What is VSWR (SWR)
What is SWR
In short, SWR is a measurement of forward power with respect to reflected power. The interactions between the forward and reflected waves produce standing waves (wave patterns that appear to stand in place) and can be visually seen in this AT&T demonstration video:
Additionally, I find the definition of SWR on Wikipedia to be accurate and informative.
The most interesting take aways from the AT&T video are:
1) reflected waves are produced when the characteristic impedance of the transmission line is different than the characteristic impedance of the load.
2) Forward and reflected waves superpose to produce standing waves and thus differences in voltage maximums and voltage minimums along the transmission line.
3) Reflected waves are re-reflected back to the load from the source in exactly the same way that power in the form of the forward wave is sent back to the source from the load. In other words, reflected waves bounce back and forth across the transmission line until they are either consumed as part of the natural resistance of the transmission line or are radiated out by the antenna.
As evidenced by this video, and research in cable TV and other industries, reflected power is not imaginary and is certainly not lost. This is an age old
debate in the amateur radio community that has been completely verified by other industries but is not fully agreed upon by the Amateur Radio
community. One difference between Amateur Radio and other industries where standing waves can present significant problems is that Radio Amateur Operators wish to utilize a wide range of frequencies, while other industry work is confined to only a few specific frequencies. It is due to this need to work a wide range of frequencies that SWR, and a slew of myths, have continued to be front and center conversations with Amateurs and authoritative authors.
So, is SWR that terrible beast that we've all heard about that burns up on transceivers, limits the amount of power that radiates from our antennas, causes our toasters to burn the toast? I'll go with the experts in this field and say that it's not. In this regard I would like to direct your attention to the work done by
Walter M. Maxwell (W2DU) (http://www.w3pga.org/Antenna%20Books/Reflections%20III.pdf) where Mr. Maxwell explains in detail the causes of SWR and why we are all overly concerned about SWR in our antenna systems.
Well, if SWR is not the bully that we all thought it was, why do we go to great lengths to reduce SWR on our antenna systems? The answer is that SWR does
lead to higher losses in our transmission lines (especially coax) and our transceivers can be sensitive to reverse power. This is one reason why modern solid
state transceivers will cut power output when high SWR is detected. Additionally, high SWR, as a result of an impedance mismatch between the transmission
line and load, will prevent the transceiver from producing its full rated output power. All modern transceivers are designed to face a 50 ohm load. If the
resistance/impedance is not 50 ohms, the transceiver may not be able to produce its full rated output. This is one reason why we need to transform
transmission line impedance to 50 ohms when connecting it to our transceivers.
SWR and Antenna Tuners
The responsibility of the antenna tuner is twofold:
1) Regardless of transmission line and antenna impedance, present a perfect 50 ohm load to the transceiver.
2) Present a conjugate match of the feed line/load impedance back to the feed line/load.
For item number 2, when looking from the tuner into the feed line, there is a specific impedance that must be matched. When looking from the feed line into
the tuner, the conjugate of the feed line/load impedance must be created in order to eliminate SWR. By creating a perfect conjugate match to that of the
feed line/load impedance, we create a condition where the impedance is now completely resistive and not reactive at the tuner. Once the tuner has done this, it's just a matter of converting the remaining resistance to 50 ohms.
From the tuner through the feed line and into the antenna SWR remains the same along the entire length of the transmission line. Forward power produces reflected signals at the interface between the feed line and the antenna, while the antenna tuner provides for an equal and opposite reflection of reflected signals BACK to the antenna. This is how 100% of supplied power, other than that lost to feed line attenuation, is eventually radiated by the antenna. Note that this ONLY happens if the tuner is completely lossless, which they are not. More on this later.