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Restricted Space Antennas

by Walt Fair, Jr., W5ALT

Quarter Wave Groundplane Vertical

Having looked at dipole antennas, it is apparent that the antenna currents are symmetric about the center of the antenna. In other words, if you divide the antenna at the center feed point, the 2 halves of the antenna look like mirror images of each other. With that in mind, it would be interesting to see if 1/2 of the antenna could be replaced by something else. This would result in shortening the antenna by 1/2. As it turns out, it is indeed possible to do that. The resulting antenna is normally mounted on or near the ground in a vertical position. In this section we will describe the resulting 1/4 wave ground plane vertical.

What is it? Let's start with a center fed vertical dipole, which was analyzed in a previous section. One way to visualize how to shorten the antenna is to progressively modify it. First we split the wire below the feed point into 4 wires. Next, keeping the 4 strands evenly spaced, we raise the ends of the wires until they are horizontal.

As shown in the following graph, when the "radials" are at 0 degrees, the impedance is about 70 ohms, since we are really dealing with a vertical dipole. As the radials are raised to an angle of 90 degrees, the impedance drops to around 20 ohms. Notice that when the radials are at an angle of about 45 degrees, the impedance is very close to 50 ohms, which is similar to the inverted Vee.

Height. It has already been demonstrated that a vertical dipole is somewhat immune to the effects of a ground on its impedance. It might be expected that the ground plane vertical would also be immune to the efects of ground.

The effects of a real ground on the resonant length and impedance is shown in the following figures. Even thugh it doesn't look like it, there really are 3 curves on the resonant length graph! As can be seen, neither the resonant length nor the impedance vary dramatically with ground conditions, but the impedance will depend upon the height above ground and range from around 20 to 40 ohms.

It was also shown, however, that the vertical dipole gain depends on the ground conditions. By analogy it would be expected that the ground plane vertical would also be affected by the same factors. As the following graphs show, that conjecture would be true. A comparison with the similar graphs presented for the vertical dipole shows that the shape and tendencies are indeed the same. The gain for the ground plane is very nearly constant and equal to an isotropic antenna at low heights and real ground conditions. Meanwhie the take-off angle appears to be in the range of 15 to 20 degrees for most cases at low heights. Note that the better ground conditions give a lower angle.

Conclusions. So what can we conclude about the ground plane vertical? In general it seems to behave much as a vertical dipole at a similar height. It's impedance and resonant length variations should make it reasonably simple to tune for resonance and the impedance should present a reasonable match to 50 ohms, even though it is a little low. In addition, since it can be mounted very close to the ground and provide a reasobably low take-off angle, it should indeed be a reasonable antenna for practical use.

Of course, hams have known that for a long, long time. But it is nice to know that the theory agrees with practice.