C U B I C A L Q U A D A N T E N N A S By: John K. Agrelius, KM6HG The Cubical Quad Antenna is basically a directional beam antenna made out of wire loops. The loops are usually square and are approximately one wavelength long, with the reflector slightly larger and the director(s) slightly smaller. It has more gain per element than a Yagi, it is less susceptible to static noise and best of all the characteristic impedance is close enough to 50 ohms that it can be feed directly with 50 ohm coax. The major disadvantage is that it's pretty bulky for HF and quite a chore to build. But, it's ideal for VHF. In fact, I'm using a 5 element Quad on 2 meters at my QTH. It has great gain, a good front to back ratio, it covers the entire band and the VSWR is flat at the Design Frequency. Building Your Antenna The support for the antenna should be made of wood, fiberglass, PVC or some other non-metallic material. I used 2" X 2" pine for the boom and 1/2" wood dowel for the spreaders. The wire loops were made of number 18 enameled copper wire (for HF I would use number 12 copperweld). You'll also need a couple of pieces of 1/8" Plexiglas about 1" X 2", a couple of muffler clamps that will fit your mast and a piece of aluminum tubing or PVC pipe about 18" long. That's it.. Take the 2" X 2" pine and using the dimensions from the printout, cut the boom. CAUTION - The dimensions are the MINIMUM length of the boom, you'll have to add a little just to be safe. I added an additional 24" because I mounted the boom to the mast behind the reflector so the antenna was not affected by the mast. This makes the total boom length about 64" (40.2" + 24"). Now, take the wood dowel and using the dimensions from the printout, cut two wood dowels per element. CAUTION - Again, the dimensions are the MINIMUM length necessary to make the wire loops. I added an additional 2" to each spreader. This makes the Reflector spreaders 32.7" (30.7" + 2"), the Driven Element spreaders 31.2" (29.2" + 2") and the Director spreaders 29.7" (27.7" + 2"). Using a drill press, drill a small hole 1" from the end of each spreader. These holes will be used to run the wire through, making the square loops. Take one of the 32.7" spreaders and one of the 31.2" spreaders and using a saw, cut a slit on one end of each spreader up to the hole you just drilled. Using care, make the slits wide enough to slide the Plexiglas into them. Drill an 1/8" hole near each end of both pieces of Plexiglas. Press the Plexiglas into the slits, center it and drill another 1/8" hole through the wood dowel and the Plexiglas 1/2" from the end of the dowel. Refer to FIGURE 3. Attach the Plexiglas to the wood dowel with a small screw and some Elmers glue. Now, using the drill press, drill a 1/2" hole 1" from the front end of the boom. With the dimensions from the printout, drill a second hole 12.4" from the first hole, and so on, until all four holes are drilled. These will be the holes for the horizontal spreaders. Rotate the boom 90 degrees on the drill press and measure 1/2" back from the middle of each hole and mark it. Drill the other four holes for the vertical spreaders. You'll also want to drill a couple of holes for mounting the antenna to the mast. I used a muffler clamp and it seemed to work OK. You may have a better way, especially if it's a horizontally polarized antenna. Drill the two holes for the clamp about one foot from the rear end of the boom. Remember, if the beam is vertically polarized then the holes for the clamp must be drilled horizontally through the boom. You'll also need to drill a small hole near the end of the boom to attach the support arm. You can now insert the spreaders in the holes on the boom. I hung the boom from the ceiling with some wire to keep it in the clear. Starting at the rear of the boom, insert the long 32.7" spreader (with the Plexiglas insulator) into the hole. Moving toward the front of the boom, insert the 31.2" spreader (with the Plexiglas insulator) into the next hole. Make sure the Plexiglas insulators are both on the same side of the boom. Continuing toward the front of the boom, insert two of the 29.7" spreaders in the next two holes. You should now have 4 horizontal spreaders lined up with the two shortest in the front of the boom and the longest at the rear. Both Plexiglas insulators should be on the same side. Using plenty of Elmers glue, center each spreader and glue them in place. Be sure to align the spreaders so that they are all parallel and make sure the holes for the square wire loops are aligned properly. After the glue dries, do the same for the vertical spreaders. Let the glue dry and put at least 2 coats of spar varnish on everything and let it dry over night. You're now ready to string the wire loops through the cross-bars. Using the dimensions from the printout, measure and cut the enameled wire to length. CAUTION - Again the dimensions are for the exact length needed, so you should add a little extra. I cut the directors one inch longer (78.4" + 1") for a total of 79.4 inches. The Driven Element and the Reflector need to be cut 4 inches longer. The extra length will be needed for adjustment of the antenna. So, the Driven Element should be 86.5" (82.5" + 4") and the Reflector should be 90.7" (86.7" + 4"). Now strip off about 2 inches of enamel from each end of the Director wires and about 6" of enamel from the Driven Element wire and Reflector wire. Feed one of the Director wires through the holes on the end of each spreader at the front of the boom. Pull the wire tight making a square loop, overlap the two ends by 1" and solder the connection. Do the same for the next Director. Now take the Driven Element wire and feed it through each of the spreaders with the ends of the wire terminating at the Plexiglas insulator. Run equal amounts of wire through the holes in the insulator, fold the wires back and wrap each one around itself a couple of times. Point the ends of the wires toward the front of the boom and solder the connection. Do the same for the Reflector. You're going to need a shorting stub on the Reflector to adjust the antenna. This can be made from a short piece of copper wire. Cut a 2 1/2" piece of number 12 copper wire and bend the ends over, making a small loop in each end. This will slide over the two ends of the Reflector wires at the insulator. Push it all the way down to the insulator for now, making the Reflector loop as short as possible. Refer to FIGURE 4. Antenna Adjustment We're now ready to hook up the feedline, mount the antenna and make some adjustments. The hard part is over. For VHF you should use low loss cable like RG-213, for short runs you can get away with RG-8X. Separate the center conductor from the shield for about 1 1/2". Twist the length of exposed shield and tin the wire. Strip back 1/2" of insulation from the center conductor and tin the wire. Bend a small loop in the end of both the shield and center conductor. Connect the center conductor to one of the ends of the Driven Element loop close to the insulator and the shield to the other end of the loop the same distance from the insulator and solder the connections. Using electrical tape, attach the coax to the spreader and the boom, routing the coax down the boom through the Reflector loop and out the rear of the antenna. You can now mount the antenna to the mast but be careful, it's pretty bulky and fragile. This is a good time to call on one of your Ham friends, they're always eager to help with antennas. Refer to FIGURE 6 so you can understand the mounting method using the Antenna Support Arm. With the antenna mounted in position, hopefully well away from surrounding objects, you can hook up the other end of the coax cable to your SWR meter and check the VSWR. First take a reading at the Design Frequency and write it down. If the VSWR is below 1.5 : 1 at the Design Frequency you might want to leave it alone and get some signal reports from a few friends. Rotate the antenna while listening to a weak station. You should see a big difference as you point the beam in different directions. If the VSWR is too high, first recheck all the dimensions, especially the Driven Element. The total length of the Driven Element should be very close to the dimensions on the printout. For our antenna, at 146 MHz., it should be about 82.5". That includes the length of coax that was stripped back. Measure from where the coax shield separates from the center conductor, all the way around the spreaders and back to where the center conductor and shield separate. That is the total length of the Driven Element. If that measures OK, then you need to adjust the stub on the Reflector for the lowest VSWR. As a last resort, you may need to move the coax connection at the feed point. If the VSWR is Higher at a Lower frequency then you need to make the Driven Element Longer. Keep the Feed Point the same distance from the insulator for each connection to the coax. Remember, the antenna is influenced by the Height above ground and Surrounding Objects, so keep it as high as possible. Remember to solder the Reflector Shorting Stub when finished. One final note - When building a Cubical Quad with only two elements, the characteristic impedance of the antenna will be well above 50 ohms. Their are several ways to compensate for this. Two of the easiest ways are to either decrease the spacing between the Driven Element and the Reflector (about one half of what the printout recommends) or to use a 1/4 wavelength piece of 75 ohm coax as a linear transformer to feed the antenna with. The rest of the feedline would still be 50 ohm coax. Remember, we're talking about an electrical 1/4 wavelength (246 divided by the Frequency times the Velocity Factor). For HF, the best way is to use a Balun. In fact, you should always use a good Current Balun with any HF antenna that uses coaxial cable as feedline. Cubical Quad Diagrams --------------------- _______ | | 1/8" Hole ------>O | | | ---------------------------------------- ( O<--- 1/8" Hole Wood Dowel ---------------------------------------- | | 1/8" Hole ------>O |<---- Plexiglass |_______| FIGURE 3 - Insulators _________________________________________________________________________ * * * * * * *<------- Reflector Loop ------->* * * X<---------- Solder Joint ---------->X* * X * * * * * * * * * ____*__ * * _ * * | * |* *| |* * Hole ------>O* *| |*| * | | | | * ---------------------- /| |\ * Reflector ( O <------ Wood Dowel ------->( | | ) *<--Shorting ---------------------- \| |/ * Stub | |<------------ Plexiglass ------------>| | * Hole ------>O* *| |*| * |___*___|* *|_|* * * * * * * * * * * * X<--------- Solder Joint ----------->X* * X * * * * * * *<------ Reflector Loop -------->* * * * * Front View Side View FIGURE 4 - Reflector Shorting Stub Page 10 Cubical Quads (continued) ------------------------- .\\ . . \\ ' . . \\ ' . . \ __ ' . _._______________________| |_______________________'_ - -----------------------|\\|-----------------------.- ' . | \\ . ' ' . | \\ . ' ' |. |\\ . '<- 2nd Director | | \\. ' Loop .\\ .| | . \\ | |' . . \\| | ' . . \| | ' . _._______________________| |_______________________'_ - -----------------------|\\|-----------------------.- ' . | \\ . ' ' . | \\ . ' ' |. |\\ . '<- 1st Director | | \\. ' Loop .\\ .| | . \\ | |' . Driven . \\| | ' . Element | . \| | ' . Feed ---> ||.________________________| |________________________'_ Point ---> |-------------------------|\\|------------------------.- ' . | \\ . ' ' . | \\ . ' ' . | |\\ . '<- Driven Element '| . \\. ' Loop . \\. | | . \\ | | ' . . \\| | ' . Reflector | . \| | ' . Shorting ||._________________________| |_________________________'_ Stub ---> \|--------------------------|\\|-------------------------.- ' . | \\ . ' ' . | \\ . ' ' . | |\\ . '<---- Reflector ' | . \\. ' Loop | | | | | | *| || \ <------- Muffler Clamp *| || / | | Muffler Clamp ------> *--| \ | | *\\| / | | \\ | |<------ Boom \\ | | \\ | | Antenna Support Arm ------> \\| | \|__|* FIGURE 5 - Top View of 4 El. Quad Page 11 Cubical Quads (continued) ------------------------- || || || || __ Reflector /----->|| | | / ______||_______________|__|__________________ / Muffler _|__|_ __ | Cross /-------->() Clamp -->(O_ __ _O) /O/ |<---- Boom / ______ _______________|__|_____________/ /__| / || | | / / Bar /----------->|| | | / / || | | / / || | | / / || | | / / || Mast ------>| | / /<----- Support Arm | | / / | | / / | | / / | | / / | | / / _|__|_/ / Muffler Clamp -------->(O_|__|/O/ | / / | /_/ | | | | | | | | FIGURE 6 - Support Arm