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G9A

ANTENNAS AND FEED LINES

- Feed lines: characteristic impedance and attenuation; standing wave ratio (SWR) calculation, measurement, and effects; antenna feed point matching

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G9A011 of 11

Which of the following factors determine the characteristic impedance of a parallel conductor feed line?

Why Characteristic impedance is set by the line's geometry and the insulating material between the conductors, not by how the line is used. For open-wire (parallel conductor) line, Z0 is approximately 276 x log(D/r) where D is the center-to-center spacing and r is the conductor radius (the dielectric constant of the insulation also factors in). Spread the wires farther apart and Z0 rises; use fatter conductors and Z0 falls.
Watch out Length and frequency are tempting because they do affect loss and the impedance you measure at the input end when the line is mismatched, but the characteristic impedance itself stays the same whether the line is 3 feet or 300 feet long.
Z0 is built in at the factory: spacing and wire size only. Frequency and length change loss, not Z0.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9A022 of 11

What is the relationship between high standing wave ratio (SWR) and transmission line loss?

Why On a mismatched line the reflected wave travels back down the feed line and is re-reflected, so power makes multiple trips through the cable and each trip suffers the cable's normal attenuation. The extra loss caused by SWR is always added on top of the matched-line loss, and it grows with how lossy the line already is. A theoretically lossless line would show no added loss no matter how high the SWR, which is why the effect is described in terms of a lossy line.
Watch out Saying there is no relationship ignores the fact that reflected power retraces the line and is attenuated again; the idea that high SWR reduces the effect of loss has it backwards, since mismatch always makes total loss worse, never better.
SWR does not create loss by itself, it multiplies the loss the line already has: low-loss line, small penalty.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9A033 of 11

What is the nominal characteristic impedance of "window line" transmission line?

Why Window line is a form of open-wire (ladder) feed line where two parallel conductors are held apart by a plastic web with rectangular windows cut out of it. The wide conductor spacing and mostly-air dielectric give it a high characteristic impedance, nominally 450 ohms. That wide spacing and low dielectric loss also make it much lower loss than coax, which is why it is popular for multiband doublets fed through a tuner.
Watch out 50 and 75 ohms are the standard coaxial cable impedances (50 for transmitters, 75 for TV and hardline); 300 ohms is the familiar twin-lead TV feed line, but window line with its cut-out windows runs higher at 450 ohms.
Ladder/window line = 450 ohms. Twin-lead = 300. Coax = 50 or 75. Wider spacing, higher impedance.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9A044 of 11

What causes reflected power at an antenna's feed point?

Why A transmission line has a characteristic impedance (commonly 50 ohms for coax used in ham stations), and power flows smoothly down it only when the load it sees equals that impedance. When the antenna feed point impedance differs from the line's impedance, part of the incident wave cannot be absorbed by the load and is reflected back toward the transmitter. That reflected wave interferes with the forward wave to create standing waves, which is what an SWR meter reports; a perfect match gives 1:1 SWR and zero reflected power.
Watch out Operating at resonance only makes the feed point reactance zero, it does not guarantee the resistive part equals 50 ohms, so an antenna can be resonant and still show reflected power. Using a balanced antenna with unbalanced line causes common mode current on the coax shield, a separate problem from reflection, and excess power damages parts rather than creating reflections.
Reflection is a mismatch, not a resonance or a power problem: line ohms must equal antenna ohms.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9A055 of 11

How does the attenuation of coaxial cable change with increasing frequency?

Why Coax loss comes mainly from conductor resistance and dielectric losses, and both get worse as frequency rises. Skin effect confines current to an ever thinner outer layer of the conductor as frequency increases, raising effective resistance, while dielectric heating also grows with frequency. That is why a cable rated at maybe 0.5 dB per 100 feet on 80 meters may lose several dB per 100 feet at UHF, and why better cable or shorter runs matter most on the high bands.
Watch out Saying attenuation is independent of frequency ignores skin effect; that is why manufacturer loss tables always list loss at several specific frequencies rather than one number. There is no 'Marconi's Law of Attenuation.'
Higher frequency, higher loss. Check any coax loss chart: the numbers only climb as you go up in frequency.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9A066 of 11

In what units is RF feed line loss usually expressed?

Why Feed line loss is attenuation, a ratio of power out to power in, and ratios of power are expressed in decibels, not ohms. Manufacturers publish coax loss as decibels per 100 feet at specified frequencies, because loss rises with frequency. Since dB is a ratio, you can simply scale it: 1.5 dB per 100 feet means 3 dB (half the power) over 200 feet.
Watch out Ohms per 100 feet or per 1,000 feet describes DC resistance of a conductor, not signal attenuation, and coax data sheets standardize on 100 feet rather than 1,000 feet.
Loss is a power ratio, so decibels; cable specs use 100-foot lengths. Think "dB per hundred feet."
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9A077 of 11

What must be done to prevent standing waves on a feed line connected to an antenna?

Why Standing waves exist because part of the forward wave is reflected back toward the transmitter. A reflection happens only when the load impedance differs from the line's characteristic impedance, so when the antenna feed point impedance equals the feed line's Z0 (commonly 50 ohms for coax) all the power is absorbed by the antenna, nothing comes back, and the SWR is 1:1. Line length, line type, and grounding do not change this: only the match at the load does.
Watch out The choices about the line being some number of quarter or half wavelengths are tempting because line length does matter for impedance transformation tricks like quarter-wave matching sections, but a length alone cannot stop a mismatched load from reflecting power. DC ground potential at the feed point relates to static and lightning protection, not to SWR.
No reflection, no standing wave. Match the load to the line's Z0 and SWR is 1:1, whatever the line length.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9A088 of 11

If the SWR on an antenna feed line is 5:1, and a matching network at the transmitter end of the feed line is adjusted to present a 1:1 SWR to the transmitter, what is the resulting SWR on the feed line?

Why SWR on a feed line is set by the mismatch between the line's characteristic impedance and the load at its far end, which is the antenna. A matching network (antenna tuner) at the transmitter end only transforms the impedance the transmitter sees; it does nothing to the antenna's feed point impedance, so the reflected wave on the line is unchanged and the line still runs at 5:1. The tuner just re-reflects that returning power back toward the antenna, and the line continues to suffer the extra loss that comes with 5:1 SWR.
Watch out Reading 1:1 is tempting because that is what the SWR meter at the transmitter shows once the tuner is adjusted, but that reading applies only to the short section between the tuner and the radio, not to the whole feed line.
A tuner fools the radio, not the feed line. SWR is set at the antenna end and stays there.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9A099 of 11

What standing wave ratio results from connecting a 50-ohm feed line to a 200-ohm resistive load?

Why SWR from a purely resistive mismatch is just the ratio of the two impedances, larger divided by smaller: 200 / 50 = 4, written as 4:1. Because the load is resistive there is no reactance to complicate the calculation. SWR is always expressed with the larger value first, so it is never less than 1:1.
Watch out The inverted form 1:4 is not how SWR is written; SWR is by definition at least 1:1, so the bigger number always comes first. 2:1 would come from a 100-ohm load on 50-ohm line.
Divide big by small, never smaller than 1:1. 200/50 = 4:1.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9A1010 of 11

What standing wave ratio results from connecting a 50-ohm feed line to a 10-ohm resistive load?

Why SWR from a purely resistive mismatch is just the ratio of the larger value to the smaller: 50 ohms divided by 10 ohms gives 5, so the SWR is 5:1. The larger number always goes first because SWR is defined as the ratio of maximum to minimum voltage on the line and can never be less than 1. It does not matter whether the load is above or below the line impedance; a 250-ohm load on the same 50-ohm line would also read 5:1.
Watch out The choice written as 1:5 is the same arithmetic stated backwards, and SWR is never expressed that way; a reading below 1:1 is physically impossible.
Divide big by small, and the big number goes first. 50/10 = 5:1.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9A1111 of 11

What is the effect of transmission line loss on SWR measured at the input to the line?

Why SWR at the input depends on the ratio of reflected to forward power seen there. The forward wave is attenuated on its way to the antenna, and the reflected wave is attenuated again coming back, so the reflected power arriving at the transmitter is reduced more than the forward power. That makes the measured SWR at the line input lower than the actual SWR at the antenna feed point, and the loss can hide a serious mismatch. In the extreme, a very lossy line reads close to 1:1 no matter what is at the far end.
Watch out The idea that loss raises the reading gets the physics backward; loss always attenuates the reflected wave and pulls the reading toward 1:1. And loss makes the reading less accurate as a measure of what the antenna is doing, not more.
Loss hits the reflected wave twice, so it hides mismatch: a lossy line always reads closer to 1:1 at the shack.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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