Introduction |
## ANTENNA NOTES FOR A DUMMY## Restricted Space Antennasby Walt Fair, Jr., W5ALT## What is SWR?
SWR or
In the best circumstances, we would use a 50 ohm transmission line to connect a 50 ohm impedance antenna to a transmitter rated at 50 ohms output impedance. In that case everything is matched and as long as we make sure there are no currents flowing on the coax shield, everything should work great. Since all parts of the system are matched, transmission line losses are minimized, the transmitter can operate at its designed efficiency and almost all of the power output by the transmitter will get to the antenna and be radiated. But what happens when we connect a 50 ohm transmission line to an antenna with a feed point impedance that is not 50 ohms? Let's say, for example, that the antenna has an input impedance of 10 ohms resistive, which is not too uncommon in short antennas. Notice that the transmission line is 50 ohms and is matched to the 50 ohm transmitter output. However, the impedance mismatch between the 50 ohm transmission line and the 10 ohm antenna causes an SWR of 50/10 = 5:1 and a substantial amount of power is reflected from the antenna back down the transmission line. More than likely the protective circuits in the transmitter will cause it to reduce power.
In the same way, the ratio of the maximum to minimum current is called the "Current Standing Wave Ratio" or ISWR, where the I stands for current. It can be shown that the ISWR is the same as the VSWR, but VSWR is normally easier to measure. Normally we just say SWR, implying VSWR. But don't forget that what is being described is the voltage distribution along the transmission line caused by the mismatch between the transmission line and the load or antenna. In our example, the SWR on the transmission line is 5:1. This is equal to the ratio of the antenna impedance (10 ohms) to the transmission line characteristic impedance (50 ohms). Thus, without knowing anything else, we know that the maximum voltage along the line is 5 times the minimum voltage. And we also know that the maximum current on the transmission line is 5 times the minimum current on the line. Since resistive losses depend on the current squared (I
Now, since in our example, there is a point on the transmission line (at the antenna) where the impedance is 10 ohms and there is also another point on the line (1/4 wavelength away) where the impedance is 250 ohms, it stands to reason that there is some point on the line in between where the impedance is exactly 50 ohms. In fact, that is correct. If we found that point and connected the transmitter exactly at the 50 ohm impedance point, the transmitter would be satisfied and transmit at full efficiency. But, So, let's see what happens with our SWR meter. If we connect the transmitter directly to the antenna and measure the SWR there, the meter will read an SWR of 5:1, just as expected. If we connect the transmitter exactly at the 50 ohm impedance point, the meter will read an SWR of 1:1. If we connect the transmitter at the 1/4 wavelength point, where the impedance is 250 ohms, the meter will again read 5:1, since 250/50 = 5. How can that be? The SWR isn't changing, because the standing waves still exist due to the impedance mismatch between the 50 ohm transmission line and the 10 ohm antenna. Yet the meter reads anywhere from 5:1 down to 1:1 and back to 5:1, depending on where the transmitter and meter are connected. What the heck is going on!?
The ARRL Antenna Book and other textbooks that describe SWR meters generally talk about bridge circuits and directional couplers. In these circuits the transmitted signal is fed across a bridge consisting of resistors that equal the transmission line characteristic impedance of 50 ohms. (Note that some professional meters may use other impedances, but they are generally expensive and not used for amateur purposes.) The meter is essentially measuring the ratio of the impedance
It is apparent that we need to keep the SWR as close to 1:1 as possible to reduce feed line losses. However, our meter cannot read the actual SWR on the transmission line and just because it indicates 1:1 does not mean that the transmission line and antenna are matched with no standing waves on the transmission line. What are we to do? At this point, it would behoove anyone interested in optimizing their antenna to grab a book on transmission lines and study the distribution of impedance along a line. The example used here showed a purely resistive load. While things are more complicated when the antenna shows a resistive and reactive load, the concepts are the same. There is one place along the transmission line where we can always guarantee that we can read the actual SWR with respect to a 50 ohm line. That is exactly at the antenna. In other words, if you want to know what the SWR is for line losses, then read the SWR using 50 ohm coax at the antenna, not at the transmitter. |

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