Practical_Antenna_Handbook_0071639586
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Practical_Antenna_Handbook_0071639586

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CHAPTER 16 Mobile and Marine Antennas Mobile operation of radio communications equipment dates back nearly as far as “base station” operation. From the earliest days of wireless, radio buffs have attempted to place radio communications equipment in vehicles. Unfortunately, two-way radio was not terribly practical until the 1930s, when the first applications were amateur radio and police radio (which used frequencies in the 1.7- to 2.0-MHz region). Over the years, land mobile operation has moved progressively higher in frequency for practical reasons. The higher the frequency, the shorter the wavelength and therefore the closer a mobile whip can get to being a full-size antenna. On the 11-m citizens band, for example, a quarter-wavelength whip antenna is 102 in long, and for the 10-m amateur band only 96 in long. At VHF and above, full l/4 mobile antennas are practical. Partially as a result of this, much mobile activity takes place in the VHF and UHF region. Interestingly, even as land mobile moved higher in frequency with the passing decades, amateur radio mobile activity has moved both higher and lower, with everincreasing interest in overcoming the challenges that very long wavelengths present. Today, enterprising amateurs in the United States are working transoceanic DX on 160 m during their daily commutes! Mobile operation in the amateur 144-, 220-, and 440-MHz bands is extremely popular because of several factors, not the least of which is the small size (and cost) of efficient vehicle-mounted verticals (or whips) for these bands. Mobile Antennas for VHF and UHF At 2 m and above, the roof of even a small coupe has enough flat or nearly flat surface area to act very much like an ideal ground plane—provided, however, that the antenna is located near the middle of the roof, and that the shield, or grounded, conductor of the coaxial feedline makes a clean, low-resistance connection to the roof at the base of the antenna. If that is done, a simple 19-in piece of tinned copper wire or plated spring steel will work as well as any store-bought whip on 2 m. Over the years, the author has had 2-m whips installed by a commercial two-way radio shop on more than one occasion, but has also done it himself a few times. If you are comfortable with snaking RG-58 or RG-8X cable between the roof and the liner of your vehicle and know how to install an old-fashioned chassis-mount SO-239 (drill a pilot hole for a chassis punch of the appropriate size, such as those made by Greenlee), you can have a 2-m mobile antenna that is hard to beat! (Don’t forget to seal against rain around the connector. Use a rubberized gasket or a caulking compound.) 375

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