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AMATEUR RADIO By GARRY CRATT, VK2YBX Antennas for the VHF & UHF bands One of the most difficult decisions the amateur is faced with today is the choice of antenna to use. This article sets out to explain the advantages and disadvantages of the various types of antennas available. One of the most commonly held beliefs is that antennas should be judged on gain alone, and that an antenna with high gain is better than one with low gain. But that's not all there is to it. Antenna gain is closely related to antenna directivity, which in turn is closely related to the radiation pattern. A high gain antenna may be unsuitable in some cases if, for example, it radiates energy in the wrong direction. In fact, it is the ability of an antenna to radiate most of the energy fed to it in a particular direction (and minimum radiation in other directions) that provides power gain. The general direction of maximum radiation when the radiation pattern is plotted is called the major lobe. The areas of lower radiation levels (as no antenna is perfectly directional) are called minor lobes. The term front to back ratio is also commonly used in discussions on antennas. This is simply the ratio of the power radiated in the preferred direction to the power radiated in the opposite direction - see Fig.1. The power gain is the ratio of the maximum radiation signal in a given direction to the maximum radiation signal produced by a theoretical reference antenna with the same power input. This theoretical reference antenna is . w -' z < w d ffi > RELATIVE POWER (uW) Fig. 2: graph showing how the angle of radiation decreases as the antenna approaches one wavelength. A lower angle of radiation gives greater gain. 68 SILICON CHIP MAJOR LOBE ~NORf LOBES Fig.I: the general direction of maximum radiation is called the major.lobe, while the areas of lower radiation are the minor lobes. Fig.3: dimensions for a discone antenna to cover 140-450MHz. The disc can be solid aluminium while the cone can be wire mesh. known as an isotropic radiator, which is a hypothetical, lossless omnidirectional antenna. In practice, antenna gain measurements are normally made in comparison to a single half-wave dipole. Because the radiation pattern of a half-wave dipole is somewhat imperfect, its power gain compared with the theoretical isotropic radiator has been mathematically calculated at 2.15dB. This means that the gain oI a practical antenna can be referred to an isotropic radiator by adding ~ COAX \::!f__c_oA_x_~ Fig.4: a vertical collinear antenna can be made by joining pieces of coaxial cable (see text). The antenna can be centre-fed or end-fed. 2.15dB to the measured gain against a half-wave dipole. All this is easy to visualise when applied to a directional antenna such as a yagi, quad etc, but takes a little more explanation when applied to omnidirectional vertical antennas. Beca u s e it is used as th e reference, it follows that a halfwave dipole has a unity gain. If we take a vertical half-wave dipole and replace its lower half with a groundplane, the image of the vertical quarter-wave radiator is reflected in the groundplane. As a result, the groundplane antenna produces similar results to a halfwave dipole. There are many other factors, too detailed to explain here, that also come into play. Suffice to say, the qua rter wave radiator is groundplane dependent, and incapable of substantial gain. In order to obtain usable gain from a vertical antenna, we need to compress the major lobes so that they have a low angle of radiation compared to the horizontal plane see Fig.2 . In fact useful gain in this type of antenna is closely related to a low angle of radiation. VHF vertical gain antennas, for example, may comprise two half wave dipoles fed in phase etc, and use compression of the major lobes to obtain a low angle of radiation. Such an antenna is called a collinear. that of a quarter wavelength radiator. Fig.3 shows the dimensions for both disc and cone to cover the frequency band from 140-450MHz. Discones are sold commercially by Emtronics, Vicom, Dick Smith Electronics, etc. Vertical collinears Vertical collinear antennas can be easily fabricated by using coaxial cable.of either 75-ohm or 50-ohm impedance. This type of antenna is made from a series of lengths of coa xial cabl e , each a halfwavelength long and fed in phase. The elements are joined together with the outer braid and centre conductors transposed at each connection. Fig.4 shows the details. COAXIAL CA■L■ SP■Cll'ICATIONS Cable No The discohe, so called because it comprises a metal or mesh cone, is a broadband vertically polarised antenna which acts as a wideband impedance matching transformer, coupling the 50-ohm input to the higher impedance of free space. The radiation pattern is similar to Nomi1111I Imp Zo(j) "-L Cable Outoide Diam 1MHz A - (d■ /1-) Velocity Factor 10MHz 100MHz c-ltanc:e RG-5/U RG-5 ■ /U RG-IA/U RG-IA/U RG-9/U RG-9■ /U RG-10A/U RG•11A/U RG-12A/U RG-13A/U RG•14A/U R(l-11/U RG•17A/U RG-11A/U RG,19A/U RG 20A/U RG,21 A/U RG-29/U RG-34A/U 0 RG-34 ■ /U RG-35A/U RG 54A/U RG-55/U RG,SSA/U RG-51/U RG-51C/U RG,59A/U RG-59■ /U RG,82A/U RG-74A/U RG-13/U RG-171/U RG-213/U RG-211/U RG, 220/U 52.5 50.0 75.0 50.0 51 .0 50.0 50.0 75.0 75.0 75.0 50.0 52.0 50.0 50.0 50.0 50.0 50.0 53.5 75 0 75.0 75.0 58.0 53.5 50.0 53.5 50.0 75.0 75.0 93.0 50.0 35.0 50.0 50.0 50.0 50.0 8.45 8.45 8.45 10.3 10.66 10.8 12.06 10.3 12.06 10.8 13.85 18 22.1 24 28.4 30.35 4.7 4.6 16 18 24 6.35 5.2 5.4 4.95 4.95 6.15 6.15 6.15 15.6 10.3 2.0 10.3 22.1 28.4 Maximum Operating (pF/10011) Voltage 1 - H • 3000MHz (mm) 0 The discone antenna Thus the sections are assembled by soldering the centre conductor of the first section to the braid of the second section and vice versa. The gain, bandwidth and radiation pattern are all governed by the number of elements used. Bandwidth can be roughly calculated as: BW = 2f/(3n + 1) where f is the operating frequency and n is the number of elements. For example, a 16 element array operating at 438MHz will have a bandwidth of 438MHz x 2 -:- 49 = 17.9MHz bandwidth. The entire array can be secured inside a piece of PVC conduit. Alternatively, the junctions of the elements can be sealed with weatherproof insulation tape or heatshrink tubing. Using PVC conduit will cause the centre frequency to be pulled low, so it is best to start construction by cutting a length of coaxial ea ble to an electrical half-wavelength (using the formula 150/f(MHz) = half wavelength (metres). Once this has been done, strip back the PVC sheath 15mm at both en d s , separate the outer braid, then twist the braid conductors together and tin them with solder. (rms ) 0.21 0.16 0.21 0.16 0.16 0.175 0.16 0.18 0.18 0.18 0. 12 0.1 0.066 0.066 0.04 0.04 1.4 0.33 0.065 0.07 0.18 0.36 0.36 0.33 0.42 0.34 0.25 0.10 0.23 2.6 0.16 0.066 0.04 0.77 0.66 0.78 0.55 0.57 0.81 0.55 0.7 0.66 0.66 0.41 0.4 0.225 0.225 0.17 0.17 4.4 1.2 0.29 0.3 0.235 0.74 1.3 1.3 1.25 1.4 1.10 1.10 0.85 0.38 0.8 5.8 0.6 0.2 0.2 2.9 2.4 2.9 2.0 2.0 2.1 2.0 2.3 2.3 2.3 1.4 1.2 0.8 0.8 0.68 0.68 13.0 4.4 1.3 1.4 0.85 3.1 4.8 4.8 4.65 4.9 3.40 3.40 2.70 1.5 2.8 13.8 1.9 1.0 0.7 11.5 8.8 11.2 8.0 7.3 9.0 8.0 7.8 8.0 8.0 .5 8.7 3.4 3.4 3 3.5 43.0 16.0 3.0 5.8 3.5 11 .5 17.0 17.0 17.5 24.0 12.0 12.0 8.6 6.0 9.6 46 8.0 4.4 3.6 22.0 16.7 21.0 16.5 15.5 18.0 16.5 16.5 16.5 16.5 12.0 16.0 8.5 8.5 7.7 7.7 85.0 30.0 12.5 8:6 21.5 32.0 32.0 37.5 45.0 26.0 18 .11 .5 24.0 76 0.659 0.659 0.659 0.659 0.659 0.659 0.659 0.66 0.659 0.659 0.659 0.67 0.659 0.659 0.659 0.659 0.659 0.659 0.659 0.66 0.659 0.659 0.659 0.659 0.659 0.659 0.659 0.66 0.84 0.659 0.66 0.665 0.66 0.66 0.66 28.5 29.5 20.0 30.5 30.0 30.5 30.5 20.5 20.5 20.5 30.0 29.5 30.0 30.5 30.5 30.5 30.0 28.5 20.5 21 .5 20.5 26.5 28.5 29.5 28.5 30.0 20.5 21.0 13.5 30.0 44.0 27.9 29.5 29.5 29.5 3000 3000 2700 4000 4000 4000 4000 5000 4000 4000 5500 6000 11000 11 000 14000 14000 2700 1900 5200 6500 10000 3000 1900 1900 1900 1900 2300 2300 750 5500 2000 5000 11000 14000 Table 1: coaxial cable specifications. Use low-loss cable for long runs and check that the impedance is correct. (Courtesy Dick Smith Electr onics). MA RCH 1988 69 MAST s (a) STACKING IN THE SAME PLANE (b) STACKING IN PARALLEL PLANES Fig. 5: additional antenna system gain can be achieved by stacking in either the same plane or parallel planes. Our diagram shows two vertically-polarised antennas, but horizontally-polarised antennas may also be stacked. Next, strip 5mm of dielectric from the centre conductor and tin the centre conductor. You can now check the resonant frequency of this length of cable with a grid dip oscillator (GDO). This is done by shorting the braid and centre conductor at one end of the element, and coupling the GDO to the other end. Because the formula used to calculate the half-wave section does not take into account the velocity factor of the cable, it is normal for the cable to be too long at first. It can be made to resonate at the correct frequency by progressively reducing its length by trial and error. An advantage of the coaxial collinear is that it is an easily reproduced design. Because of this, it is used by many commercial antenna manufacturers. Cables and connectors Having selected a suitable antenna, care must be taken to maximise the available antenna system performance by selecting a suitable feedline. As can be seen from Table 1, the main factor with which we are concerned is attenuation. The difference in attenuation between, say, RG-58C/U and RG-213 over a length of 30 metres at a frequency of 100MHz is 3dB. Clearly, for long coaxial runs, we need to select a cable with low loss. For mobile installations, where the length of 70 SILICON CHIP coaxial cable used is less than three or four metres, the loss incurred by using a smaller diameter cable is a worthwhile tradeoff compared to the ease of installation. Of course, the selection of cable is also determined by price. Lowloss cable can be expensive, particularly for UHF work. Connectors also form an important part of any antenna system, particularly at VHF and UHF. Of prime importance is the impedance of the connector, which is largely determined by the physical construction and design. The materials used in manufacture are also important, particularly the dielectric insulation. An inferior connector will create an impedance mismatch, causing a high SWR (standing wave ratio). It may also have a high insertion loss, thereby further reducing antenna system performance. Connectors should be chosen carefully, as there are many inferior types available which are quite unsuitable for RF work and which should really be limited to video use. Many connectors are also available in both 50-ohm and 75-ohm variations, so it is important to choose the correct type. or vertically polarised antennas. By stacking yagi antennas (or any derivative of a yagi), between 5/8 wavelength and 1-1/4 wavelengths apart, an additional 2.5 to 3dB of gain can be realised. Stacked antennas must be fed in phase, and because a typical VHF or UHF yagi is a 50-ohm device, a matching transformer must be used. To feed the two antennas in phase they must be fed in parallel. This means that to obtain a reasonable match to 50-ohm feedline, we must transform the impedance of each antenna to 100 ohms, so that the two antennas in parallel have an impedance of 50 ohms. This is easily achieved by using a coaxial line transformer, made from 75-ohm coaxial cable. The formula used to calculate this is as follows: Zs = Zq x Zq/Zl where Zq is the impedance of the matching transformer, Zl is the feed impedance, and Zs is the required antenna impedance for parallel operation. If we substitute figures from the above example, we get: 100 = Zq x Zq/50 Thus, Zq x Zq = 5000, so Zq = 70.7 ohms. In practice 75-ohm cable presents a minimal mismatch. Because each antenna must be fed with one of these coaxial transformers, the physical construction should look like Fig.6. Note also that each impedance transformer must be a single quarter wavelength in length, or an odd multiple of a quarter wavelength. With that background, you LINES PARALLELED ' ----50r! TAIL Stacking antennas Additional antenna gain can be achieved by stacking two identical, directional antennas, in either the same plane or in parallel planes. This applies equally to horizontally TO MAIN FEEDLINE Fig.6: phasing harness for stacked antennas. Each section must be an odd multiple of a quarter wavelength. should now find it easier to select an antenna for a particular application. Let's take a look at a few examples. Base station operation For maximum directivity, either horizontally or vertically, a yagi antenna is a good choice. A rotator will then enable 360° operation. For omnidirectional operation, a five-eighths wavelength antenna could be used provided a good groundplane is available. If no suitable ground plane is available, a vertical collinear antenna could be used instead. Note that a suitable location is required for good omnidirectional operation (eg, the top of a hill). The discone anternia is suitable for omnidirectional operation on a variety of VHF & UHF frequencies, provided that 3dBd gain (dBd means gain referred to a dipole) is acceptable. For directional operation on a variety of VHF & UHF frequencies, a log periodic antenna can be used with a rotator. Mobile operation The size of the antenna and the mounting method on your vehicle are the two main considerations here. As virtually all mobile applications require vertical polarisation and omnidirectional operation, the choice of antennas can be reduced to the following: (1). Quarter-wave whip - generally requires drilling a hole in the middle of the car's roof for best results. Advantages: (a) physically short; (b) relatively high angle of radiation which gives better results in city or hilly locations, particularly at UHF. Has marginal gain of ldBi. (2). Five-eighths vertical - requires a ground plane as above, but has a low angle of radiation, providing better gain than a quarter wave whip (almost 3dBi). Offers good performance on flat terrain (but probably worse in hilly terrain). (3). Half-wave radiator - ground independent and so can be easily gutter mounted. Has ldB of gain over a quarter-wave whip. (4). Two half-wave radiators end fed - ground independent, improved gain (almost 5.2dBi), but SAFETY WATCH Lt Safety Watch will be an occasional feature in SILICON CHIP drawing attention to issues of electrical s afety in the workshop and home. VCRs and water don't mix Vases of flowers should never be placed on top of TV set or near a video cassette recorder. If the vase is knocked over, the water could do a lot of damage to the internal circuitry of the TV or VCR and may even cause the picture tube to crack. Worse, if splashed water comes into contact with mains wiring inside the VCR, it could create a path between the mains and chassis. Because most VCRs are double-insulated (ie, they only have a two-core power flex), any leakage between the mains and chassis could mean that the VCR exterior is live and lethal. Moral: keep all vases, drinks and other containers of liquid away from your VCR and TV set. Keep it well away from your stereo equipment too. sign of deterioration. They're not cheap but then neither is a fire in the kitchen. Safety with the iron Hazardous power cords We recently came across a power cord fitted to a vertical grille which had shorted at the point where the cord entered the grille base. When the sheath was removed from the cord, the insulation surrounding each of the three leads was found to be badly perished. The short occurred between Active and Neutral. When more closely examined, the outer rubber sheath of the cord was noted to be shiny from continued exposure to grease and had tell-tale signs of cracking and perishing where the cord entered the moulded 3-pin plug. Moral: carefully examine power cords used for frypans and vertical grilles. Such cords are prone to perishing because of their exposure to grease and cooking oil. Replace the cord at the first Avoid placing appliances on stove tops. This is what can happen. Some people put their electric iron on the stove to cool off before it is put away in a cupboard. This photo shows what happened when the iron was dislodged slightly, onto an adjacent hotplate which had been inadvertently left on. As you can see, a great deal of damage was done to the iron in only a few seconds. The iron had to be replaced. Moral: if you put your iron on the stove to cool down, place it well away from the hotplates. Better still, leave it on the ironing board to cool down and then put it way. continued on page 93 M ARCH 1988 71 Line Grabber for Phones sweep mode is also available. The GOS-522 has a genuine 20MHz bandwidth and has a 20 nanosecond/division sweep range to make timing measurements at high frequencies much easier. For further information on the GOS-522 contact your GW Instruments stockist or the Australian distributor, Emona Instruments, 86 Parramatta Road, Camperdown, NSW 2000. Phone (02) 519 3933. check this by measuring the voltage across the limiting resistor) but the LED is not illuminated, it is likely that the LED is installed the wrong way around. If you can't get the LED to illuminate for supply voltages above 20 volts, try shorting out the SCR. This will indicate whether the SCR and its associated components are faulty or not. Repeat these tests for your other Line Grabbers. You should be sure that they are working correctly before you connect them to the phone lines. A further wrinlcle There is another variation of the Line Grabber you could use if you Amateur Radio tronics Pty Ltd, 5 7 Vore Street, Silverwater, NSW 2141. Phone (02) 648 3488. continued from page 21 wanted to be clever. Say you had a phone which you use a lot and you don't want to fit the Line Grabber to it. That's OK. All you do is fit Line Grabbers to all the other extensions but not to your phone. This will allow your phone to grab the line at any time but if another extension is picked up before yours, you can still listen in. That could be useful in situations when another extension answers a call intended for you. ~ Acknowledgement: we thank Arista Electronics Pty Ltd for giving us the idea for this project. They will have a commercial version available shortly. Antennas physically long. Suitable only for UHF due to physical instability. Next month, we'll describe a few practical antennas that you can build yourself. Corrections In Table 1 on p.77, January 1988, the location of the VK4RAT transmitter should have been listed as Townsville (not Brisbane). Also, the vision input signal should read 426.25MHz (not 444.25MHz). Would readers also please note that the address of the Sydney ATV continued from page 71 Group is now 24 Larra St, Guildford, 2161, NSW. The repeater operates from 6.30-9pm on Mondays, Tuesdays and Thursdays, and from 12-5pm on Saturdays and Sundays. We thank the two readers who contacted us with the above information. Finally, the author would like to acknowledge the following amateurs who provided information on amateur TV for the January issue: VK2BTV, VK2ZZO, VK2AAK, VK3PC, VK3BFG, VK5AWA and VK5KG ~ High, Low, Sink & Source Strobe warning light This self-contained Xenon strobe light can be used as a warning beacon on boats or cars, as an attention-getter for shop displays, as a Christmas or party decoration, or as an external indicator for a domestic burglar alarm. It runs from 12V DC, battery or mains plugpack, and draws about 150mA. Flash rate is about two per second. The screw-on lens cap is available in red, orange or blue. The unit is weatherproof and has a screw mounting base. It retails for $35 from Arista outlets. source up to about 18 milliamps but depending on the output voltage it can sink only about one milliamp. Some logic circuits can sink a lot more current than they can source. The prime examples of this aTe TTL (transistor-transistor logic) devices which can typically sink about 25mA, or a lot more in the case of Schottky devices, when their outputs are low (ie, close to 0V) but can source virtually no current when their outputs are high (5V). These examples of devices which have unequal source and sink current capability invariably have output stages which are essentially non-symmetrical. In some cases, they may have open-collector out- continued from page 65 puts which means that they can sink quite a lot of current but can source no current at all unless they have an external "pull-up" resistor to pull their outputs high. Finally, before we leave this discussion, there is another definition of high and low which is relevant to comparators and logic circuitry. A signal is said to be high if it is high enough to cause a comparator or logic gate to change state. In this definition, high means above the positive threshold of the device's input. For example, in a logic circuit running at 15V, high may be any voltage above + 7.5V. Similarly, low many be any voltage below + 7.5V. ~ MARCH 1988 93