|
|
Post by rmc on Nov 20, 2012 3:29:15 GMT
I doubt this is actually a myth, because I've personally seen it myself. But -- I cannot find a clear explanation as to why this apparently is possible -- other than, perhaps this: en.wikipedia.org/wiki/Body_capacitanceSo, the next time you have that old radio all tuned into the station of your choice, only to notice that whenever you step away, (notice: I said step away - not "let go": meaning there was no physical contact apparently creating the preferred antenna situation) its signal becomes nothing but intermittent static, how do you explain it? Can standing near an appliance antenna actually help reception? If so, WHY?? |
|
|
|
Post by GTCGreg on Nov 20, 2012 3:37:16 GMT
There could be a number of reasons, all related.
First, your body could act as a reflector. Radio waves can bounce off any conductive surface (which your body is) and be directed back to the radio. If the distance is just right, the reflicted signal will be in phase with the directly received signal and cause it to be stronger.
Another reason could be because your body blocks an out of phase signal that is canceling out the directly received signal. This is called multi-path. By blocking the multi-path signal, the radio could receive the desired station better.
Just because you are not directly in contact with the radio, doesn't mean that your body can not become part of an antenna system.
|
|
|
|
Post by rmc on Nov 20, 2012 4:02:53 GMT
I've recently been told of a concept called: "Stray Capacitance"
Not too sure I understood that explanation.
Thanks to GTCGreg for the reflectance explanation, however!
|
|
|
|
Post by GTCGreg on Nov 20, 2012 4:09:39 GMT
Any two conductors, such as your body and the ground or your body and the radio's antenna, separated by an insulator (air) forms a capacitor. However, the amount of capacitance coupling between your body and the radio's antenna a few feet away would be so small that it wouldn't cause much effect. If you were holding the radio in your hand, capacitive coupling could become a factor.
Where you see your body's capacitance come into play is when a static charge builds up on you. It's the electrical charge on your body's capacitance that causes you to get zapped when touching an object after walking across a carpet in a very dry environment.
|
|
|
|
Post by freegan on Nov 20, 2012 5:12:21 GMT
I believe that this effect is the result of the same basic principle that allows us to use trees as antennas.
All living organisms consist of a predominance of a dielectric fluid; water.
This fluid is capable of absorbing and re-radiating electromagnetic waves.
You may have noticed that vertical organisms near an antenna or in the path of of its signal produce a 'ghost image'.
When you adjust your antenna, standing close to it, you are optimizing the reception with your 'ghost image' present.
As you move away your 'ghost image' no longer produces the same constructive interference that it did when you were close, hence the signal deteriorates.
|
|
|
|
Post by silverdragon on Nov 20, 2012 9:54:55 GMT
Hmmm... I was addressing this the other day... the Kitchen Radio tends to be in a bit of a "Radio blackspot" and looses or gains the signal if we are moving around....
My Final answer was "Lump of damp string" antennae... as in the "T" section you get with Hi-Fi equipment?... it was nicknamed such as someone somewhere once said it was "about as much use as"...
Then someone ELSE decided to test that theory, and found out that a lump of damp string actually WOULD work as a radio antenna?....
who knew?....
Or is that just "Urban myth"?...
Ok, so, does this add an extra section to the myth....?
We all know of the Kite flying myth of Electricity and thunderstorms, so, was that (above) experiment valid?... WILL a lump (or length, 27 inches to be exact..) of damp string work as a valid radio antenna?....
As far as I know, yes, but a metal areal will work better, of course, and doesnt need to be damp.
|
|
|
|
Post by OziRiS on Nov 20, 2012 22:23:51 GMT
Will a lump of damp string work as an antenna? Maybe, though probably not very well, depending on the frequency you're trying to receive/transmit on. But a length of it? Sure. I've done it.
When I was in the army, I was a radio op. GTCGreg's explanation of the human body working as a reflector was exactly the reason why, when we had to make our own antennas, we had a length of cable of at least 30 feet between the radio (and the ops) and the antenna itself. We also tried to place the antenna so that any other things (trees, buildings, vehicles etc.) in the immediate vicinity would work to our advantage as reflectors, concentrating the strength of our signals in the direction we needed.
|
|
|
|
Post by c64 on Dec 5, 2012 18:20:31 GMT
All antennas interfere with each other. In RF technology, you differ between "near field" where two antennae become "one", the far field and the Fresnel Zone in between.
Just look at a Yagi antenna for example. It's nothing but a simple dipole but a lot of "dead" stuff in front of it and some more on its back. The purpose of all those grounded metal rods is to affect the dipole and to concentrate the RF from a certain direction on the dipole. This gives you an "antenna gain" of up to 35dB. And every 3dB, the power is doubled so you can tune your radio into a source which is several thousand times weaker than you need using a conventional antenna.
And the antenna affects the resonance frequency of a radio oscillator. So change the parameters of an antenna by coming close or going away and the radio tunes to a different frequency. That's the main reason why radio sets used to have external aerial even if it wasn't bigger than one you could built in. The super simple oscillator circuits couldn't keep their frequency with people wandering around it on close range. With a built in antenna, you couldn't leave the set after properly tuning it to a station. This only had changed by using much more complicated circuits like the superhed tuner or crystal stabilized ones.
There is a very cool device to adjust radio sets and which is very useful for other purposes, it's called the "grid dipper". It contains a very stable oscillator running its power through a simple unstable one which is exposed to stuff close to it. When there is an oscillator - no matter if active or passive - with the same frequency, you get a "dip" on your meter indicating that something steals energy from your dipmeter. The modern reincarnation of the "grid dipper" (measuring the amplitude on a vacuum tube grid) is the transistor dipper which doesn't work the same way but gives you the same result. Those things had also vanished in favour to ultra high speed frequency counters. Nobody uses those classic dippers any more - at least with knowing it! The decent of those ancient grid dippers is the modern RFID technology! Hooking a grid dipper to the Gameport of a PC gives you a superb RFID reader.
|
|
|
|
Post by srracing on Jan 28, 2013 3:45:31 GMT
Well you sure killed that thread with more than the OP wanted to know. (or understand) ;D
The answer is YEP. A human body can certainly aid or decrease the signal to the radio.
|
|
