There are several ways to reduce the length of a dipole and still use it as an effective antenna. Remember it is the electrical length that determines resonance. The physical length can be considerably less than a half wave length on your desired frequency as determined by 468/f mHz.
A shortened antenna will always be less efficient and cover a smaller bandwidth than a full sized antenna, but that does not mean the short antenna will not work.
Loading coils my not be the most efficient solution but they can be very effective. For instance, consider a rotatable dipole made from a couple of center loaded, quarter wave mobile whips mounted horizontally, base to base, on a section of 2×2 lumber. A pipe flange can be used to attach the assembly to pipe which can be clamped into a small antenna rotator. The whips can be fed at their bases as a balanced system using a 4:1 balun. The balun will have to be connected in a step-down configuration to match the approximate, 12 ohm impedance of the antenna to a 50 ohm coaxial feedline. The inherently low efficiency of such an antenna will be offset by the possible increased height and rotatability.
The only problem with loading coils is reduced antenna efficiency. Not only does the radiation effiency drop off due to smaller size, but there are also resistive losses in the coil itself.
Traps allow operating one dipole on more than one frequency. As an example consider an 80/40 meter trap dipole. On 40 meters it works like a normal 40 meter dipole. On 80 meters the inductance in the traps allows each antenna leg to be only 50 feet overall length to be electrically resonant on 80 meters.
At the lower frequency the trap acts as a loading coil and efficiency drops off as a result. Some folk don’t like traps because of the lower efficiency. Some trap style wire antennas use traps that don’t hold up well to the legal power limit. These are all valid points, but not reasons to avoid using traps. Keep in mind that a large number of tri-band beams have used traps for years without problems.
The classic dipole is current fed. The center feedpoint is at a maximum current node. This current node is responsible for most of the radiation. A dipole will maintain a reasonable radiation pattern even if its ends are allowed to hang vertically. In the case of a 75 meter dipole, not much efficiency will be compromised if only 40 feet of each leg is run horizontally with the remaining 20 feet of each leg allowed to hang down vertically. The ends of the dipole are at high voltage nodes. Take care to insulate the ends properly and ensure they are still high enough to prevent guests and pets from coming in contact with those high voltage points.
It is important to remember that the classic dipole is a balanced antenna. If you expect a predictable radiation pattern from a balanced antenna, you need to install it in a balanced configuration. If you let one end droop, make sure the other end droops the same way.
Linear loading can be considered an extreme form of creative routing. Linear loading is where you take a full sized antenna and fold the elements into a smaller physical space maintaining four to six inches seperation between the folded elements. There is interaction between the folded elements and the main radiator. The interaction has an effect on feed point impedance. Normally it lowers the feed point impedance. The lowering effect can be overcome (somewhat) by adding capacitive loading in the form of ‘top-hats’ to both ends of the dipole.
Linear loading is more efficient than using traps or loading coils while still reducing the physical size of the antenna. Linear loading also has the least effect on radiation efficiency. Linear loading is used on the GAP vertical and the KLM KT-34 beam. Both of these antennas have a good reputation as effective radiators.
This configuration of the dipole is a favorite because it only requires one central support. It also requires less horizontal space from end to end. The included angle at the top of the antenna between the two legs should be at least 90 degrees. At 90 degrees the legs of the antenna can be approximately 1.4 times physically longer than the horizontal distance between the ends. This would allow a 130 foot, 75 meter antenna to fit into a horizontal distance of about 95 feet. In this configuration the dipole becomes a vertically polarized antenna. At least the vertical components tend to add while the horizontal components tend to subtract.
I recently modeled a 75 meter inverted vee using EZNEC. The results were dissapointing. The radiation launch angle was straight up. Thats fine for use up to a few hundred miles out, but not good for more distant contacts. The inverted vee has been retired. Replaced with a shunt feed on the tower. The 55 foot tower now acts like a quarter wave vertical on 75 meters and performs better than the old inverted vee.
Well, I thought the shunt feed tower was better than the inverted vee, but I was mistaken. See the followup paragraph at the end of the shut feed tower article.
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