Low-Profile Mobile Antennas
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en cc, ii) -c Low-Profile 0 ;: • • C Z Mobile Antennas <{ C '= ver the years there have been many tech· '"ec niquesfor hidingantennason avehicle.One Q) ~ Z O way back in the 1950s and '60s was to use +- w the "curb feeler: As you can see in photo A, this >"" was a flexible spring. much like a rubber duck with· C '" out the rubber. andusuallyjust onespring.The leel er was mounted on the passenger side of the car near the rear tire. The feeler would make a scrap 0 ing noiseas you parallel parked yourcarand let you know mat any closer to the curb would scratch up your hub caps. You almost have to be over 50 to remember curb feelers. and I had a heck of a time finding one to photograph. However, back in the 19505 the Dallas Police used to replace the curb feelers on their unmarked police cars with a short antenna. They also used 450 MHz. Very few home receiverscould listen to 450 MHzbackthen, so they were pretty good secret channels. One hidden antenna that I have used on my van is the AMlFM radio antenna. Of course, lhese Photo B- Electrically-isolated trunk hinges. worked better in the days when you really had a whip out there, not conductive glass in the wind- shield. By adding a diptexinq filter, the antenna could be used on 6 or 2 meters without the trans mitter blowing out the AMlFM radio. In photo B is a trunk hinge on my 1992 Tempo, but one Japanese auto manufacturer used to mount its trunk lid on plastic bushings. Now the entire trunk lid was electrically isolated from the car body, and it was the trunk lid that was used as the AM/FM antenna. Either the trunk or the hood could be used as an antenna with this technique. Another hidden antenna used on some govem ment vehicles is the trunk-lid gap in photo C. Thi s is an antenna design we will be talking more about '1626 Vineyard. Grand Prairie, TX 75052 e-mail: <[email protected]> Photo C- Using parts of the vehicle as a slot Photo A- Remember curb feelers ? antenna. www.cq-emaleur-redlo.comMey2009. c a • 73 - ------------ ------- --- o Cente c Ecnsaat.1 TIE in __.I n l _ "'1 __. 1~10-a Fee .... __ I_·.'U 0--......, ....... (- Of set ~ ~ { - F ed Licensed 1984 Or Before? QCWA invites you t o join wit h Fig. J- Slot antennas andconvention those distinguished amateurs al dipoles. licensed 2S yea rs or longer. Request an application from: network, so these temporary ATMs QCWA. Inc.. Dept. CQ have a cell phone in there and calls in POBo x 3247 your card number and waits for an Framingham, MA 01705-3247 authorization. I came up with a dual www.qcwa.org band slot-antenna design for both the Stop by our booth at Dayton 850 MHz and 1900 MHz cell-phone bands tha t just looked like the back of May 15-17, 2009 the ATM machine. You would need a few minutes with a sledge hammer to vandalize that ATM's antenna. Electrically Conductive Glass Many electrical accessories are now being hidden in the glass itself. During in coming months- the slot antenna. As manufacturing. a layer of tin oxide is lay shown in fig. 1, a dipole is a half-wave of ered on the glass. Tin oxide is electri elect rical conductor in a large insulator. cally conductive and almost invisible all ;nsula~ A slot antenna is a halt-wave of by itself. Next, the glass is reheated Photo D- Camouflage antennas. tor in a large conductor. Therefore, the almost to its melting point. The tin oxide gap between the trunk lid and the car bakes into the glass, making an electri body can be used as an antenna . The cally conductive "trtt" thatis nowjust part nant just below the a-meter band, so a engine hood will work as well, but the of the windshield . Electric window quick trim and it's now a 2-meter anten trunk-lid gap is usually easier. heater elements, AM , FM, cell-phone, na. The lip is bent back as a safety pre In this example. the coax is fed across and Bluetooth® antennas are now just caution. It seems that just about any the slot. The coax shield goes to the car part of the transparent glass and you commercial antenna can be body, and the center lead goes to the are looking through your antennas. trimmed/pruned/modified to a ham trunk lid. I have this one in the center of While fine for local radio broadcasts, I'm band-not always the one you were the slot, but you can also feed the slot not sure I would want to pump 100 wans hoping for. but if they are cheap enough off center. That is , the coax can be on 20 meters into that tin-oxide layer. it's worth a shot. That sounds like a good attached most anywhere along the slot, Does anyone know if they have suc topic to expand on in a future column. and the feed point can be optimized for cessfully used these tin-oxide layers as RadioShack used to sell a CB antenna your favorite band. Plan on using an the GPS antenna? I've never heard of that looked like an early cell-phone antenna tuner 10 getthe SWR down, but anyone gening one of these tin-oxide antenna and these were easily retuned when you can design the slot instead of antennas to work with GPS, and with to 10 meters. just using something that is already the poor conductivity and poor place there. a good match without the anten ment, a GPS antenna would be quite a na tuner is easy. challenge. Other Multiband The extremely low profile of slot I'm not sure if I would call the anten Vertical Techniques antennas also makes them very popu na in photo 0 a hidden antenna. This With commercial ham verticals, one lar on high-speed aircraft. I also took antenna is advertised as having lots of common way to makethe antenna work advantage of the low/zero profile of a gain on the Soo-MHz and 1900-MHz on more than one ham band is with a slot antenna to design one for an ATM cell bands, and with a TNC coax con "trap." As shown in fig. 2, a trap is just machine. Ever see an ATM machine at nector it is not going 10 fit many cell a parallel tuned circuit tuned to a ham a sporting event or a concert? No phones these days. However, a quick band. At its tuned frequency, this trap chance to connect it to a high· speed frequency sweep showed it was reso- looks like about a 10.000 ohm resis- 74 • co • Mav 2009 Visit Our Web Site -r- - r- Ail used All used Tuned to on on - 28 MHz 27 MHz 15 Meters ""~ "'" Section used Inductance on 10 MeIers -: Section used T for 160 MHz 10-1 5 Meter Trap Vert. 160-27 MHz Vert. Fig. 2- Diagram ofa dual-band trap vertical. Fig. 3- Position of the band-separating inductor. Photo E- Band-separating inductor. Fig. 4- Parasitic capacitance and self resonance. tance, or reactance, and keeps the 2Q-. 17-, 15-. 12-, and 1a-meter trap ver contains a parasitic capacitor and at radio wave from using the top part of the ticals are available, a 4Q- , 2Q-, 1Q-meter some frequency forms its own parallel antenna. For those of you who can pick model would be about the best you can resonant circuit. By putting just the right fly specks out of pepper. yes, that reac dousing onlyinductors. 1would beinter number of tums in this band trap, it is tance is going to vary with the design ested in hearing from any readers with possible to make a tuned trap out of just and "0- (Quality Factor) of the trap and experience in this area . There is a a coil of wire. This is a bit trickier way to where it is tuned in the band. For the another effect at work here, though. make a multiband fiberglass vertical rest of you, the trap looks like a very In photo E are a fewtums of wireform than the aluminum-foil trick we went high-resistance, and the signal pretty ing a band trap on a fiberglass rod over last time, but easy to make in the much goes up to the trap and stops. The antenna. In this case the coil gives the thousands, especially with an automat trap is tuned to one frequency; on lower 27-MHz rod a 160-MHz resonance. As ic coil winding machine. Next time I'll frequencies the trap looks like an induc we mentioned last time. many CB show a combination of these two tor or a loading coil. Thus, on the sec radios loday also have a 160-MHz techniques to make a three-band fiber ond band the antenna becomes sort of weather-band receiver, and truckers glass whip. a center-loaded vertical using the coil in etten like 10 listen to local weather the trap as its loading coil. reports. By putting this coil about a quar Another way 10 add bands is 10 use ter wave above the base, as shown in As always, we welcome your ques just an inductance. I have used both a fig. 3, the anlenna now has both a 27 tions and topic suggestions.