Why does a remote car key work when held to your head/body?
This is a really interesting question. It turns out that your body is reasonably conductive (think salt water, more on that in the answer to this question), and that it can couple to RF sources capacitively. Referring to the Wikipedia article on keyless entry systems; they typically operate at an RF frequency of $315\text{ MHz}$, the wavelength of which is about $1\text{ m}$. Effective antennas (ignoring fractal antennas) typically have a length of $\frac{\lambda}{2}=\frac{1}{2}\text{m}\approx1.5\text{ ft}$.
So, the effect is probably caused by one or more of the cavities in your body (maybe your head or chest cavity) acting as a resonance chamber for the RF signal from your wireless remote. For another example of how a resonance chamber can amplify waves think about the hollow area below the strings of a guitar. Without the hollow cavity the sound from the guitar would be almost imperceptible.
Edit: As elucidated in the comments, a cavity doesn't necessarily need to be an empty space; just a bounded area which partially reflects electromagnetic waves at the boundaries. The area occupied by your brain satisfies these conditions.
Edit 2: As pointed out in the comments, a string instrument is significantly louder with just a sounding board behind the strings, so my analogy, though true, is a bit misleading.
Edit 3: As promised in the comments, I made some more careful measurements of the effect in question, using a number of different orientations of remote position and pointing. I've posted these as a separate answer to this question.
As promised in the comments to my answer, I went out and measured the effect in a number of different configurations (a couple of days later than promised :-)). For those of you who just want the conclusions, here they are:
The remote seems to work better when held to the head though the improvement isn't as marked as one might have expected from a google search of the topic. The best possible orientation seems to be to hold the remote flat against your temple. If you aren't willing to hold it to your head, pointing it at the vehicle seems to work better than pointing it up, and there doesn't seem to be much dependence on how high you hold it. Finally, holding the remote to your chest is worse than just holding it at arm's length.
The Experiment
I chose six different positions in which to hold the remote, and in each of those positions I held the remote in two different orientations (described in the list below). In each position/orientation I clicked the remote 3 times, waiting a few seconds between clicks. I recorded the number of times out of 3 that the car responded to my click.
The car, a 2009 Volkswagen GTI, was parked sideways. Temperature: 70.5$^\circ$ F; Barometric Pressure: 29.75 inHg; Humidity 86%; Winds: ~5 mph. There were no large structures around accept for the concrete encased stainless steel vacuum tube of the LIGO Livingston Interferometer which runs parallel to the measurement axis and extends for kilometers in both directions. The battery in my remote is a bit old, but I tried to keep my clicks evenly spaced and began with several discarded clicks to try and cancel out battery effects.
The different orientations are documented in the picture below, but here is a description
- Low (Foward/Up): Held down by my leg pointing the remote towards the vehicle or pointing it directly up into the sky.
- Middle (Forward/Up): Held my arm extended to the right pointing the remote towards the vehicle or pointing it directly up into the sky.
- High (Forward/Up): Held my arm high above my head pointing the remote towards the vehicle or pointing it directly up into the sky.
- Chin (Pointed/Flat): Held against my chin either pointed up into my chin or flat against my chin.
- Temple (Pointed/Flat): Held against my temple either pointed into the temple (like a salute) or held flat against my temple.
- Chest (Pointed/Flat): Held at the center of my chest pointed towards my chest or held flat against it with the remote pointing up.
The Results
In table form:
and graphically:
Remote "key fob" designers intentionally limit size so they conveniently fit in your pocket.
However, the convenience comes at a big price - the tiny loop antenna inside is extremely inefficient, transmitting less than 10% of the energy pumped into it, while the rest is simply converted into heat.
When holding your remote to your head, your arm, shoulder and head form a much larger "body loop" antenna which is almost 100 times more efficient than the remote's antenna.
Then, just like in a transformer, the small single "winding" of the small loop magnetically couples with the larger, nearby single "winding" of your "body-loop".
The magnetic coupling between these two antennas is not great, but it's good enough to make the combination antenna around 2x to 3x better than the remote alone, resulting in a notable improvement in operating range.