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

Proper decoupling: the key to good vertical antenna design
³]­p¤@¤ä¦n««ª½¤Ñ½uªºÃöÁä: ¾A·íªº°hÃÂ

FIGURE 1 Electric field in vicinity of an isolated center-driven dipole antenna

The need for decoupling a vertical antenna from its coaxial feedline can be appreciated by referring to Figure 1. Antennas which are driven by 2-conductor transmission lines always have two terminals, insulated from each other, between which the high frequency AC voltage generated by the transmitter is impressed. Figure 1 shows a typical center driven dipole antenna, with terminals a-b, across which the AC voltage generated by the transmitter is connected. During each cycle of the AC voltage, current flows out on one leg of the dipole and in on the other, leaving positive charge on one extremity and negative on the other. The current then reverses, and the polarity of the charges on the respective extremities is reversed. An electric field is created in the vicinity of the dipole between the separated charges, as shown in Figure 1. The electric field bulges out from the terminals as the charge spreads out toward the extremities. The following charge constitutes the current on the antenna, and associated with the current is a magnetic field, encircling the wires of the antenna. The AC current is zero at the extremities of the dipole and maximum at about a quarter wavelength in from each end, in what is called a "standing wave" current distribution. The separation of the oscillating charges on the conducting members of the antenna produces the mysterious phenomenon of electromagnetic radiation. If the total length of the vertical center driven dipole of Figure 1 is one-half wavelength, the resulting radiation pattern is well suited for many communication needs. Maximum radiation intensity is directed toward the horizon, and the pattern is omnidirectional in azimuth.

FIGURE 2 Electric field in vicinity of a whip antenna connected to a coaxial line

The problem remains of connecting the feedline to the antenna terminals. We might try to bring the coaxial line up from below, connecting the center conductor to the lower extremity of the antenna. What will we do with the outer conductor? Suppose that we just let it terminate at the base of the antenna as shown in Figure 2. The AC voltage from the transmitter is piped up to the antenna within the coaxial cable, and appears between terminal a, the base of the antenna, and terminal b, the lip of the outer conductor.

The resulting electric field is shown in Figure 2. On each half-cycle, field lines originate on positive charge on the antenna and terminate on negative charge on the outside of the coaxial line. Then, the polarity reverses as the antenna becomes negatively charged, and the upper part of the outer conductor positive. An electromagnetic field is guided down the outside of the outer conductor as shown in Figure 2. The outside of the outer conductor is hot with radio frequency currents all the way down to the transmitter. The actual antenna really consists of a dipole, one side of which is the vertical leg, the other leg being the outside of the outer conductor, the metal chassis of the transmitter and other associated wiring. The radiation pattern of the system is generally unpredictable, and generally bad! The length of the upper leg of the dipole might be 1/4 wavelength, 1/2 wavelength, or something else. In every case, the electromagnetic field will "spill" out from the end of the coaxial cable, exciting current and charge on both the intended "antenna" and the outside of the coaxial line.

This phenomenon can be readily verified by detecting the presence of current on both the antenna wire and the outside of the coaxial line, using the simple pick-up loop described at the end of this booklet. Simply attach a 1/4 wave, 1/2 wave or 5/8 wave whip antenna to the end of a length of coaxial line of arbitrary length (6 or 8 feet would be fine) and connect the other end to a VHF transmitter. Ten watts of power is quite sufficient for test. Support the antenna and coaxial cable above the ground, and hold the pick-up loop close to the antenna. Maximum coupling occurs when the plane of the loop lies in any plane containing the antenna wire (or coaxial line). This current is not leaking through the coaxial line. It emerges from the feed of the coaxial line and flows back over the outside of the outer conductor due to the lack of any form of decoupling.

FIGURE 3 Impedance Matching network at base of whip

Some people might say, "Ah. but you don't have any impedance matching between the antenna and the coaxial line!" Impedance matching has nothing to do with the currents spilling out over the coaxial line, as the following example will show.

Figure 3 shows an impedance matching network of a design used on certain end-driven vertical antennas. The current Ia at the base of the antenna flows into the upper terminal of a tapped inductor. Current IL flows from the tap into the center conductor of the coaxial line. A current IL of equal magnitude, but opposite direction, flows on the linear surface of the outer conductor. Current Ib flows in the bottom section of the inductor, below the tap. Finally, a spill-over current, Is, flows on the outer surface of the outer conductor. Kirchoff’s current law requires that: Ia = IL+ Ib and Is = Ib + IL. We can solve for Ib in the first equation and substitute it in the second to obtain Is = Ia. The spill-over current must be the same as the antenna current, independent of whether the antenna is matched to the coaxial line or not!

Antennas with no decoupling
Unfortunately, from the standpoint of the end user, there are many coaxially driven VHF vertical antennas in use today which totally lack any form of decoupling. These antennas fall into two classes: automotive whips mounted on other than vehicles with metal roofs, and antennas, sold for fixed, base station use.

Automotive whips are frequently used by amateur radio operators as temporary base station antennas, connected to the transmitter through a length of coaxial line. While the radiation pattern will be horrible and much of the transmitter power wasted, the amateur operator will usually still be able to communicate with at least some other stations. The situation is more serious when automotive whips are mounted on the mastheads of sailboats for use in the marine VHF band. The spill-over current will flow down the stays and shrouds, as well as the outside of the coaxial line and the mast itself, if the latter is of metallic construction. All of these conductors are long in the wavelengths and will radiate with many narrow lobes in the vertical plane, which rock with the motion of the boat, causing fading, so often experienced in marine communications. The advantage of masthead height to achieve greater VHF range will be lost.

The above examples represent misuse of the product. Whip antennas, which give excellent performance when mounted on the horizontal metallic surfaces of automobiles, are simply not intended to be thrust into the air at the end of a length of cable.

A more serious situation exists in the case of non-decoupled antennas which are sold for base station and other non-automotive uses. End driven vertical half-wave dipoles with absolutely no decoupling are on the market specifically for marine masthead use and a number of base station antennas with absolutely no decoupling are offered to the amateur radio market. Advertising claims made by manufacturers of some of these antennas simply cannot be substantiated.

Figure 4 shows measured radiation patterns on three antennas, two with absolutely no decoupling and a third with excellent decoupling.

All three antennas were measured with an attached 8 1/2 ft. length of coaxial line in a straight line from the base of the antenna.

Antenna 1 is a half-wave whip with a tuner at the base, advertised for small boat masts. Note the butterfly pattern with major lobes of radiation at high and low angles and a loss of signal toward the horizon. (See Figure 4)

Antenna 2 is a 5/8 wavelength whip and its pattern is even more horrible than the first,

FIGURE 4 Radiation patterns of 3 vertical antennas

with stronger lobes at high and low angles. (See Figure 4)

Antenna 3 is a base driven, half-wave whip with a quarter wavelength decoupling sleeve. Note that the main lobe of the pattern is directed toward the horizon. This pattern is virtually unaffected by the length of cable leading up to the antenna. The slight asymmetry in the shape of the lobe is a result of the small current spilling over onto the outside of the decoupling sleeve, which is actually in phase with the current on the antenna and produces a small, omni-directional gain over an ideal half wave whip. (See Figure 4)

FIGURE 5 Fields around a sleeve dipole

How to properly decouple the antenna
The first step in achieving good decoupling of a vertical whip is to bring the coaxial line up through a quarter wave sleeve, as shown in Figure 5. This antenna, called a "sleeve dipole" has been widely used for many years. The sleeve is connected to the outer conductor of the coaxial line at the feedpoint, and extends down around the coax for a quarter wavelength. The center conductor extends above the feedpoint for a quarter wavelength. The antenna behaves like a vertical, center-driven half-wave dipole. The bottom of the sleeve is a point of high charge concentration, yielding considerable electric field coupling to the outer conductor of the coax line, as shown in Figure 5. A certain amount of "spill-over" current will flow down the outside of the outer conductor. While this effect is minimized by employing a large ratio between the diameters of the sleeve and of the coax line, much more complete decoupling is achieved by installing a second quarter wavelength decoupling sleeve below the first, as shown in Figure 6. The radiation patterns of these antennas are substantially independent of the length of the coaxial line, or of any metallic mast on top of which the antenna may be mounted. As a matter of fact, radiation from the small current existing on the lower decoupling sleeve combines fortuitously with the radiation from the vertical dipole to produce a broad lobe in the vertical plane, centered on the horizon.

Another effective form of decoupling is shown in Figure 7. In this design, the radiating element consists of a half-wave whip base-driven through a quarter-wave decoupling sleeve. Since the current along the whip is distributed in the form of a standing wave with maximum amplitude in the center, the base of the whip is a point of low current and high voltage. As explained in the previous section, the magnitude of the current spilling over the outside of the decoupling sleeve must be equal to that of the current flowing into the base of the whip.

Since this current is small, the spill-over current is also small. The spill-over current is also distributed as a standing wave, with maximum amplitude at the top, where the sleeve connects to the outer conductor of the coaxial line, and minimum amplitude at the bottom or open end of the sleeve. The current distribution on the entire antenna is sketched in Figure 7. The current coupled onto the outside of the coaxial line below the sleeve is extremely small, as can be experimentally verified with the RF detector described at the end of this booklet. Since the input impedance of this antenna is high, a tuner is necessary to transform the impedance to 50 ohms. An L-C network for matching purposes can be built-in at the feedpoint.

BACK TO ARTICLE HOMEPAGE ANTENNA ANALYST SERIES CABLEMATE ANALYST ISOPOLE HOT RODS PRICING

E-Mail
aea@aea-wireless.com

AEA
A Division of Tempo Research Corporation
1221 Liberty Way
Vista, CA 92083
Phone ( 760 ) 598-9677 * FAX ( 760 ) 598-4898