Key Fobs and my Head

[dibblebill]

So I saw this years ago on Top Gear and today, not wanting to walk back to my car, got curious and put my key fob to my head. It wasn't working from where I was, but when I touched it to my head, it did. The car locked!

Top Gear never got an answer, but it still appleis to key fobs. Does anybody know WHY putting your fob to your head increases its effective range?


Is this answer accurate?

** Definitive **
Name: Colin, Stamford Hill
Qualification: Electronics City and Guilds
Answer: When you put the transmitter against the body, the body acts like an amplifier and amplifies the RF (radio frequency wave) signal towards the car. The fluid in the body transmits the signal as your body acts like an aerial. Like when you made a crystal radio when you were younger and it worked better by the radiator.

From: http://www.lbc.co.uk/why-do-car-key-fobs-work-better-if-theyre-by-your-head-54965

 

[Speed Daemon]

When I think "amplifier", I think of an active circuit that adds gain by adding electrons. Holding a key fob to your body can't do that.

When you hold a key fob to your head, you're doing several things that can improve RF transmission:

1. You're elevating the transmitter/antenna. Radio and TV stations spend big money to put their transmission antennas on top of very tall structures to be able to "see" a larger area of customers. Compared to holding the fob "down low" at your waist, holding it higher accomplishes the same basic effect.
2. You're giving the antenna a ground plane. Most antennas, especially really tiny ones, rely on tricks like this to make them seem longer than they really are. You can achieve the same effect by holding the fob in your hand and raising your arm. But for reasons unknown it seems to work better when you look silly doing it.
3. Most key fobs transmit in the 300MHz range, with a wavelength of one meter. the quarter wavelength (λ/4) just happens to be roughly the size of an adult male's head. So there may be some resonant coupling going on.

IMO it's mostly #1.

 

[dibblebill]

Ah. I was holding it at head height initially. Thanks for an in-depth response.

 

[SamuraiBigEd]

And consequently it fries the few remaining brain cells many of us rely on!

 

[Speed Daemon]

Worry not, SamuraiBigEd! Thanks to the skin effect, the UHF frequencies aren't able to make it through your cranium, so your brain is safe from your key fob. The real potential for brain damage comes after we put it out on the road with multiple semi-guided, multi-ton rolling projectiles in play.

 

[dibblebill]

That high-velocity plastic-aluminium emergency surgery is a pain.

 

[SiempreTuna]

1. You're elevating the transmitter/antenna. Radio and TV stations spend big money to put their transmission antennas on top of very tall structures to be able to "see" a larger area of customers. Compared to holding the fob "down low" at your waist, holding it higher accomplishes the same basic effect.
2. You're giving the antenna a ground plane. Most antennas, especially really tiny ones, rely on tricks like this to make them seem longer than they really are. You can achieve the same effect by holding the fob in your hand and raising your arm. But for reasons unknown it seems to work better when you look silly doing it.
3. Most key fobs transmit in the 300MHz range, with a wavelength of one meter. the quarter wavelength (λ/4) just happens to be roughly the size of an adult male's head. So there may be some resonant coupling going on.

That all make senses to me.

Given this works for keyfobs, how come it doesn't appear to work for cell phones? Don't 1 and 2 apply there, too?

Maybe even 3, depending on your network the size of your head

 

[dibblebill]

No clue. It might and we just never notice, since they're up against our heads frequently.

 

[SiempreTuna]

Receptions lousy round my way - I'm going to try holding the phone at head height before I try to dial.

 

[Speed Daemon]

That all make senses to me.

Given this works for keyfobs, how come it doesn't appear to work for cell phones? Don't 1 and 2 apply there, too?

I don't know. Cellphones have to work on a wide and increasing range of frequencies, sometimes more than one band at a time. So all bets are off. The patch antennas that are most likely to be used in the current generation of phones have their ground planes built in, so #2 probably doesn't apply. And since the cell site that your phone sees best might be "looking" through a small aperture of foliage, buildings and what-have-you, the ideal place to hold one could be literally anywhere.

With a key fob transmitter, you have a good general idea of where your car's dashboard (where the receiving antenna is hidden) is located. With a cellphone you have no clue where the closest cell site might be concealed. :dontknow:

 

[Davdi]

I know where the nearest cell to home is, I can see it from the bedroom window! It's a micro-cell about 25 yards away

 

[Speed Daemon]

I know where the nearest cell to home is, I can see it from the bedroom window! It's a micro-cell about 25 yards away

You are lucky!

Back when I had an AT&T (pre-SBC) phone, I was delighted when I learned that AT&T Mobility was building a cell site on top of my apartment building. Although I ultimately had a major cell site about 25 feet away, because it was directly overhead and with highly directional antennas, it put me in a null zone where I got no reception at all! Ironically the cell sites that I had been able to get a signal from had their power in my direction turned way down because of the cell site above me. D'OH!!!

 

[SiempreTuna]

I know where the nearest cell to home is, I can see it from the bedroom window! It's a micro-cell about 25 yards away

With that kinda reception, I'm guessing the key fob trick's probably not required

A tin foil hat for all those radio waves, maybe