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AMATEUR RADIO BY GARRY CRATT, VK2YBX Build this simple turnstile antenna for weather satellite reception Interested in listening to signals from the polar-orbiting or geostationary weather satellites? This simple antenna can be built for -a few dollars yet will give good results. Most amateur operators appreciate that, for ground based communications , vertically polarised antennas with a low angle of radiation perform best. By stacking dipoles vertically, the radiation pattern is compressed so that the usable "gain" is radiated towards the horizon , thus theoretically encompassing any receiving stations located on the ground. Conversely, for airborne communications, an antenna having a higher angle ofradiation is desired, since the signal must be radiated upwards instead of hugging the ground. The same applies for the reception of weather satellite signals. The easiest weather satellite signals that can be received using simple equipment are those radiated by the polar orbiting APT satellites. These satellites broadcast circularly polarised signals located around 137MHz and have typical output powers of DIPOLE 2 / FEEOPDINT '-DI )J4 PHASING LINE Fig.1: basic layout of a turnstile antenna (groundplane not shown). It consists of two 1/2-wavelength dipoles mounted at 90° and fed 90° out of phase. Note the 1/4-wavelength phasing line used to feed the second dipole. about 5W or so. Due to their polar orbit, they pass quite close to the Earth's surface at heights ranging from 110-BOOkm. The signals received on Earth from these satellites are much stronger than those received from geostationary satellites, which orbit at 37,000km. In any case, the optimum antenna for the reception of weather satellite signals is one which combines circular polarisation and a high angle of radiation. The turnstile antenna This photograph shows the general construction of the turnstile antenna. The reflector was made from small mesh chicken wire attached to a wooden baseplate, while the dipoles consist of threaded steel rods attached to a wooden mast. A right-angle metal bracket is used to secure the mast to baseplate. Although a simple groundplane antenna having a 1/4-wavelength radiator can be used in applications requiring a high angle of radiation, a far more useful antenna is the "turnstile" - so named because of its resemblance to a supermarket turnstile. A turnstile antenna consists of two 1/2-wavelength dipoles mounted at goo and fed goo out of phase. When mounted above a suitabl e reflector as NOVEMBER 1991 53 This close-up view shows how the dipole elements & the phasing harness are fitted to the mast. The threaded rod dipoles make it easy to attach the phasing harness using solder lugs & nuts. shown in one of the photographs, the radiation pattern fulfils both our requirements (see Fig.2). The response of the antennc!, directly above th e dipoles is circularly polarised, while at angles between 45° and 0°, the response is elliptically polarised. In this regard, the reflector spacing is an important consideration, as it determines both the radiation pattern and the feed point impedance of the antenna. Reflector spacing As can be seen from Fig.3, if the reflector spacing is increased from 0.22 to 0.37 of a wavelength, the vertical response is reduced while the response at lower angles (30-50°) is improved. A further consequence of this reflector spacing is that the characteristic impedance of each dipole is very close to 100 ohms. In order to feed the two dipoles out of phase, they must be connected via a 1/ 4-wave phasing harness. Fig.1 shows this arrangement. As with other phasing harnesses, the physical length of the harness is reduced below a 1/4wavelength according to the velocity factor of the coaxial cable used. By selecting a 0.37 wavelength reflector spacing, this arrangement can be fed with standard 50-ohm coaxial cable, thus greatly simplifying the construction compared with other reflector spacings requiring a matching section. In practice, the antenna response is also improved for receiving polar orbiting satellites. The accomp anying photographs show the general construction of the antenna. The reflector can be made from small mesh chicken wire or phosphor-bronze m esh, while the dipoles can be made from aluminium tubing or steel or brass rods. To protect it 1 Fig.4: this diagram shows the dimensions required for a 1/4wavelength phasing line. SILICON CHIP Fig.3: increasing the reflector spacing of a turnstile antenna to 0.37 wavelength reduces the vertical response but improves the response at lower angles (30-50°). This makes the antenna suitable for satellites passing at relatively low angles but still gives useful pickup for satellites passing almost directly overhead & for geostationary satellites. Construction 260mm 54 Fig.2: shown at top is the typical radiation pattern from a groundplane antenna, while immediately above is the pattern for a turnstile antenna with a reflector spacing of 0.22 wavelength. Note the dramatically improved vertical response of the turnstile antenna. from the weather, the entire antenna should be sealed using "Estapol" or some other plastic paint/sealant. As constructed, the antenna has a modest gain of around 6dB , which is quite sufficient for the reception of polar orbiting weather satellites. Also, there is no specific requirement for the antenna to be elevated to any great degree above the ground, so long as the "view" is largely unobstructed from about 20° above the horizontal. We built the prototype out of scraps of "chipboard" and softwood planks. However, for a long-lasting antenna, chipboard is no good as it deteriorates rapidly in wet weather. If you want it to last, use good quality timber and These photographs show the results that can be obtained by connecting the turnstile antenna to suitable receiving equipment. All four images are from GMS-4 but you can also tune into the polar-orbiting satellites. prime and paint it for weather resistance. The main mast, which holds the dipole elements in place, is centrally located on the baseboard by a right angle bracket, available from most hardware stores. The dipole elements are made from zinc-plated threaded steel rods and are connected to the coaxial cable using solder lugs. Mounting the dipoles The dipole elements are mounted by drilling horizontally through the mast at 10mm intervals. The elements are then mounted by passing them through the timber and securing them using steel nuts and washers. Another advantage of using this method is that the 1/4-wavelength phasing section can be pre-fabricated on the workbench. We used heatshrink tubing to seal the jacket of the coaxial phasing section. As the fe ed point impedance of the dipoles at 0.37 wavelength reflector spacing is about 100 ohms, the phasing line must be made from 75-ohm coaxial cable. Fig.4 shows the dimensions of this part of the antenna. The coax feed can be standard 50ohm type and solder lugs are recommended for the connections to the dipoles. The reflector uses two overlapping layers of small diameter chicken wire, available from most hardware stores (as is- the threaded rod). As this wire is only available in 900mm wide strips, a total length of 2.6 metres is required to cover all of the base plate. We used galvanised staples to secure the wire to the baseboard and overlapped the screen to give the fullsized reflector dimensions. Perhaps this simple antenna, which can be used with an inexpensive scanning receiver or a modified 2-metre receiver, will encourage readers to further explore the world of weather satellite reception. Just to whet your appetite, we have included several photos of full-screen video pictures, obtained from the more sophisticated GMS weather satellites, just to show the remarkable results that are possible. Of course, you'll need some extra equipment in addition to your scanning receiver to obtain pictures like these , including a computer interface and, possibly, a video printer. We'll have more to say about this in a future article. In the meantime, have fun tuning into those satellite signals. Further reading (1). Australian Electronics Monthly, July 1986. (2). The ARRL Antenna Book. (3). Weather Satellite Handbook, by Ralph E. Taggart. SC NOVEMBER 1991 55