Just in time for the 3-day U.S. Labor Day weekend, I have the modification finished to my VK2ABQ, now replete with a rotor and on the new, sturdier mast. There was no time to spare today as the rain (and thunder!) swept in as I was relaxing on the balcony having just run the coax to the second floor and having given a last SWR check on the MFJ-259.
Though, by all means, we must give deference to Fred Canton, VK2ABQ, for coming up with the fundamental design, I wish I could name this something else besides a [some adjective]-VK2ABQ, which is rather awkward at best. Why wasn't it named the Canton Antenna in the first place? Hmm?
Actually, this design is probably more closely related to the hexbeam, which is really where I got my inspiration, specifically the G3TXQ hexbeam with its "perimeter reflector" design that makes the original back-to-back 'M's version of the hexbeam more broad-banded. In fact, if you dig into G3TXQ's incredibly informative site you'll run across a diagram of the very design I have here, though he only refers to it in passing as "S2", one of many configurations in this family of antennas. I honestly don't remember seeing that diagram before concocting my design, but you know what they say about great minds! :)
I'd pondered the hexbeam many times over the years. It's a compelling, compact design. Here I was again, thinking about basically throwing away the VK2ABQ, since I'd have to re-do the hub and find two more spreaders. I hardly felt that ambitious, however. Alternatively, I already had the "morphed" VK2ABQ in my pocket, but the diamond-shaped yagi design looked tricky on its own, let alone the 17 meter open-sleeve and I was still feeling skittish from the failed 20 meter phased beam. So, I took a different tack of trying to "square the hex" as it were.
The diagram at the top of this post is what I came up with and when I modeled it I was pleasantly surprised. In summary, these are what I saw as the advantages:
- Simple change. This was an easy mod to make as I didn't have to move the corners of the original VK2ABQ. I only had to disconnect the loop, bring the center of the driver to the center post and add some length to the sides. The reflector stayed put, I only had to add some wire to each end. Note that the driver is slightly longer than the reflector.
- Better gain. 10 dBi at the bottom end of 20 meters.
- Better F/B. The original VK2ABQ can produce about 12 dB of F/B, but this one promises 20+ dB.
- 50 ohm feedpoint. It really surprised me that I got this thrown in for free. The nominal feedpoint impedance for a standard VK2ABQ is 160 ohms. There is some reactance, but as it's a fraction of the resistance it can be dealt with easily.
- More broad-banded. The antenna tunes very flat and the reactance stays low across 20 meters. Gain tapers off very slowly, F/B tapers moderately. The gap distance between the ends of the elements appears to be very non-critical.
- Improved structure. The "5th leg" of a VK2ABQ, which mine didn't have, is usually a spar on one side to support the feed-line connection and balun, if any. Since the feedpoint is now in the center that spar is unnecessary.
One of the great advantages of the VK2ABQ over its cousin, the Moxon, is that it's trivial to multi-band due to its symmetry. Multi-banding the design I now have seems possible as 4nec2 shows, but the devil may be in the details. That's for another day.
Below are photos of the new mast and the antenna in place. I hope to use the weekend to put theory to the test when the lightning isn't crashing. I think WSPR would be a great way to get an inkling of its behavior. I am 20 feet short on the rotor control cable, so will have to scrounge that up to see how the directivity is. In an upcoming post I'll discuss the modeled characteristics of this whatever-you-want-to-call-it antenna are, but you can preview all of it wrapped in a zip file on the Antenna Files page.
|Mast and antenna in the ground position|
|Top of the mast folds to keep the antenna horizontal|
|Counterweights on the bottom end of the mast|
|The hinge, latch, and adjustable stop|
|TV rotor and a refurbished ferrite core balun|
|I use this jump-start unit to run the winch (it's also great for portable QRP operations!)|