Standing Wave Ratio (SWR) is an important concept that describes how good of a match exists between a transceiver and antenna system. Recall that a perfect match corresponds to an SWR of 1.0, with higher values indicating some degree of mismatch. (See this article on SWR for more information.)
We often use the presence of low SWR as an indication that an antenna is tuned properly and that it is operating well. Sometimes you’ll hear radio hams say something like “I know my antenna is resonant because the SWR is close to 1.” Or they might say “a resonant antenna always has an SWR of 1” or “My antenna is radiating well because it has an SWR of 1.” All of these statements are a bit misleading, so let’s take a look.
SWR is all about impedance match, telling us how well the antenna or antenna system is matched to the transmitter. Our ham radio transmitters generally have a 50 Ω output impedance so a 50 Ω antenna provides a good match and an SWR of 1. We also normally use 50 Ω transmission lines to maintain a constant impedance throughout the system. SWR does not tell us (directly) how well the antenna is working, it only indicates the impedance of the antenna.
Terminology
There are many definitions of resonance, depending on the particular application area. From a physics point of view, a good analogy is a swing as described here. The behavior described is that a system may exhibit a very strong response when stimulated at or near a particular frequency, known as the resonant frequency. We often see this behavior in antenna systems, when they produce a stronger output at a particular frequency.
The definition of resonance for an antenna is that its impedance is purely resistive. This means that the voltage and current are in phase at the antenna feed point. See this radio-electronics.com article for more information on this.
Antenna efficiency is defined as the ratio of power actually radiated (in all directions) to the power absorbed by the antenna terminals. In other words, efficiency is how much of the power delivered to the antenna actually gets radiated, which is the purpose of an antenna. Some antennas are better than others for having the RF energy radiated.
Some Examples
Let’s start with a simple example: a dummy load. A dummy load is basically a resistor that replaces the antenna so we can test transmitters without actually radiating a signal. For ham radio use, a dummy load will be a 50 Ω resistor, consistent with the fact that most ham radio systems operate at 50 ohms. A good dummy load provides excellent impedance match at all frequencies of interest and it is not resonant. I say it's not resonant because the load looks like 50 Ω for all frequencies (within a specified range). (I suppose you can argue that the dummy load is resonant everywhere because it is resistive at all frequencies. But it doesn't have a peaked response that is usually associated with resonance.) Its antenna efficiency is zero, because it does not radiate any of the power. So here we have a great example of a low SWR but no resonance and no radiated power.
Now let’s look at the classic center-fed half-wave dipole in free space. At the resonant frequency, the antenna has an impedance of 73 Ω, purely resistive. The SWR can be calculated by taking the ratio of the impedance to 50 ohms, giving SWR = 73/50 = 1.5. (By the way, for impedances less than 50 ohms, the SWR is calculated using SWR = 50/R.) Dipole antennas generally work well, so the antenna efficiency will be high and depends on the actual construction of the antenna. Note that the SWR is not equal to 1 at resonance, it is a bit higher. However, an SWR of 1.5 does represent a good match and is normally considered just fine.
Now let’s take a look at an antenna that is nowhere near 50 Ω at resonance, the half-wave folded dipole antenna, described in this article. This antenna has an impedance of about 280 Ω at the resonant frequency. If we connect this antenna to a 50 Ω transmitter, the SWR is 280/50 = 5.6. So here is an example of a resonant antenna that has a high SWR. At the resonant frequency, this antenna will radiate efficiently but will present a difficult impedance to a 50 Ω transmitter. The impedance match is poor and we will struggle to deliver power from the transmitter into the antenna. While we might choose to accept this high SWR, a more practical approach is to add a matching network to produce a 50 Ω impedance.
Many of the antennas we use are designed to be close to 50 Ω (SWR = 1) when they are resonant. For this case, the SWR is a good indicator that the antenna is resonant, which is why most hams associate low SWR with resonance. Low SWR does not tell us anything about how well the antenna is working (antenna efficiency). A dummy load has excellent SWR but fails to radiate. Some antennas are like that, too.
-- Bob KØNR