G-9
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SUBELEMENT G9 – ANTENNAS AND FEED LINES
[4 Exam Questions – 4 Groups]
9A – Antenna feed lines: characteristic impedance, and attenuation; SWR calculation, measurement and effects; matching networks
50 and 75 ohms are the typical characteristic impedances of coaxial cables used for antenna feed lines at amateur stations. 300 ohms is the characteristic impedance of flat ribbon TV type twinlead. The distance between the centers of the conductors and the radius of the conductors determine the characteristic impedance of a parallel conductor antenna feed line. A difference between feed line impedance and antenna feed point impedance might cause reflected power at the point where a feed line connects to an antenna. If a transmission line is lossy, high SWR will increase the loss. The attenuation increases in coaxial cable as the frequency of the signal it is carrying increases. Decibels per 100 feet is how RF feed line loss is usually expressed. The antenna feed point impedance must be matched to the characteristic impedance of the feed line to prevent standing waves on an antenna feed line. The effect of transmission line loss on SWR measured at the input to the line is, the higher the transmission line loss, the more the SWR will read artificially low.
If the SWR on an antenna feed line is 5 to 1, and a matching network at the transmitter end of the feed line is adjusted to 1 to 1 SWR, 5 to 1 is still the resulting SWR on the feed line.
4:1 standing wave ratio is the result of connecting a 50 ohm feed line to a non-reactive load having 200 ohm impedance.
5:1 standing wave ratio is the result of connecting a 50 ohm feed line to a non-reactive load having 10 ohm impedance.
1:1 is the standing wave ratio that will result when connecting a 50 ohm feed line to a non-reactive load having 50 ohm impedance.
2:1 is the standing wave ratio that will result when connecting a 50 ohm feed line to a non-reactive load having 25 ohm impedance.
6:1 is the standing wave ratio that will result when connecting a 50 ohm feed line to an antenna that has a purely resistive 300 ohm feed point impedance.
G9B – Basic antennas
A disadvantage of a directly fed random-wire HF antenna is 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 sloping downward. Slope the radials downward is a common way to adjust the feed point impedance of a quarter wave ground plane vertical antenna to be approximately 50 ohms. The radial wires of a ground-mounted vertical antenna system are placed on the surface of the Earth or buried a few inches below the ground.
The radiation pattern of a dipole antenna in free space in the plane of the conductor is a figure-eight at right angles to the antenna. The feed point impedance of a 1/2 wave dipole antenna steadily decreases as the antenna is lowered below 1/4 wave above ground. The feed point impedance of a 1/2 wave dipole steadily increases as the feed point is moved from the center toward the ends. If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional.
Lower ground reflection losses is an advantage of a horizontally polarized as compared to a vertically polarized HF antenna
32 feet is the approximate length for a 1/2 wave dipole antenna cut for 14.250 MHz.
131 feet is the approximate length for a 1/2 wave dipole antenna cut for 3.550 MHz.
8 feet is the approximate length for a 1/4 wave vertical antenna cut for 28.5 MHz.
G9C – Directional antennas
The direction of maximum radiated field strength from the antenna is the “main lobe” of a directive antenna. When antenna gain is stated in dBi, it compares to gain stated in dBd for the same antenna as 2.15 dB higher.
The physical length of the boom, number of elements on the boom and spacing of each element along the boom are Yagi antenna design variables that could be adjusted to optimize forward gain, front-to-back ratio, or SWR bandwidth. Gain increases when increasing boom length and adding directors to a Yagi antenna. Larger diameter elements would increase the bandwidth of a Yagi antenna. 1/2 wavelength is the approximate length of the driven element of a Yagi antenna. The director is normally the shortest element in a three-element, single-band Yagi antenna. The reflector is normally the longest element in a three-element, single-band Yagi antenna. In a Yagi antenna the power radiated in the major radiation lobe compared to the power radiated in exactly the opposite direction is the “front-to-back ratio”. To match the relatively low feed point impedance to 50 ohms is the purpose of a gamma match when used with Yagi antennas. An advantage of using a gamma match for impedance matching on a Yagi antenna to match 50 ohm coax feed line it does not require that the elements be insulated from the boom. The gain is approximately 3 dB higher when two 3-element horizontally polarized Yagi antennas spaced vertically 1/2 wavelength apart typically compare to the gain of a single 3-element Yagi. It narrows the main lobe in elevation is an advantage of vertical stacking of horizontally polarized Yagi antennas.
The reflector element must be approximately 5 percent longer than the driven element in a two-element 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 quad antenna of any shape is moved from the midpoint of the top or bottom to the midpoint of either side the polarization of the radiated signal changes from horizontal to vertical.
It is approximately 1/4 wavelength long each side of the driven element of a quad antenna. The forward gain of a two-element quad antenna is about the same as the forward gain of a three-element Yagi antenna. Slightly more than 1/4 wavelength is how long each side of the reflector element in a quad antenna.
The gain of a two-element delta-loop beam is about the same as the gain of a two-element quad antenna. 1/3 wavelength is how long each leg of a symmetrical delta-loop antenna is approximately.
G9D – Specialized antennas
NVIS means Near Vertical Incidence sky-wave as related to antennas. High vertical angle radiation for working stations within a radius of a few hundred kilometers is an advantage of an NVIS antenna. Between 1/10 and 1/4 wavelength is height above ground an NVIS antenna is typically installed.
To permit multiband operation is the primary purpose of antenna traps. They have poor harmonic rejection is a disadvantage of multiband antennas.
Wide bandwidth is an advantage of a log periodic antenna. In a log periodic antenna length and spacing of the elements increase logarithmically from one end of the boom to the other.
A very long and low directional receiving antenna describes a Beverage antenna. Beverage antenna has high losses compared to other types of antennas is why it is not used for transmitting. Directional receiving for low HF bands is an application for a Beverage antenna.
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