# Vtx power is measured in mW to and the regulations talk about milliVolts per meter (mV/m) How to convert?

I’m looking at page 33 of the document: “Licence-Exempt Radio Apparatus: Category I Equipment”

It has this table:

I see the 500mV/m next to the 5.8GHz range, so I can see where people got the idea that you need a license for anything over 500mW. But that doesn’t appear to be what the document actually says.

I’m looking for the maximum power in (milli)Watts a transmitter can be before needing a license, and they’re instead telling me “harmonic emissions measured at 3 m” in millivolts per meter.

I asked Wolfram Alpha and it’s telling me that you can’t convert between the two: how do you convert milliwatts to millivolts per meter? - Wolfram|Alpha

Am I looking at the wrong document?

Before even trying to determine field strength at a given distance one has to know the gain of the TX antenna. Google is your friend.

1 Like

1.6dbic

Thank you for sharing your expertise. Although this is basic for you, I am at a loss here. Is there a formula? Is the axial ratio important? It’s 0.99. I don’t know what these terms mean. I don’t think I even grasp the concept of “volts per meter”. I mean, I know V=IR and W = V x A, but I don’t know how you get meters in there.

I mean, I read this: Basics of Radio Waves - dummies but it just seems to be about frequency and wavelength.

I had a quick look at the legislation and from what I can gather it is a matter of the field strength measured at 3m distance from the device.

So, if you have a somewhat powerful transmitter (in hobby transmitter terms) and you exceed the value mentioned, than, providing the antenna of the device can be replaced, you could simply attached a less effective antenna to the device and you might then be able to get within specified range (mV/m).

The whole legislation appears to be a bit odd because then as a consequence of doing the low performing antenna thing, you now have to comply with other parts of the legislation in regards to RF interference created by the transmitter.

I guess it’s a bit like having an imported vehicle which doesn’t comply with local noise regulations and you replace the muffler in order to comply. …But in doing so you now may have a problem in regards to fuel consumption being pushed higher above the legal limit for specific class of engine.

Yeah it seems like the people making the rules didn’t really think about the physics.

Is there a way to plot this? Like a line graph? Like, if you take gain as a fixed value, and have mW on the X axis vs. mV/m at 3 meters on the Y axis.

Is there an equation for this? Like an “Ohm’s Law” or “ideal gas law” but for whatever this is?