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Author Topic: radio reception at a road junction.  (Read 3788 times)

paul.fr

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radio reception at a road junction.
« on: 07/05/2007 10:06:33 »
whilst coming home this morning with the radio on, we approached a junction. upon crossing the junction the radio station changed, then went back to the origional one once we had gone past/through the junction.

it was as if the station was broadcasting straight down one road )east to west) but not on the other road (north to south)

what was going on?


 

Offline Soul Surfer

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radio reception at a road junction.
« Reply #1 on: 08/05/2007 16:47:45 »
This could in theory happen if someone was using a leaky feeder transmission system to distribute local radio information on the same channel that you were listening to.  However I do not think that these are used in the UK.  On the other hand if you were using a RDS receiver, cables in or around the road could cause a local null and persuade your receiver to change channel briefly.

The only time you are likely to notice this is with a station like  UK radio 4 which frequendly splits to give local news programmes at various times and you managed to briefly switch between regions.
« Last Edit: 08/05/2007 16:49:54 by Soul Surfer »
 

another_someone

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radio reception at a road junction.
« Reply #2 on: 08/05/2007 19:59:41 »
What I find more commonly is not that the radio changes station on a junction, but sometimes the radio gets overpowered (I suspect in the IF band rather than the RF band) by some other transmitter that is totally out of channel (often local taxi radio comms).
 

Offline JimBob

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radio reception at a road junction.
« Reply #3 on: 08/05/2007 21:33:20 »
Paul, I think it is because of the gallows that once were there. The ghosts still linger and messes with your radio for something to do.
 

Offline daveshorts

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radio reception at a road junction.
« Reply #4 on: 09/05/2007 13:14:29 »
Depending what frequency the radio was, and what the buildings were made of, I guess you could get the buildings altering the way that radio waves travel. Possibly funneling them down the road, or if there was a transmitter along the perpendicular road it may have been blocked by hte houses on either side.
 

paul.fr

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radio reception at a road junction.
« Reply #5 on: 09/05/2007 13:19:16 »
Depending what frequency the radio was, and what the buildings were made of, I guess you could get the buildings altering the way that radio waves travel. Possibly funneling them down the road, or if there was a transmitter along the perpendicular road it may have been blocked by hte houses on either side.

The frequency was in the Fm range 96 to 98. The strange thing is not only does this happen in built up area's, are you suggest Dave, but also whilst travelling along the motorway (M180)
 

lyner

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radio reception at a road junction.
« Reply #6 on: 14/05/2007 10:28:42 »
There is a well known phenomenon called the 'capture effect' with frequency modulated  (fm) transmissions.
Broadcast fm uses quite a wide deviation (i.e. the frequency of the carrier wave is swept over a wide range of frequencies by the modulating sound signal). This is what gives fm a better audio signal to noise ratio  than amplitude modulation- your  receiver, effectively, presents your ear with only a small amount of noise but a big audio signal - nice to listen to.  The same demodulation process tends to latch on to the bigger of two received signals and reject the smaller.
In many areas, there are two signals at similar levels but the receiver will just present your ear with one programme; it has 'captured' it.
As you go across your junction, the gap in the buildings / local hills / etc  allows the interfering signal to dominate, briefly. It only needs to be a bit higher than your wanted signal for it to be captured. The change over is quite dramatic - like a switch.
This could never happen with amplitude modulated signals (am) - you would just hear the two signals- one on top of the other - and you would have re-tuned long ago due to the high level of interference.
Wide deviation fm is great but, below a certain 'threshold' level of interference, you lose it completely . This makes it unsuitable for long distance communications, unless you use very sophisticated receiver techniques. Narrow band  fm has better 'carrying' properties but worse sound quality; it is very popular for  mobile  radio communications as the transmitter is simpler  to build.
 

Offline techmind

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radio reception at a road junction.
« Reply #7 on: 25/09/2008 17:03:34 »
Could be "physics" effects as discussed, but if you have an auto-retuning car radio, it could be that misbehaving. There's been a recent discussion among the techs that BBC Radio 4 from Oxford transmitter is particularly susceptable to causing momentary breakthrough of incorrect stations:
http://groups.google.com/group/uk.tech.broadcast/browse_thread/thread/27a26625a495a98d#

Apparently Oxford R4 rebroadcasts the Sutton Coldfield transmitter, which transmits that alternative Radio 4 frequencies include various ones around 103.5-104.5MHz (as used in Wales). When applied verbatim to the Oxford region, these could point to various other BBC Local services... Apparently older radios retune first, then realise what they've found is not what they were looking for, and retreat back. Better radios have two receivers internally, and only hand-over the speakers once everything has checked out.
« Last Edit: 25/09/2008 17:06:38 by techmind »
 

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radio reception at a road junction.
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