Antennas for Mobile Radio
August 1968 Radio-Electronics

August 1968 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

When this "Antennas for Mobile Radio" article appeared in a 1968 issue of Radio-Electronics magazine, the term "mobile" referred mostly to Hams, commercial dispatch operators, law enforcement, and the military. Nowadays, "mobile " is heard and seen all the time in reference to cellphones and notebook computers. The funny thing is that the vast majority of people do not think of their phones and Wi-Fi-connected notebook computers as a form of radio. They just have some sort of invisible cord connecting them to anywhere in the world. It is only when a good connection cannot be had that the concept of radio waves might occur, but even then, it is pre-Millennials who grew up having to adjust TV and/or radio antennas, or who used walkie-talkies, that readily relate to the "wireless" concept from personal experience. Early cellphones (waaayyyyy back in the 1990s) had retractable antennas, so they "felt" like a form of radio; however, once antennas were buried inside the phone case, all external indications that a radio was at work was lost.

Antennas for Mobile Radio

Is your sky-piece first class? Match antenna performance with today's high-quality transceivers.

By D. Blacklock

Are you having trouble working your mobile units? Do you spend five minutes trying to get a message to a mobile unit and end with the operator calling you by land line? Does he disappear into a dead zone and stay lost for a half hour? If so, the trouble may be in your antenna system.

CB and other two-way transceiver equipment now on the market includes some of the best communication equipment ever designed. Transceivers have to meet FCC technical requirements and stiff competition in the market for the two-way radio dollar. Receiver sensitivity, adjacent-channel rejection, transmitter power output and modulation percentage are all that the law and cost engineering allow.

However, there can easily be a weak link in the communication chain: the antenna system. If the antenna fails to transmit and receive efficiently, you cannot take full advantage of this equipment.

Types of Antennas

A study of manufacturers' literature shows a bewildering array of antennas available, from simple and inexpensive quarter-wave verticals to elaborate and expensive multi-element beam antennas. Antenna selection for your base station requires a familiarity with the capabilities of the basic antenna types.

The quarter-wave end-fed vertical is the simplest of the vertical antennas. It consists of a simple vertical radiating element cut to one-quarter wavelength. This antenna must work against a ground plane (Fig. 1), cut to one-eighth or one-quarter wavelength.

Advantages of this antenna are its light weight, ease of mounting and low cost. Disadvantages are no relative gain, high radiation angle (Fig. 2-a) and a radiating element that is not at de ground, allowing static buildup and increasing receiver noise.

Half-wave vertical antennas are identical to the quarter-wave except they are cut for one-half wavelength. They have the same advantages and disadvantages as the quarter-wave, with an improved radiation angle and better gain (Fig. 2-b).

The five-eighths-wavelength electrically extended vertical is the most commonly used and one of the most efficient antennas available today. It consists of a vertical element which is electrically extended to represent five-eighths wavelength. This antenna also has a ground plane of three or four radials of one-quarter or usually one-half wavelength. This is the longest vertical (just under 20') that may be used for CB under current FCC rules.

Development of this type of antenna was a great stride forward in making available more efficient antennas for two-way frequencies. The five-eighths wave has several advantages over quarter- and half-wave verticals.

Due to the five-eighths wavelength radiator, the antenna has a very low radiation angle and, as a result, exhibits considerably more gain than the quarter-wave vertical (Fig. 2-c).

This radiation angle may be made even lower by using short radial elements at the top of the vertical element. The use of a loading coil allows this antenna to be shunt-fed, which in turn enables the entire antenna to be operated at dc ground.

With the five-eighths wave it is possible to achieve a gain of 4 to 5 dB over quarter- and half-wave verticals. This is the most powerful nonrotating antenna available. The disadvantages are high cost and the heavier mounting required because of increased weight and wind resistance.

The three antennas discussed so far are of the omnidirectional type. They provide uniform coverage in all directions. Directive antennas have the ability to concentrate the signal into a narrow path, which may then be directed toward a specific point or area.

Multi-element rotatable beam antennas consist of two to five elements mounted on a boom. There is one radiator or driven element, along with one to four reflector and director elements. Since this type of antenna is normally rotated to provide 360° coverage, two identical sets of elements are often mounted on a cross boom, balancing the array and providing a central point of rotation. Two antennas side-by-side will produce a very narrow horizontal beam.

An advantage is effective power gain due to concentration of the radiation. (This may be 6 to 10 dB, depending upon the number of elements.) Also, high rejection (18-26 dB) of noise and unwanted signals not in the beam path is achieved. Disadvantages are high installation costs - array, support tower and rotator - and large physical size requiring a clear area up to 30 feet in diameter for rotation. Strong support is needed because of weight and weather.

Phased or electronically steerable arrays are a relatively new type of antenna having the advantages of the beam antenna while eliminating the disadvantages of size, weight and mechanical rotator. Mechanically more simple, this antenna system is more complex electronically. These arrays are bidirectional and unidirectional.

Bidirectional arrays are the simplest of the phased-array antennas (Fig. 3). Typically, two vertical antennas are spaced one-half wavelength apart. The antennas are fed through an impedance-matching transformer and equal-length feed lines. With these feed lines the signals arrive at the antennas in phase. With the antennas excited in phase and a half-wavelength apart, field cancellation takes place. The signal is therefore radiated along a path at right angles to the plane of the antennas. The pattern extends to the left and right of the antenna plane. This is the broadside mode of operation, and for half-wave spacing produces a beam width of about 60°, a gain of 3.8 dB over a single vertical antenna and 30 dB attenuation in the plane.

If the feed line to one antenna is lengthened by a half wavelength (Fig. 4) the antennas are excited 180° out of phase, and the consequent field cancellation results in radiation along the plane of the antennas. This is the end-fire mode, producing a beam width of around 80° and a gain of 2.3 dB over a single vertical. Signals broadside to the plane are attenuated by 20 dB.

By addition of a simple switching device, it is possible to shift the array from broadside to end-fire modes. Switching modes allows 360° coverage.

Advantages of this array are a reasonable gain over a single vertical, some rejection of noise and unwanted signals and 360° coverage with mode switching.

Disadvantages are nonuniform 360° coverage in or near maximum range-some dead spots exist due to beam patterns.

The cardioid array is a unidirectional phased array that differs from the bidirectional array in the spacing of the vertical antennas. The verticals are 'spaced' one-quarter wavelength apart. Additionally, the antennas are fed 90° out of phase. The resulting field cancellation produces a cardioid (heart-shaped) radiation pattern (Fig. 5). This pattern is centered about the plane of the antennas and is directed toward the out-of-phase antenna. The beam is approximately 120° wide, with 30 dB rear attenuation and 20 dB side attenuation. With the addition of a simple switching device to switch the 90° phase shift from one antenna . to the other, it is possible to shift the direction of radiation 180°.

Advantages of this array are a broad beam covering almost 180°, switching capability to provide 360° coverage, 4.5 dB gain over a single vertical and smaller space requirements than those of the bidirectional array. Disadvantages are that a 360° coverage will have two dead spots.

A steerable cardioid array is the basic cardioid array improved to provide an array which can be positioned in six discrete positions of 60° each. thus providing complete 360° coverage. See "Electronic Antenna Rotation," Radio-Electronics, Aug. 1967. This requires the use of a third vertical antenna. The three verticals are oriented at the points of an equilateral triangle which is a quarter wavelength on a side. The array is fed through a switching unit that selects the pair of verticals being excited and the antenna which has a 90° phase shift. This antenna and phase-shift selection permits the direction of radiation to be stepped through six positions. The array produces a pattern having a 60° beam width with 20 dB side attenuation and 30 dB rear attenuation.

Advantages are uniform 360° coverage and a gain of 4.5 dB over a single vertical. Disadvantages are the cost of the three verticals and a more complex switching unit.

With phased arrays it is possible to achieve the same gain and side-back rejection as with a stacked-beam array less mechanical disadvantages.

Antenna Selection

Choice of an antenna must be governed by requirements of your communication system. Factors to be weighed are area to be covered (both range and terrain), weather conditions, rf interference (nature and man-made) and cost. Generally, limit use of simple quarter- and half-wave antennas to applications where the area to be covered is small and relatively free of rf interference. For areas of larger coverage; a five-eighths wave extended antenna is advisable. Highly directive antenna systems should be reserved for coverage over large areas, or areas where shadows and masks occur because of tall buildings, hills and valleys.

Remember that a directional antenna can cover only one small segment of a large area. A directional system is definitely appropriate when the communication traffic is in one direction from the base station. Why scatter the signal to the "four winds" if it can be aimed where it does the most good?

When an evaluation of requirements indicates a directional system can be used to advantage, consideration should be given to the phased array. This type of antenna array can give the same performance as the stacked beam without the mechanical and space problems. In areas of the country with high winds or severe icing conditions, the stacked beam can be easily damaged.

Furthermore, the phased array is unobtrusive when installed. Large stacked beams always give the appearance of a ton of pipe hung in the air. A neighbor is less likely to object to two or three vertical antennas than to what appears to be a "30-foot psychedelic mobile!"

Ideally, antennas should be installed on the highest object in the area. Unfortunately, this is usually impossible. The location should be as free as possible of obstructions. If the installation is a phased array or rotary beam, there of course must be sufficient room to allow for correct spacing or movement. The location should also be as free as possible of manmade noise:

Consideration must be given to the distance from the transceiver, as long feed line(s) will introduce loss and attenuate the signal. Large leafy trees in the signal path should be avoided as they cause great rf attenuation.

It is almost impossible to formulate general rules or requirements on choosing an antenna location, as each installation will have its own problems. If general rules can be stated they are these: Keep the antenna as high as possible. Keep it as clear as possible. Keep it as quiet as possible.

A word of caution. Pick a location clear of power lines for two good reasons. First, power lines can generate noise. Second, if your antenna is blown down during a storm, the resulting display of fireworks may be exciting for a moment, but after the sparks stop flying it will surely be unpleasant. Power companies become very unhappy about other people's things in their power lines. Buying candles for the neighborhood can be expensive!

Mounting the Antenna

When it comes to mounting the antenna, too much is better than not enough. Remember the antenna and supports will be exposed to the elements and will deteriorate. Use good mechanical construction practices. All hardware should be galvanized if made of steel, and anodized if made of aluminum. Use the mounting hardware supplied with the antenna and follow the manufacturer's instructions.

A mast or short tower can be used. One thing to remember, do not use a chimney. Stack gas is highly corrosive, and will reduce the most expensive antenna to junk in a short time.

Vertical antennas are designed for mast mounting. This may be either pipe or tubing, as long as it is capable to taking the load. The mast sections and hardware used for TV antennas are suitable if they are heavy-duty.

Rotary beam .antennas are best supported with a tower, because of beam and rotator weight. The support for a beam antenna is subjected to, not only the loading of antenna weight and ice, but also a torsional or twisting load caused by the rotator moving the beam and wind stress on the beam. This requires a strong support structure. It's possible to use TV components, but they must be suitable for very heavy-duty use.

Roof structure must be taken into consideration when choosing mounting hardware. Flat roofs are much easier to attach the mast or tower to than steeply pitched roofs. With masts, an adjustable mounting foot can be used that will follow the roof pitch, while allowing the mast to be vertical. Towers present a more difficult problem on pitched roofs. The legs of the tower must be cut to fit or a tower with adjustable legs must be used. Often the best solution is to build a wooden plat-form to match the pitch of the roof.

The roof must be capable of supporting the antenna load. This is important in the case of the beam type of antenna as total weight of antenna tower and rotator can be 100 pounds or more. With some types of construction it may be necessary to add bracing in the attic under the mounting base. In any case, the mast or tower base must be securely attached to the roof. Wind pressure and vibration can cause the base to "walk" and one day your antenna might be flat on your roof.

Guy wires mayor may not be required with the mast support. This will depend upon the type of mast. The short tripod base does not require support, unless the mast extends several feet above the top of the tripod, or wind conditions are high.

The tower requires guy wires in almost all cases. If guy wires are used, three are the minimum. Spacing should be symmetrical and must not interfere with the rotation of the beam. Tension on the wires must be uniform. The manufacturer of the tower can supply instructions on "guying" and these should be followed.

Waterproof all points of attachment to the roof to prevent leakage. This can save the expense of ruined insulation or ceilings.

In mounting an antenna, keep in mind that inclement weather will try to take it down, so put it up to stay.

Before you charge out and buy enough mast or tower to rival the local broadcasting station, remember the FCC has some definite ideas about the maximum height of an antenna. So, before you start cutting pipe, read and heed FCC Rules and Regulations, Vol. VI, Part 95, Section 95.37. A small advantage gained from an unlawful installation will not make up for a citation and possible fine.

A few words about something of extreme importance, and one of the most neglected items of antenna installation. This is grounding the antenna and mast or tower. This is important for two reasons. First, with the five-eighths wavelength extended vertical, the ground plane and the base of the vertical must be at dc ground to take full advantage of this design. The second reason is safety. You've just stuck about 20' of metal into the air, and if you're lucky it's the highest object in the surrounding area, or at least the highest on the roof of your buildings.

A grounded mast or tower will not prevent a lightning stroke, but it will act as a lightning rod and provide a path for the bolt. The ground wire must be of heavy-gauge cable, No.4 or heavier. The ground must be a good earth ground: a metal stake at least 6' long driven 5' deep. In rocky ground it should be driven even deeper. In all cases, it must go deeper than the frost line. Use this ground for the entire system. A ground placed on the transceiver may cause a ground loop and resultant trouble. The feed line shield will ground the transceiver.

Antenna Connections

Connecting the antenna to the transceiver is another area where many two-way radio systems fall flat. This connection must be of the highest quality. By now you have a large investment in your transceiver and the installation of a good antenna. If you skimp here, your money will be wasted.

Feed lines, or to be more accurate, rf transmission lines, are a very special type of cable. It is not lamp cord and cannot be handled like an extension cord. There are many types of coaxial cable, but the antennas discussed all require 52-ohm coax.

The most common 52-ohm coaxial cables are RG-8/U and RG-58/U. Either may be used. Choice is determined by the length of the feed line required: RG-8/U has the lowest loss, 1 dB/100', at 30 MHz while RG-58/U has a loss of 3 dB/100' (Fig. 5). RG-8 is approximately 1/2" in diameter. RG-58 is 1/4" in diameter. For feed lines up to 30' (at 30 MHz), RG-58 is satisfactory but this should be considered its maximum length. The loss for 30' is almost 1 dB, and remember a 3 dB loss is equal to one-half the power. For top performance, RG-8 should be used for all feed lines because of the lower loss. For CB cable runs over 100' and all vhf applications, RG-17/U should be used.

Bargain surplus coaxial cable should be avoided unless it carries a guaranty. First, it may not be 52-ohm impedance. Second, coaxial cable must be stored and handled carefully to avoid damage. Surplus cable may be damaged internally yet show no external damage. Also, it is of utmost importance that coaxial cable be kept dry. Even a small amount of moisture leaking into the cable can seriously increase losses, and once moisture has seeped into the cable it is very difficult to dry it out. If the cable has been rolled into a small tight coil, the dielectric insulation may have cracked, again increasing loss. It may be hard to find surplus coaxial cable in the length needed and splicing, which should be avoided, may be necessary.

So, spend a little bit more and go first class. Buy new, top-quality coaxial cable in sufficient length to make the feed line(s) in a continuous run without splices or connectors.

Most antennas available today are fitted with coaxial connector type SO-239, the mating plug being a PL-259. This or an equivalent connector should be used. Unfortunately, this connector is not waterproof, and all exposed connecters must be waterproofed.

Routing of the feed line(s) should follow the most direct path to keep the line as short as possible. The coaxial cable should be secured to keep it out of the way and from moving around. Use clamps that will not damage the outer covering or crush the cable. Do not make sharp bends in the cable; bends should have a minimum radius of 6". Leave a strain relief at the point where the cable connects to the antenna. If you are using a phased array, remember all feed lines must be the correct length.

Antenna Checkout

After installation of the antenna, the final step is to make an electrical check of the system. This will require the use of a standing-wave ratio (SWR) bridge, an instrument capable of measuring the ratio of incident or forward power to reflected or reverse power. This ratio is an index to the performance of an antenna system. Ideally, you would have 100% incident power with 0% reflected power, but like all ideal conditions this is not possible. The maximum ratio allowable should be 1 to 1.5.

If measurement shows the SWR to be greater than 1 to 1.5,. something has gone wrong. The first step is to isolate the trouble to the feed line or antenna. This is done by disconnecting the feed line at the antenna and terminating the line with a 52-ohm non-inductive resistor. Remeasure the SWR. If it is still high, the trouble is in the feed line, the most likely place. Check all soldered joints at the connectors. Check the line for obvious physical damage. If this doesn't correct the problem, chances are the line has moisture in it and must be replaced. With phased arrays check each antenna in this manner.

If the line checks out, the problem is with the antenna. Check the connector for dirt or metal chips. Check to see that the vertical element was assembled properly. Follow the manufacturer's instructions.

The antenna and feed line are exposed to the elements and are subject to deterioration over a period of time. The amount of deterioration will, of course, depend upon the weather conditions in your area. High winds, ice coatings, salt, and industrial fumes all have damaging effects. The antenna and feed line should be inspected at least every 6 months.

If your area has high winds, inspection should be made after each wind storm, especially of large beam antennas.

Check the VSWR once every 6 months. If the VSWR is increasing, it's your first sign of electrical trouble.

Don't put up your antenna system and just forget it, for when something goes wrong it will do so at the most inopportune time. It's nicer to be up on the roof on a sunny day than in the middle of the night during a storm.

All this may seem like a lot of trouble just to put up an antenna, but a two-way radio system that does not work reliably both ways is next to useless. It will do nothing but cost you money in lost time. A two-way radio system should save both. R-E