There was only one change to this section. The wording of question G9B02 was changed so that it makes a little more sense.
There are many different types of antennas, including:
- random-wire antennas,
- dipole antennas, and
- vertical antennas, including ground plane antennas.
As the name implies, random-wire antennas are a random-length. To match the antenna to the transmitter, you’ll need an antenna tuner, which is normally located in the shack. Because of this, there may be high RF levels in the shack when you are transmitting. One disadvantage, therefore, of a directly fed random-wire antenna is that you may experience RF burns when touching metal objects in your station. (G9B01)
A vertical antenna is a quarter-wavelength long and operates against ground or a set of radials. The approximate length for a 1/4-wave vertical antenna cut for 28.5 MHz is 8 feet. (G9B12) The radial wires of a ground-mounted vertical antenna system should be placed on the surface or buried a few inches below the ground. (G9B06) This is to reduce ground losses
When mounted above ground and used with radials, the vertical antenna is called a ground plane antenna. The natural feed point of a quarter-wave vertical is 35 ohms, but the feed-point impedance of a ground-plane antenna increases when its radials are changed from horizontal to downward-sloping. (G9B03). A common way, therefore, to adjust the feed point impedance of a quarter wave ground-plane antenna to be approximately 50 ohms is to slope the radials downward. (G9B02)
The half-wavelength dipole antenna is perhaps the most common amateur radio antenna because it is simple to build and operate. Its feedpoint impedance is approximately 72 ohms, making it a good match for 75-ohm coax and a good match for 50-ohm coax.
When building a dipole antenna, make sure that the feedpoint is at the center of the antenna. The feed-point impedance of a 1/2 wave dipole steadily increases as the feed-point location is moved from the center toward the ends. (G9B08)
Dipole antennas are usually mounted horizontally. An advantage of a horizontally polarized HF antenna, as compared to a vertically polarized antenna, is lower ground reflection losses. (G9B09) The approximate length for a 1/2-wave dipole antenna cut for 3.550 MHz is 131 feet. (G9B11) 32 feet is the approximate length for a 1/2-wave dipole antenna cut for 14.250 MHz. (G9B10)
Ideally, a dipole antenna should be mounted a half-wavelength up off the ground. The low angle azimuthal radiation pattern of an ideal half-wavelength dipole antenna installed 1/2 wavelength high and parallel to the Earth is a figure-eight at right angles to the antenna. (G9B04) If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional. (G9B05)
Antenna height also affects the feed point impedance. As the antenna is lowered from 1/4 wave above ground, the feed-point impedance of a 1/2 wave dipole antenna steadily decreases. (G9B07)
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