Arliss Thompson W7XU 7314 SW28th Ave. Portland. OR 97219
After several years of hohum openings and low levels of activity, conditions are finally improving on the 28 MHz band. And while a DXer can work the world with ten watis and a dipole when that band is open, an antenna with some gain can really payoff when conditions are marginal. Not only does gain help the transmitted signal, it also increases the strength of received signals from the desired direction and Knocks down potentially interfering signals coming from otner areas. That combination of features often gives the station with a gain antenna quite an advantage over the operator using a dipole or ground plane and high power.
Gam antennas come in a variety of sizes and configurations The one described here ts a spider quad The 2-element spider quad has gain comparable to a 3-ele-ment yagi. is constructed from readily available materials, and doesn't cost an arm and a leg. Although t thought primarily of the Novice operator when I built my version of this antenna, thedesign is also adaptable la the 12 and 15 meier bands.
Most of hamdom es familiar wiih the cfassic quad antenna shown in Figure iAr The spider quad, Figure 1B, is a less common version of this antenna- El has
Figure 1. (a) Conventional cubical quad, Illustrated method of feeding is applicable lo other designs as well several advantages over the classic configuration, such as constant spacing between efe-ments in multiband designs and no boom, which represents a potential savings in weight, wind loading, and cost. The chief disadvantages of the spider quad are the increased complexity of designing a spider mouni. compared to mounting convential quad spreaders, and the need for longer spreaders in the boom-less design I can t argue with the need for longer spreaders wdh boomless quads, but the fashioning of a spider mount need not be difficult,
The mount I used for my antenna was described by Lynda Crowley KP4DIP in a "Hints and Kinks" column some years ago (QST, December 1970), Her hub was low cost and easy to replicate, I haven't seen it repeated elsewhere, however, so it may be new to many hams..
The hub is constructed from a short length of steel pipe (f used a 2.5 foot section of 1.5 inch diameter water pipe that I had on hand). Make two cuts in one end of the pipe, each approximately twelve inches long, across the diameter of the pipe. The cuts should be at right angles to one another. Simh lar cuts are made at the other end of the pipe, taking care to make these cuts align with the first ones (see Figure 2A). This is quick work with a bandsaw, but less than fun (trust me) with a hacksaw. Leave about 6 inches ol undisturbed
Figure 1. (a) Conventional cubical quad, Illustrated method of feeding is applicable lo other designs as well
Figure U (b) Two-band spider quad. The tuning stubs for each band are two wires 12* long\ spaced 3 apart pipe in the center in which to drill to allow attachment of a hub-to* mast clamp.
Next bend the cut ends of pipe outward to form the spider itself (Figure 2B), The desired angle is 108 degrees. This angle simultaneously controls the distance between the tips of the spreader arms and the element spacing. However, the angle is not especially critical since there is ample spreader material for 10- and 12-meter band antennas and good performance is possible wilh a variety of spacings in the 0.15 to 0.20 wavelength region. The builder can measure the angle with a protractor or approximate the correct angle with a wooden form in the shape of a triangle 10 7.16 x 3 H x 11 inches. Once the spider is complete, it needs a few coats ol paint to protect it from the weather.
For spreaders, I purchased 8 ten-foot Lengths of 0.75 inch diameter Schedule 40 PVC pipe. The price is hght at well under $2 each. These spreaders are long enough to support a 15-meief band quad, but aren t rigid enough for an antenna that size One-inch diameter PVC (particularly with some heavy monofilament or lightweight nylon cord for added bracing, as shown in Figure 1B) should be adequate for a 15-meter model. The 0 75-inch diameter material suffices for a IOmeter band antenna, Figure 3 gives the length of the spreaders and other dimensions for each band.
I attached each spreader to its corresponding spider arm with two staintess steel hose clamps. They can also be attached by fastening spreaders to the arms with glass filament shipping tape The shipping tape snouid be over-wrapped with black vinyl tape to protect it from sunlight. While I have noi tried the tape method1 it was apparently used successfully by Mitchell N0ARQ. on his "Optimum Gain Boomless Quad"' (Afl-RL Antenna Compendium L Vol 1, pp 11-17), and is wodh consideration, particularly for keeping expenses down.
Loop dimensions are listed in Figure 3. These dimensions are
for uninsulated wire. Wire insulation may cause the resonant frequency of the antenna to be lower than the calculated value (Boucher, N., "Cubical Quad Antenna Design,1' ARRL Antenna Compendium, Vol. 1, pp. 41-45), Stranded wire is better to solid in this application since it has less tendency to break wrth flexing of the spreader arms. Number 14 or 16 wire is adequate. There are many ways to attach the wire to the spreader arms. The simplest is to pass the wire through botes drilled in the spreaders. An additional short piece of wire can be wrapped around the ioop wire on enher side o< ihe point where it passes through the spreader arm to hold the loop in place. Loops may also be strung through lightweight insulators that have been attached to the spreaders.
Figure 4 shows two ways the quad may be fed for horizontal polarization. To match the impedance of the driven loop to that of 50Q coax, feed the antenna either through a quarter-wave transformer of 75ft coax, or through a gamma match. These are shown in Figures 1A and 5. A quarter-wave transformer will require a separate feedline for each band, it's a good idea to use a separate feedline for different bands in the case of the gamma match, but if necessary, feed the loops for several bands in parallel with a single length of coax (Figure 5).
I 1/2" QiA STEEL PIPE
Figure 2. (a) Cuts to be made in T .5" pipe to form spider hub.
I 1/2" QiA STEEL PIPE
Was this article helpful?