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Apologies for the lousy photo. The 'BEC' ...... Ratings... 7.5-350PF, 10KV, 40A |
OBSERVATIONS FROM SCOTLAND 28 May 2008 GM1SXX
With Love from Russia.
In pursuit of a better STL (Small Transmitting Loop) that can handle more than QRP power, I've treated myself to a better tuning cap. Here's a rather fuzzy photo of it perched on top of my dinky little FT817 for scale.
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Apologies for the lousy photo. The 'BEC' ...... Ratings... 7.5-350PF, 10KV, 40A |
Yes, it *is* big, and heavy. At a guess, it weighs as much as a bag of spuds! It's a hefty lump of glass, copper and brass.
It's construction is very ingenious. A 'corrugated' copper sleeve preserves the vacuum while a threaded plunger that runs inside it moves the cup shaped moving 'vane'. |In essence, it's nothing more than a pair of copper cans that fit inside each other in an evacuated envelope. This one is ex-Russian military surplus and appears to be brand new. Considering the sheer quality of construction I didn't pay a lot of money for it. In fact it was about a tenth of the cost of a comparable US made one.
Here's what Reggie's software has to say about a 1.9*1.9M loop at 7M high on 20 & 80M.
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The capacitor is a huge and heavy brute of a thing, good for around 100W PEP in a mag-loop. Yes, I know it looks good for at least 1 KW PEP, and it *would* be with 'normal' aerials, but with small transmitting loops, the voltages and circulating currents are such that you need such a capacitor for a 100W PEP system. This is the price you pay in trading size for convenience. The STL or 'mag-loop' has a very narrow bandwidth so when moving frequency you normally need to re-tune the loop. In time this becomes second-nature. For those used to older equipment with high-q pre-selectors, tuning the mag-loop will feel very similar to 'flying' your pre-selector. Racal and Collins fans should take to it readily!
This capacitor has a half inch drive shaft and needs 31 turns to go from end-to-end. Turning it more unscrews the drive mechanism in turn allowing the thrust bearing and shaft to come adrift, something you don't want, so I need to devise a system that only allows me to rotate the spindle through 31 turns. After a fair bit of thought my preferred fix is to use a piece of screwed rod to drive the spindle from a geared motor. The rod will have a 'rolling nut' that travels to and fro between a pair of lever-arm microswitches to limit the shaft rotation. The screwed rod will be fitted with an insulating sleeve at the drive end, good for 10KV since it needs to be isolated from the 'hot' capacitor connection. That at least is the theory. I need to make an insulated shaft coupler and a few other bespoke parts for this.
The 'control system' will be a simple shack mounted box containing nothing more than a pair of pushbuttons.. FWD / REV and a FAST/SLOW selector switch that will supply either 12V or 3V to the motor. A twisted pair of wires will handle the control side of things.
The only really practical alternative to a vacuum cap for STL's is to use a 'butterfly' variable. At the 100W level you simply can't use wiping contacts like with a normal air variable. They are just too lossy. They will work at a pinch for low power use, but any more requires a good capacitor. Butterflies are rare in the winter :-)
to be continued......
73 Al.
GM1SXX
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