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

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

It has this table:

vtx
vtx816×1025 199 KB

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?

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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
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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.

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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.

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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?