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Author Topic: Why does where I stand affect my radio reception?  (Read 5704 times)

Offline annie123

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I hope this is the right section for this. Why is it that my physical presence affects my radio? When I am near it the sound is fine but often when I move away the reception becomes inferior with lots of static. And this doesn't happen with every station or with any consistency of station, distance, weather, etc etc.This is not a portable radio but land line.
« Last Edit: 14/07/2008 11:02:27 by chris »


 

Offline Soul Surfer

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Re: Why does where I stand affect my radio reception?
« Reply #1 on: 14/07/2008 10:24:21 »
The radio signal being picked up by your radio is a very complicated thing because all sorts of conducting and dielectric materials reflect radio signals to it is not just the signal coming in a direct line from the transmitter to your receiver. To that is added all the locally reflected signals.  When you add radio signals together if they are in phase they add to the signal if they are out of phase they subtract from it.  This amplification and subtraction depends on the path length differences of the signals and the wavelength of the radio signal used.  My guess is that you are talking about VHF FM radio signals at around 100MHz.  The waves of these signals are about 3 meters long so a path difference of 1.5 metres would cause the reflected signals to subtract from the direct signals

This mix of direct and reflected signals produces a signal with what is called Rayleigh Fading.

Now as far as the radio signals are concerned you are a conducting object that is a significant fraction of a wavelength long so you are quite a good picker up and reflector of VHF FM signals so if you are close to the set you create an improvement in the signal but if you stand the right distance away you can null the signal and nothing will be received.

When you say your radio is operated by a land line I presume that you mean that it ia operated from the electrical mains and not batteries and not that your signal is coming from a telephone or internet cable (in which case I have no idea what is going on)  Even mains operated radios have to receive their signals from the air around them and usually have a wire about  75cm long (one quarter of one wavelength) to do this.  This is sometimes a telescopic rod but might just be a dangling wire or the mains cable itself.
« Last Edit: 14/07/2008 10:33:50 by Soul Surfer »
 

lyner

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Why does where I stand affect my radio reception?
« Reply #2 on: 15/07/2008 11:26:05 »
If you look at TV antennae, you will see a lot of elements ('rods') on a long 'boom' (at least, most TV antennae look like that). Only one of those rods is actually connected to the down-lead. The others are either 'directors' or 'reflectors'. When you stand next to your radio, you are acting in just the same way as one of those 'parasitic' elements; you can effect the directivity in just the same way. Wherever you are standing, there are currents flowing up and down your body, in response to all the radio and TV signals going past / through you. When you are next to your radio, those currents are affecting the way the radio antenna 'sees' the world. You are changing the 'radiation pattern' of the antenna.
 

Offline Pumblechook

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Why does where I stand affect my radio reception?
« Reply #3 on: 15/07/2008 11:53:40 »
Several people in the early days of radio, including Marconi, noticed that moving objects, particularly metal ones, could make radio signals fluctuate (by a reflection adding to or reducing the direct signal...depending on the time delay/phase)and that led to RADAR.  You can use two receiving aerials and partly 'phase out' the direct signal and make the system more sensitive...Passive Radar.    That was done in 1935 at Daventry by Watson Watt to demonstrate Radar but usually Radar involves sending out short pulses and then looking for the reflections with the transmitter off.

 
 

Offline annie123

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Why does where I stand affect my radio reception?
« Reply #4 on: 15/07/2008 21:49:59 »
Thanks everyone. but why doesnt this effect happen when someone else is near the radio?
 

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Why does where I stand affect my radio reception?
« Reply #4 on: 15/07/2008 21:49:59 »

 

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