HELP NEEDED what size tuneing capacitor for loop antenna ???

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WARLOCK
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HELP NEEDED what size tuneing capacitor for loop antenna ???

Post by WARLOCK » 04 Mar 2017, 14:48

i am hopeing to build a magnetic loop antenna can anyone please help as to what type /size of tuning capacitor i will need to cover as many of the ham bands as possable ???

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ch25
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Re: HELP NEEDED what size tuneing capacitor for loop antenna ???

Post by ch25 » 04 Mar 2017, 20:23

Depends on loop diameter. No doubt it must be vacuum variable capacitor.
You can't have too many antennas...

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Re: HELP NEEDED what size tuneing capacitor for loop antenna ???

Post by nay27uk » 06 Mar 2017, 23:18

My Shortwave Mag Loop is 1 m diameter with a feed loop of about 12" diameter, my Cap is a Duel Gang Air Variable out of an old valve signal generator.

I could not say the puff range of my Cap as I don't have the kit to test it but because it was the original main tuning capacitor from a HF/VHF Signal Gen it was what I went with and It works fine over the whole HF Spectrum.

Try and get an Air Variable from an old SW Radio or old valve Signal Generator if you can.
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Re: HELP NEEDED what size tuneing capacitor for loop antenna ???

Post by nay27uk » 08 Mar 2017, 00:06

Forgot to add that I used 10mm copper tubing the stuff you buy in a role like this stuff http://m.ebay.co.uk/itm/YORKSHIRE-TUBE- ... v=searcher
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Re: HELP NEEDED what size tuneing capacitor for loop antenna ???

Post by DX-Digger » 08 Mar 2017, 00:35

I have a couple of programs for calculating Loop dimensions and for making Home brew capacitors (not written by me)

https://www.dropbox.com/sh/gu4awerx2g9c ... sh5Ta?dl=1

The main consideration with regards to the capacitor is RF power applied to the loop.
If you plan on running 100w then the plate spacing has to be large to stop flash over, Vacuum capacitors are preferred with high power.
For QRP powers spacing of the plates in the capacitor can be closer but not too close.
Also be aware there are VERY HIGH Voltages carried in a mag loop and if you are not careful you will get Burned.

The biggest problem with mag loops is resistance loss usually in the connections from the capacitor to the loop.
Hope that helps
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nay27uk
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Re: HELP NEEDED what size tuneing capacitor for loop antenna ???

Post by nay27uk » 08 Mar 2017, 02:22

Some good points there DX-Digger and I was just assuming he wanted it for RX only not thinking it may be for TX also, so yeah if for TX also then as DX-Digger said spacing is somewhat crucial.
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RTL-SDR 100 KHz - 2.0 GHz V-UHF/HF

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