NA7U's Cubex 5-Band Quad
After many years of using a modified Lightning Bolt quad for 20-15-10 meters, I upgraded to a Cubex 5-bander. The LB quad was always a bit of a maintenance problem due to the less expensive construction (I got what I paid for!). I'd heard good things about the Cubex models from other hams and Norman at Cubex was tremendous at answering my many questions and helping me to decide on the best model. In the end, I stayed with 2 elements, but got 5 bands in their MK-II-PT5 model. I opted for the continuous spreaders and pre-configuration of this over one of their less expensive Skymaster kits. I believe it was worth every extra penny. Below are some shots of the quad during construction and up on the tower with a few pictures of construction details.
One modification I made was to use Budwig HQ-1 center connectors, which have a built-in SO-239. There are four coax runs here (hard to see, I know). 10, 12, and 15 meters have 1/4 wavelength 75 ohm tuning stubs (RG-11/U). 17 and 20 meter loops are naturally close to 50 ohm impedance (due to the relatively close spacing of 8 ft.), so they have a straight coax run to them. I already had a 4-position antenna switch box, so tying 17 and 20 meter loops saved me buying a new relay box.
The 20 and 17 meter loops require a bit of reinforcement at the corners, seen in the center picture. On the right is a picture simply showing the routing of all that coax to the center of the boom. Each coax has a 1:1 ferrite bead current balun in-line. In the case of 15 meters, I used a pre-made coax loop as the balun, since it was lying around unused.
You might also notice in these pictures the method I used for construction of the antenna itself. I set the boom on braced poles. I assembled the spreaders to the spiders on the ground, then attached them to the raised boom. From there I "windmilled" the entire antenna as I ran each wire (10 of them!) through the pre-drilled holes in the spreaders.
You can also see if you look closely the setup for getting the quad up on the tower. There is a doubled-back rope going from the top of the poles to a point high on the tower. These acted as a ramp, supporting the quad's weight (about 40 lbs.) as we raised it up to the top. The antenna was clamped to the mast, then the mast and antenna raised a couple of feet for its final position, about 1/2 wavelength on 20 meters above ground.
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Unfortunately, it was an all-hands-on-deck affair hoisting the tower in place, so there are no pictures of that process. I thought later wouldn't it be cool to have a head cam while I was on the tower top! Anyway, it's a beautiful sight, once on top, and a big relief. Most projects seem to take twice as long as you figure, but when it involves tower work it is more like four times as long! Even after the quad was secured up top, there were many, many small jobs to complete, such as adding the rotator, routing cables, sealing connections, etc., etc.
In fact, it was approaching dusk by the time all was in place, and I only had time enough to hook up the coax to the shack junction and run the MFJ 259 over the bands to see how the measurements up the tower compared to those I took when it was still just a few feet above the ground. The SWR measurements are summarized in the table below. Note that I simply constructed the antenna according to the instructions, making the wires just "snug" not tight. The placement of the pre-drilled holes govern the geometry of the antenna and thus I made no adjustments.
|On the Ground - low SWR point||On the tower - SWR Low||On the tower - CW band point SWR|
|1.7:1 @ 28.080||1.0:1 @ 28.480||1.7:1 @ 28.000|
|1.3:1 @ 24.755||1.1:1 @ 24.325||1.8:1 @ 24.910|
|2.0:1 @ 21.730||1.3:1 @ 20.980||1.4:1 @ 21.050|
|1.2:1 @ 17.930||1.4:1 @ 18.240||1.5:1 @ 18.075|
|1.2:1 @ 13.980||1.1:1 @ 13.980||1.2:1 @ 14.060|
Preliminary receive tests on 20 and 15 meters yield a consistent 3-4 S-Units difference when the quad is rotated 180 degrees from a steady signal of known location. That corresponds to 18-24 dB F/B, which is close to as-advertised. I'm confident the other bands are performing well, but time and experience will tell. Quads have great gain, are quieter, and have a lower take-off angle relative to a yagi of the same number of elements and same boom height. Their construction is a bit of a challenge, but some planning mixed with patience will overcome the obstacles and you'll have a thing of beauty to marvel at and with which you can bust those DX pile-ups!
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