VSWR = Voltage Standing Wave Ratio
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.
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.
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?