How a QFH works
A quadrifilar helix is two bifilar loops, twisted half a turn and set at right angles on a common mast. The large loop is about 2.6% longer than a wavelength and the small one about 2.5% shorter, so they're slightly inductive and capacitive. That puts their currents 90° apart, which produces circular polarisation without a separate phasing line. The pattern is a broad dome, so the antenna hears satellites from horizon to horizon with no rotator.
The dimensions follow the method used in the widely known calculator by Jan Coppens ON6JC, including its correction for conductor thickness and the extra length of bends.
Building it
- Materials: 10 mm copper pipe, thick coax braid or 4 to 6 mm solid wire all work. Use PVC pipe for the mast and booms.
- Feed: run the coax down the inside of the mast and connect it at the top, with the braid to one loop and the centre to the other. Coil a few turns of the coax inside the mast as a choke.
- Twist direction: for satellites, wind the helix for right-hand circular polarisation as seen from below. Most guides show the direction.
- Add an LNA at the antenna for the cleanest pictures.
The weather satellite tracker shows when the Meteor satellites pass over, and the helical antenna calculator covers the axial helix for higher frequencies.
Questions
What frequency should I build a QFH for?
137.5 MHz covers the Meteor-M satellites on 137.1 and 137.9 MHz. NOAA's analogue satellites were switched off in 2025.
Can I build a QFH from coax?
Yes. Many people use the braid of thick coax, or the coax itself, as the conductor. Enter its outer diameter.
Do I need a rotator for a QFH?
No. It receives from almost the whole sky, which is why it's popular for weather satellites.
Why are there two loops of different sizes?
One is slightly long and one slightly short, which shifts their phases by 90° and gives circular polarisation.