|A Homebrew Quad|
Updated: March, 2011: See spreader attachments comments.
During one of our weekly TARC DX Net conversations, quad antennas came up as being a good DX antenna that works well at lower heights. Many of us do not have the space or budget for a tall tower and a quad will work well at 40ft. and on a light duty, less expensive tower. A quad is also readily home brewed and potentially less expensive than many commercial antennas.
This quad plan is a combination of ideas and designs from several sources: Cubical Quad Notes, Volumes I & II, by L.B. Cebik, W4RNL, All About Cubical Quad Antennas by William I. Orr, W6SAI, and a January/February 2008 article in ARRL- NCJ (National Contest Journal) by Steve Root: Design and Construction of a Quad That Will Last. In the end it was Steve's article that became the basic plan with a few changes picked up in the Cebik and Orr books. Steve also answered several questions for me via email. Some time was spent with antenna modeling software which helped confirm element sizes and spacing relative to impedance matching.
There is not space to have both my AV-640 vertical and the quad up at the same time. Any comments made about performance will necessarily be anecdotal based upon having used a vertical over the last 2 years and using the quad going forward. Chasing DX using the same equipment and location gave me some understanding of what the AV-640 will do from my QTH.
Thank you to members of the Tamiami Amateur Radio Club and in particular Bob Avrutik, N1RA, Konrad Owens, WA3RRS, and Jack Sproat, W4JS. Without their help this project would not have gotten off the ground.
Building the Quad
The spreaders are built from two 8 ft. sections of fiberglass tubing (Max-Gain Systems, Inc.) telescoped, glued and clamped. Approximately 13 ft. is need to accommodate the 20m reflector.
The excess tubing from the smaller section is glued into the end of the larger section to reinforce the area that will be clamped to the spider.
The spiders are aluminum angle stock attached to the boom with 2 u-bolts. The u-bolts in this photo were replaced with 2 in. stainless steel u-bolts from DX Engineering. The clamps are stainless also.
This was a test assembly of the boom, spiders and spreaders. No question that a 20m sized quad is a handful.
Attaching the wire elements to the spreaders requires a large flat space and some patience to get the sides equal and parallel. The short piece of aluminum tubing is a place-holder for where the spider will eventually attach to the boom. It also made handling the assembly easier.
I looked at quite a few methods of attaching the wire to the spreaders before settling on using heavy duty UV rated wire ties and stainless hose clamps. This allowed for easy adjustment while configuring the wire. I will have to inspect the wire ties over the next few months to see how they are holding up.
March, 2011 Update: Do NOT use this method. It worked very well for ease of construction and adjustment however the "heavy-duty", "UV-protected" wire ties last about a year and a half in Florida sun. I am in the process of re-working these attachments. This mistake is labor intensive since the quad will have to be lowered and re-raised.
New attachment method:
The tubing is air brake tubing and can be acquired from a truck service shop. It's inexpensive in the small quantity that is needed. The shop I found gave it to me. This approach looks like it will be sturdy and will still be
adjustable. Drilling through the fiberglass spreaders and running the element through the spreader does not allow for adjustment.
The wire ties in this photo are placeholders where the driven element feedpoints will eventually attach to the baluns.
This is a shot of the boom assembly. This quad is fed with separate feedlines between the 20, 17 and 15 meter elements and a remote antenna switch. A single feedline runs from the switch to the shack. The feed points have individual baluns. The decision to use baluns was influenced by readings indicating that maintaining the antennas inherent balancing helps with a more uniform radiation pattern and front-to-back performance. It comes at the cost of the extra weight and additional support tubing. Most of the information I found advised strongly against a single coax feed.
The underside of the relay box has the lightning arrestors attached. These had to be on the output side of the relay box because the switch I bought requires DC voltage between the control box and the relay box. Had I bought a switch with a separate control cable I could have used one arrestor in the main feedline at the base of the tower. The arrestors are connected together with aluminum strap and grounded to the boom.
The underside of the relay box after attachment to the boom and weatherproofing.
The weight of the baluns, separate feedlines and balun support had to be offset in order to balance the boom above the rotor. Two, five-pound ingots of plumbers lead bolted on the boom came pretty close and slightly shifting the boom om the boom-to-mast plate balanced it out.
A static diffuser tops off the mast above the rotor. I don't know how much this helps but I figured it can't hurt.
Attachment of the coax and rotor control cables to the top of the telescoping portion of the tower mast. This provided just enough clearance to keep the cables from rubbing on the lower section.
Raising the Quad: You don't have to be crazy to try to put up a quad at my QTH but it helps! The guys hung in there despite all the obstacles encountered. We finally got under way June 4th after a couple of weeks of rain delay.
The antenna raising crew.
The boom was pre-installed along with the rotor and cables. The orientation had already been done without the elements being in the way. Actually, short of a cherry-picker or helicopter, this is the only way we could have done the installation.
The mast section in my left hand was used to help maneuver the elements from the front yard to the back. Holding the elements vertical by the spreaders proved to be too unstable. There is not room in the back yard to assemble the elements.
Bob is making sure the balun support clears the roof line as we raise the tower with just the reflector in place in order to rotate the boom into position to attach the driven element.
Konrad is making sure the element clears the bamboo as we continued raising the reflector.
Half-way there. You can see from this picture why it was necessary to raise the tower, rotate the boom and then lower it again to fit the driven element.
Attaching the driven element.
Success was not assured....
We made it! Fortunately the guys took a "never-say-die" attitude.
The last step was to attach the driven element wires to the baluns.
At full height the boom is at 41 ft. SWR readings were a bit long; all 3 bands were right around 1.2-1.3 : 1 in the CW section. Of course Bob, a CW jock, readily approved! We're sure not taking it down to tweak the SWR for the phone bands as the tuner will take up that slack. The benefit of a directional antenna with gain has been readily apparent in the first week of use.
Thanks for the help guys!