ANTENNA NOTES FOR A DUMMY
Restricted Space Antennas
Inverted Vee Dipole
Having looked at both horizontal and vertical dipoles, one way to reduce the area needed for a dipole is to droop the sides. This popular design is called the inverted Vee and is a proven performer.
What is it? An inverted Vee dipole is a dipole with both legs slanting down towards the ground in the shape of an upside down V. The obvious advantages of this design is that it takes up less horizontal space than a horizontal dipole and only requires 1 support to hold up the center of the antenna, instead of 2 supports for the ends of a normal dipole.
The obvious parameter to look at for this antenna is the height of the center or peak of the antenna. Since we are interested in limited space antennas, for modeling purposes, we will keep the ends 10 ft off the ground (so people don't stumble into to them) and use #14 AWG wire as for the previous dipole models. Notice that the center can't be any higher than ?/4 or the wires will be vertical and we no longer have a dipole. In the model, if the height of the center is 10 ft, then we have a 10 ft high horizontal dipole with all the characteristics shown in an earlier section.
Length and Impedance. Notice that in the above figure the inverted Vee resonant length and impedance varies quite a bit depending on the ground conditions. The total length can vary up to about 3 feet and the length of each leg is half the total length. The impedance also varies from 10 to 70 ohms, but except for the case of a perfect ground, the low end of the range is around 30 ohms.
One attribute often touted for an inverted Vee is that the drooping of the wire ends causes a decrease in the impedance. It is often claimed that at around a 45 to 60 degree angle a 50 ohm impedance is obtained. While it is generally true that the impedance decreases, except over a perfect ground, the actual impedance can be higher or lower, but normally not too far from 50 ohms. It appears that center heights between about 25 and 35 feet may be a good compromise to obtain a reasonable match, but in any case, the impedance is never too bad.
The effect of ground on the inverted Vee can be seen in the above left hand figure. As seen, the less perfect grounds give a gain that is in the 3 to 4 dBi range, comparable with a horizontal dipole of similar height. But as the right hand figure shows, except for the case where the antenna is nearly vertical, the maximum radiation is always straight up at 90 degress.
So what can we conclude about the inverted Vee dipole? In general it seems to behave much as a horizontal dipole at a simlar height. It's impedance and resonant length variations should make it reasonbly simple to tune for resonance and match to 50 ohm coax. The fact that it needs only 1 support is a plus in terms of installation. Often that consideration makes it the only feasible type of dipole that can be installed and it's nice to know that it is not too much of a compromise.
Next - Bent Dipoles