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ANTENNA NOTES FOR A DUMMY

Restricted Space Antennas

by Walt Fair, Jr., W5ALT
 
 

Ground vs. Radials

 

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.

Now, since there is current flowing on the radials, there will also be radiation from the radials, but we want to minimize the radiation in order to maintain the desireable properties of the vertical antenna, including low angle of radiation. One way to do that is to arrange the radials symmetrically about the base of the vertical. In the case of symmetric radials, the current in each radial is flowing in an opposite direction (away from the center) to the current on the radial directly opposite to it and the total radiation in the horizontal plane will cancel. Therefore, in that respect, the radials will have little effect on the low angle radiation.

But not all is perfect. There will also be radiation vertically from the radials and some of that will interact with the ground. Of the part that interacts with the ground, some radiation will be reflected and some ground currents will be induced, leading to ground losses. But that's not what we wanted!

So what can be done? One obvious possibility is to mount the antenna as high as possible, thereby minimizing the interaction with the ground and avoiding ground losses as much as possible. Hence the guideline "The higher the better". The other possibility is to add as many radials as possible in order to minimize the current on each radial. The current on each radial will be equal to the total current on the vertical element divided by the number of radials, so "the more the better".

Another way to look at the effect of radials in a vertical mounted above ground is that the radials are shielding the antenna from ground. In effect we are trying to create an "artificial ground" that is better than the real ground that mother nature gave us to work with. From that viewpoint we would like to have as many radials as possible, as long as possible. Again, consistent with the guidelines commonly quoted by amateurs. However, in my opinion, that viewpoint is too simplistic, since it ignores the fact that we can never completely shield the antenna from the ground in practice. No matter what, there will always be a potential difference between the radials and the ground, so there will be some interaction. It seems much better to forget about the analogy of shielding and just treat the antenna and radials as a complete antenna system that will interact with the ground to some extent. The important point is that, whether we want to think about them separately or not, the radials are part of the antenna.

Radial Angle. It has been stated many times that angling the radials downward at a 45o angle will improve an antenna. Let's see what happens when the radials are not horizontal, as in the ideal case above.


The above graph shows the impedance, the take off angle and the gain of a ground plane vertical as a function of the radial angle. In all cases, the lowest part of the radials was 10 ft above an average ground, which would represent mounting the antenna so the radials don't cause problems for people walking nearby. As can be seen, the gain doesn't vary much at all and neither does the take off angle. Certainly we probably could not detect the small differences in gain and take off angle shown. However, the impedance does vary from some 70 ohms for a vertical dipole to about 25 ohms when the radials are horizontal.

The implication of this graph is that the angle of the radials will have a minimal effect on the antenna perfomance, but it will change the feed point impedance. The minimal effect on the radiation can be understood by noting that the radials are symmetrical and their radiation in the horizontal plane cancels, as previously noted. However, somewhere around 45o the feed point impedance is very close to 50 ohms at resonance. So from an impedance matching standpoint, there is a reason to make the radials slope downward at an angle of about 45o. Changing the angle on the radials may make the antenna perform a little better, but it will also be somewhat easier to match.

Bent Radials. Since we're interested in limited space antennas, one common problem is what to do when you don't have room for the radials. After all, the radials for a 40m groundplane vertical require about 33 ft of space around the antenna.

Fortunately, the exact position of the radials isn't all that important. Just as we noted that we can bend a dipole all around and it will still work, so we can bend the radials around, too. In fact, as long as we keep the radials symmetric, there will be little effect on the antenna performance, since radiation from the radials will still cancel. Although the computed performance isn't shown here, it is even possible to arrange the radials in a spiral pattern around the base of the vertical and still maintain performance and impedance characteristics.

So, just as for the dipole, the ground plane vertical can be modified within reason and still be made to work under less than ideal circumstances.

 

Next - Short Verticals