# Tag Info

0

I don't think that holding the fob to the head does much good, but what does make a huge difference is holding it high up. The simplest flat plane multipath reflection model predicts that the received power is proportional to $$\left(\frac {h_1 h_2}{R^2}\right)^2$$ where $h_1$ and $h_2$ are the heights of the transmit and receive antennas and $R$ is the ...

2

Yes, physics covers sound. The SI unit of frequency is Hertz ($\mathrm{Hz}$), which is the same as $\mathrm{s^{-1}}$. The intensity of sound is measured in decibels ($\mathrm{dB}$), which are not an SI unit, but they are in common use and accepted by many standards bodies.

0

Unfortunately I can't remember where I read this, but I recently read a theory/explanation that it's merely because the key is being held higher up.

-1

Are you sure holding it to your head really makes it better? Your experiment is very poorly designed because you have only sampled instances where the remote wasn't working when your head wasn't in the picture. This is blatant selection bias. No doubt, putting the remote near your head (or anything else conductive) will significantly alter the ...

5

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 ...

5

The way it works has nothing to do with your body. Remotes have their antenna as a more or less circular trace on the board (a loop antenna). The strongest signal is when the top or base of the remote is pointed at the receiver. The weakest signal is when the fob is pointed 90 degrees away, such as when pointing it like a TV remote. Guess which way most ...

44

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 ...

1

Hints: The derivative is linear, so for any two functions $f$ and $g$ one has $(f+g)'' = f''+g''$. Let $x = x_1 - x_2$, and try to manipulate your equations into one equation for $x$.

-1

You can say $$\lim_{t \to \infty} \frac{1}{t} = 0 ,$$ so when the period tends to infinity, in the limit the frequency is 0.

3

Yes. For example, the frequency of times you go to space is zero.

2

A quantity oscillating with frequency equal to zero would simply be static or constant. EDIT When $T$ goes to infinity, it is not possible for an observer to see that the phenomenon is periodic. Think about $T=\text{a few times the age of the universe}$, for instance. If there is no observable periodicity the concept of frequency is not physically ...

0

Frequency of a wave is equal to the inverse of the period. If the period goes to infinity, that means that the wave has only a single crest and trough. In some sense in this limit it stops being a wave, but just a single "disturbance". This is certainly possible, at least in principle, in some media.

Top 50 recent answers are included