- Note: Some material here is by Author: R.J.Edwards G4FGQ © 6th January 2003
than its counterpart on 40m. A half-wave dipole may also be used on odd-multiple harmonic frequencies without impedance mismatch. For example, a 40m dipole cut for the lower end of 40m will usually work on 15m, where it becomes a 3/2-wavelength dipole antenna. Dipoles (and verticals) may also be designed for operation on higher frequency bands through the use of "band traps" (or just "traps"), resonant coil/capacitor devices that act as automatic switches, isolating the end-most sections of the antenna above their design frequency band. An interesting variant form of the dipole is the inverted-V, which is simply a half-wave dipole with the center feedpoint forming the apex of an upside-down V (see figure). This is a great way to mount a dipole if there are no convenient end-supports available. Ideally, the legs at the feed-point apex should form a right-angle, but this isn't critical.
Also having a feedpoint impedance of close to 50-ohms, the inverted-V requires less horizontal space and is easier to install than a horizontal dipole, but provides essentially the same performance as a horizontal version at the same apex height. In the picture below is the author's 30m/40m inverted-V made from 12-gauge stranded/insulated wire.
The 30m antenna hangs below the 40, suspended by 4" spacers made from readily available 3/8-inch polybutylene water supply-line tubing, cut & drilled at each end to allow threading the antenna wire through. The coax feedline attaches to a 1:1 balun, which also supports the antenna at the apex (a better angle view of the feedpoint may be seen below in the picture of the Quad). Note: A great all-band wire antenna is a 1/2 wave 80m inverted-V with apex at a height of at least 40 feet, fed with ladder-line through a tuner. In the early 1990s, I used one with 100w to work DXCC on 12m, 17m, 40m, and 80m (as well as on 30m before the award was offered for that band). Several variants of the dipole, usually touted as "multiband" antennas, are the G5RV and Windom. Each has its merits, but be aware that an impedance matching tuner is required for effective multiband performance. - Vertical antennas: The most commonly used vertical antenna is the 1/4-wave vertical with ground radials. It can be very effective low-angle radiator even at ground level, it is particularly appealing when residential restrictions are an issue, and can be readily concealed in flagpoles, trees, alongside chimneys, etc.
A 1/4-wave vertical antenna is theoretically equivalent to a vertically-mounted 1/2-wave dipole antenna with the lower end buried below ground. The earth then acts as the "missing" half in which the antenna currents flow, mostly at the surface. Theoretically, a metal stake would provide a sufficient ground connection for operation; however, ground conductivity is seldom good enough. One must provide a bed of conductive media, or radial system, at the base of the vertical in order to minimize ground losses and for it to perform with low-angle radiation as expected. If it is possible to elevate the antenna and radial system at least 1/4-wavelength above ground, then performance will be enhanced, but the radials must be a resonant 1/4-wave in length. However, ground-mounted vertical antennas will perform very well with radials on, or buried just below the surface. Ground-level radial wires may be of any length (typically 1/8 - 1/4-wavelengths are used), but should be as symmetrically distributed and as plentiful as possible. By using a radial system to act as a faux-Earth, verticals may be mounted at any height above ground. The main difference in elevated-vs.-ground-level mounting is that the radials then need to be 1/4-wave resonant lengths. When mounted above ground with elevated radials, a 1/4-wave vertical is known as a "ground-plane" antenna. For those who prefer multiband antennas, there are commercially available multiband verticals. Just remember that low angle radiation from a 1/4-wave vertical antenna is contingent upon an effective radial system. An interesting vertical antenna is the 1/2-wave vertical dipole. Since this is a true dipole, no radials are needed and the radiation angle is essentially that of the 1/4 vertical with radials. This is the basis for some of the commercial multi-band "no-radial" type verticals, which are good compromise antennas that offer the smallest footprint of any antenna and the possibility of low-angle radiation. Antennas for 80m and 160m are a problem for most of us, as they usually require more space than we have available, especially for antennas that have low radiation angles, much less gain. For DXers, it is an acute problem because we not only want to be able to work stations, we want to be able to work DX! Well, there are solutions. Home-brew or commercially available compact vertical radiators for 80/160 can provide good results, especially when used with a good bed of radials. If one has a tower for a beam antenna on the higher bands, it may be possible to use the tower itself as a vertical radiator with the beam providing a "top-hat" capacitance loading that can improve efficiency. Reference Websites
|
)
)
)
)
|
|