The speed of light

[Harry Dalek]

Why do we say the speed of light if all electromagnetic radiation travels at the same speed ...why does light get the glory ?

Why not the speed of the long wave where it all starts mmmmmm

[M3DVQ]

because they're all light...

It's not the "speed of visible light"

Really we should just call it "the speed of massless particles and their associated fields in free space" but that's a bit of a mouthful

[Dave Moll]

I agree that that the above is something of a mouthful, but we do tend to over-abbreviate by leaving out the "in free space" bit.

It is important to remember that electromagnetic radiation (also known as light, as mentioned above) travels more slowly through a medium - otherwise refraction would not be possible.

As this subject has been raised, I wonder whether I could ask a question that has puzzled me for some time. Relativity states that a photon (or any other massless particle that may exist) must travel at "c", the same as any particle with a real mass must travel less than "c" (and any particle with an imaginary mass - so-called tachyon - would have to travel faster than "c"). Therefore, how is it possible for a photon to travel more slowly when it passes through a medium? Or is it that the photon is absorbed and re-emitted after a given delay - therefore travelling at "c" until it briefly ceases to exist before continuing on its way at the only speed possible for it?

[Klaas Robers]

It is not that the light (which is the first electro-magnetic fenomena that we observed, thus we speak of the speed of light) travels slower in a medium. It is that the path length inside the medium is longer than we measure outside the medium. Think of it as if the path is meandering in stead of being in a straight line. The light travels with the same speed.

[Metallica Man X]

Speed of Energy. Pure and simple lol.

[Steve Anderson]

This 'slowing down' of the speed of light is readily observable in coaxial cables (and other transmission lines). Quoted in a coax cable specification will (should be) its 'Velocity Factor', most often around 70%.

This means that the signal travels at 70% of 300,000,000m/s or 210,000,000m/s within this coaxial cable example, the inverse of which is 4.76ns/m. A properly terminated coax cable of 100m will introduce a delay of 476ns, around 0.5µs.

In the 'Bad-Old-Days' of PAL and NTSC this was often used to ensure that when two colour sources were mixed, not only where the syncs synchronous but so was the colour sub-carrier. Tweaking the length of one of the two (or more) feeds into the vision mixer or editor via adjusting the cable length was often used.

Later this became redundant when equipment had electronic means of adjusting all these various parameters.

In fast analogue oscilloscopes a coax delay line was often used to delay the signal to the vertical deflection, the timebase was triggered from the un-delayed signal thus allowing observation of fast leading (or trailing) edges for ringing etc..

A notch or comb filter can be made by mixing/adding the delayed and un-delayed signals. At a certain frequency the delay will introduce a 180° phase shift thus zero output at this particular frequency when summed. This will also happen at 3f (540°) and 5f(900°) producing a comb filter. There's almost endless data on this 'out there' so I won't go any further...to the relief of many.

All of the above assumes perfect cables but in reality an amplifier would be needed to make up for any losses which in itself introduces a delay...and so on, and so on....

But at NBTV frequencies you may as well forget all of the above...

Steve A.

[Harry Dalek]

I am wondering about very low electro magnetic radiation ELF...which i never really thought about ,i knew about the band below AM frequencies but 3 HZ up is a new one for me we can hear this low band but i never knew they were used to transmit via antennas ...i suppose you could use a mains transformer and have a simple transmitter .

[Steve Anderson]

Communications with submerged submarines depends on VLF, on diving the antenna is extended out of the stern of the vessel and two-way communication can be performed. It's (these days) of a binary nature and with the bandwidth available of quite slow speeds. But it's better than nothing.

I recall seeing a UK band-plan document a few years back (try the RSGB) where below 9kHz was 'unallocated'. Above 9kHz was mainly 'Marine', "Frequency Standards' (MSF in the UK on 60kHz) and atmospheric propagation studies (SIDs and the like...a fascinating field of study that amateurs are large contributors to).

Harry, in your own back-yard...13.000kHz, Royal Australian Navy, Gippsland, Ex-Omega Navigation, listed as active,

Here I get a strong signal (during the day) from VTX3, 18.2kHz, Katabomman, South Vijayanarayanam, India. Uses high-speed Morse, very useful for SID monitoring...

But here's one I cannot confirm...

00000.082kHz:Russian Navy ELF communication facility, Kola Peninsula, RUSSIA:MSK
ACTIVE
Transmission format: 82 Hz N0N and (multichannel?) MSK. Prior to message is a presumed "bellringer" or message-follows signal: 0.4 Hz shift down to 81.6 Hz for 8 minutes, then up to 82.7 Hz for 4 minutes. This initial signal sequence is directly followed by a downshift to 81 Hz for 38 seconds and a up-shift to 83.3 Hz for 22 seconds. After this comes the message, using these shifts, for a total of 16 minutes. A complete message sequence lasts for 29 minutes.
"Zevs" is reported to have an operational frequency range of 00000.031 to 00000.166 kHz. This Russian ELF transmitter is nicknamed �ZEVS�.

Steve A.

P.S. SID = Sudden Ionospheric Disturbance.

[Harry Dalek]

Communications with submerged submarines depends on VLF, on diving the antenna is extended out of the stern of the vessel and two-way communication can be performed. It's (these days) of a binary nature and with the bandwidth available of quite slow speeds. But it's better than nothing.

I recall seeing a UK band-plan document a few years back (try the RSGB) where below 9kHz was 'unallocated'. Above 9kHz was mainly 'Marine', "Frequency Standards' (MSF in the UK on 60kHz) and atmospheric propagation studies (SIDs and the like...a fascinating field of study that amateurs are large contributors to).

Harry, in your own back-yard...13.000kHz, Royal Australian Navy, Gippsland, Ex-Omega Navigation, listed as active,

Here I get a strong signal (during the day) from VTX3, 18.2kHz, Katabomman, South Vijayanarayanam, India. Uses high-speed Morse, very useful for SID monitoring...

But here's one I cannot confirm...

00000.082kHz:Russian Navy ELF communication facility, Kola Peninsula, RUSSIA:MSK
ACTIVE
Transmission format: 82 Hz N0N and (multichannel?) MSK. Prior to message is a presumed "bellringer" or message-follows signal: 0.4 Hz shift down to 81.6 Hz for 8 minutes, then up to 82.7 Hz for 4 minutes. This initial signal sequence is directly followed by a downshift to 81 Hz for 38 seconds and a up-shift to 83.3 Hz for 22 seconds. After this comes the message, using these shifts, for a total of 16 minutes. A complete message sequence lasts for 29 minutes.
"Zevs" is reported to have an operational frequency range of 00000.031 to 00000.166 kHz. This Russian ELF transmitter is nicknamed �ZEVS�.

Steve A.

P.S. SID = Sudden Ionospheric Disturbance.

bellringer

Hi steve
looking into it has surprised me its much lower than i was thinking ...yes these very low frequencies are fine for data...as you say if you don't mind waiting bit like my internet at times !

It would be Just fine for morse code yes.

Oh i will check out the navy frequency .

One thing that confuses me about the RF frequencies at audio frequencies is as with frequencies of light if the frequency is strong enough we see it ...if the RF at ELF at those frequencies were strong enough would it vibrate the air so we could hear it ?

[Dave Moll]

Wouldn't these transmissions be similar to the emanations from an Audio Frequency Induction Loop System (hearing loop) where a hearing aid (or other suitable transducer) is needed to convert the electromagnetic waves into audible sound?

[Harry Dalek]

Wouldn't these transmissions be similar to the emanations from an Audio Frequency Induction Loop System (hearing loop) where a hearing aid (or other suitable transducer) is needed to convert the electromagnetic waves into audible sound?

Makes you wonder if the Cochlear implant could pick up radio transmissions..at those frequencies only thing is i think they use FM for the implant but i suppose being to close to a transmitter of some sort might play hell with their new hearing aid
I think that would be a good one for the old Mythbusters team !

[DrZarkov]

Upgrade to human 2.0, whith help of Cybus Industries...
http://www.cybusindustries.net/

[Harry Dalek]

Upgrade to human 2.0, whith help of Cybus Industries...
http://www.cybusindustries.net/

Yes Doctor Z its getting that way ! i could be even more interesting i used to joke with my younger sister inlaw that the day will come and you will have the internet on a Internet-Contact Lens

http://www.psfk.com/2011/03/internet-en ... rfing.html

might not be to far away .

[gary]

Now if they can only get that up to 32x48....

[gary]

Harry! don't get interested and have a go at this!

http://www.youtube.com/watch?v=qHECpEhJdB8