The distance between the centers of the conductors and the radius of the conductors are factors that help determine the characteristic impedance of a parallel conductor antenna feedline. The characteristic impedance of flat ribbon TV type twin lead is 300 ohms.
The typical characteristic impedances of coaxial cables used for antenna feedlines at amateur stations are 50 and 75 ohms. The attenuation of coaxial cable increases as the frequency of the signal it is carrying increases.
A common reason for the occurrence of reflected power at the point where a feedline connects to an antenna is a difference between feedline impedance and antenna feed point impedance. The antenna feed point impedance must be matched to the characteristic impedance of the feedline to prevent standing waves on an antenna feedline.
A reason for using an inductively coupled matching network between the transmitter and parallel conductor feed line feeding an antenna is to match the unbalanced transmitter output to the balanced parallel conductor feedline.
RF feed line losses are usually expressed in dB per 100 ft.
The standing-wave-ratio that will result from the connection of a feed line impedance given in the first column to a load indicated in the second is given in the following table [SWR is biggest impedance ÷ smallest impedance : 1]:
Feed Line | Load | SWR |
---|---|---|
50-ohm coax | Non-reactive 10-ohm | 5:1 |
50-ohm coax | Non-reactive 50-ohm | 1:1 |
50-ohm coax | Non-reactive 200-ohm | 4:1 |
50-ohm coax | Vertical antenna with 25-ohm feed-point | 2:1 |
50-ohm coax | Vertical antenna with 50-ohm feed-point | 1:1 |
50-ohm coax | Folded dipole antenna with 300-ohm feed-point | 6:1 |
If the SWR on an antenna feedline is 5 to 1, and a matching network at the transmitter end of the feedline is adjusted to 1 to 1 SWR, the resulting SWR on the feedline will be 5 to 1 [The 1 to 1 matching network has no effect].
One disadvantage of a directly fed random-wire antenna is that you may experience RF burns when touching metal objects in your station.
The feed-point impedance of a ground-plane antenna increases when its radials are changed from horizontal to downward-sloping. The radial wires of a ground-mounted vertical antenna system should be placed on the surface or buried a few inches below the ground. An advantage of downward sloping radials on a ground-plane antenna is that they can be adjusted to bring the feed-point impedance closer to 50 ohms.
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. If the antenna is less than 1/2 wavelength high, the horizontal (azimuthal) radiation pattern of a horizontal dipole HF antenna is almost omnidirectional. The feed-point impedance of a 1/2 wave dipole antenna steadily decreases as the antenna is lowered from 1/4 wave above ground. 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.
An advantage of a horizontally polarized as compared to vertically polarized HF antenna is that it has lower ground reflection losses.
The approximate length for a 1/2-wave dipole antenna cut for 14.250 MHz is 32.8 feet [468 ÷ 14.255 = 32.84].
The approximate length for a 1/2-wave dipole antenna cut for 3.550 MHz is 131.8 feet [468 ÷ 3.55 = 131.83].
The approximate length for a 1/4-wave vertical antenna cut for 28.5 MHz is 8.2 feet [234 ÷ 28.5 = 8.21].
A Yagi antenna consists of a driven element and some combination of parasitically excited reflector and/or director elements. A reason why a Yagi antenna is often used for radio communications on the 20 meter band is that it helps reduce interference from other stations to the side or behind the antenna. In reference to a Yagi antenna, the "front-to-back ratio" the ratio of the power radiated in the major radiation lobe compared to the power radiated in exactly the opposite direction
The direction of maximum radiated field strength from the antenna
is called the "main lobe" of a directive antenna.
The approximate maximum theoretical forward gain of a 3 element Yagi antenna is 9.7 dBi.
The purpose of a "gamma match" used with Yagi antennas is to match the relatively low feed-point impedance to 50 ohms.
The SWR bandwidth of a Yagi antenna is increased by using larger diameter elements. The approximate length of the driven element of a Yagi antenna is 1/2 wavelength. The director is normally the shortest parasitic element of a three-element single-band Yagi antenna. The reflector is normally the longest parasitic element of a Yagi antenna. One effect of increasing the boom length and adding directors to a Yagi antenna is increase its gain.
ALL of the following is a Yagi antenna design variable that could be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth:
NONE of the following describes a common method for insulating the driven element of a Yagi antenna from the metal boom when using a gamma match:
The gain of two 3-element horizontally polarized Yagi antennas spaced vertically 1/2 wave apart from each another compared to the gain of a single 3-element Yagi is typically approximately 3 dB higher. The advantage of vertical stacking of horizontally polarized Yagi antennas is that it narrows the main lobe in elevation.
A cubical quad antenna is a directional antenna typically constructed from 2 square loops of wire each having a circumference of approximately one wavelength at the operating frequency and separated by approximately 0.2 wavelengths. Each side of a cubical-quad antenna driven element is approximately 1/4 wavelength. The forward gain of a 2-element cubical-quad antenna compare to the forward gain of a 3 element Yagi antenna is about the same. Each side of a cubical-quad antenna reflector element is slightly more than 1/4 wavelength. The reflector element must be approximately 5% longer than the driven element of a cubical-quad antenna for the antenna to operate as a beam antenna, assuming one of the elements is used as a reflector.
When the feed-point of a cubical quad antenna is changed from the center of the lowest horizontal wire to the center of one of the vertical wires the polarization of the radiated signal changes from horizontal to vertical.
The gain of a two element delta-loop beam compare to the gain of a two element cubical quad antenna is about the same. Each leg of a symmetrical delta-loop antenna driven element is 1/3 wavelengths.
As related to antennas, the term "NVIS" means Near Vertical Incidence Skywave. An advantage of an NVIS antenna is high vertical angle radiation for short skip during the day. An NVIS antenna is typically installed between 1/10 and 1/4 wavelengths height above ground.
A log periodic antenna is one for which the length and spacing of elements increase logarithmically from one end of the boom to the other. An advantage of a log periodic antenna is wide bandwidth.
A Beverage antenna is a very long and low receiving antenna that is highly directional. It is generally not used for transmitting because it has high losses compared to other types of antennas. An application for a Beverage antenna is for directional receiving for low HF bands.
The primary purpose of traps installed in antennas is to permit multiband operation. A disadvantage of multiband antennas is that they have poor harmonic rejection.