and thickness of the insulation. It will require different dimensions that I have not modeled and tested.
If we could just starch the wires and toss them in the air, the quad would be a very simple antenna. However, we need a supporting structure, shown by the dotted boom and arms in Figure 1. The support structure should be nonconductive, although a metal boom usually does not affect antenna tuning. For 10-meters, the arms can be strong fiberglass, quality bamboo (with lighter wire elements), or thin-wall PVC.
The sketch shows what one might do with an all-PVC structure. The boom can be about 4-5' 1" or 1.25" nominal Schedule 40 PVC. (1" nominal PVC is closer to 1.25" in diameter, while 1.25" nominal PVC is closer to 1.5" in diameter.) The arms can be thinner-wall SDR-135 PVC, 0.5" diameter nominal. Each of the 4 arms per element should be just about 6.5' long, but lets add another half foot to each.
Since PVC comes in standard 10' lengths, we shall need 8 pieces of the half-inch diameter stock, along with 4 end-to-end couplers (plus a 5' piece of fatter Schedule 40 for the boom). You can either drill the ends of the arms for the wires or use half-inch Tee fittings--although the smoother fittings add weight to the assembly.
Assuming a 5' boom, use a 1 1/8" drill-mounted hole cutter to cut two pair of aligned holes in each end of the boom. The outer pair at each end should be about 6-9" in from the end, and the inner set another 1.5" further in. The hole-pairs should be at right angles to each other at each end, and the hole sets at each end should be very closely aligned with each other.
Now we can make the wire supports. First, place an end-to-end coupler in each hole and run a stainless steel nut and bolt through the boom and coupler to fasten the assembly. Using PVC cement, add 7' lengths of the half-inch thin-wall PVC to each coupler end. Add Tees to the ends, or drill out and deburr generous holes for the wire about 1/2" in from each arm end.
Use a length of cord marked the same length as the wire that will replace it--with some excess. Run the cord through the holes and stress the arms outward toward the boom end until the cord matches the element size. Tie it off. If you use fresh nylon cord, you can leave it in place even after you add the wire--it will not hurt anything. Since cord stretches, you may experience some flex reversals, but after a few of these, you will learn how to keep the arms flexed in the correct direction.
Now add the wires. Make the reflector loop solid by twisting the ends of the wire together and soldering. The driven element requires a small insulated plate and a coax connector.
Assuming you have or can build a boom-to-mast mounting plate and have the U-bolts to mount the structure, you are almost ready to go. But let's not hurry.
Performance. The 2-element quad we have just built was designed for best front-to-back ratio from about 28.25 through 28.75 MHz. Below 28.25 MHz, the front-to-back ratio decreases rapidly to a little over 10 dB. Above the target frequency range, the front-to-back ratio decreases more slowly so that the 10 dB figure is not reached until about 29.4 MHz.
Forward gain of this antenna is maximum at the low end of the band and decreases about 0.1 dB per 100 kHz. Gain is best in the first half MHz of 10 meters, rivaling the gain of the 8'-boom Yagi. It remains as good as any 2-element Yagi all the way to the top end of 10 meters.
Since I am not trying to sell you anything, notice that I have told a story about what happens to the gain and front-to-back ratio rather than citing peak figures for each. Figure 2 shows the azimuth pattern at the elevation angle of maximum radiation when the antenna is modeled at 1 wavelength up (about 35' high).
Figure 2 shows not only the variation in front-to-back ratio and the rear pattern, but also the gradual reduction in gain with increasing frequency.
Feeding the antenna. The feedpoint impedance of the antenna alone varies from about 70 to 150 ohms across 10 meters. since we anticipate feeding the antenna with standard 50-ohm coaxial cable, we need to add one more component: a matching system.
The simplest and most effective matching system for this antenna is a 1/4-wavelength section of 75-ohm coaxial cable, such as RG-11 (for higher power) or RG-59 (for lower power levels). Both cables have a velocity factor of 0.66, which means that a full wavelength of cable is 0.66 of the wavelength in free space. Let's use 28.5 MHz as our design frequency. A wavelength at this frequency is just about 34.5' long. A quarter wavelength is a little over 8.6' or 103.5" long. The quarter-wave matching section is 0.66 of this or 68.3" long.
A perfect quarter wavelength will transform an impedance higher than 75 ohms to a lower impedance, which is just what we need. In fact, when fed with a 50-ohm cable to the shack, the quarter-wave section shows under 2:1 SWR all across the 10- meter band. Figure 3 shows anticipated SWR curve.
Do not operate this antenna without the quarter-wave 75-ohm matching section or an equivalent matching circuit.
If a 2-element quad meets your space and operating needs, this
one will do the job about as well as it can be done. Feel free to
alter the support structure using materials with which you are
comfortable in your shop. (For me, PVC is tinker toys for
adults.) A TV rotator will easily turn this light antenna. For best
results, be sure the bottom wire is at least 20' up, and a tower
or mast height of 35' is a good minimum target height for
Updated 10-2-98. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.