Sunday, May 16, 2021

Low cost loop antenna for 40m from VK2BLQ

Stephen, VK2BLQ, and I test the Sydney - Melbourne path using WSPR and we have been comparing our noisy external antennas with loops in various positions. 

Today he sent details of his latest loop made mostly with bits of Bunnings.

He writes: "The loop is approx. 900 mm diameter as it was a 3m coil of copper from Bunnings, when it was cheap.

I let it run on 40 m wspr yesterday and this morning and pretty much was only received along the east coast but not by Peter. Only the wire antenna which is 3 m above the loop reached USA Canada and all over other places.

WSPR is ok but the only real test is for two way communications; I could hear VK3XU at good  level this morning and VK2ARZ a bit later. I need a small changeover switch between keyer and key.

The tuning capacitance is the double sided pcb, the actual value of capacitance depends on the thickness and type of the fibreglass, so it was a case of measuring a piece and cutting off small pieces until I can see the minimum swr on a Nano VNA. The PCB has a coat of lacquer and soldered to the loop, it probably is not too resilient outside.

The Coupling loop is a 1 m length of RG 11 75 ohm TV cable with only the braid and foil shield being used. It is longer than the customary 20 %, and perhaps that is why the Q is low.

Some heat shrink to cover the joints and definitely only temporary.

Next version will be a 6 m perimeter octagon as I have four 1.5 m lengths of copper. I read somewhere that the capacitor should be at the bottom with the feedpoint at the top for a better angle of radiation. This is possible if I re-solder the PCB capacitor".

Stephen is certainly covering Australia on 40m WSPR but oddly missing the bit of Melbourne where I am located.

We are travelling to MayHam at the end of the month and if it fits in the car we'll bring it along for the Home Brew display.

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