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Author Topic: magloop tuning  (Read 864 times)
WD4KPD
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Posts: 60




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« on: May 29, 2008, 03:08:45 PM »

have nice magloop for 80/160m with vacuum cap.  its all manual tune now.

how can i move the cap indoors near the rig. i do not want to mess around with remote tune.  am guessing some type of shielded/balanced line with proper voltage rating, but wouldn't this add capacity to the system ?

david/wd4kpd

wd4kpd@suddenlink.net
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K4ZN
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Posts: 26




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« Reply #1 on: May 29, 2008, 03:58:38 PM »

You need remote tune. With even low power, the voltages on a small loop antenna are very high, in the thousands of volts.

Also, the radiation resistance of the antenna is very low - so, it is essential that the connection to the capacitor be as short and direct as possible.

If it were possible to run some type of cable or ribbon from the antenna to the shack and somehow get the antenna to tune, the loss would be so great that what you would have would be something like a dummy load. And yes, sometime you can make contacts with a dummy load - this fact does not make it a good antenna.

Here are a couple of links on small loop antennas

http://www.standpipe.com/w2bri/

http://www.aa5tb.com/loop.html

This second link has many links at the bottom of the page. There is lots of info on the web on small loop antennas. Hope this helps.
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W8JI
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« Reply #2 on: May 30, 2008, 04:48:34 AM »

Not only are voltages very high, so are currents in the capacitor.

This is why you can NEVER move the capacitor away from the loop. It has to be right at the gap, and the gap has to be exactly opposite the grounded point of the loop where the feedline is.
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N3OX
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« Reply #3 on: June 01, 2008, 03:06:33 PM »

Yep, no choice but to do remote tune somehow.

The simplest way to do it is to just wire up a gear motor and limit switches (with reversing diodes) and wire a reversing DPDT switch.

The limit switches will keep you from ruining the capacitor by over-torquing it, and the noise in the antenna is all the indication you need that you're getting close to tuned.

It really doesn't have to be too fancy.

Max Gain systems has gear reduction drives with limit switches included for about $40.  That might get you past most of the mechanical hassle; then you just need an insulating shaft of some kind and a couple of couplers.  If you use two couplers and a shaft you'll have sort of a driveshaft arrangement that takes all sorts of misalignment, just like a car's driveshaft does.

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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