## Friday, September 30, 2011

The need for radials with a ground mounted vertical has invoked lots of discussion among amateurs over the years. The literature contains many references to how many radials are needed, how long they should be and what affect they will have on the performance of a vertical antenna. And yet lots of confusion still exists. In this section we will take a look at ground mounted and above ground mounted vertical antennas, especially with respect to the radials and try to make some sense out of the subject.
Ground Mounted Vertical. First, let's look at a ground mounted vertical antenna. As shown in the sketch, it consists of a vertical radiator that is mounted directly on the ground and fed at the base. As should be apparent, in the case of a perfect ground, the potential (voltage) with respect to ground is precisely zero on the side of the feed point attached to ground. That means that the entire voltage of the source is applied to the vertical radiator. This is different than a dipole, where the voltage swing is applied to both sides of a dipole.
In a dipole, the voltage with respect to ground is equal and opposite on both sides of the feed point. In a ground mounted vertical with a perfect ground, the voltage on the ground side of the feed point is always zero with respect to ground. This is inherently an unbalanced antenna and there's not much that can be done to change that. It will also have a take off angle of zero degrees and an impedance of 36 ohms at resonance.
Note that a perfect ground has zero resistance and reactance. Therefore there can be no voltage differences, no matter how much current is flowing in the ground, and therefore no losses. So far so good.
But what happens in the "real world"? In reality, there is no such thing as a perfect ground with zero resistance and reactance. Real ground conditions do indeed induce losses and there are voltage gradients caused by ground currents around an antenna. So what can be done?
One approach is to make the ground as close to perfect as we can. That means putting a metal plate or mesh or a large number of radials at the surface of the ground to decrease the ground resistance and impedance. Obviously, the more metal we can put down, the better it will approach a perfect ground and the more efficient the antenna will perform. That's why we often hear the guidelines that "the more radials, the better." An alternative is to mount the antenna over salt water, which has a very low resistivity and makes an excellent ground. We are simply trying to turn our real ground into something as close to a perfect ground as possible.
Above Ground Verticals. In a vertical antenna mounted above ground, the situation is a little different. As shown in the figure, the antenna is usually fed at the base of the vertical element, however, the radials are not directly connected to the ground and there is nothing to keep them at ground potential. In this situation, the radials will have current flowing on them and at the feed point the current on the vertical element will be balanced by the current flowing on all of the radials. This is still not a balanced antenna, though, since the currents are not symmetrical around the feed point. In fact they flow vertically on the vertical element and horizontally (or at some other angle) on the radials.