The majority of installations end up with a beam of sorts on a 50 foot tower with dipoles installed as inverted vees hanging below the beam. The inverted vees will radiate and receive signals, unfortunately they will not perform as well as a properly installed dipole.
A standard dipole has a 75 ohm balanced feedpoint, and about 2.7 db gain over an isotropic radiator and is one of the less expensive antenna choices. Those are some nice features but there are conditions which must be met in the installation to take advantage of these features. Proper installation is not always inexpensive.
The dipole must be installed as a horizontal wire at an even height of at least one half wavelength. If it is installed at greater height, it must be in multiples of half wavelengths.
Such an installation will result in a 75 ohm feedpoint. It will also get us most of the 2.7 db gain as the ground reflects radiated energy skyward to combine with the energy being radiated upward by the antenna. How much gain we get depends on the quality of the ground and its ability to reflect RF energy. The half wavelength spacing of the antenna above ground will ensure that the reflected energy will be in phase with other energy radiated by the antenna so that there will be an addative affect rather than a cancellation.
Half wavelenght at 40 meter is 65 feet. Half wavelength at 80 meters is 130 feet. Half wavelength at 160 meters is 260 feet. Standard dipole installations cannot be made for these three bands if the suport structure is a 50 foot tower.
One of the more common compromises is to install the lower band dipoles as inverted vees using the peak tower height of 50 feet to support the apex of the inverted vee. This works fairly well, probably because it raises the high current feedpoint as high as possible. The result is an antenna that is more or less omnidirectional and pretty much vertically polarized. Sort of a high angle vertical radiator without the need for ground radials.
A sometimes overlooked solution is the delta loop and right angle loop. This is a full wavelength of wire at the lowest frequency of interest, typically mounted apex up with base around 15 feet off the ground. It can be fed with open wire line and makes a good all band antenna if used with a good antenna tuner. If it is fed at the center of the base, it becomes a horizontally polarized, cloud burner. Sort of like the inverted-vee but with lots more gain (up to 6db depending on frequency).
It can be fed at a point about 25 percent up from one corner to transform the loop onto a very effective vertically polarized radiator. Low angle radiation, no need for radials, and a 50 ohm match to coax for a single band SWR of less than 1.5 to 1.
If we go back and study our beam atop the 50 foot tower we might notice that it has a good sized boom. My KLM-34 has a three inch diameter 20 foot long boom and all the elements are insulated from the boom. Normally the boom merely supports the elements of the beam, but there is no reason the boom could not be used as an antenna. Perhaps a rotatable dipole. Twenty feet is a little short for 40 meters but it is in the clear and the boom is a good bit larger in diameter than any of the elements of the beam. Large elements can be shorter and still reach resonance. Add ten feet on either end of the boom, use capacitive and linear loading to get to resonance and you could have a decent rotatable dipole for 40 meters. Why stop there? A little more effort could turn the boom into a 40/30 meter rotatable dipole nicely elevated and in the clear. The boom can remain grounded in the center. Feed it with a gamma match.
More on this later when I get a chance to try it. If it works well I will include it as a project with enough data to duplicate the effort.
I recently found this....http://lists.contesting.com/_towertalk/199710/0676.html
It presents a way of using a yagi boom directly, without modifications, by relying on the reflector and director of the beam to load the ends of the boom. Then an omega match is used to match to coax on the desired frequency. In the case of an antenna like the KLM-34 where all elements are isulated from the boom, the two elements closest the ends of the boom may need to be electrically tied to the boom to make this work.
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Last updated Sep-2006
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