QFH Antenna (The Quadrifilar Helical antenna) provides circular polarization and complete hemispherical reception, which is precisely what is needed to receive the polar orbiting weather satellites, and as a 2 meter antenna it will receive horizontal, vertical and clockwise circular polarizationís from all directions. Fig 1. illustrates the antenna which consists of two vertical loops at right angles to each other, resonant at slightly different frequencies, twisted into a half turn helix. Correctly made it will provide true all round coverage with circular polarization at all angles. The usual crossed dipoles and reflectors used for satellite reception only provide circular polarization directly upwards when the signal strength is high anyway, at low angles when maximum sensitivity is required the polarization is linear, losing 3 dB on the circularly polarized satellite signal. In addition Quadrature feed of crossed dipoles to achieve circular polarization is beautiful in theory and almost impossible to achieve in practice.
Bob Cobey G0HPO an avid weather satellite fan first made one in this area and reported phenomenal results, even inside the roof space of his bungalow. Good noise free pictures were received from Greenland through to Algeria.
The R.I.G articles by Mark Pepper Rig 37 and Chris Van Lint (Rig 44) and also in QST are based on the original work by NASA featured in the ARRL Antenna handbook. There are a number of practical problems in the design which have been addressed and solved over many weeks of computer modelling and prototype construction.
The computer generated (Azimuth & Elevation) polar diagrams are of course perfection and a series of actual measurements of the horizontal and vertical patterns were undertaken on the final design to confirm the computer results.
The measured vertical and horizontal polar diagrams using clockwise polarization, were produced using the latest version of a program by Bob Freeth ( G4HFQ ) called PolarPlot. The Windows 95/98 & 2000 version of the program is now available for download by visiting his page using the links in the right hand column.
Vertical and horizontal polarizationís are 3dB down on clockwise as expected and the horizontal pattern is still circular. The polar diagram in the vertical plane is particularly useful, showing very little reception from below, thus reducing the formation of nulls caused by ground reflections. There are no nulls in the horizontal pattern at any polarization. Allowing for slight distortion in actual measurement of antenna performance, caused by the usual site problems the measured plots and measured gain were identical to the computer generated plots which was particularly satisfying, given the inherent difficulty in modelling a helix in XYZ format for the computer.
The major problem is how to make the beast, and particularly how to make it weatherproof. Firstly the dimensions given by the NASA based articles need to be adjusted for weather satellite and 2 meter frequencies. The diameter of tubing used in the ARRL handbook works out to 19mm at 137 MHz and dimensions show the INSIDE diameter of the helix. Conversion to 8mm tubing which is a much more sensible size at 137 and 145 MHz was not just a question of simple scaling. As stated in the book a quadrature-phase current relationship is required between the two loops. This is achieved by making one loop larger than the desired frequency resonant length and therefore inductive, while the other loop is smaller and therefore capacitive. Correct resonance of each loop means that a circular pattern may be achieved by simply connecting the two loops in parallel. This is confirmed in practice when the SWR dips once only at the required frequency. It is all too easy to get a double hump response which is incorrect, and will show a figure of eight pattern rather than a true circular horizontal polar diagram. The sizes shown have been confirmed by computer modelling and actual measurements on the prototype, and do not conform with the book figures. In particular the helix should be the same diameter throughout its length and therefore the length of the arms forming the helix is important. The formula for this length is where 'r' is the radius and 'h' the height of the helix.
The major problem was how to make weatherproof the connections at top and bottom of the helix. The connections at the top of the helix in the original design were almost impossible, as the connection had to be made to the ends of four tubes, two of which were 24mm below the top two, with all inside the support mast. Using the computer modelling program, various connection methods were tried to establish a sensible method without compromising the performance. It proved possible to slope the tops of the small loop so that all four tube ends were in a common plane at the top of the mast where they can be reached. The final method uses a small piece of PC board very simply made, shown in Fig 2, which gives good mechanical and electrical connections.
Fig 3 illustrates the original infinite balun arrangement which provides an
elegant way of bringing the feeder away from the antenna at a voltage null.
This is totally impossible to make if the antenna is made using a central non
support mast (as it must be at 137 MHz) when the connections would have to be
inside the mast.
Cut lengths in mm for 137.55 MHz, using 8mm Central Heating tube & 90 deg elbows.