Home > Antennas, Theory > Examples of Signal Strength Related to Power

Examples of Signal Strength Related to Power

The Answer to Antenna Puzzle #2 was based on the idea that signal strength is related to power by squares.

If you remember the power formula from ohm’s law, you know that:

power = voltage squared / resistance

and signal strength is most often measured in voltage per unit length (for example micro-volts per meter.)

To show this relationship, here are a few photos of signal strength measured on the 10M band at four different power levels. This was done by connecting an oscilloscope to a nearby monitor antenna, then snapping a photo of the screen.

Each image is twice the power of the previous image.

For 10 watts we see 0.39 volts (peak-to-peak):

10 watt signal

For 20 watts we see 0.542 volts. Notice that we don’t see twice the 0.39 volts, what we see is the square root of two times the square of 0.39.  So, thats:

0.39 squared = 0.152
0.152 x 2 = 0.304
square root of 0.304 = 0.551 (roughly)

20 watt signal

For 40 watts we see about 0.846 volts. (Notice that it’s not precisely what we expected. That’s because I don’t have precise control over the radio’s power output. It’s just approximate.)

40 watt signal

For 80 watts we see 1.192 volts:

80 watt signal

So, when you go from 10 watts to 80 watts you don’t see your signal strength increase by a factor of 8, you only see it increase by about 3!  (Kind of seems like a rip-off, doesn’t it?)

That’s worth keeping in mind if you ever need to measure the signal strength of your antenna or figure out strange antenna puzzles posted to miscellaneous ham radio sites on the web.

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