line, or drops slowly and uniformly along a line with losses. This is the matched condition,
represented by a 1 to 1 SWR. The impedance at the load end of such a line is the same as that at the
generator end. When adjusting a matching network properly, the way to do it is to observe the
SWR and tune for as low a ratio as possible.
The SWR is also an indication of the value of resistance at the load end. The ratio is the same as
the ratio of load resistance to line characteristic impedance. This ratio can mean that the load is
either greater than or less than the line's impedance. For example, if the SWR on a length of 50 ohm
line is 3 to 1, the load resistance is either 150 ohms or 16.7 ohms (3 times 50 or one-third of 50). This
is only accurate with pure resistive loads.
It can be shown mathematically that a 2 to 1 SWR in a system which has the transmitter output
impedance equal to the line impedance delivers 89% of the measured forward power to the load.
This relates to a power loss of half a decibel - hardly noticeable in signal strength. At a 3 to 1 ratio, the
loss becomes appreciable with 75% of the measured forward power delivered. So in adjusting
antenna tuners, it is a nice feeling if you achieve a 1 to 1 match, but in reality, anything below 2 to 1 is
satisfactory. Line losses do increase a bit also with increasing SWR, but it is still a small fraction of a
dB at 2:1.
OVERALL SUMMARY
1. Any antenna can be represented as an equivalent resistive/reactive impedance whose
resistive component, termed radiation resistance, is a measure of the power radiated.
Reactance can be either inductive or capacitive.
2. Antenna impedance is a function of frequency, configuration, selection of feed point location,
height above ground and nearness to surrounding objects.
3. The reactive portion of the impedance does not absorb power but limits the amount of power
radiated by the resistive component. It is best to eliminate the reactive component, by
inserting an equal value reactance in series, but of the opposite type.
4. Best system performance is attained when antenna impedance is purely resistive with value
equal to transmission line impedance, which in turn equals transmitter output impedance.
5. Since antennas seldom present matched impedances to line over a band of frequencies and
from band to band, a partial solution to using these mismatched systems is to convert the
impedances at the transmitter end of the line to what the transmitter is designed for, with an
antenna tuner.
6. The transmission line will change the antenna impedance in both resistive and reactive values
at the transmitter end, depending on the line's electrical length, frequency and characteristic
impedance.
7. Due to slowing down of the current flow in the transmission line from that in free space, the
electrical length of a line will be longer than the physical length.
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