Two questions–G9A14 and G9A15–were added to this section. The other questions are unchanged.
Feedlines are the cables used to connect antennas to receivers and transmitters. The most important characteristic of a feedline is its characteristic impedance. 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. (G9A01)
50 and 75 ohms are the typical characteristic impedances of coaxial cables used for antenna feed lines at amateur stations. (G9A02) The reason we use cables with these impedances is that they closely match the impedance of commonly used amateur radio antennas, such as quarter-wave verticals (35 ohms) and half-wave dipoles (72 ohms). 300 ohms is the characteristic impedance of flat ribbon TV type twinlead. (G9A03)
A difference between feed-line impedance and antenna feed-point impedance is the reason for the occurrence of reflected power at the point where a feed line connects to an antenna. (G9A04) A measure of this mismatch is the voltage standing-wave ratio, or simply SWR. The SWR is equal to the ratio of the impedances.
A standing wave ratio of 1:1 will result from the connection of a 50-ohm feed line to a non-reactive load having a 50-ohm impedance. (G9A11) This is the best possible case. When the SWR is 1:1, we say that the feedline is “matched” to the load. To prevent standing waves on an antenna feed line, the antenna feed-point impedance must be matched to the characteristic impedance of the feed line. (G9A07)
When the two impedances are not matched, an SWR greater than 1:1 will result, and the SWR will be equal to the ratio between the two impedances. For example, a 4:1 standing wave ratio will result from the connection of a 50-ohm feed line to a non-reactive load having a 200-ohm impedance. (G9A09) A standing wave ratio of 5:1 will result from the connection of a 50-ohm feed line to a non-reactive load having a 10-ohm impedance. (G9A10)
If you feed a vertical antenna that has a non-reactive 25-ohm feed-point impedance with 50-ohm coaxial cable, the SWR will be 2:1. (G9A12) If you feed an antenna that has a purely resistive 300-ohm feed-point impedance with 50-ohm coaxial cable, the SWR will be 6:1. (G9A13)
In order not to damage your transmitter, it’s important that the impedance its output “sees” is 50 ohms. To accomplish this, we often use devices called antenna tuners, and when adjusted properly, they transform the impedance at the end of the feedline to 50 ohms. That makes the transmitter happy, but the SWR on the feedline is unchanged. 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, the resulting SWR on the feed line is still 5 to 1. (G9A08)
When the SWR on a coaxial cable feedline is greater than 1:1, it will attenuate the signal, and there will be some interaction between high standing wave ratio (SWR) and transmission line loss. If a transmission line is lossy, high SWR will increase the loss. (G9A14) To transfer the greatest amount of power from the transmitter to the receiver, the SWR on the feedline should be 1:1.
Even when perfectly matched, a coaxial cable will attenuate the signal somewhat, depending on the frequency of the signal. Coaxial cable attenuation increases as the frequency of the signal it is carrying increases. (G9A05) RF feed line losses are usually expressed in decibels per 100 ft. (G9A06)
This transmission line loss will have an effect on the SWR measured at the input to the line. The higher the transmission line loss, the more the SWR will read artificially low. (G9A15)