Tiny was here!
VSWR
VSWR = Voltage Standing Wave Ratio
or
ISWR = Current Standing Wave Ratio
As you can see the Standing Wave Ratio (SWR) can be measured by Voltage or Current.
The results will be the same. The SWR is derived from the measurement of RF
power supplied by the transmitter/amplifier (forward power), compared to the
power reflected back to the transmitter/amplifier (reflected power). The reflected
power is created by the impedance bump resulting from a mismatch between the
antenna and it's feed line. For our purpose we can define impedance as (RF)
resistance. However, in the real world it also involves reactive components.
The SWR is an indication of how well the feed line impedance matches the antenna
impedance at a particular frequency. No change other then to the antenna or
feed line affects SWR. Normally the antenna is adjusted to match the feed line.
My Antenna Tuner Changes My SWR!
NO,it does not. An Antenna
Tuner/Matcher adds either Capacitance or Inductance either in series or parallel
which counteracts the existing mismatch. It fool's the radio, allowing it to
dump it's full power into the antenna system. All the while the mismatch remains
unchanged and the components inside the turner dissipates a portion of your
RF power as heat.
Why is SWR important?
It is commonly assumed that a good swr match is required inorder to put out
the strongest possible signal. While this is true, many old timers will tell
you of using antennas with SWR of 5:1 or 7:1 or even higher with great success.
However, if you get more detailed information, you will find that they were
using very rugged vacuum tube transmitters, rugged antenna tuners and open wire
feed lines.
As the SWR and the reflected power increases there will be points alone the
feed line where the maximum voltage of the forward power and the maximum voltage
of the reflected power are in phase, these points may see as much as several
thousand volts even at very low power levels. While open wire lines tolerated
this with only an occasional arc over or two, it is a killer for coax with it's
closer spacing. Another major consideration is that the reflected power must
be dissipated somewhere. If an antenna tuner or matcher is being used it will
see not only the forward power but the reflected power as well. If it is rated
for 100 watts and you are transmitting 100 watts connected to an antenna with
a 2:1 SWR , approx. 11% of the power is reflected back resulting in the tuner
having to deal with 111 watts of total power. It is obvious that the tuner or
the transmitter final output transistors, if no tuner is being used, become
subject to damage. Most modern transistor equipment's have ALC circuits built
in that reduce the output power if a high SWR is encountered. I consider the
possible damage to be far more important then
the signal strength issue.
Signal Strength
When you consider that the "Signal Strength Meters" used in ham receivers
are normally calibrated to 6db per S-unit, the signal strength vs. SWR issue
becomes far less important. This 6db calibration means that inorder to go from
a S5 to an S6 the signal strength must increase by a factor of four. If you
are receiving a
station running 500 watts at S5 inorder for that signal to increase to an S6
2000 watts would be required. The same station would have to reduce power from
500 to 125 watts to go from S5 to S4.
SWR 1:1 = 100% efficiency
SWR 1.5: 1 = 96% efficiency
SWR 2:1 = 89% efficiency
SWR 3:1 = 75% efficiency
Even with an SWR of 3:1, 75% of the power is accepted by the antenna, while
25% is reflected back into your equipment.
RELATIVITY
What does the ratio
mean?
A transmitter output circuit built and adjusted for a 50 ohm match, using 50
ohm coax cable, connected
to a 50 ohm load = 1:1 SWR
The same equipment connected to a 100 ohm load = 2:1 SWR
The same equipment connected to a 25 ohm load = 2:1 SWR
double or 1/2 of 50 ohms equals a 2:1 SWR
Do you see the trend?
Tiny was here!