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AMATEUR RADIO BY GARRY CRATT, VK2YBX How glass mount antennas work One of the status symbols on cars today is a small antenna mounted on the rear window for a mobile \elephone. These antennas have the advantage that they can be installed without the need to drill holes for cables but are they as effective as conventional body mounted antennas? This article gives the answer. One of the dilemmas facing today's amateur is how to mount mobile antennas without permanently marking or modifying the exterior of a motor vehicle. In the past, tradition has dictated that a 1/4-wavelength whip antenna mounted in the centre of a vehicle roof provides optimum performance. .These days, people are far more reluctant to drill a hole in the centre of the roof of a brand new motor vehicle for the sake of antenna performance, particularly when they are informed that the performance of a suit- ably sited glass mount antenna can be just as good. Glass mount antennas are available for a wide range of frequencies, including the now more commonly used cellular bands (830-890MHz). This kind of mounting system has considerable appeal to operators using rental or company vehicles where any modification to the exterior of the vehicle would be out of the question. Many communications retailers now stock glass mount antennas for VHF, UHF and other radio bands. This type of antenna is unobtrusive, easily This photograph shows a typical glass-mount antenna coupling box that has been modified to form a test probe. The centre conductor of the coaxial feedline is connected to a conductive plate whose area is the same as that of the mounting foot of the antenna on the other side of the glass. 78 SILICON CHIP removed, weatherproof, broadband, and an ideal alternative to bumper or boot mounted systems which cannot provide a good omnidirectional radiation pattern. If an antenna is to be used without a groundplane, then theory dictates that it must be a 1/2-wave antenna rather than a shorter 1/4-wave whip which requires a groundplane to form the symmetrical half of the antenna. Another consideration when designing a glass mount antenna is the mechanical "foot" that must be used to mount the radiator. This must be mechanically stable and easily affixed to any window of a motor vehicle. Ideally, it should also allow some adjustment of the angle of elevation of the antenna to compensate for the slope of the window, ensuring that the antenna remains vertical in all mounting situations. How they work To analyse the theory of glass mount antennas, it is best to consider the external radiator first. The whip section and the mounting foot comprise a 1/2-wave resonant radiator. However, due to the loading capacity caused by the mounting foot and the dielectric loading resulting from mounting the foot on glass, the physical length of the radiator will be somewhat shorter than the theoretical value. However, the radiator will still be presented as an electrical 1/2-wavelength with voltage loops occurring at both ends. The foot can be mechanically mounted onto the windscreen or rear window in the motor vehicle using a suitable adhesive. This has negligible effect on the performance of the antenna. The impedance of the exterior radiator is around 2-3kQ. As the patent theory of coupling through glass is much the same as that of an interstage coupling capacitor, and since the coupling is done at a high impedance point, very little loss occurs, and only a small amount of capacitance is needed for efficient coupling through the glass. Fig.1 shows the equivalent circuit of a glass-mount antenna. In most modern applications, a 2-element colinear radiator has been chosen which provides some gain over a standard 1/4-wave whip , and also radiates a fairly omnidirectional pattern. Theoretically, two 1/2-wave radiators spaced approximately 0.7 of a wavelength between centres produce some gain above that of a 1/4-wave radiator if they can'1be kept in phase. Practical experience has shown that the airwound phasing coil is the best trade off between the electrical requirement and mechanical robustness. Most designs use a 5/8-wavelength upper radiator and a quarter wavelength lower radiator, separated by an airwound phasing coil. This design is known as a "3dB" gain antenna. By stacking and correctly phasing these two elements, the radiation pattern is compressed, resulting in a lower angle of radiation. The major lobe of a 1/4-wavelength antenna is approximately 30° above the horizon, while a "3dB " antenna has an angle of radiation of 8°. Under test conditions , it has been shown that a properly sited "3dB " glass mount antenna can be made to perform ldB better than a roof mounted quarter wavelength antenna and, at the same time , provide a radiation pattern with relatively low distortion. Interestingly, one of the design criteria is that a glass-mounted antenna should be able to survive going through a car washing system. The RF energy is coupled through the glass by placing a coupling box cl I SOURCEl I GENERAWR I I L- A---------,.1---]l . iJI, _ _ _j inside the window and then affixing the mounting foot precisely over the same area on the outside of the window. The coupling box contains a tunable LC network, with a high-Q air dielectric capacitor, formed from a piece of brass sheet, providing the means for varying the resonant frequency. A non-conductive screw is adjusted to distort the brass sheeting, thereby varying the capacitance. The bottom end of this sheet connects to the coaxial cable braid. The centre conductor of the coaxial feedline is connected to a conductive plate whose area is the same as that of the mounting foot on the other side of the glass. These two parallel 9onductive plates, together with the glass of the window, form the capacitor which couples energy through to the outside antenna. Typically, the value of capacitance is about 2 to 3pF. At the resonant frequency, the "hot" end of the parallel LC circuit is at a high impedance, whilst the coaxial cable is tapped to an impedance point of 50Q. Fig.2 shows the circuit represen- · tation of this arrangement. Locating the antenna Since we know that the optimum capacitance for good coupling of RF energy through the glass is around 23pF, it is obvious that incorrect loca- TRANSFORMER I I I L-=, . , J G-_1-S-H-IE-LO-- y I-' .,. GROUND "'I" Fig.1: equivalent circuit for a glass-mount antenna. Only a small amount of capacitance (2-3pF) is required for efficient coupling through the glass. a,----, z l I I I I : : 11 11 11 50tl TRANSMISSION · LINE TO TRANSMITTER AND/OR RECEIVER Fig.2: the circuit representation of a glass-mount antenna. The energy from the antenna (A) is coupled through the glass by placing the mounting foot over the coupling box located inside the window. OATLEY ELECTRONICS IFOR No.1 KITS I ~ ~ I INFRA RED NIGHT VIEWER A very small complete ki t • Includes both an adjustable lens and an eyepiece • The matching tu bes, lenses an d eyepieces were removed from 5 ~~1/iTHE 2mW PHILIPS head, 12V supply kit, warning labels, ballast resistor and the instructions: KIT No. LK13. 299 EXPERIMENTAL E.H.T. POWER SUPPLY Includes the IR tube, lens eyepiece, electronics kit and th e case ki t: KJT No. tRNW5. 75mm round IA plastic fil te r: $16. o 2mW PHILIPS LASER HEAD WITH 240V POWER SUPPLY Uses a brand new visible red He-NE LASER HEAD: Encapsul ated He-Ne tube • Ve ry tight beam (0.95mR ): More suitable for most applications • Head dimensions: 37mm diameter by 260mm long • Supplied with a very small pro fessional 240V power supply, that even has a TTL inte r lock: Needs 3-5V <at> approx 3mA, ac ross two isolated term inals to swi tch the laser on: Opto isolator built into the supply• Use it for surveying , science experiments, laser displays, holography, etc.• Priced at about 1h of the ex pected price and ONLY WHILE STOCKS LAST AT: s210 ~~1/i!THE 2mW PHILIPS head, the com mercial power supply, warn ing labels, ball ast resistor and the instr uct ions: KIT No. LK14. VISIBLE LASER DIODE Similar story to the IR diode but this one is very visible: 3mW <at> 670nM ON LY $210. For a 3mW visi ble laser diode, collimator/heatsink assembly, a suitable drive r/digital switch kit and the instructions. Same head as the LK14, but thi s unit is supplied with our newest 12V lase r inverter kit , which will power most laser tubes • The supply is easy to con struct and comes wilh a totally prewound transformer, and simple instructions • The sw itched mode power supp ly wit h MOSFET outpu t is very efficien t and a gu aranteed design • THE INCREDIBLE PRICE? s175 new Ge rman night vision equipment • Simple construction: Instructions provided • For a limited time only the kit is supplied with a high quality mi litary inverte r: Draws on ly 25mA from the 9V battery • IR ill umin ation is req uired in the dark, and good ranges are possible with just med ium powered foc ussabte to rches, fitted with an IR fil ter • You wou ld normally pay over $3000 for a view like this! DON ' T MISS OUT! 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IR LASER DIODE A 5mW/780nM laser diode mo unted in a prec ision made adjustable coll imator asse mbly • Removed from new and near new equ ip ment • Very ti ght bea m and is just visible to humans in a darkened room • Requi res only a constant current source to operate : Approx 80mA/2V • Use ii for communications, securi ly. medical research. scientific experiments, invisible lase r pointer: Very visible 10 an IR Nig ht Viewer! • Suitab le low voltage dr ive r/d igi tal kit is inc luded in our SPECIAL INTRODUCTORY PRICE: ONLYsg g For t he collimated assembly illustrated, and a su itable expe rimental driver/dig ital switch kit and the instructions. Melbourne Distributor: Electronics World (03) 723 3860 or (03) 723 3094 SE PTEMB ER 1991 79 Amateur Radio - glass mount antennas CHRISTIAN BLIND MISSH{~ ·J Re,$totiK! :;~~t, be1K This view inside the coupling box shows how the adjustment screw is used to distort the brass plate to change the tuning capacitance. She is just one of the 2.5 million blindnessthreatened people treated every year by CBMI health care workers and eye specialists. Millions more are still waiting for sightsaving treatment. tion of either the antenna or coupling box would severely degrade the performance of a glass mounted antenna. In an effort to accurately determine the optimum location for both coupling box and antenna mounting foot, a simplified test set up has been devised. This allows the measurement of the capacitance of the glass at any location on the vehicle. If the capacitance of the glass falls within the set limits, the location is suitable for the mounting of a glass mount antenna. If capacitance is high, and this can be the case if the area of glass to be measured contains demister wires, mesh or sun screen tinting, the location will not be optimum. Test set-up Please help CBMI to transform your gift into eyesight! COUPON Please cut and send to: CHRISTIAN BLIND MISSION INTERNATIONAL, P.O. Box 5, 1245 Burke Road, KEW, Vic. 3101 Phone: (03)817-4566 D = Please send me further information about CBMl's work. As long as i_t is possible for me, I will help: D monthly D quarterly D annually D to prevent blindness D to restore eyesight D to rehabilitate the blind Enclosed is my gift of$ _ _ __ __ Mr/ Mrs/ Miss: _ _ _ __ _ __ _ Street: _ _ __ __ __ _ __ City: _ 80 _ _ __ _ Pnstcode: _ SILICON CHIP _ _ The test set up itself is made from the coupling box of a standard glass mount antenna. The box has been modified so that the centre conductor of the coaxial cable is fed directly to the PC board coupling plate and the braid of the coaxial cable goes to the brass foil sheath inside the coupling box. This box may then be connected to a digital multimeter which measures capacitance. Typically, the DMM should be switched to its 200pF range, as this will provide resolution of 0.1 pF. An additional test lead connected to the braid of the coaxial cable at the multimeter end and terminated with an alligator clip at the other end may be used as an optional vehicle earth connection. With the probe connected to the digital capacitance meter, the meter will give a particular reading in free space. By placing the test probe against the glass, this reading will increase by several pF. Placing the probe across demister wires or window tinting, or even close to the chrome surround of vehicle windows, will cause the capacitance to be higher than optimum for good RF performance. Hence the probe can be used to verify that the preferred mounting location is unaffected by any of these factors. If a genuine coupling box from a glass mount antenna is unavailable, the test probe may be made from a zippy box with a PC board in place of the lid. Patent protection Glass mount antennas are protected in many countries of the world by patents. The principle of coupling RF energy through glass dates back as far as 1929, and developments of this technique have transpired since that time until the 1980s. US Patent 4238799 covers a glass mount antenna system designed for the 2 7MHz CB bands. Since that time, other variations and improvements have been made for virtually all radio bands utilised today. In particular, Australian patent 535273 covers the application of this technology for cellular use. The information presented in this article is for technical interest only. Readers should be aware that any use of the information contained in this article on a commercial basis may breach intellectual property rights protected by these patents. A commercial version similar to the glass mount capacitance tester described in this article is available from the Antennas Specialist Company, Cliffland, Ohio, and possibly from their representative in Australia, RF Industries Pty Ltd - phone (02) 749 1488. Radio Convention Radio amateurs who will be in the "top end" during September should make it their business to drop in for the 1991 Radio Convention presented by the Townsville Amateur Radio Club. The venue will be the James Cook University in Townsville, on 27th, 28th and 29th of September. For further information phone (077) 73 3487 or (077) 74 0211. SC