How long 2.4GHz antenna should be? The experiment with an antenna meter

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NS1201SA VSWR meter: bit.ly/3m23hmx
Antennas for the win! Hell yeah... At least in a few places in internets you can find something like "FrSky shipped wrong antennas (for XM+, XSR, R-XSR, you name it), they should be 31.25mm while they are shorter!!"
It kind of makes sense, after all, with a speed of light, "correct" quarter wavelength for 2.4GHz is 31.25mm. Too bad it is not true, and all those "pseudo-experts" (damn, I think I'm one of them now) tell the only small piece of truth. Equation "c/f/4" is correct but only assuming antenna is made from the void. And we all know that this is not the case.
In this part, it is time for some practical exercises. Let's take a VNA antenna analyzer and first make a "proper" full quarter length monopole without including Velocity Factor into the equation. And then let's include Velocity Factor. Guess which one will be better...
Oh, I can also show you what happens when you mount antennas touching quadcopter arms!
In a wire, things like dielectric constant, Velocity Factor (Propagation Factor) starts to do their things and as a result, those 31.25 for 2.4GHz antenna length is only a myth.
On top of that, the 2.4GHz band does not mean this is exactly 2.4GHz. Frequency hopping adds something from itself!
This video shows why and what goes where.
en.wikipedia.o...
en.wikipedia.o...