Maximum transmission efficiency occurs when the load impedance
matches the generator impedance. Engineers do this to eliminate
standing waves, which can severely damage equipment.
Transmission lines in AM stations are more likely to be mismatched
than those in FM transmitters. In FM, the desired
transmitter-to-antenna match is accomplished using pre-established
standard antenna and transmission line impedances that are designed to
match each other, and separate antenna tuning units are not required.
Most FM antennas provide some form of final tuning and adjustment to be
made once the antenna is installed on its tower. When problems of
abnormal VSWR occur, they generally result from mechanical or
electrical damage due to the transmission line or antenna, and
frequently to heavy ice formation on the antenna. When ice is allowed
to build up the VSWR increases and the protection circuit trips the
transmitter before any serious damage can occur.
The view in general
FM engineers are likely to experience more severe VSWR problems than
AM engineers. Almost every FM transmitter has a built-in VSWR meter
associated with a protection circuit to turn the transmitter off if the
VSWR becomes excessive. The considerably higher operating frequency,
and consequently shorter wavelength, means that path lengths can become
critical and it is easier to develop unexpected standing waves.
Figure 1. The voltage nulls appear at
half-wavelength points, coinciding with the short in the Lecher
In AM work misadjustment of ATUs and other devices in the
transmission line circuit can result in a severe mismatch that goes
unnoticed. Sometimes a bad mismatch can exist for years in a
nondirectional AM operation that has sufficient transmitter power
output to overcome the losses produced by the mismatch. In AM
operations, coaxial transmission lines normally have a 50Ω nominal
impedance. However, other values can be used without diminishing
antenna operating efficiency, because the ATU matches the transmission
line impedance to the antenna impedance. VSWR, although important, is
usually less critical in AM transmission than in FM, and a small amount
is often tolerated without noticeable problems. Over the years as the
ATUs and phasor are adjusted, small errors are often introduced and
standing waves begin to occur, which can cause problems such as heating
in the transmission line and ATU components.
Long before electronic frequency measuring devices, radio engineers
had to measure frequency using Lecher wires, which amounted to a
yardstick. There weren't any pocket frequency meters that would tell
show the frequency at the press of a button. Instead, it was necessary
to measure the distance between the brightest (strongest) or the
weakest (null) indications on a pair of long straight parallel wires
supported on insulators and (transmission line) spaced two or three
inches apart. The indicator was a neon bulb that tended to extinguish
suddenly and made it difficult to find absolute nulls. Any simple
indicator that does not load down the Lecher wires can be used. Areas
of high voltage are normally quite broad while nulls are narrow. This
is why the engineer should measure at the nulls.
Lecher wires provided a good understanding of standing wave ratios
and their effects on antenna transmission lines. The basic principles
demonstrated by the Lecher wires helped to develop high-power
transmission lines. Early single-wire antenna connections soon gave way
to balanced and unbalanced transmission lines, and eventually to
Figure 1 shows a Lecher wire with the far end shorted. Zero volts
appear at the far end. If the probe is moved back toward the origin, a
maximum will be found one quarter wave back from the end. Moving on
from the quarter wave, a null is found at a half-wavelength point.
Moving a quarter wavelength farther will bring us to another maximum at
a half wavelength point. The distance between two maxima, or two nulls
is a half wavelength. Multiplying this distance by two gives the
distance for one full wavelength. From this we can calculate the
frequency by converting to kilohertz. The reflection coefficient is -1
because the signal is completely reflected back and there is no power
absorbed in the short.
When a signal reaches the short circuit a certain amount of signal
will be reflected back and the remainder dissipated in the short if
there is dissipative resistance. The phase of the reflected signal is
controlled by the coefficient of reflection.
Figure 2 shows the effect of an open circuit at the receiving end.
The receive end voltage is maximum. Measuring between two adjacent
nulls shows a half wavelength. In the case of an open circuit, or a
short circuit, no energy is taken from the line (provided there is no
dissipative resistance). Instead, the signal is reflected back in the
opposite direction. Because the reflected signal has the same amplitude
and phase as the outgoing wave, both voltages are added and the voltage
is at a maximum at the open end.
Figure 2. With the short in the Lecher wire
removed, a high voltage appears at the open end.
Should the line be terminated by a reactance that has absolutely no
resistance, no power will be absorbed in the receiver end or load. In
the case of a high-powered transmitter this could have disastrous
effects. Putting power into a line that is not terminated will have the
same effect because in both cases there is no resistance across the
terminal, and the transmitter could be badly damaged by the high
If a line is not correctly terminated the outgoing and reflected
waves will pass each other, going in opposite directions. The two
voltages will add in some places and subtract at others, and the
resulting voltage will be lower or higher than the original signal.
Because the two signals go in opposite directions and at the same
speed, the nulls and the maxima will not move around but will stay in
fixed positions. This produces the familiar and sometimes troublesome
An RF transmission line is not an ordinary circuit. AM lines should
never have problems with delay effects because the longer wavelengths
of the AM frequencies are large compared to the dimensions of items
that can cause delay. Delays are caused by the finite time required
when energy moves along a transmission line.
E-mail John Battison at email@example.com.