Post by shcm on Feb 5, 2021 9:35:28 GMT
It's an old one that.
1. You're providing a better antenna structure for the fob to radiate with.
2. In the process of doing that, you're actually holding the fob slightly higher anyway.
I expect you yourself will know that for max power transfer, the load impedance (antenna) has to be matched to the source impedance (transmitter) (one impedance has to be the complex conjugate of the other, and you also have to worry about the characteristic impedance of any transmission line between the two).
An "Electrically Short" antenna (i.e. one that is significantly shorter than the wavelength of the frequency being used, i.e. much less than lambda/4) usually has a very low input impedance (and also not a great radiation pattern). In practice, that low antenna input impedance means it's not easy to match the antenna to the transmitter, so you end up with a system that doesn't radiate particularly well and a lot of the power gets reflected back into the transmitter. Wasted power!
(Very efficient, electrically short antennas are a bit of a mythical holy grail. Ultimately to radiate well, you have to move charge a significant proportion of the wavelength involved and also not have a structure than cancels out part of the field in the process).
@ 434MHz the wavelength is approx 70cm. You can see that any antenna @ this wavelength in a fob, will be an electrically short one .i.e. pretty darn poor. By placing the fob under your chin, you're capacitively coupling the signal into the skull/skeleton, which provides a far better match/antenna structure. Not a great one, but far better than the fob itself.
Possibly also the skull is acting as a resonant structure (cavity?) in some way. While you can make some rough approximation as to how such structures will radiate, often you have to attempt to computer model (simulate) such things, to get an accurate understanding and that modelling is not straight forward. The maths is quite involved. It has been tried with this "phenomenon", to varying degrees of success.
So, a better match, radiate more power, obviously you'll get a bigger signal at the receiver. However, if the receiver has already been driven into compression (hit the "end stops" and distorting a bit like you might get in an audio amplifier) by the presence of another huge signal getting into the receiver, it becomes increasingly difficult to extract your tiny fob signal on top of that, even if you've upped the effective radiated power a bit. It may help under some circumstances, but it depends on the level of interference.
It will kill no brain cells. It's non-ionising radiation. It's about 10uW-ish at the most. If you think that is a problem, then I'd stop using you're phone right now! (and that's almost certainly not a problem either). At worst it's a very small heating effect. If it's anything else (which it isn't) then maybe people should also stop eating food out of microwave ovens! (Oh gawd, what false rumour may I have started, with that being quoted out of context!)
