One phenomenon common to transmitter installations does not always
receive the attention it warrants. Voltage standing wave ratio (VSWR)
can wreak havoc in a transmitter when it suddenly increases and dumps
the transmitter in the middle of the night.
The average AM transmitter is seldom prone to severe VSWR problems
unless there has been a substantial change to the antenna tuning unit
(ATU) or common point (CP) input tuning, or the transmission line is
damaged. FM transmitters are just as susceptible to VSWR problems due
to antenna problems or transmission line damage. It behooves the
engineer to be aware of the VSWR and to ensure that antenna-system
components are properly tuned for optimum VSWR.
VSWR facts of life
A transmission line is considered to be a long series of inductances
with parallel capacitors (as shown in Figure 1). Each pair of
capacitors and inductors represents a very short piece of coaxial
cable. Capacitance is produced by the spacing between the wires or
inner and outer conductors in a coaxial cable. Inductance is
determined by the size of the conductors. As current enters the line,
it is retarded by the inductance of each tiny section. The consequent
limiting of the charging rate of the capacitive element of the line
determines the characteristic impedance of the line. The equation for
characteristic impedance can be found in RF Engineering, June 2000,
pg. 20. A rough approximation is given by the taking the square root
of L/C, where L and C are per unit length of the line.
Another element in the antenna system is the effect of the dielectric
in the coax on the speed of the signal. As a result, the signal
travels more slowly through the cable than through the air. This
difference may be as great as 65 percent. Thus, when the electrical
lengths of transmission paths are important, it is essential to take
velocity factor into consideration.
For efficient transmission of power, the load impedance must match
that of the line and the generator. When these impedances match
correctly, no power is reflected back along the line from the antenna
to the generator and the only power lost in the system is that due to
I superscript 2R losses and possibly some skin effect loss. But when
the impedances do not match each other, losses occur and the power
delivered to the antenna is less than that which enters the
transmission line. If this occurs, the transmitter output will see
either capacitative or inductive reactance. In such cases, the output
stage would be detuned to an extent depending on the degree of
mismatch. In severe cases it is possible for the final amplifier
stage to be damaged.
When a line mismatch occurs, a standing wave caused by the reflected
power appears on the coax. Every 180 degrees along the line major
current and voltage points will occur. Obviously, halfway between
each of these points minimum current and voltage points will occur.
The voltage standing wave ratio is the ratio of V subscript max/V
subscript min, measured at these points. The current standing wave
ratio is the ratio between the I subscript max/I subscript min.
The basic VSWR meter consists of a line section with a short wire
parallel to the inner conductor. The voltage developed on this wire
is rectified and passed to a DC voltmeter, which is calibrated in
terms of VSWR ratios. A value of 1.0 is shown at the minimum reading
point on the scale and, as detected voltage increases, the meter goes
upscale showing increases in the VSWR. In many FM transmitters, the
output from this or a similar probe drives a PA breaker to protect
the stage in the event of massive VSWR increases.
A VSWR reading of 1.1:1, or about 5 percent reflected signal, is
usually the maximum that can be safely handled, although most
situations are different. Remember that excessively high VSWR can
cause high voltage breakdown of cables.
VSWR and AM antenna systems
Most AM antenna systems use 50V transmission line but, regardless of
line impedance, the same rules apply. AM transmission lines are often
quite long, perhaps 300 feet or more, and carry comparatively low
power in most instances. Line overheating due to high VSWR is rare.
The operating frequency is low and the bandwidth of the signal is
basically very narrow. Transmitter final stages are, in general,
amenable to output load impedances of about 50V, although some modern
transmitters are finicky regarding output loads. In many stations, it
is unusual to find 50V Ohms j0 when an OIB is used to measure the
line input impedance at the transmitter and at the ATU input. In many
of these cases, measurement of line current showed a very small
difference between the two ends of the transmission line, despite the
apparent mismatch and expected high VSWR and corresponding power loss
in the line.
In stations where a directional antenna is used, common point
impedance is of great importance, but even there I have found more
than a few stations with mismatches in CP and individual tower lines.
Sometimes, checking RF power in the antenna has shown a discrepancy.
It appears that, in general, unless a finicky transmitter is used,
reasonably high VSWR ratios can be tolerated in terms of RF radiation
efficiency. But this definitely does not mean that such kinds of
operation should be endured, nor are they up to the Standards of Good
VSWR and FM
The average FM installation is far more susceptible to the effects of
high VSWR. Again we find that 50V is the usual line impedance, and
transmitters and antennas expect to see this value. VSWR by itself
does not reduce signal coverage, but it can do nasty things to your
Although not especially common in lower-power FM, operations
discontinuities caused by transmission line coupling bullets and
severely sharp bends in flexible lines can introduce VSWR effects and
should be borne in mind. A number of problems can be linked to high
VSWR. For example, stereo separation is affected. So is synchronous
AM noise, and intermodulation products can increase.
Some antennas exhibit fairly narrow bandwidths. It is a good idea to
check your operating bandwidth from time to time. If possible, the
transmission system should exhibit flat response beyond the 200kHz
spacing. If you don't have the necessary equipment, you can use your
FM transmitter and VSWR meter if your transmitter has an exciter with
a variable frequency control adjustable in small increments.
Finally, extreme cases of high VSWR appear as hot spots on the
transmission line. These cases lead to melted insulation and spacers,
fire and off-air crises.