SWR
Many of us use an SWR meter to measure the Standing Wave Ratio of our antenna at regular
intervals. But just what is that measurement telling us and to what use should we put
that information.
Is it necessary to get your SWR down to a 1:1 reading? To what end
shall we go to achieve a low SWR? This article attempts to answer these questions and
provide some other useful information on feeding antennas.
When power is delivered to a transmission line by a generator (transmitter) the power
travels up the line until it reaches the load (antenna). If the load has a pure
resistance value equal to that of the transmission line, then all the power is absorbed by
the load. In the case of an antenna it would all be radiated. If, however, an impedance
mismatch exists between the load and the line, some of the power will be reflected back
down the line to the transmitter causing standing waves of voltage and current. VSWR is
the ratio between the maximum and minimum voltage on a transmission line. Normally VSWR is
simply referred to as SWR.
VSWR = SWR = Emax/Emin = Ef+Er/Ef-Er
Where: Ef = E forward and Er = E reflected
An SWR meter is a RF power meter, connected as a bridge, and called a reflectometer. They
normally consist of a toroid transformer through which a transmission line is inserted.
The output of the transformer is connected to a meter movement via a rectifier diode. A
switch reverses the connections to the transformer and allows a voltage reading related to
the both the forward and reflected power on the line. A typical SWR meter is shown in the
ARRL Antenna Book, 16th Edition, Chapter 27, pp. 16-17.
SWR meters are designed for a
specific impedance. Therefore, for accurate readings, they must be used only with lines
of the same impedance. Normally, SWR meters sold for use in the Amateur Radio Service are
designed for use in 50 ohm systems. In use, the SWR meter is calibrated for each
measurement by adjusting the forward power for a value of 1 on the meter; the meter is
then switched to the reflected position and a reading taken. This reading, say .2 for
example, is not the SWR, but the reflection coefficient, called by the Greek letter rho
(p) in engineering texts. The SWR is obtained by using the equation SWR = 1+p/1-p. In
our example the SWR is 1.5:1. Many SWR meters sold today take both measurements
simultaneously using a single dual movement meter and display the results directly at the
point where the meter indicators cross. Meter scales are also often calibrated in watts
for a 50 ohm system and will indicate the forward and reflected power.
SWR readings can be taken anywhere on the line between the generator and load; the
readings will be identical. The reason is that SWR is a ratio and is not dependent upon
exact values of voltage, current or impedance, all of which are different at different
points on the line. Therefore, there is no need to hang off a tower or ladder to make the
measurement exactly at the antenna feed point when an SWR measurement made at the
transmitter will do.
The SWR on a 50 ohm transmission line that is terminated in a 50 ohm dummy load is 1:1.
All the power put into the transmitter end of the line is absorbed by the load; none is
reflected back to the source. It is good to remember this fact if your antenna shows a
flat SWR across the lower bands (40, 80 & 160 meters). Chances are it is acting like a
dummy load and may not be radiating much RF. Normally, an antenna will show some
variation of SWR across the band of operation, with the lowest at the point of antenna
resonance. If the antenna is resonant at say 7.15 MHz, expect to see a low (close to 1:1)
SWR there, but have the SWR rise to a higher (3:1) at the band edges. Recording SWR
readings of an antenna across a band or bands of frequencies provides some very useful
information. If you are operating a solid state transceiver, without an antenna tuner, it
will tell the limits of your operating range on that band or bands, as most solid state
(no tune) transceivers reduce power as the SWR increases beyond 2:1.
Keeping a record of
SWR readings over time will allow you to see changes in the antenna that point to a need
for maintenance.
Is having a high SWR bad? Not necessarily, and in fact most multiband antennas
operate with high standing wave ratios. By tolerating a high SWR we permit operation over
entire bands or even unrelated bands with a few antennas. But how about all that power
that is reflected back from the mismatched antenna? The power is not lost. It is added
to the power from the transmitter and reflected back up the line to the antenna. However
the transmitter power output will be reduced by exactly the same amount of power that was
reflected (added). The total power does not change. The transmitter power is reduced
because it does not see a matched condition at the point where the transmitter feeds the
line. [1]
Since we want as much of the available power as possible to reach the antenna and be
radiated, a matched condition is needed. This can be accomplished with an antenna tuner.
The antenna tuner will provide a complex impedance (R+-jX) that transforms the 50 ohm
output of the transmitter to the impedance at the transmitter end of the
transmission line. For instance, if the impedance at the transmission line is ZL=RL-jX
the tuner will provide an impedance ZT=RT+jX such that the transmitter will see 50 ohms
non-reactive (Z=50+j0). If the R values are equal, the jX values will be equal and
opposite; this condition is called a conjugate match. Normally the R values are not the
same. Never-the-less, the antenna tuner provides an impedance transformation which allows
the antenna to operate as if it were resonant. The antenna can also be matched at the
antenna using tuning stubs, an reactive (LC) network or a remote antenna tuner. It turns
out that it makes no difference where the matching takes place, along the transmission
line, at the antenna or at the transmitter; the results are the same.
So far we have assumed that the transmission line was lossless. But how about the real
world and the loss in the transmission line as a result of all that reflected power
traveling up and down? Actually, amateurs go to great expense to provide low loss
transmission lines, so the lossless line assumption is almost valid for most amateur
applications. Losses, especially on the low bands are very low, even with a high antenna
SWR. For instance, assume an 80 Meter antenna operating on 75 Meters with an antenna SWR
of 4:1, fed with 100 feet of RG 8, and matched at the transmitter. The additional loss
due to mismatch at the antenna would be only .4 dB; not enough to really matter.[2]
The key points to remember are: SWR is the ratio of the maximum to minimum voltage or
current on a transmission line. It is the same everywhere on the line. An SWR of 1:1
indicates that the antenna system is resonant. A high SWR is not necessarily bad
especially if low loss transmission line is used. A high antenna SWR can be transformed
to a low value at the transmitter with an antenna tuner. Try your dipole on some of the
other bands, or your Yagi on the WARC bands and don't worry too much about SWR.
73, Kevin/W3DAD
------------------------[1] M. Walter Maxwell, W2DU, Reflections - Transmission Lines and Antennas, (Newington,
CT: ARRL, 1990), pp 6-3,4
[2] Gerald Hall, K1TD, The ARRL Antenna Handbook, 16th Edition (Newington, CT: ARRL,
1990), p 24-10