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AMATEUR RADIO By GARRY CRATT, VK2YBX Mobile antennas for VHF and UHF amateur operation Last month we looked at the general types of antennas available to the radio amateur, and the reasoning behind the selection of an antenna for a particular application. This month we look at some of the more useful VHF and UHF mobile antennas, and suggest some practical construction methods. The choice of a mobile antenna for VHF or UHF is dependent on several factors. As the frequency of operation increases, the capture area of the antenna decreases. This means that at VHF and UHF, higher gain antennas are required to overcome the smaller aperture of the antenna and the higher path loss at these frequencies compared with HF. Normally, increased gain means increased antenna size, and this can be a mounting nightmare. If we try to mount a large antenna as high as possible for best performance, there may be height restrictions when entering a garage. If we mount the antenna at a lower point on the vehicle, the radiation pattern may be distorted, causing loss of signal. Also large signal variations will be experienced as the vehicle moves, not only due to obstacles in the signal path, but also due to the tilt of the vehicle as it is driven over terrain that is not perfectly flat. As explained last month, whilst the benefit of using an antenna with a Photo 1: the loading coil for the VHF 1/2-wave whip antenna, together with the 20mm-diameter PVC housing and metal end fittings. The whip section is screwed directly onto the top fitting at right. low angle of radiation can be realised over flat terrain, this low angle of radiation can be a distinct disadvantage when traversing hilly terrain. It is this dilemma which forces the amateur to compromise and use an antenna that has reasonable gain in the right direction, a usable angle of radiation in most conditions, and reasonable physical size. Practical antennas There are, however, a number of mobile antennas that do provide reasonable performance within the limitations discussed above. The 1/4-wave whip can provide up to ldBd of gain if it is mounted in the centre of the roof. At VHF, the 1/4-wave whip balances simplicity and reasonable size (49cm], against meduim aperture and a tendency to have a medium angle of radiation. It provides quite useful performance provided a good groundplane is Photo 2: both ends of the loading coil for the 5/8-wavelength antenna are soldered to the metal end fittings. The bottom metal fitting, shown at right, allows the completed coil assembly to be screwed into a standard TPI base. Al'lllL '1988 67 7 I en e .li "' N 9 TURNS, 1.6mm COPPER WIRE, 9.5mm INSIDE DIAMETER -..._ son COAX son COAX Fig.1: internal construction of the VHF 1/2-wave antenna. The coaxial cable enters through the end fitting and its centre conductor soldered to a short length of teflon wire which is then wrapped a:round the top of the loading coil. Fig.2: internal construction of the UHF 1/2wavelength antenna. Keep the lead length between the top of the loading coil and the bottom of the whip as short as possible (it's shown exaggerated here) and tune the antenna after installation with a GDO. available and it is mounted as high as possible. ½-wave whip The difference between mounting an antenna in the centre of the roof and on the rear guard or boot of a vehicle can be as much as 3dB! A logical progression from the 1/4-wave whip is the 1/2-wave whip which, when mounted on a minimal groundplane, has a high input impedance. If the radiator is extended to 3/4 of a wavelength, the impedance closely matches 50 ohms, but the radiation pattern is compromised. At about 0.6 of a wavelength, maximum forward gain is achieved. The solutions to both problems provide us with two usable antenna types. 68 SIUCON C/111' Photo 3: the completed VHF (left) and UHF 1/2-wave loading coils, prior to installation in the PVC housing. The end fittings may be secured using epoxy resin. At top is the completed UHF 1/2-wave antenna. LIST OF SUPPLIERS 26 TPI mounting base Mobile One Communications, PO Box 166, Randwick, 2035. Phone (02) 516 4500. Dick Smith Electronics, PO Box 321 , North Ryde , 2113 . Phone (02) 888 3200 . Santronic Corporation, PO Box 12, Kingsgrove, 2208. Phone (02) 758 1522. Half-wave coil fittings Space Electronics, Unit 1, 27 Garema Circuit, Kingsgrove, 2208. Phone (02) 758 2154 . Gutter grip mounting bracket Hatadi Electronics Corporation , 19 Wilmette Place, Mona Vale, 2103 . Phone (02) 997 7077. Santronic Corporation, PO Box 12, Kingsgrove, 2208 . Phone (02) 758 1522. Dick Smith Electronics, PO Box 321, North Ryde, 2113. Phone (02) 888 3200 . Captain Communications, 28 Parkes St, Parramatta, 2150. Phone (02) 633 4333. Standard Communications, 6 Frank St, Gladesville, 2111. Phone (02) 816 4755 . Stainless steel or fibreglass whip sections Mobile One Communications, PO Box 166, Randwick, 2035 . Phone (02) 516 4500. RF Industries, 7 4 Sydenham Rd, Marrickville, 2204 . Phone (02) 519 5188 . Polar Electronic Industries, 2 Commercial Rd, Highett, 3190. Phone (008) 33 5633. Loading coils If we add a series coil to the lower extremity of a 1/2-wave whip, ground the bottom end of the coil, and capacitively couple the feedline, the result will be an antenna having twice the gain of a 1/4-wave whip (twice the aperture), and which can be matched to a 50-ohm feedline. At VHF, the length of this arrangement (around 102cm) may be excessive, but at UHF it is quite manageable. The antenna requires only a minimal groundplane and so can be "guttergrip" mounted, eliminating the need for a hole to be drilled in the roof. A 3/4-wavelength whip provides the answer for those requiring an antenna with higher gain than a 1/4-wave whip, provided they are prepared to tolerate the increased length of such an antenna, particularly at VHF. We know that a 3/4-wavelength radiator can provide a close match to 50-ohm cable, but that the radiation pattern is not optimum for omnidirectional operation. If we reduce the length of the radiator by 1/Bth of a wavelength, and feed it with a coil equivalent to 1/Bth of a wavelength, we can produce an antenna having a close match to 50 ohms, a lower angle of radiation than a 1/4-wave whip, and gain approaching 4dB. So it is possible to design several antennas that can be used to overcome the physical limitations of vehicle mounting, and provide usable gain at VHF or UHF. Building your own Most amateurs will be familiar with the standard "Scalar" or "Belling-Lee" type antenna base, which utilises a 5/16th~inch diameter 26 TPI stud mounting system. These bases are made by many companies in Australia, and are used on almost all commercial 2-way radio installations. The base is normally fitted through the roof, or can be used on a metal gutter grip type bracket, allowing a variety of mounting positions. Photo 2 shows a metal fitting which can be used, together with a 6 TURNS. 1.6mm COPPER WIRE, 9.5mm INSIDE DIAMETER ~ Fig.3: internal construction of the 518th wavelength antenna. Both end of the loading coil are soldered to the metal end fittings and the completed assembly then screwed to a standard TPI base. length of 20mm diameter PVC conduit, to form an ideal base which is compatible with the standard 26 TPI base. This can be used to house the series coil for a 5/8-wavelength antenna, and simply screws onto the standard base (see Fig .1). The whip section can then be screwed directly onto the top of the series coil assembly. Construction of the 1/2-wavelength whip utilises a slightly different but equally inexpensive metal component, which allows easy mounting onto a gutter grip bracket. It also allows the coaxial cable to be fed inside the coil. There is a particularly clever method of capacitively feeding the coil. This involves soldering the centre conductor of the coaxial cable to a short length of teflon-covered wire, continued on page 96 Al'lll L 1988 69 T CEli'I' Cash in your surplus gear. Advertise it here in Silicon Chip. Advertising rates for this page: Classified ads - $7.00 for up to 15 words plus 40 cents for each additional word; Display ads (casual rate) - $20 per column centimetre (max. 10cm). Closing date: five weeks prior to month of sale. If you use a PO Box number, you must include your permanent address and phone number for our files. We cannot accept ads submitted without this information. To run your own classified ad, put one word on each of the lines below and send this form with your payment to: Silicon Chip Classifieds, PO Box 139, Collaroy Beach , NSW 2097. PLEASE PRINT EACH WORD SEPARATELY, IN BLOCK LETTERS 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ($7.00) 16 ($7.40) 17 ($7.80) 18 ($8.20) 19 ($8.60) 20 ($9.00) 21 ($9.40) 22 ($9.80) 23 ($10.20) 24 ($10.60) 25 ($11.00) Name ....... .. .... .... ..... .......... ..... ......... .. .. ........ .. .. .. .. .. ........ ..... .... ..... ...... .......... ... ...... . Address .. .. .... ... ... .. ... .. .. ....... ........... .. ... ..... ..... ....... ..... .......... .... ..... ........ ..... .. .... ..... . Suburb/Town .. .. .. .. . .. .. .. . .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . .. .. Postcode ........ .. Enclosed is my cneque or money order for$ ........ ... .... .. .. ........ .......... or please debit my Bankcard □ Visa □ Advertisers Index Our advertisers are vital to the success of SILICON CHIP. Please give them your support. Altronics .. .. ................ .... 70-73 Arista Electronics .............. .. . 1 9 Avtek ................................ IBC Dick Smith Electronics .... 52 ,53, 84 ,85 Elmeasco .. ..... .. ......... ....... . IFC Geoff Wood Electronics ... 41 -4 7 Jaycar Electronics .......... 20-27 Kenwood Australia ..... ....... OBC RCS Radio .......................... 92 Rod Irving Electronics .. .... ..... .. 9 PC Boards Printed circuit boards for SILICON CHIP projects are made by: • RCS Radio Pty Ltd , 651 Forest Rd, Bexley, NSW 2207. Phone (02) 587 3491. • Jemal Products, 5 Forge St, Kewd.ale, WA 6105 . Phone (09) 451 8726 . Card No ...... ....... .... .... .... ... ..... ... .. ..... .... ..... ........ .... ...... ..... ....... ..... ..... .. . Signature ..... .... .. .. .... ..... ............ .... ....... ................ ....... ... .... ..... .. .... ......... ............. . FOR SALE NEARLY 1500 PRINTER buffer kits now sold. Prices start at $39 for a 256K short form kit. All items advertised are in stock. Dealer enquiries welcome . Bulk discounts. Schools, Govt. Depts. orders accepted. Oh yes!! IBM compatible . Australian designed and manufactured. Ideal project for user groups or students. For a free catalog send a 37c stamp to : Don McKenzie, 29 Ellesmere Crescent, Tullamarine 3043. SHACK CLEAROUT . R1000 Kenwood Receiver with box and instructions. As new- $300 . HX 2000 Regency scan ner $200; Ranger AR3300 HF transceiver, 26-30MHz, 25W PEP, AM-SSB-CW-FM, $250; Marconi signal generator TF995 with spares $100; 2 x UHF CB Uniden Sundowner with CTCSS modules fitted , $200 each; 9dB base station antenna $90; 6dB base station antenna $60; Ratcliff 96 SILI CON Cl-Ill' signal generator Model 205 (45 - 180MHz) $50. All prices negotiable. Ring (02) 487 1439 after 8pm evenings and weekends . Garry VK2YBX. FOR SALE: ETI SERIES 5000 preamplifier, $320. 1/3-octave graphic equalisers, $160 each . Phone (02) 542 3628 after 5pm . Amateur Radio continued from page 69 which is wrapped around the top of the loading coil. Figs.1 & 2 show the details. This construction technique must be followed exactly, otherwise the resonant frequency will be other than that which is desired. In any case, a GDO (grid dip oscillator) should be used to verify the correct frequency of operation, after the antenna is mounted on the vehicle. After the coil has been wound .and the coupling "capacitor" installed, the PVC shroud can be glued in place and the top of the coil soldered to the top metal fitting. As the 1/2-wave whip mounting technique is quite strong, it is suitable for either VHF or UHF antennas. ~ Negative Feedback continued from page 14 pected to have the theory as a set of differential equations. Let's leave our young enthusiasts before they get too far ahead of us. Clearly they must be the brightest building site observers ever. One point is clear though. When they have fully developed their theory of feedback systems, it will be a truly general set of equations. Next month, we will show you more of this fascinating stuff, but we will naturally accent the electronic side of this beaut story. And we will be very down-to-earth and practical to boot. ~