QFH Antenna for VHF and UHF

The QFH (or quadrifilar helix)m besides being a very nice postmodern sculpture for the inexperienced, is an antenna that suits excellently to the reception of signals from satellites or from aircrafts without the need of pointing. Actually, its radiation pattern is almost constant up to very low elevation angles over the horizon. Further, this antenna has a circular polarisation and this fits with signals transmitted from satellites (using circular polarization, too) or from aircrafts in linear polarisation.

The design of our QFH is largely based on the assembly manual written by Paul Hayes and on the software made by G4UNU and recalled by Paul Hayes in his manual.

In order to check the goodness of the design we did some simulations using the software 4NEC2 by Arie Voors. The results obtained for the 145 MHz antenna, in terms of radiation patterns and of VSWR are provided in the following figures. The radiation patterns are provided for three different azimuth angles: 0°, 45° and 90°. These repeat simmetrically.

Radiation patterns in elevation for azimuth angles of 0°, 45° e 90°

3D view of the radiation pattern and VSWR and return loss behaviours

As can be seen, the radiation patterns are almost hemispherical: the gain variation as function of azimuth is negligible, while in elevation the -3dB lobe width is higher than 120°. The peak gain (in vertical direction) is about 2dB, while the VSWR is better than 2:1 between roughly 140 and 153 MHz.

Up to now, we built about ten antennas both for polar satellites (i.e. at 137MHz) and in radioamateur band at 144MHz to connect the international space station, and we never found problems.

Here below you can find the original manual from Paul Hayes to build the QFH, the one from Bob Thorpe G4UNU to build the balun, the manual written by us based on our experience and the software to compute the dimensions of the antenna in case you want to change its frequency or you need to use pipes with different diameter.

In case you are not able to run the software to compute the antenna parameters due to compatibility with DOS, at the two following links you can find the dimensions obtained for an antenna at 137,5 MHz and for one at 145 MHz.

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