Mobile antennas: Evolution Predominant antenna types change as commercial users migrate to higher frequencies and adjust to spectrum availability, but technical refinements in antenna materials and manufacturing now focus on aesthetic considerations.
Expansion of commercial mobile radio use into higher frequencies, different applications and environments has meant a coincident evolution in the design, construction and placement of mobile antennas. The user no longer adjusts to the available equipment. Instead, it is a question of matching the equipment to the application.
Major technical innovation in antenna construction for land mobile and cellular use has pretty much reached a plateau, although behind the scenes there is constant improvement in range testing to meet targeted specifications. Frequencies have gotten higher, and antennas have gotten shorter. Although there has been some flirtation in recent years with serpentine arrays, patches, helixes, ring radiators and other antenna types, the mast, in one form or another, seems here to stay. Technology has become more finely tuned, so manufacturers are looking at aesthetics. Like champion pidgeon breeders, antenna makers are all dealing with the same basic bird, but each strives for a unique mixture of color and feathering.
The choices facing the user are selecting an antenna appropriate to the frequency and application, and taking advantage of available embellishments that significantly improve communications system performance.
Gain The most basic antenna for mobile use is a quarterwave vertical antenna with an omnidirectional pattern. Its gain is the same as a dipole, 0dBd (or about 2.1dBi).
It should be pointed out that cross-comparisons among different antenna makes should not be taken at face value, because the apparent gain may be higher than the desired reference. When determining antenna suitability for a particular application or topography, check the reference for the antenna gain as quoted by the manufacturer. Most cellular antennas, for example, offer 3dB gain compared to a reference halfwave dipole (3dBd), which is the best comparison and was the original AMPS industry standard. Comparisons to a quarterwave ground plane or to an isotropic antenna (dBi) are less-telling. A “5dB gain antenna” may be referenced to a dipole, but if the reference is to an isotropic antenna, the gain is actually 2.85dBd.
Gain has tradeoffs. As gain increases, the beamwidth decreases, but less power is required at the transmitter. A high-gain mobile antenna directs the radiation pattern vertically, concentrating energy toward the horizon (making it suitable for Kansas, but not downtown Chicago).
Gain of 3dB is adequate for general mobile use. Higher gain does not enhance the reception level (and the total amount of energy radiated from the antenna does not increase). Moreover, the elevation level for scattered signals in cities with tall vertical structures is higher than in suburban or rural areas.
Antenna types A mobile antenna should be omni-directional and, ideally, mounted at the vehicle’s highest point, the roof, to obtain clear reception. A roof-mounted 3dB collinear antenna will provide a better pattern than a single-element quarterwave antenna. A glass-mount antenna produces a pattern roughly equivalent to a roof-mounted quarterwave antenna in free space.
When a quarterwave vertical element is mounted on metal roof, the roof acts as a ground plane, creating a “mirror image” forming the complement of a halfwave dipole with an omnidirectional pattern. Non-metallic roofing (vinyl, composite, or fiberglass) disqualifies the roof as a site. Placing the quarterwave elsewhere, such as a fender or bumper will distort the horizontal pattern and increase the directionality of the gain.
Low-band VHF (25.01MHz-49.6MHz) for land mobile uses a quarterwave whip, usually a base-loaded antenna with a coil that reduces the mast requirements to about four feet.
High-band VHF (150.8MHz-173.4MHz) also uses a quarterwave antenna, which at 150MHz is about 1.5 feet tall. Halfwave and 5/8-wave antennas with 3dB gain are also used (with a coil to match impedance to the receiver anntenna input).
UHF (450MHz-512MHz) requires a quarterwave about 7″ tall. Collinear UHF antennas use two 5/8-wave elements joined by a phasing coil and have about 5dB gain.
Antennas for the 800/900MHz band (806MHz-940MHz for land mobile) are generally either unity-gain quarterwaves or 3dB-gain collinear antennas combining a 5/8-wavelength element, a phasing coil and a quarterwave element.
Glass-mount antennas are indisputably the most popular cellular antenna, because they avoid marring a vehicle with a hole and the attendant problem of corrosion. Common configurations are a vertical halfwave element collinear with another halfwave, or a vertical 5/8-wave element collinear with a quarterwave; in both cases a phasing coil connects the two vertical elements and keeps the RF energy in phase.
For PCS applications at 2.2GHz-2.9GHz, factory-tuned antennas offer about 100MHz bandwidth at 3dB gain or 75MHz bandwidth at 5dB gain, with a VSWR of about 1.5:1.
Refinements The following is a partial list of improvements and equipment options for mobile antennas introduced in recent years (“from the ground up,” so to speak):
*Ground planes — Magnetic-mount antennas for cellular and PCS are available with integral ground planes imposed at the feedpoint to overcome ground decoupling at higher frequencies.
*Cabling — Improvements in feeder cables include low-loss, braided cables. Even corrugated cables now have a greater degree of flexibility. Lower surface current on cabling minimizes weak signals and prevents the escape of RF signals that might interfere with vehicle electronics or engine diagnostic computers.
*Mounts — For body-mounted antennas, shielded mounts decrease interior radiation and reduce noise compared to open-junction mounts. Coatings are now being used on base springs to avoid noise generation from the coil. Durable materials such as Neoprene are being used in seals and gaskets to avoid moisture intrusion into the vehicle body and into antenna coils. Automobile styles continue to be more aerodynamic, so for glass-mount antennas, flexible plastic bases are being used that conform to the concavity of windshields and windows. Improved adhesive formulations for stickers and tapes hold antennas more securely. Fold-over hinges and swivels for both body-mounted and glass-mounted antennas accommodate hatchback body styles, car washes and low overhead clearances. Cellular phone use is no longer vehicle-dependent. Available to the mobile worker are several couplers and temporary antennas (primarily window-clip and magnetic versions) to augment use of a portable cellphone when using a rental vehicle on location. Vinyl-coated clips avoid damage to glass. Applicable to SMR, cellular AMPS or GSM, the antennas can be obtained in unity, 3dBd and 5dBd versions with open or enclosed coils.
* Multiplexing — Duplexers and triplexers are available with insertion losses ranging from 0.1dB to 0.4dB. Triband mobile antennas, resembling elevated feed antennas, can serve UHF, VHF and cellular frequencies.
* Couplers — Non-mechanical, solid-state coupling boxes for glass-mount antennas straddle window defogger wires.
* Loading coils — Loading coils can be obtained with chroming for durability. (Likewise, brass fittings provide durability and resistance to corrosion.)
* Phasing coils — UV-resistant materials are being used for many antenna parts, including non-fade housings for phasing coils. The materials also deter corrosion. Wind noise aesthetics for coil enclosures have led to straking, or channels, that direct wind more efficiently around the mast and eliminate whistling.
* Whips and masts — Solid encasing or sheathing of whips (in plastic, composites or polycarbonates) reduces noise and the build-up of static charges. Antennas can get taller–with high masts now being stocked to accommodate sport utility vehicle and truck installations where more height is required to clear the roof line, or shorter–with low-profile, three-inch globe and spike glass-mount antennas for cellular use. Broadband disguised antennas, or AM-FM “look-alikes,” with 125MHz bandwidths, are available for surveillance work requiring inconspicuous vehicles.
* Modularity — Interchangeble base coils, springs and whips allow quick replacement or modification for migrating to a different frequency. Inventory space can be reduced as well with modular systems.
The antenna is, paradoxically, the least expensive and most important component in a mobile system. The sophistication of a mobile radio system is irrelevevant if there is a problem at the first point of reception and last point of transmission: the antenna.