It’s because the antennas in fobs lack an effective ground and are typically unbalanced. The body acts as an RC choke, creating a short on the ground side of the antenna (a typically intentional part of the design). The closer the fob is to your body mass, the better it will perform. Bone may also act as a reflector to some degree, and height can play a role.
Why does holding a key fob to your head increase its range?
91–100 of 161 posts
Re: Why does holding a key fob to your head increase its range?
#92From the top answer:
> 1/2 m ≈ 1.5 ft
Thanks US units for the quote of the day: "Meters are for waves, feet are for antennas"
Re: Why does holding a key fob to your head increase its range?
#93your skull acts as an antenna
I've started using a HAT running the TINFOIL distribution, then I've configured ingress with request token validation. I bill everyone around me on a per-request basis for UNLOCK calls to their car. I've found the system works pretty well at keeping unwanted GET requests to my MIND database by various bad actors.
Re: Why does holding a key fob to your head increase its range?
#94A more likely theory is that the fob transmitter (and antenna) is tuned to put out max signal when near the body. This is it's normal use. If however you tested it with the fob somewhat isolated it would then be out of tune. eg in the factory it would be tuned at a set distance from a "dummy body". It's the same with a walkie-talkie radio. The antenna must be tuned when held in the hand, as your body provides the mis…
Not really - antenna is tuned to the maximum allowed emission power per local regulations (e.g. big differences between EU, US and JPN regulations).
Certification tests do not include any human handling, since it's really unpredictable and varies.
Therefore, tests are just the fob with a (per specifications) positioned Rx antenna (highly controlled environment).
BTW similar effect is observed with the fob being next to the bottle of water. There are peer reviewed studies on these topics, that my RF engineer shared with me when I asked him way back about it, but are heavy on the RF/antenna theory, beyond my understanding.
Re: Why does holding a key fob to your head increase its range?
#95Earlier quoted context omitted.
i suspect the torso's cavity isn't as amplifying as the liquid in your brain. an interesting test would be to try a full bladder vs empty bladder...
We could then measure how the signal range changes as you pee, particularly with the length of the urine stream. I'm serious. This would be an analogue to those navy experiments with shooting streams of sea water to fashion temporary antennas of arbitrary length, instead of having permanent ones that have fixed length and stick out.
Thanks for the liquid antenna pointer, I had no idea this was actually tried! Apparently not only the navy thought about this, but there are idea floating around about using liquid metal for variable length antennas.
One of the links I found: https://www.mwrf.com/technologies/embedded/systems/article/2...
Re: Why does holding a key fob to your head increase its range?
#96A more likely theory is that the fob transmitter (and antenna) is tuned to put out max signal when near the body. This is it's normal use. If however you tested it with the fob somewhat isolated it would then be out of tune. eg in the factory it would be tuned at a set distance from a "dummy body". It's the same with a walkie-talkie radio. The antenna must be tuned when held in the hand, as your body provides the mis…
Not really - antenna is tuned to the maximum allowed emission power per local regulations (e.g. big differences between EU, US and JPN regulations). Certification tests do not include any human handling, since it's really unpredictable and varies. Therefore, tests are just the fob with a (per specifications) positioned Rx antenna (highly controlled environment). BTW similar effect is observed with the fob being next…
I read it in the sense that they would retune or reengineer it if it was discovered that holding it in a hand resulted in poorer performance - ie. they need to factor in the environment it operates in (a hand beside a body) after they design it on paper.
Re: Why does holding a key fob to your head increase its range?
#97top gear (UK) demonstrating this in practice (potato video quality) https://www.youtube.com/watch?v=_jACSPipPSE
[deleted]
Re: Why does holding a key fob to your head increase its range?
#98An explanation I read somewhere is that the radio waves reflect from the inside of your skull, the curvature acts as a parabolic antenna.
Given the conductance of flesh and the frequencies at question the skin depth would only be a few cm at most. The RF energy never makes it through the brain.
Re: Why does holding a key fob to your head increase its range?
#99Earlier quoted context omitted.
I’ve been using this trick for many years. Things like when in a car park and trying to confirm I’ve locked my car at some distance, this has worked. It’s been pretty reliable for me.
One more anecdote, this has not consistently worked for me. Raising the height of the receiver may have an effect but I'm not sure.
We need to cross analysis people across the axes of tall/short and large/small head for posterity
Re: Why does holding a key fob to your head increase its range?
#100Can we now discuss it in the context of 5g? That would be so much more interesting.