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| The general form of my support for the vacuum capacitor. |
OBSERVATIONS FROM SCOTLAND 28 May 2008 GM1SXX
Building an HF loop aerial. Part 1
At a fairly early stage, I decided that the optimum size of loop aerial for me was one that was square and of side 1.9M. This allows a single tuning capacitor to cover four amateur radio bands with varying (but generally good) levels of efficiency. Since a true Mag-Loop does not exceed 1/10th Lambda, this loop cannot be fairly described as a mag-loop on at least two bands, so as far as I'm concerned, it's simply a resonant loop structure. Most of the problems with tuned loops tend to be of the mechanical sort and this one is no different. Since it'll be hidden in the attic, there's no need for it to be aesthetically pleasing, fortunately for me. Providing a suitable means of holding, aligning and insulating the awkward vacuum cap has proved to be somewhat of a challenge. Although I do have access to an engineering workshop, I decided that a 'kitchen table' approach might allow others to copy the concept.
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| The general form of my support for the vacuum capacitor. |
I needed a way to hold the capacitor, insulate it and provide a level platform on which to mount a drive system. I also need to be able to detect the ends of the mechanical travel. The mount for the capacitor was simply formed from two squares of plastic faced chipboard. Because I wanted the option of re-using the capacitor if it all goes horribly wrong, (they are expensive!!) I wrapped the metal end caps in three layers of PVC insulating tape before I epoxied it into the wooden 'cheeks'. Means I should be able to remove it at a later date should the need ever arise. The end caps are 60MM diameter and I cut the holes with a cheap hole-saw.
The two cheeks were coated with 2 ton Epoxy, the capacitor pressed into place and the whole lot set aside for a day to cure. The cheeks were then glued and screwed onto a pair of parallel lengths of solid mahogany timber to act as a base-plate. On this will be mounted the drive motor and limit switches. A pair of connections formed from 8MM copper micro-bore copper pipe will connect the capacitor to the loop proper. All connections MUST be as low resistance as can be managed because of the heavy circulating currents in the loop (tens of amps). The drive motor is made by MAXON and is a geared (148:1 ratio) unit.
If you do build yourself a loop aerial, don't use the braid from large-diameter co-ax to connect the capacitor to the loop. It's lossy and you will fare much better with some flattened pipe or even flat copper strip. I used flattened 8MM microbore pipe because I had some lying around. Unfortunately I forgot to make a photo of them. Perhaps in the next installment.
The most difficult parts to fabricate on the kitchen table are the pair of spindle adaptors needed. The capacitor has a half-inch shaft that will connect to a length of M8 screwed rod. The other end of the screwed rod will connect to the drive motor and so needs to have an M8 thread to quarter inch adaptor. These adaptors were fashioned from some short lengths of DELRIN of one inch in diameter. In order that they are accurately machined, I used a cheap (£30) electric drill press and a slightly modified machine vice. In order to hold cylindrical objects vertically for drilling, I closed the jaws and drilled a 10MM diameter hole through the dead centre of the jaws. Being constructed of cast iron, the vice was very easy to drill.
This may be a cheap and nasty way to make adaptors, but it does allow cylindrical objects to be held vertically for drilling. So long and you don't move the vice (I clamped it to the drill table), you can drill nicely concentric holes so long as you do the smallest ones first! I drove a 1/4 drill bit right through the piece of DELRIN then replaced the drill bit with a larger metric one to drill the hole I'd tap later to take the M8 Screwed rod. The holes are nicely concentric although the adaptor 'body' does visually 'wobble' a bit when it turns because of my crude method. All that REALLY matters is the concentricity of the holes drilled through the plastic. You can see the vice in the photo above, with the M8 tap still embedded in it. The idea worked rather better than even I had hoped for!
The other adaptor for the capacitor end has a half-inch hole in one end and an M8 threaded hole in the other. The reason for using screwed rod is that I can add a piece of threaded bar that can close a pair of limit switches to detect the end of travel for the vacuum cap. It takes 31.5 turns to go from end-to-end and a 'rolling nut' that 'walks' down a slot and opens a pair of micro-switches (one at each end) is the way I plan to manage this.
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| Here you can see my modified vice that was used to make the plastic adaptors |
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| Here you can see the half-inch drive spindle of the Vacuum Capacitor. |
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| Unscrew the capacitor too many turns and this happens! |
As you can see in the photos above, I made a simple band-type clamp to hold the motor-gearbox assembly. This is nothing more than a Jubilee-clip soldered to a strip of heavy gauge copper sheet. The motor gearbox will sit on top of three strips of mahogany glued together to reach the required height. Once I have the electro-mechanical bits sorted out, the remainder of the construction will be very straightforward. The loop will be fed via a toroidal transformer, mainly because I like to be 'different, and to buck the usual trend of using gamma matches with these loops. Yes, the toroid will introduce more losses than a gamma match might, but I like to be different :-)
I'll have to build the loop as two 'legs', lift it into the attic and solder the halves together 'in-situ'. The loop will be attached to the roof timbers using 15MM plastic saddle type pipe clips.
More as it happens.
73 Al.
GM1SXX
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