I am disappointed to see no mention of "passive radiator" nor "parasitic element" on that page anywhere: https://en.wikipedia.org/wiki/Driven_and_parasitic_elements#...
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Why does holding a key fob to your head increase its range?
61–70 of 161 posts
Re: Why does holding a key fob to your head increase its range?
#62A 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…
Even simpler, tune in an FM radio and move your hand near the antenna. It will sound better or worse depending on your hand position and how close it is to the antenna.
So a homemade theremin?
Re: Why does holding a key fob to your head increase its range?
#63Earlier quoted context omitted.
RF is harmless below the point where it directly heats tissue. Having a full strength cell tower transmitter inside your home is not safe due to the intensity of the transmission, it will directly heat tissue inside your body.
> RF is harmless below the point where it directly heats tissue. Would it heat tissue anymore than the Sun (in warm places) or your heater or air-conditioner set to heat already does 10x stronger?
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Re: Why does holding a key fob to your head increase its range?
#64Wow, I first heard about this in the early 90's in high school! I was told to hold it under your chin. It seemed to work, but I never formally did any experimentation nor did I have the statistics base back then. I still did it up until I got a proximity fob a few years ago. However, I always felt silly because I thought it was an old wives tail from my childhood. Very cool to learn there's actual science behind it.
Re: Why does holding a key fob to your head increase its range?
#65Not sure I agree with the cavity theory. I suspect it’s you, you big ole bag of salt water, acting as a capacitive ground plane for the antenna. It’s like mounting a ham or CB antenna on a car and having the metal of the car not be in direct contact with the ground, but it acts as a capacitive ground plane. You can do something similar with a vertical antenna using ground radials in a star pattern on the ground.
Re: Why does holding a key fob to your head increase its range?
#66Earlier quoted context omitted.
RF is harmless below the point where it directly heats tissue. Having a full strength cell tower transmitter inside your home is not safe due to the intensity of the transmission, it will directly heat tissue inside your body.
> RF is harmless below the point where it directly heats tissue. Would it heat tissue anymore than the Sun (in warm places) or your heater or air-conditioner set to heat already does 10x stronger?
Microwave RF that is somewhat directional behaves a lot like laser light.
If you were to step into the beam carried by a waveguide for something like a powerful TV transmitter, it will cook you through like a death ray out of science fiction. Fortunately it's not actually collimated, so just a few feet away the energy is barely enough to cook your retinas faster than your blood supply can cool them (which can cause blindness). Otherwise, yes, it's much like infrared heating effects. If infrared penetrated a few centimetres deep, anyway.
Re: Why does holding a key fob to your head increase its range?
#67A 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…
Even simpler, tune in an FM radio and move your hand near the antenna. It will sound better or worse depending on your hand position and how close it is to the antenna.
Back in the early 00's when we still had analogue VHF TV signals, we wanted to watch the football world cup at school. Of course there was no cable or anything at the school and no VHF antenna and streaming was not a possibility yet so we had to be creative. We found an antenna cable and tried to manipulate the cable and use whatever as an antenna. We tried the sheet metal roofing, but that didn't work very well, we tried some metal rods and that didn't work great either. By chance we found out that if someone held the cable in one hand and leaned out of the window with their other hand stretched out we got a signal that was good enough to watch the game. Well, for everybody except the poor bastard that had to stand behind the TV halfway out the window.
Re: Why does holding a key fob to your head increase its range?
#68Earlier quoted context omitted.
That was my first thought too, but the OPs tests had the range only improving when held to the head.
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.
Re: Why does holding a key fob to your head increase its range?
#69I mean, it's electromagnetic waves. The right wavelength will bounce off things and increase likelihood of receiving a coherent signal. Works for a lot of things; RF, sound, light. RF has low energy and huge wavelengths. IIRC keyfobs are in the 200-600MHz range.
Ha, found some (kind of) evidence:
Radiowave Effects on Humans - March 28, 1980 / T. Neil Davis (https://www.gi.alaska.edu/alaska-science-forum/radiowave-eff...)
One reason the question is unanswered is that the energy absorbed by a human from radio waves depends upon the relationship between the size of the human and the frequency of the radio waves. Just as a TV antenna of the right length and orientation picks up the best signal (the most energy) from a transmitted wave, so it is with a human being. It appears that the cranial cavity of a mammal will resonate at specific radio frequencies determined by the size of the brain cavity. At these resonant frequencies the human head will absorb vastly more radiowave energy than it will at other nearby frequencies.
An adult's head will resonate at a frequency between 350 and 400 MHz (megahertz). Being smaller, a child's head will resonate at a higher frequency, somewhere between 600 and 850 MHz. Since each individual may have his or her own resonant frequency, a particular frequency radiowave might affect one person more than another. Consequently, testing on humans--even if people are willing to let this happen--can be rather complicated.
Re: Why does holding a key fob to your head increase its range?
#70Earlier quoted context omitted.
> RF is harmless below the point where it directly heats tissue. Would it heat tissue anymore than the Sun (in warm places) or your heater or air-conditioner set to heat already does 10x stronger?
Microwave RF that is somewhat directional behaves a lot like laser light. If you were to step into the beam carried by a waveguide for something like a powerful TV transmitter, it will cook you through like a death ray out of science fiction. Fortunately it's not actually collimated, so just a few feet away the energy is barely enough to cook your retinas faster than your blood supply can cool them (which can cause b…
>Microwave RF that is somewhat directional behaves a lot like laser light.
While fob frequency is microwave, isn't it also much lower energy compared to, say a microwave oven, powered by mains power to cook food?
And also the fob operating in the low 100s of Mhz, not GHz, wouldn't that make it less dangerous?