A QFH antenna from copper pipe and patience
A quadrifilar helix is the classic antenna for weather satellites: circularly polarised and reasonably omnidirectional towards the sky.
I used 15 mm copper pipe for the mast, 4 mm copper wire for the elements, and a calculator to get the loop lengths for 137.5 MHz with a bending radius that fits the pipe.
Dimensions
| Large loop | Small loop | |
|---|---|---|
| Total length | 2270 mm | 2156 mm |
| Height | 526 mm | 504 mm |
| Internal diameter | 299 mm | 284 mm |
Two failures first
The first version used aluminium wire, which refused to keep its shape. The second one was electrically fine, but I swapped the direction of the twist and got the wrong circular polarisation, losing about 15 dB on every pass. The third one finally works and has survived a winter on the roof.