Thursday, March 17, 2011

The Sideways Quad or VK2ABQ

Note: If you haven't already, be sure to also read my later posts (starting here)  that describe how I modified this basic VK2ABQ ("folded" it) to get a 50 ohm match and, I think, better gain and F/B.

You saw in my previous post the last antenna I built, a VK2ABQ (and the dog's name is Lily, btw). I also noted my disappointment in it, which I'll endeavor to explain later, but for now, let me just describe the antenna itself and share some of the shade-tree construction details. 

Named after the call sign of Fred Canton, the VK2ABQ is, in essence, a quad full-wave loop turned on its side and then split in the middle of two opposite sides in order to form two elements. The feed line attaches to the middle of the forward element. The reflector can optionally have a series inductor inserted opposite the feed line attachment to tune out reactance.

Schematically, a 6-meter version of this antenna looks something like this:


Note that in this diagram the driven element is very slightly shorter than the reflector. That's a feature that you'd expect if your mind is in Yagi mode. It's actually not necessary for this antenna and the benefits are minimal in trying to emulate a Yagi in that way from what I've read. The key feature to a VK2ABQ is the close (capacitive) coupling of the ends of the two elements. I've never seen a definitive rule on this point, but the spacing is something on the order of 5 mm, the distance between the holes of a large coat button, which are the insulators that Fred Canton used for his own antenna.


As most of you likely know, Les Moxon, G6XN, improved the forward gain and F/B of the VK2ABQ by using a more rectangular shape and slightly more gap on the element ends. L.B. Cebik, W4RNL, with the benefit of computer modeling, further optimized G6XN's model making it even more rectangular, with more element gap, to create the now familiar Moxon antenna


The Moxon wins vs. its square brother with respect to gain and F/B ratio, but the VK2ABQ wins the match for construction simplicity and the ease with which one can turn it into a multi-bander. Multi-banding is a feature I always look for. That and the reported gain of about 4 dBd seemed sufficient reasons to give this antenna a whirl. I soon began collecting parts to make a 20 meter version, figuring I'd add a couple more higher bands later.




For the diagonals I ordered four 4.5 meter collapsible fiberglass fishing rods off eBay. I get a lot of things off eBay, such as electronic components, because most of the Chinese suppliers have much more reasonable shipping charges to Costa Rica than do the U.S. sellers (Their customer service is better in general than the U.S. sellers but that's a topic for another day and another blog). Although I was a little wary about what these poles would look like once in hand, I needn't have worried. They seem to be of good quality. They are incredibly lightweight. The length of the diagonals (using 248/14.1 MHz per side as the basis calculation) were to be 12' 4". I knew the poles would be very thin on the far end, which is why I ordered ones about 15' long. 


Once I had the poles I began making the hub with materials on hand: plywood scrap, steel scrap, PVC pipe. Since the poles are tapered I did a reverse split on the PVC pipe and pushed through the rods from the center outward. Muffler clamps hold them in place.

The weight was already a bit more than I was shooting for, but I carried on. Next step was to set up a temporary jig with some scrap pipe and sawhorses to put the hub at about 3 feet off the ground. I extended the poles and started stringing #20 insulated hookup wire for the elements. Two problems cropped up, one minor, the other a showstopper. The minor one was that I hadn't really thought through how I was going to attach the wires to the corners so that the tension could be adjusted later. I thought I could just use some stiff wire to attach the corners directly to the fishing rods, but in practice this seemed even too shade-tree for me. The other problem was that despite collapsing inward the last 3 feet or so of each rod they were still far too flexible. First they'd bow up, add some tension, and one or more bow downwards. It was impossible to keep things in one plane.



Thus, was born Hub 2. The significant change here (besides the new colors!) are the 5 foot fiberglass tubes I dug out of a pile in the "attic" (saved for a 40 meter quad project I'd dreamed up years ago when I must've been smokin' some good stuff!). I attached these to the hub in place of the fishing poles. 


The butt end of a pole was not really a close fit inside the fiberglass tube, but I had a tiny brain spark on how to solve that. I slide off the butt end section from the fishing pole, reversed it, and slid it back on. Now it was like a cylindrical wedge that fit very snugly inside the yellow tube. The remainder of the pole slid into that wedge and the entire joint is held snug with some self-tapping screws.


Later, all joints were bound with electrical tape. From this point on I'm only using a bit over half the fishing pole length for each diagonal. The entire structure is quite stiff and the total weight about 20 lbs. 


I further enhanced the rigidity with a center mast of 3/4" PVC pipe which has a coupling on top that rotates. On the coupling are four more of those self-tapping screws (I love these things) driven in about halfway. The screws are attachment points for 20 lb. fishing line that runs down to the end of each diagonal. After the wire is in place and tensioned, the final adjustment is to rotate the coupling, which wraps the guys around itself adding upward tension to the ends of the fishing rods.




Despite the 50 ohm coax feed shown in the schematic of the 6 meter loop at the top of this posting, the VK2ABQ has roughly a 170 ohm feed impedance. I have a couple of 4:1 voltage baluns around, but they're clunky so I decided instead to copy one of these.  




One last detail are the attachment hooks, one for each end of the diagonals. Rather than attach them directly to the fishing rod, which gets a little fragile out towards the ends, I put on a sleeve made from clear tubing. Around this goes a SS hose clamp that holds a home-made hook made from copper wire and soldered for stiffness. 




Once the element wires were strung I could adjust the overall tension by loosening the hose clamp and sliding the hook towards the outer end of the rod. Voilá!


Notice that the reading on the MFJ is about 13.400 MHz at the lowest SWR point (about 1.2:1). Instructions elsewhere said that the resonance point would probably move up about 300-500 Hz from what is measured on the ground. Though that meant the elements were still too long, I thought it was close enough to try. 

I'll continue in a later post the mounting of the loop, attemps to match feed line and what I thought of the overall performance. Until then, ...


73,

Casey TI2/NA7U



2 comments:

  1. Hello Casey, the moxon type antenna is a interesting design as it can be used in a small space. You did a nice job so far building this 20m version. I think it is incredible you have the ability to build such an antenna over there, I would be a lot more difficult without the help of the modern times eBay via the internet.
    To get back on my horizontal quad length. It is not a coïncidence I choose 84 Mtr, following the advice of a fellow dutch amateur PA0FRI it seems to be the best lenght for multiband use. I had the luck it just fitted around my garden. I think his site will interest you as he tested/built a lot of HF antennas and tuners.

    ReplyDelete
  2. Thank you Bas for that link. I've added it to the blog list of links. Unfortunately, PA0FRI seems to have constructed the pages such that the ones in Dutch can not be translated by Google Chrome. There is plenty in English, however.

    73!

    ReplyDelete

Thanks very much for your comment! 73, Casey

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