One of the two QFH antennas on NOAA-17 (Photo courtesy and (c) Lockheed Martin
The quadrifilar helicoidal antenna - quite a mouthful - is easy to
construct (even though it does look complicated), and the results are
excellent. In contrast with the omnipresent turnstile antenna (two
crossed dipoles with a reflector) the circularity is almost perfect
in any direction. I've also received quite a few reports from people
who built the antenna using the data from this page, who were very
In it's original form, the relation between diameter and height is 0.44, but decreasing this relation slightly, a still better coverage of for satellites near the horizon is obtained. In literature and on the web, extreme versions can be found.
If the left column of this page (under 'Pictures') are links to pictures of several versions built by me and sent to me by several happy readers. The first link shows images generated by POV-ray.
Alfred... There is no such thing as gain in an antenna. This is one of the most difficult things to teach my students. This term was established many years ago, probably for commercial reasons...
The correct term is directivity - i.e., the capacity of antenna to direct the available power into one preferred direction. This looks as a gain in the correct direction, but the power was 'stolen' from elsewhere.
The QFH is an excellent antenna for satellites, as it receives from 'the entire northen hemisphere', from horizon to horizon, with all the sky in between. Such an antenna cannot have any significant 'gain', as it doesn't have directivity. Such an antenna is not useful for hot spots, as you probably won't have any clients 'in the sky'.
Moreover, your clients, except if you give them new, circularly polarizaed antennas, will probably be using vertical antennas from their laptops or notebooks. This means an extra loss of 3 dB because of the polarization.
I'll put this text on the page, as it seems to be a regular issue.
|(c) John Coppens ON6JC/LW3HAZ|