Probably the first question that should be asked is, what is SWR? Simply put, SWR is the ratio of the maximum voltage (resulting from the interaction of incident and reflected voltages along a transmission line)to the minimum voltage. In a correctly terminated line (that is where the load is equal to the characteristic impedance of the line) all the power supplied by the transmitter will be delivered to the load (except for a fraction of power lost in the line itself). In this situation our SWR meter will give a reading of 1:1. Neglecting loss if we were to measure the voltage at any point along our transmission line the voltage would be the same at all points along the line. In practice the voltage will gradually decrease as we move further from the transmitter along our transmission line due to line losses.
Maximum efficiency will be obtained when the feed line is perfectly matched to the antenna, when we have no reflections on our transmission line and we have an SWR of 1:1. Nothing here will disagree with this statement , however I do wish to attempt to dispel a few myths about what our SWR meter is actually telling us.
Modern transceivers generally require that they are coupled to a load that has a low SWR to prevent damage due to high voltages or currents resulting from high SWR. Almost all modern transceivers will start to reduce their output power as SWR increases in order to protect their output transistors from damage. Any damage caused is not due as many think to reverse power flowing back into our transmitter but in fact due to the unusually high voltages or current resulting from the mismatched impedances.
The Foundation license syllabus asks us to Recall that an SWR equal to, or less than 1.5 indicates a satisfactory antenna match. The Standard and advanced Syllabus asks us to recall that an SWR of 1.5:1 is acceptable. These statements are indeed correct but unfortunately do not explain the whole story. The most unfortunate situation regarding the understanding by a large number of amateur operators is that many believe that a low SWR reading is an indication of antenna efficiency, nothing could be further from the truth.
An antenna that is resonant at a particular frequency will present as purely resistive, with no reactive component. This situation will occur at one, and only one single frequency. Above this frequency the impedance will be inductive and below it will present as capacitive. Most practical real life antennas including the simple dipole do not naturally present an exact 50 ohms and will not give an SWR of 1:1 at any frequency. Beam antennas use various matching methods such as the gamma match hairpin match etc, to transform their natural impedance to something more acceptable to be fed by a coaxial line.
We can often be fooled into believing that we have an exact 1:1 match due to a number of reasons. The first being that our SWR meter has diode detectors that require a small voltage to forward bias them and at low power levels and low levels of SWR there may be insufficient voltage to forward bias the diode to give a reading, also the diodes do not have a linear conduction curve. We have probably all noticed that we generally get a higher reading of SWR at higher power levels due to this. The second most common reason for low SWR readings is cable loss, very few of us measure the SWR at the input to the antenna, we measure it at the input to our feed line in the comfort of our shack. The reflected wave from any SWR is attenuated by the feed line loss so again at low values of SWR feed line loss combined with the diode biasing issue of our less than perfect SWR meters may result in a zero reflected indication giving us a reading of 1:1.
We have all heard on air comments like “My antenna system really gets out because my SWR is near 1:1 across the whole band” As I mentioned earlier a low SWR is no indication of efficiency in fact if your antenna exhibits an unusually low SWR across a wide frequency range you should actually be concerned. You may in fact have a very efficient dummy load. Antennas with high resistance conductors lossy loading coils and poor ground systems will all present with low SWR over broad frequency ranges.
An example of obtaining a good SWR for all the wrong reasons; a quarter wave vertical antenna above a good ground plane consisting of 120 buried radials will exhibit a base resistance of approximately 36 ohms this will give an SWR (in a 50 ohm system ) of 1.4:1, however if we remove most of the radials the base impedance will rise to approach 50 ohms to give an almost perfect match. The additional 14 ohms is now made up of ground loss resistance resulting in overall poorer performance for the antenna but with a 1:1 SWR. If you were concerned about an SWR of 1.4:1 ( you actually shouldn’t be)an L network matching network could be constructed to give you a near perfect match.
Can anything be gained by reducing our SWR from 1.5:1 to a lower value? and Is reflected power lost power? The answer to both of these questions is probably not. The 2012 edition of ARRL handbook in the chapter on transmission lines has this to say “what actually happens to the energy reflected back down the line? This energy will encounter another impedance discontinuity ,this time the generator .Reflected energy flows back and forth between mismatches at source and load. After a few such journeys ,the reflected wave diminishes to nothing, partly as a result of finite losses in the line, but mainly because of absorption at the load.” Many amateurs believe that reflected power is lost power and that the reflected power is subtracted from the forward power to give the radiated power; this is simply not the case. Looking closely at the above statement from ARRL handbook we see that most of the reflected power eventually gets absorbed by the load. Let’s say that another way, most of the reflected power gets radiated by the antenna. The only part that doesn’t get radiated is that which is lost in the feed line losses. This feed line loss increases as SWR increases but for most practical lengths of good quality coax at HF this loss does not become an issue until we reach SWRs well in excess of 5:1. Feed line losses also increase with frequency so it will be prudent to keep SWR as low as reasonably possible at VHF and UHF.
By now It should be apparent that the need to keep SWR for HF at or below 1.5:1 is only to prevent damage to the output stage of our transceiver or to prevent our transceiver from reducing output power, not to increase the radiation of energy from our antenna. It is very likely that reducing SWR from a value of 2.5:1 will not actually result in any discernible increase in radiated power. With an antenna tuner in the shack to keep our rig happy successful contacts can be enjoyed on antennas with remarkably high SWR’s. Please note that while an SWR meter placed between your radio and your antenna tuner will indicate a low value of SWR once the antenna tuner is tuned, the SWR on your feed line will remain the same, the standing waves on your transmission line will remain unchanged, The only place the SWR has changed is on the short piece of cable between your transceiver and your antenna tuner. The tuner in this case has simply transformed the mismatched impedance to a value closer to the 50 ohms your transceiver expected.
Now to address some myths about SWR, to quote the ARRL handbook “High SWR does not by itself cause RFI, or TVI or telephone interference” A second myth often stated in the same breath High SWR will cause excessive radiation from the transmission line. Again ARRL Handbook states “SWR has nothing to do with excessive radiation from the line. Imbalances in feed lines cause radiation, but such imbalances have nothing to do with SWR”. The most prevalent myth is that you can’t get out if your SWR on your transmission line is higher than 1.5:1 or 2:1 or some other arbitrary number. The ARRL handbook gives this example, a multiband dipole 100 foot long fed with 450 ohm open wire feeder will have an SWR on 80 meters of approximately 19:1, however with a balanced tuner in the shack this antenna is a great performer.
So to answer the question “what does your SWR meter tell you” It tells you nothing more than how close to 50 ohms the thing is that you have connected to the terminal marked antenna/load on your SWR meter and how happy your solid state transceiver feels , it certainly does not tell you how good your antenna is or how well it will perform.