Friday, 23 January 2015

VHF DF Loop - with Integral Sensing

For some time now ive been planning building a 2m DF loop, something that can stay in the car as a ready to go device for tracking down interference and/or abusive/pirate transmissions on 2m. But as with a lot of stuff ive never quite got around to doing it!

Recently though I came across a simple design on the 'net, of a 1/2 wave loop made from coax. A 1/2 wave DF loop has a figure of eight radiation pattern, meaning the target can bearing can be found using one or other of the sharp nulls in the pattern, but leaving you with no clear idea which direction along that bearing is correct! A 1/4 whip would then be added to 'fill-in' one of the nulls, and allow the bearing to be established, this being known as a 'sense' antenna,

Where this design differs, and which lends itself to easy and simple construction, is that the 1/2wave loop is formed from the coax inner conductor, and the 1/4wave sense element is formed from the outer braid. The braid itself then also acts as a matching element, allowing the antenna to be tuned for best match to the 50 ohm feedline.

And it really is that simple. Add a bit of feedline to the radio, or a suitable connector, plus some hardware to make the whole lot rigid and provide a handle, and your away.

Im considering making one using some offcuts of Heliax, which will allow me to make the whole shebang - loop and handle, out of coax, and still be suitably rigid. A piece of 1" dia Heliax for the handle, and some half inch Cellflex for the loop, plus a pigtail length of RG-58 with a BNC connector on it. Perhaps, a single hole chassis mount BNC socket can be 'engineered' into the end of the Heliax handle...

The joints can then be coated in liquid rubber to seal them.

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