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ANTENNA NOTES FOR A DUMMY

Restricted Space Antennas

by Walt Fair, Jr., W5ALT
 

Vertical Dipoles

Now that we understand how a horizontal dipole performs, we can look at the same antenna mounted vertically. Recently there has been quite a bit of interest in vertical dipoles and in this section we'll see why.

What is it? A vertical dipole antenna is simply a dipole antenna that is mounted vertically instead of horizontally. Because of the orientation, it will have some characteristics different than a horizontally mounted dipole. The length will still be approximately 1/2 wavelength, so the same formula can be used to estimate the size.

Notice that without a ground reference, there is nothing to determine whether an antenna is mounted horizontally, vertically, or at some other orientation. Therefore, the free-space performance of a vertical dipole is the same as for a horizontal dipole. Without the earth for a reference, we have no way of telling them apart. That means that the comments on wire size from the previous section apply here and need not be repeated. The following evaluations were done using a vertical dipole made from #14 AWG copper wire. The height indicates the height from ground level to the bottom of the antenna. The feedpoint is halfway up the antenna.

 


Length and Impedance. Notice that in the above figure the vertical dipole varies much less in both resonant length and impedance than the horizontal dipole. In addition, the variation does not depend very much on the quality of the ground. That is very different than its horizontal counterpart. Note that at low heights, which is usually the rule since this antenna is some 66 feet tall by itself, the impedance is somewhat higher than the horizontal dipole, varying between 70 and 100 ohms.

It seems that by mounting the dipole vertically, we have reduced its sensitivity to ground effects and that could be important. However, not all is perfect.

 


First the good news. As the above figure on the right shows, the take off angle is generally low. In fact, with a perfect ground it's zero degrees, so this antenna might be excellent for working DX. Over an average ground, putting it up too high even raises the take off angle, so we don't want it more than about 1/4 wavelength up unless the ground is very poor.

Then comes the bad news. Many advertise that the antenna is not affected by the ground and the figures we have looked at show that to be true for the most part. But the above left hand figure shows the effect on gain - that's a different story. As can be seen, the difference in gain between the antenna over a perfect ground and over a real ground is large. At low heights the antenna radiates about like an isotropic radiator at low radiation angles, with about a 7 dB loss from the perfect case. Investigating why clearly shows that the difference is losses to the ground.

So, after looking at these performance indicators, it seems aparent that mounting the dipole vertically reduces the sensitivity to the ground conditions, but the ground still has a profound affect on the performance. However, comparing it to a horizontal dipole, the radiation at low angles should still be better. The figures indicate that this type of antenna should be easy to adjust for resonance, no matter where it is mounted.

Next - The Inverted V