Skull + Key fob = Extended range. Why?

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
It may seem silly but I'm pretty convinced that holding your car's key fob remote to your chin extends the range maybe 30%. I know I'm not alone in this.

What I can't figure out is, why? If anything you might predict that tissue would absorb some of the energy and reduce range, not increase it.
 

SLK001

Joined Nov 29, 2011
1,549
Increased height? I don't know what the freq of these things are, but I imagine that they are in the ISM range of bands (1-3GHz). Also, a key dangling from the fob might provide an additional parasitic antenna element.
 

GopherT

Joined Nov 23, 2012
8,009
It may seem silly but I'm pretty convinced that holding your car's key fob remote to your chin extends the range maybe 30%. I know I'm not alone in this.

What I can't figure out is, why? If anything you might predict that tissue would absorb some of the energy and reduce range, not increase it.
Remember the old rabbit-ear antennas? Holding the antenna improved reception. A big bag of salt water is an antenna.
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
I guess the problem is that I don't understand antennas in general. A wire held up on a tower I get. How is a nearly grounded bag of salt water useful?
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496

nsaspook

Joined Aug 27, 2009
13,086
I've Googled around for a while and found many related links but with more speculation than solid analysis. There seems to be agreement that holding the fob to your head causes your head to become part of the antenna through capacitive coupling. Just why that extends range seems to be vague, although there are plenty of hypotheses:
Human tissue resonates in the 40-80MHz
Back of the skull is a parabolic reflector
Some actual measurements of the effect
Water is just as good - video
IMO it's less capacitance coupling into a 'real' antenna and more of a 'image' antenna using the body/water as a ground plane.
https://www.ece.nus.edu.sg/stfpage/...e Notes/Supplementary Notes/Image Theorem.pdf

 

ci139

Joined Jul 11, 2016
1,898
Counterpoise.
the loops in receiver of FM broadcast antenna that select/prefer the certain station are typically at the size of 30cm/1foot×10cm/4" . . . for about 100MHz ((personal experience atemping to receive FM in bad spot × atm.-c conditions × distant/weak station))
 

Thread Starter

wayneh

Joined Sep 9, 2010
17,496
IMO it's less capacitance coupling into a 'real' antenna and more of a 'image' antenna using the body/water as a ground plane.
https://www.ece.nus.edu.sg/stfpage/elehht/Teaching/EE4101/Lecture Notes/Supplementary Notes/Image Theorem.pdf

So this says the RF energy is reflected, and therefore a bit more of it shows up at the receiver? That makes some sense. The last video I linked seemed to make sense until I thought longer about it. He essentially said the water (or your head) absorbs and re-radiates the energy, but that doesn't explain to me why more energy arrives at the receiver.

If the reflection theory is correct, the range should be reduced if you face away from your car? I'll have to try that. I'll watch that water jug experiment again.
 

nsaspook

Joined Aug 27, 2009
13,086
The 'radiation resistance' of your typical Key Fob closed-loop antenna is very low. This limits the actual power that's transmitted to a low level due to the large mismatch with the near field EM impedance near it. The low antenna resistance is usually RF matched internally to the transmitter chip output impedance to deliver max power but most of it is wasted in the wire resistance instead of being radiated into space to the far-field. Any sizable ground-plane affects the radiation pattern, near field EM impedance and efficiency of these small antennas.

http://pdfserv.maximintegrated.com/en/an/AN3621.pdf
 
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