Most of the Quadrifilar Helix antennas I've built have suffered from loose mounting where the coax cable meets the copper elements. I also use self-tapping screws to connect the coax to the copper. Due to the loos mounting (push it through the hole and hope it doesn't move) the antennas develop weak reception spots due to their non-symmetrical nature.
After considering this for a while and after seeing someone use a printed circuit board for a centre, I decided to go with a variation on this for the latest tall narrow QFH.
Basically a circular disc is cut from a piece of single layer fibreglass PCB. The disc is about 4cm in diameter but the size isn't critical. I used a knife to create an isolating channel across the centre of the disc. Next I drilled a centre hole in the disc to allow the coax cable to feed in from below. I then used a circular tool (like a dremmel) to cut slots down the middle of the copper elements so that they would fit snugly over the PCB disc.
When all cut and assembled the pieces are soldered together. Finally the coax is introduced and soldered, inner to one side and outer to the other.
IMPORTANT - ensure that the order of connecting the copper elements and coax is correct. Otherwise you'll incorrectly polarise the antenna and results will be poor. There are plenty of links via google images to show how it's arranged.
Here's a picture of the unit assembled inside a waterproof electrical box and with black 8-12mm cable cleats firmly supporting the 8mm copper and preventing any movement after assembly. You can just make out where I labelled the Long and Short loops on the inside of the box - so I didn't get it wrong.
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Mon Dec 23, 2019 2:03 pm