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SWR OR FORWARD AND REFLECTED POWER

Most operators use a SWR (Standing Wave Ratio) meter to monitor their antenna condition. However, some use a watt meter that measures forward and reflected power. What is the difference?

Analyzing the Standing Wave Ratio of an antenna system can get quite complex. However, for my sake we will keep it simple.

To try and put physical characteristics on SWR, imagine holding a rope that is tied to a fence post. Take the free end and move it up and down. Looking at it from the side, you will see the rope developing a wave on it. A bunch of lazy S's going down the rope. You will observe the wave moving from where you are oscillating the rope to where it is tied. When the wave reaches the end of the rope, the wave has to go somewhere. So it returns to you. This is called the reflected wave.

If enough of the wave is reflected back to you, what you see from the side is the wave going up and down, but not moving anywhere. It is like it is standing in one place going up and down. This is a Standing Wave. The amount of wave going away from you as compared to the reflected wave is your Standing Wave Ratio.

This can be measured by measuring the voltage or current on the transmission line. It is the same reading for either one. The direct reading on the scale of the meter displays the ratio or SWR.

This type of meter, if not a cross needle, is calibrated to full scale on forward. Then the reflected is read normally by a switch on the meter to read the opposite phase of the RF wave.

The lower the ratio, the closer your antenna is to resonance and impedance match (normally 50 ohms).

However, what if you are using a watt meter? It reads forward and reflected POWER, not voltage or current. Does it read the same? No.

The scale on a watt meter is calibrated in wattage, not a ratio like the volt meter. So, what are you seeing?

There are ways to calculate forward and reflected power into SWR, but it can get quite involved. An easier way is by using a graph like the one below.

Notice, that a forward power of 10 watts compared to 1 watt reflected equals a SWR of about 1.9:1. If it was a voltage reading it would be 1.1:1. Therefore, if you are using a watt meter, you will need a much lower reflected power to have a lower SWR reading. For example, you would need a .1 watt reflected only to equal about a 1.23:1 SWR with 10 watts forward.

Either instrument is important to use in your antenna line. Just keep in mind, if it is a watt meter you are using, the reflected power indicates a higher SWR than the voltage or current SWR meters.

73,

Ralph WD0EJA

04-16