All specifications are subject to change without notice or obligation. All rights reserved. Part No. 5022-3001-A

FIGURE 6 Sleeve dipole with additional decoupling sleeve	FIGURE 7 End-driven half-wave dipole with quarter wave decoupling sleeve

Achieving omnidirectional gain
The most practical way to obtain omnidirectional gain in a vertical antenna system is to stack a number of separate antennas to form a vertically oriented colinear array. It is relatively easy to obtain 3 dB of onmidirectional gain with respect to a vertical half-wave dipole. Higher gains require much more complex structures, multiple feedpoints, power dividers, phasing sections, impedance matching networks, etc. While 3 dB of omnidirectional gain can be obtained in antennas costing little more than simple decoupled half-wave whips, higher gain antennas are generally much more expensive.

One of the most popular ways to obtain 3 dB of omnidirectional gain over a vertical half-wave dipole is to employ a center driven dipole whose total length is 1 1/4 wavelengths, that is, 5/8 wavelength on each leg. The properties of such a dipole have been known for many years. When oriented vertically and well decoupled from the coaxial feed cable, this antenna produces almost exactly 3 dB of omnidirectional gain with respect to a half-wave dipole, yet requires but one feed point. The 1 1/4 wavelength (or "twin 5/8") design produces the maximum possible gain for a single dipole. If the antenna is either shorter or longer, the gain drops off.

FIGURE 8 Center-driven twin-5/8 dipole with twin decoupling sleeves

Figure 8 shows how this dipole can be employed in a vertical orientation. Notice that the coaxial line is brought up to the center of the dipole, where the center conductor excites the upper 5/8 wavelength element. The lower 5/8 element consists of the outside of the outer conductor, down for a distance of 3/8 wavelength from the feedpoint, and then the outside of a 1/4 wavelength decoupling sleeve, The "spill-over" current, which must be equal to the current at the base of the upper element, constitutes the current entering the lower element of the dipole. The lower 5/8 element terminates at the open end of a resonant quarter-wavelength sleeve. Tests have shown the presence of a small amount of spill-over current below this sleeve. Accordingly, a second resonant sleeve is necessary to produce a high degree of decoupling. The current distribution along the antenna is sketched in Figure 8. Note the standing wave distribution, and the fact that the upper and lower current "loops" are in phase with each other. This is the desired action which produces the gain. Note also that there is a phase reversal of the currents near the feed point. The effect produces small "side lobes" at high and low angles - a characteristic of most gain-type antenna arrays.

The decoupling sleeves are filled with Styrofoam cores with a central hole to accommodate the coaxial line. An L-C network is used at the feed-point to match the antenna impedance to 50 ohms. The entire antenna is fitted snugly inside a stiff, tapered fiberglass tube, and weather sealed.

Gain
The stated gain of an antenna must use a reference to have any meaning. Commercial antenna manufacturers use a free space half-wave dipole as the gain reference and the gain will be shown with the subscript "d" (for dipole). So the number 3 dBd indicates 3 dB more power in the direction of maximum radiation than a dipole.

Another common reference is gain over the mythical isotropic radiator. This is a non-existing, but mathematically convenient antenna that radiates with the same power intensity in all directions. The gain of a half-wave dipole is 2.15 dBi and our 3 dBd antenna is 5.15 dBi. In order to make the gain numbers larger, some manufacturers use gain numbers that could theoretically be achieved if the antenna were placed over a perfect, infinitely large ground plane so that all the power delivered to the antenna would be above the ground plane. This theoretically would double the power above the ground plane and add another 3 dB. Our dipole could have an "advertised gain" of 5.15 dB and our 3 dBd antenna could be advertised to have a gain of 8.15 dB. Note that there is no reference in these numbers. The prospective antenna purchaser should insist on the gain reference.

Vertical base station antennas should have their direction of maximum radiation on the horizon. Some commercial antennas intended for amateur use cannot control their pattern and may be several dB down on the horizon. Gain is, unfortunately, measured in the direction of maximum radiation; not on the horizon. Two antennas with 3 dBd of gain may be miles apart in performance due to their differing angles of maximum radiation.

An antenna whose maximum radiation is at zero degrees elevation (like the IsoPole) is an important consideration.