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Author Topic: The Penetration of Aunty Mary !! (an amplitude modulation/fm question)  (Read 11023 times)

Offline neilep

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Hi All.....luff ewe !!

awwwwwww !!!


Erhmm.....

See my tunnel ?



Nice eh ?...being delivered next Tuesday..

but before I decided to purchase said tunnel..I had to take it for a test drive.....one thing I found was that AM (amplitude modulation) had no problem penetrating through and being received by happy me !!....FM had no chance !!


Why is this ?.....why is the penetration of AM so much better than that Of FM ?

I so wish I knew !!...I do...
*sheepy feels life is pointless unless an answer forth-comes!!*...I don't want to commit Sheepicide !!


If ewe help me , we can all play in my tunnel when it comes....and we can have a partayyyyyyyyyy !!!!!

YAYYYYYYYYYYYYYYY !!!!















 

another_someone

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AM and FM has nothing to do with it, but MW and VHF have much more to do with it.

The use of the word AM as a euphemism for MW/LW, and FM for VHF is erroneous and confusing.  It just so happens that we mostly transmit commercial signals in the VHF band in FM, while the older transmissions on MW and LW are generally AM.

The longer the wavelength, in general, the more it will penetrate through semi-permeable materials, or leek around them (a wavelength of half a kilometre will take less notice of a few metres of concrete than a wavelength of half a metre).

This does not mean that a deep tunnel will be able to receive MW signals, but a shallow tunnel is more likely to be able to allow some penetration of MW than of VHF signals.
 

Offline neilep

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AM and FM has nothing to do with it, but MW and VHF have much more to do with it.

The use of the word AM as a euphemism for MW/LW, and FM for VHF is erroneous and confusing.  It just so happens that we mostly transmit commercial signals in the VHF band in FM, while the older transmissions on MW and LW are generally AM.

The longer the wavelength, in general, the more it will penetrate through semi-permeable materials, or leek around them (a wavelength of half a kilometre will take less notice of a few metres of concrete than a wavelength of half a metre).

This does not mean that a deep tunnel will be able to receive MW signals, but a shallow tunnel is more likely to be able to allow some penetration of MW than of VHF signals.

THANK EWE GEORGE.

Despite the incongruity of my am/fm nomenclaturial terminology ewe still managed to understand what I mean.......respect !

Can ewe help me further as to explain to me why a wavelength of 0.5km is not phased by a few metres of concrete ?....my tunnel is over a kilometer in length !...this has no bearing yes ?...it's the depth of the wall instead ?


I think I need a piccy to help me here !!
 

lyner

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It all depends upon the conductivity of the ground. LF and MF can be screened quite effectively by the steel frames of some buildings where shorter wavelengths can often bounce their way through.
When an e/m wave hits a surface, the atoms are affected by it. Metals have loads of free electrons in them and these move readily. They re-radiate the wave (which explains why metals are shiny and they reflect radio waves. BUT, because of the resistance (a friction effect) some energy is lost and this actually causes the wave to penetrate a little way into the metal. This is the skin depth and is  a small fraction of a wavelength for metals.
Good insulators do not absorb much energy and the atoms are distorted a bit by the e/m fields. The wave just tends to change speed and direction a bit (refraction) due to the 'permitivity'.
'Earth' tends to be a relatively poor conductor and shows both behaviours. The skin depth is a bigger fraction of a wavelength for poor conductors. For mf (wavelength of, say, 300m), the skin depth can easily be several metres.

Seawater is a pretty good conductor so it has a small skin depth and radio waves hardly penetrate at all. Submarines communicate at very low frequencies and use an enormous antenna, trailing behind fairly near the surface.
« Last Edit: 17/03/2008 16:53:54 by sophiecentaur »
 

Offline neilep

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It all depends upon the conductivity of the ground. LF and MF can be screened quite effectively by the steel frames of some buildings where shorter wavelengths can often bounce their way through.
When an e/m wave hits a surface, the atoms are affected by it. Metals have loads of free electrons in them and these move readily. They re-radiate the wave (which explains why metals are shiny and they reflect radio waves. BUT, because of the resistance (a friction effect) some energy is lost and this actually causes the wave to penetrate a little way into the metal. This is the skin depth and is  a small fraction of a wavelength for metals.
Good insulators do not absorb much energy and the atoms are distorted a bit by the e/m fields. The wave just tends to change speed and direction a bit (refraction) due to the 'permitivity'.
'Earth' tends to be a relatively poor conductor and shows both behaviours. The skin depth is a bigger fraction of a wavelength for poor conductors. For mf (wavelength of, say, 300m), the skin depth can easily be several metres.

Seawater is a pretty good conductor so it has a small skin depth and radio waves hardly penetrate at all. Submarines communicate at very low frequencies and use an enormous antenna, trailing behind fairly near the surface.

THANK YOU VERY MUCH !


A lot for me to digest but I am getting there......Fascinating the explanation why metals are shiny !!
 

Offline techmind

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Can ewe help me further as to explain to me why a wavelength of 0.5km is not phased by a few metres of concrete ?....my tunnel is over a kilometer in length !...this has no bearing yes ?...it's the depth of the wall instead ?

I would be very surprised if you receive a 500kHz signal very far (ie more than 10-20metres) inside a 1km long tunnel. Are you sure it's not being deliberately re-broadcast via a "lossy feeder" cable running along the tunnel wall somewhere?

In general you can only propagate wavelengths which are roughly equal to or smaller than the tunnel diameter through a (substantially conducting-walled) tunnel. If you have a highly conducting tunnel and a diameter close to the wavelength then you get a "waveguide" which is a very efficent "pipe" for RF signals, and widely used (especially 30 years ago) for microwave equipment. See also http://en.wikipedia.org/wiki/Waveguide_%28electromagnetism%29
 

Offline neilep

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Can ewe help me further as to explain to me why a wavelength of 0.5km is not phased by a few metres of concrete ?....my tunnel is over a kilometer in length !...this has no bearing yes ?...it's the depth of the wall instead ?

I would be very surprised if you receive a 500kHz signal very far (ie more than 10-20metres) inside a 1km long tunnel. Are you sure it's not being deliberately re-broadcast via a "lossy feeder" cable running along the tunnel wall somewhere?

In general you can only propagate wavelengths which are roughly equal to or smaller than the tunnel diameter through a (substantially conducting-walled) tunnel. If you have a highly conducting tunnel and a diameter close to the wavelength then you get a "waveguide" which is a very efficent "pipe" for RF signals, and widely used (especially 30 years ago) for microwave equipment. See also http://en.wikipedia.org/wiki/Waveguide_%28electromagnetism%29


THANK YOU for this...

The tunnel in question is the Hatfield Tunnel

What follows is blatnatly nickedborrowed from this site http://www.iht.org/motorway/hatfield.htm  as you can see...it's 1150m long......can you see any in the following that enables me to hear the radio all the way through it ?

The Hatfield Tunnel
Hatfield tunnel

The 1150m long tunnel is a cut-and-cover structure constructed east of the existing A1 between Cavendish Way and the Green Lanes Roundabout. The tunnel accommodates the full motorway formation width with a continuous separating wall along the motorway centre line. Raised walkways along both sides of each carriageway accommodate the extensive cabling and service ducts required for tunnel services and motorway communications.

The construction of the tunnel is of traditional reinforced concrete, the roof, walls and foundation bases being continuous to form a two bay portal. The tunnel is founded on the glacial gravels with spread footings, but where additional heavy loads from the development structures affect the central wall its foundations are strengthened locally by insitu bored piles founding in the hard chalk sub strata.

It is estimated that nearly 10,000 tonnes of reinforcement and over 80,000 cubic metres of concrete were used to construct the tunnel.

Tunnel Services

Services include power supplies, lighting, ventilation, environmental control, plant monitoring, pumping, communications, fire fighting, security and traffic surveillance. Automatic control systems will continuously monitor and report to central control and maintenance on the operational state of the plant and information transmitted will be stored on computer disk.

The lighting system for each bore comprises two sub-systems. Basic lighting comprises a near continuous row of twin 65 watt fluorescent Iuminaires over the centre of each of the near-side and off-side lanes. Boost lighting comprises 135 watt and 180 watt low pressure sodium luminaires arranged over catch entry and exit for optimum visual adaptation.

The tunnel is longitudinally ventilated by 52 jet fans in each bore, located above the walkways at roof level and automatically controlled.

Panels at 39m intervals on the nearside walkway, provide emergency telephone facilities connected to police control, and automatic fire hose reels. Radio aerials for emergency and maintenance services are mounted in the tunnel roof.

Closed Circuit television System is provided in each bore of the tunnel and is remotely controlled from Police Control at Hertfordshire County Police HQ at Stanborough.

Sensor loop systems in the carriageway surface at intervals throughout the motorway detect vehicle movements. The data obtained is used to provide the police control room with information of traffic flow patterns and potential problems.
 

lyner

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Reinforced concrete - pretty impenetrable by mf but Hatfield is bang next Brookman's Park BBC mf transmitter so the local fieldstrength would be quite high you could even be getting some funny effect through cables underground. It will only be shallow - cut and cover, there could be all sorts of currents flowing through the ground so near to a main transmitter with 150kW sloshing about.
I think that's your answer, neilep, you mediumwaveologist; near field effects!

Crazy, isn't it - I was Googling the subject and the first hit was the rubbish I had already spouted!
« Last Edit: 17/03/2008 21:54:01 by sophiecentaur »
 

Offline turnipsock

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I had a dream about going into a tunnel once.
 

Offline neilep

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Reinforced concrete - pretty impenetrable by mf but Hatfield is bang next Brookman's Park BBC mf transmitter so the local fieldstrength would be quite high you could even be getting some funny effect through cables underground. It will only be shallow - cut and cover, there could be all sorts of currents flowing through the ground so near to a main transmitter with 150kW sloshing about.
I think that's your answer, neilep, you mediumwaveologist; near field effects!

Crazy, isn't it - I was Googling the subject and the first hit was the rubbish I had already spouted!

Yes..yes ewe are absolutely right....Brookmans Park has a great big transmitter type thingy !.......Thank ewe very much...I am sure this must have something to do with it...

I do like my mediumwave !!......well.....I don't have DAB in the car and I like my Radio 5 live !!
 

Offline neilep

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I had a dream about going into a tunnel once.


oooh !!......did ewe come out the other side ?
 

Offline techmind

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Reinforced concrete - pretty impenetrable by mf but Hatfield is bang next Brookman's Park BBC mf transmitter so the local fieldstrength would be quite high you could even be getting some funny effect through cables underground. It will only be shallow - cut and cover, there could be all sorts of currents flowing through the ground so near to a main transmitter with 150kW sloshing about.
I think that's your answer, neilep, you mediumwaveologist; near field effects!

Crazy, isn't it - I was Googling the subject and the first hit was the rubbish I had already spouted!

Yep - according to the map http://www.streetmap.co.uk/newmap.srf?x=522500&y=207500&z=4&sv=522500,207500&st=4&ar=N&mapp=newmap.srf&searchp=newsearch.srf&ax=522803&ay=207635 the Brookmans Park MW transmitter is just 6km south-east of the tunnel. I think therein lies your answer. With the tunnel being cut-and-cover, enough signal probably leaks in through the roof.
 

Offline neilep

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Reinforced concrete - pretty impenetrable by mf but Hatfield is bang next Brookman's Park BBC mf transmitter so the local fieldstrength would be quite high you could even be getting some funny effect through cables underground. It will only be shallow - cut and cover, there could be all sorts of currents flowing through the ground so near to a main transmitter with 150kW sloshing about.
I think that's your answer, neilep, you mediumwaveologist; near field effects!

Crazy, isn't it - I was Googling the subject and the first hit was the rubbish I had already spouted!

Yep - according to the map http://www.streetmap.co.uk/newmap.srf?x=522500&y=207500&z=4&sv=522500,207500&st=4&ar=N&mapp=newmap.srf&searchp=newsearch.srf&ax=522803&ay=207635 the Brookmans Park MW transmitter is just 6km south-east of the tunnel. I think therein lies your answer. With the tunnel being cut-and-cover, enough signal probably leaks in through the roof.


Voila !!!!...with the aid of this rather spiffy site and the generosity of the real heroes here (ie: the clever peeps)...we have been able to solve the mystery as to why I can listen to Radio 5 Live without interruption for the 10 seconds or so I am in the Hatfield Tunnel !!

THANK EWE ALL   *end of ingratiation mode*

 

Offline Pumblechook

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I have found the opposite in other tunnels.  Depends how powerful the transmitters are and whether the tunel is in line with the VHF/FM one(s). 

Communications in tunnels is pretty tricky.  I gather to fully equip London Underground for mobile phone use could cost well over 1 Bn.  By comparison communicating with someone on the Moon could be done for a few hundred pounds. 
 

lyner

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Quote
communicating with someone on the Moon
But this would be communicating with several million people!

The inverse square law makes life very convenient when communicating in space. Doubling the distance only takes the power down to 1/4. Doubling the distance with cable could involve a factor of thousands over a large distance. Then there's the problem of total bandwidth needed for a telephone network.

mf really doesn't do well through steel whereas VHF and UHF can find its way into all sorts of nooks and crannies. A tunnel is several wavelengths in diameter and so it 'may' act like a waveguide on occasions. The Mersey and Dartford Tunnels seem to be served well (not just by chance!).
Someone mentioned 'leaky feeders' as a technique of comms in tunnels; that would be for, essentially, vhf frequencies. They started trying to use a feeder in mines with dipoles connected every so often then found that it worked just as well without the dipoles. The cheaper the feeder, the better it worked.
mf is much harder to arrange; it has to involve loops and you lose a lot of power in the ground - the problem is that mf is well below the waveguide cut-off frequency.
« Last Edit: 24/03/2008 22:29:59 by sophiecentaur »
 

Offline Pumblechook

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Indeed.  Radio Hams including a pal of mine bounce signals off the Moon with relatively modest equipment.

I gather frequencies over 1000 MHz can be effective if fired at highish powers from Yagis into tunnels.   Lower frequencies don't seem to work as well even though the tunnels in theory are big enough to support waveguide modes.   
 

lyner

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Quote
unnels in theory are big enough to support waveguide modes.   
Yebbut the walls of a tunnel are not highly conducting. I have a feeling that the losses at the walls may be more when the tunnel width is fewer wavelengths because, at shorter wavelengths, a lot of the energy is carried by waves 'in the middle' and away from the walls as if they were in free space.
Below cut-off frequency there is no waveguide mode for perfectly conducting walls but, with finite resistance, the waveguide effect may extend a bit lower than this frequency. The two effects will, sort of merge into one another over what could be a wide range of frequencies.
 

Offline Pumblechook

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I think you are right on all that.
 

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