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ANTENNA NOTES FOR A DUMMY

Restricted Space Antennas

by Walt Fair, Jr., W5ALT
 
 

Bent Dipoles

One way to fit a dipole antenna in a restricted space is to bend the legs. They can be bent horizontally, drooped downward, or any other imaginable configuration to fit in the space available. If the resonant length doesn't change much and the impedance remains close to that for a straight dipole, we would expect a dipole with bent legs to work OK. Remember, whatever energy is fed to the antenna will radiate if it isn't lost in resistance.

 

What is it? A bent dipole is simply a dipole antenna that has its legs bent in order to fit in the available space, instead of running in a straight line. The number of ways that a dipole can be bent is limitless, so we can't look at all the possibilities. Here we'll look at 3 fairly common cases using our 40 meter dipole.

Drop Legs. The first case is where we simply allow the legs of the dipole to drop straight down. Of course the dipole must be high enough that the legs don't touch the ground. We'll keep the top section of the dipole 30 ft above ground and drop the legs and see what effect that has on the resonant length and impedance characteristics of the antenna.

 

The above figure shows the effect of dropping the legs of the dipole straight down. Notice that the total wire length increases a little as more of the wire is dropped straight down, but for all of the cases is between 66 and 70 feet. The long dashed line shows the horizontal length needed for the antenna. At 25 feet of drop length, the antenna will fit into about 20 feet of horizontal space, a significant space reduction. As can be seen the input impedance drops also, but even the smallest horizontal and longest drop lengths it stays above 25 ohms, so matching to 50 ohm feedline would not be a problem.

If we want to build an antenna like this, the procedure would be to cut the antenna wires somewhat longer than a horizontal dipole and then trim them to resonance. A 10% increase in wire length would be a good conservative estimate of the wire needed. Continued shortening of the horizontal section shows a decreased input impedance and in the limit the antenna would look like an open 1/4 wave section of twin lead transmission line.

Partially Horizontal Inverted Vee 1. The second bent dipole we'll look at is an inverted Vee with the ends of the legs running horizontal. This would be used to accomodate an inverted Vee when there isn't enough space or height to build the normal antenna. The legs slant down as in a normal inverted Vee until they are 10 ft off the ground, then run horizontal. In this case the legs run off in the same direction, representing what would be done if the antenna is erected at the edge of a property, with the peak next to the house and the legs running away from the house.

 

As shown in the above figure, as the horizontal legs are lengthened, the height and width required to install the antenna decrease and the total wire length increases slightly. The increase in wire length is nearly the same as for the drop leg dipole we looked at before. For all cases, the total amount of wire is between 66 and 70 feet, so the same guidelines for constructing it would apply as for the previous bent dipole - cut the wires about 10% longer and then trim with the antenna in place.

Note that the impedance for this antenna is a little lower, just as for the inverted Vees we looked at in an earlier section. Until the horizontal legs get over about 20 ft long, the impedance is still above 25 ohms, so matching should be fairly easy. Note that with 20 ft horizontal legs, the antenna can be installed in a space about 20 ft by 17 ft and is only 15 feet high.

Partially Horizontal Inverted Vee 2. The third bent dipole is also an inverted Vee with the ends of the legs running horizontal, but in opposite directions. This would be used to accomodate an inverted Vee when there isn't enough space or height to build the normal antenna. The legs slant down as in a normal inverted Vee until they are 10 ft off the ground, then run horizontal. In this case the legs run off in opposite directions, representing what would be done if the antenna is erected in the middle of a backyard and run diagonally.

 

As shown in the figure, the width required to install the antenna is less than the case above, although the needed heights are similar. Since the horizontal wires are closer together, there is more interaction and the impedance drops faster. For horizontal run lengths of 15 feet or less, the impedance still stays above 25 ohms, so matching should be OK. For longer leg lengths, though, the impedance is lower and matching will be more difficult. In addition, more wire is needed to bring the antenna to resonance, but not much more. Note that for 20 foot horizontal legs, the antenna can be installed in a space that is about 20 ft by 20 ft and 18 feet high.

Summary. Based on these analyses, it appears that the dipole is pretty forgiving. You can bend the wires and decrease the space required to install the antenna quite a bit without affecting the performance too much. In general, bending the legs of the dipole will decrease the impedance slowly, so matching shouldn't be a big problem except in the extremes. In general the wire will need to be somewhat longer, but not overly so. In cases of extreme contortions, the impedance will be less, the wire needed for resonance will be longer, and it may be necessary to use a tuner or some sort of matching network.

 

Next - Shortened Dipoles