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Sunday, 13 April 2014

G4PTH Mag Loop - Clive M5CHH (SK) version

Was asked to give some info on Clive's build as given this away....

He had made his up so that it could be used on all bands between 40m - 10m.

Follow the rest of the G4PTH article first from RadCom October 2008 (copyright so will not repeat it here). Pretty self explanatory really.

The build quality might not be quite as good as the commercial product but for us it worked fine all the same (it worked well). I look mine out to ON and was qrv with it from apartment out there. The mag loop ability to null out noise or pick up a station just by simply rotating it through the vertical axis has to be heard to be believed.

10 Ali rods 400m x 12mm x 2mm Pretty sure B&Q was the source

Stainless M6 Blots, split washers and wing nuts to fit together (Cambridge Mackays) these MUST be tight and the ends of the ali rods you may need to clean with a light sanding. You need LOW resistance all the way around the loop.
Clive and I had messed with radio and antenna setups since our little DX trip to EU-011 in 2005 when we took two homebrew dipoles and a mag loop. So we had a few IP56 enclosures for use for all things external in the junk box. You can use almost anything PLASTIC as long as rigid.  In it Clive fitted a 8-150pF capacitor. Not this is not one that stops you can go around and around.... so marking the outside of the IP56 box was essential for use. To be honest you tune using an analyser or VSWR meter not selecting a capacitor setting. The rods connect to the two M6 bolts that seem too long.

The straps were made from brass shim, RG58/RG213 coax braid (fresh) would have done, and did on another version... the object is minimum resistance.

Coupling baluns

You couple the RXTX (FT817 in our case) using RG58 coax to the max loop. The feed is using BNC sockets. For other SDR projects we had bought a load of insulated ones... normal ones are fine as long as can connect to BOTH the centre and the shield. The insulated ones are bit more expensive but easier to use.

40m FT150A ferrite (what was available) 5 turns of the green wire from 3 core 13 amp mains flex.
10m FT150A ferrite and 5 turns of the green wire from 3 core 13 amp mains flex.
 Common alignment

It used some small 8cm x 6cm x 4cm ABS potting boxes.... not important what they are as long as plastic.  You drill a hole right through so that the ali rods will slide through (do not do with the Ferrite in place!)

The best location was to have this hung from a telescoping fishing rod so bottom about 4 feet AGL, on the beach.

In use to tune was using either MFJ269, KD1JV antenna tuner or a simple vswr, tuning was sharp the band width not more that 5-6 khz.

1 comment:

M1KTA said...

Extra note:

Normal wisdom is put the capacitor at the top, feed point at the bottom.... In practice what often happened was feed point at bottom and capacitor to one side.... so could access the tuning capacitor without pulling the lot down. If this effected performance never noticed.

The loop IS effected by anything conductive (including you) so use about 20 feet of coax and do not put up near anything else.

Salt water spray will corrode the connections but 2 mins with a strip of emery paper and they are good to go. I had used for a w/e by the sea on cliff in Cornwall once. I fitting together and then daubed sota beams liquid tape all over the connections (Not the one I gave away) so they stayed good.