Wireless Communication

Posted by PITHOCRATES - December 18th, 2013

Technology 101

When a Current passes through a Wire it creates an Electromagnetic Field around the Wire

Wireless communication.  A pretty amazing technology.  Allowing ships at sea for the first time in history to communicate with people on land.  While at sea.  Using Morse code.  Those dots and dashes that take a little translation to understand.  But we could translate anything we said into Morse code.  Most people may even know the universal distress signal.  SOS (· · · – – – · · ·).  Which sent ships racing to the ship in distress.  Instead of simply disappearing from the face of the earth.  Something we owe a deep gratitude for to Nikola Tesla.  Or Guglielmo Marconi.  Depending on which side you’re on in the great patent dispute.  Was Tesla first?  Or was Marconi?  Suffice it to say they were both great inventors.  And the world is a better place because of them.

Electromagnetic field and waves.  Fascinating technology.  But one that is a little difficult to understand.  Because they’re invisible.  You can’t see them.  But we can use them to do incredible things.  If you enjoy using a smartphone you can thank electromagnetic field and waves.  For this technology is what makes wireless communication work.

When a current passes through a wire it creates an electromagnetic field around the wire.  Which can induce a current in an adjacent wire.  This is how transformers work.  By electromagnetically coupling one circuit with another.  Wireless radio transmission is similar.  Only the two circuits are pretty far apart from each other.  Being far apart, though, requires a lot of power.  And the further apart the two circuits are the more power they require.

A Radio Transmitter takes the Source Signal and Modulates it on the Carrier Frequency

When you tune into your favorite radio station you’re tuning into the carrier frequency of that station.  Which is just a powerful sinusoidal wave at one frequency they pump out on an antenna.  If you listened to just this carrier frequency you would hear a single, constant tone.  Sort of like the sound you hear on the television when they show a test pattern.  It’s not interesting or entertaining.  But it is powerful.  And the antenna they broadcast on can create one powerful electromagnetic field.  Such that the antenna on any radio receiver in or near the same city that radio transmitter is in can tune into that frequency and ‘hear’ it.  Basically with a tuner that allows only the station frequency you want to hear to pass.  While blocking the myriad of other carrier frequencies in the atmosphere.

These two antennas are the two circuits electromagnetically coupled together.  The transmitter is basically sending a current into the transmitter antenna to ‘vibrate the air’ while the receiver antenna ‘picks up those vibrations’ and induces an electric current.  And the more powerful the transmitter the farther you could pick up those ‘vibrations’.  Ships at sea had powerful transmitters.  Powered by large generators driven by their powerful steam engines.  Which allowed these signals to travel from the middle of the Atlantic to a shore receiver.  But if you did not have access to a power source you could ‘plug into’ you greatly reduced the effective range.  Because you had to use batteries.  Walkie-talkies kids play with have a small battery.  So they can’t be too far from each other to talk to each other.  The first light-weight solid state radio the Army used—AN/PRC 77 (aka ‘prick-77’)—had a much greater range.  About 5 miles.  And a much, much heavier battery.  It was so heavy that soldiers wore it like a backpack.  Which was another reason to not want to carry it.  The other being that the enemy tried to shoot the people with the radio.

Of course, these radios just didn’t transmit those carrier frequencies.  For that wouldn’t be any fun for kids.  Or useful for soldiers in combat.  And it’s just not pretty music.  No, it’s what we ‘add to’ the carrier frequency that is fun, useful and pretty.  A radio transmitter takes the source signal (voice, music, data, etc.) and modulates it on the carrier frequency.  To better understand what this means without any technical explanation listen to the Rod Stewart song Mandolin Wind.  After he sings, “I don’t have much.  But what I’ve got is yours.  Except, of course, my steel guitar.  Ha, ’cause I know you don’t play.  But I’ll teach you one day.  Because I love ya” there is a brief steel guitar solo (starting at 2:36 on the above link).  It’s a rapid picking of strings as he slowly fingers different frets.  Changing the frequency of the rapidly picked strings.  Reproducing the ‘slower’ melody on the ‘faster’ vibrating strings.  This is basically what modulation is.  Imprinting a low-power signal (voice, music, data, etc.) onto a high-power signal (a carrier frequency).  The receiver then demodulates the original signal from the carrier wave.  So we can hear or use it.

Having Cellular Towers all over the place Greatly Reduces the Amount of Power our Mobile Devices Need

The AN/PRC 77 and walkie-talkies are half-duplex devices.  They use the same carrier frequency to transmit and receive.  So only one person can talk at a time.  Which required people to say ‘over’ when they finished what they were saying to let the other person know they could start talking.  When the person said all he or she was going to say they said ‘out’ to let the other person know they were done with this communication (they NEVER said ‘over and out’.  That was only in movies with poor military consultants).  It was a great system.  Far better than earlier battlefield communications.  Such as the telegraph.  Or the messenger.  It changed the way we fought wars.  But it didn’t translate well to cellular phones.  Because this isn’t the way we talk in social situations.

Also, people just aren’t going to throw something heavy like an AN/PRC 77 on their back when they leave the house.  For the thing weighed nearly 14 pounds.  Because of the batteries.  And what would this funny way of talking and this heavy weight give you?  The ability to talk to someone 5 miles away.  There’s a reason why people don’t use these half-duplex devices for our mobile telephones.  Because there’s something better.  Cellular technology.  Where they made the use of mobile devises more user-friendly by greatly expanding the cellular infrastructure.  That thing our mobile devices talk to.  Instead of requiring a powerful transmitter and receiver (and a large antenna) in our mobile phones we built cellular towers all over the place.  So we are no further than 5 miles (approximately) from a cellular tower.  Which is all the distance our wireless signal needs to travel.  For once it reached a tower your call switched over to the landline system.  And could reach anyplace in the world.  Even to another mobile device.  As long as it is within 5 miles (approximately) of another cellular tower.

Having cellular towers all over the place greatly reduces the amount of power our mobile devices need.  Allowing a small and very light battery to power them.  Making these mobile devices very light weight.  In fact, the batteries are so light that these devices can transmit and receive on two carrier frequencies.  Allowing full-duplex communication.  Where both people can talk at the same time.  Just like in casual conversation.  They are so user friendly and convenient that today many people use their mobile phone far more than any landline telephone.  Something that would no doubt bring great satisfaction to Nikola Tesla and Guglielmo Marconi if they were alive today.  Who have given us the gift of wireless communication.  Well, at least one of them.



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