Standing wave ratio (you might see this mentioned as SWR), is the measure of how closely impedance is matched between your antenna, the radio transmitter and the transmission line. If the impedance does not match on your devices, you could risk having a standing wave.
Standing wave ratio is thought of in terms of the minimum and maximum voltage on the line. Ideally you would want your standing wave ratio to be 1:1 meaning that the maximum voltage’s peak value on the line is one times greater than the minimum voltage on the line as long as the transmission line is at least a half of a wavelength long.
Checking standing wave ratio is a standard thing to do and every amateur radio enthusiast should learn how to do it. SWR is measured in ohms and is super easy to do once you have an SWR meter. The same information can also be found using an impedance analyzer, but the SWR meter was made for just this and will be easier to use.
What exactly is SWR?
We all know that impedance and resistance both oppose current. So the greater the resistance or impedance, the less current flow there is. Kind of like a flowing, unobstructed river. If a beaver comes along and builds a dam, the current will slow down.
Say you find that your setup has 1:1 SWR, this is comparable to having a clear and free flowing river. The impedance of your transmission line and the impedance of your antenna perfectly match up. But what happens when you want to try a different antenna with a higher impedance?
So now you have your transmission line feeding an antenna with a higher impedance. Your SWR becomes 1:2. That’s like having a river flowing into the ocean and then setting up a dam just before it flows out into the ocean. The higher impedance of the new antenna won’t allow as much current to pass through to it and the excess current gets reflected back down the line.
So now that extra current that was sent back down the line goes right to the transmitter and now the transmitter is seeing more current than what it is expecting. The result of this is what’s called a standing wave in the line. A standing wave in the line results in greater line losses and is worse for higher frequencies and in longer transmission line.
How to Avoid Standing Waves
Something to note is that if your source impedance (transmitter) matches your load impedance (antenna), then it doesn’t matter what the impedance of the transmission line is as long it has an electrical length of ½ wavelength (or 1/2x wavelengths). A line of any other length will result in a mismatch of impedance.
Lucky for us HAMs, most RF sources such as transmitters and signal generators are made with the impedance of common transmission lines in mind. Coaxial cable generally will have an impedance of 50 to 75 ohms which is what most radio transmitters will also have.
So now you know that matching up the transmitter’s impedance with the transmission line’s impedance will be an easy task. Now all you have to do is make sure that the load impedance matches up with the line’s impedance. This will result in a 1:1 ratio no matter what electrical length the transmission line is because they will all have the same impedance.
When the antenna and the feed line have mismatching impedances, then the transmitter sees an unexpected impedance. This could result in a loss of power or even damage the transmitter.
Standing wave ratio or SWR is a ratio of impedances between loads and the transmission line. It is important for functionality and the safety of your equipment to get this ratio as close to 1:1 as possible.
A standing wave is created when the signal flowing from the transmitter to the antenna encounters signal that was reflected back from the antenna due to higher antenna impedances. This results in higher signal loss and more HAM frustration.
An SWR meter will help to figure out what the ratio is for your rig. You can count on any radio transmitter and the coax feed line to have a close impedance match but you really need to be diligent when matching the feed line to the antenna impedance.
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