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How exactly do you transmit?

Discussion in 'General Electronics Discussion' started by Zander, Sep 15, 2010.

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  1. Zander

    Zander

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    Sep 11, 2010
    How do I say transmit sound?

    And on the other end receive the sound?

    Like in the case of radio station to your small hand held radio.

    (I've transmitted sound before through a two transistor oscillator and a transformer but it was only a short range)

    How is the range increased with out use of a great amount of power?


    How is a remote control toy car working?





    Another form of transmission I'd like to know about is.

    How is something like video transmitted and then decoded on the other end?

    What type of circuit is it running through?

    Is it 3 different signals for red blue and green, and lastly one for audio?

    How does the circuit know where to put everything on the TV's grid?
     
  2. Mitchekj

    Mitchekj

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    Jan 24, 2010
    Google up "superheterodyne."

    I have no idea if they're using newer technologies/methods these days, but that's what I had learned some years ago.
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    time for you to start lots of reading, buy some books on the subjects of radio and TV transmission. As Mitchekj said, use google put some of your questions in there and start learning :)

    I will answer just one of your easy questions ;)
    How is the range increased with out use of a great amount of power?

    by the use of high gain antennas
    just an example .... a dipole antenna has unity gain ie. a gain of 1
    if I transmit 10Watts of power into it it will radiate most of that power minus a few losses.
    If I use an antenna with 10dB of gain over a dipole (dBd) then that 10Watts I transmit into the antenna the radiated signal will be effectively multiplied by 10 (minus those small losses)
    Now I have an effective radiated power (ERP) of ~100Watts.... Thats going to give my transmitter much better coverage over the countryside.

    I could answer many of your other questions but I would be writing a book to give you good answers. I havent got time :) so go read the books that others have written already :)

    cheers
    Dave
     
  4. Militoy

    Militoy

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    Aug 24, 2010
    You might also want to point out that there isn't really any "free lunch" using a high-gain antenna - Gain in one direction (or directions) is the result of a decrease in gain in other directions. Conservation of energy - physics 101. The fortunate thing about a high-gain antenna, is that there are always ample directions where you don't need to transmit (or receive).
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    Unless of course you are using a Co-linear :) a nice way of producing gain with an omni-directional radiation pattern. It doesnt really have anything to do with conservation of energy, you are still producing a given ERP for a given gain and input power. Its just the style of antenna you choose as to whether it beams that energy in primarily a single direction as in a yagi or dish or in a 360deg pattern. :)
    the gain figure over a dipole or isotropic radiator is still valid and is easily proved in field tests

    I have several co-linear antennas ... a base station one for 2m and 70cm bands it has 4.5 and 7 dBi gains, respectively.
    A triband mobile one that has 3.5dBi on 2m, 6.3dBi on 70cm and 9.7 dBi on 23cm bands.
    A slotted waveguide antenna on 3cm (10GHz) produces an easy 10dBi with an omni. radiation pattern

    cheers
    Dave
    VK2TDN
     
    Last edited: Sep 16, 2010
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    ok Zander
    I'm in the mood to give you a few basics on transmitters and receivers :)
    after which you can go do some reading ;)

    ok a real simple transmitter say an FM (Frequency Modulation) one
    the transmitter may consist of a crystal oscillator lets say 48MHz
    a microphone is connected to a low level audio amplifier circuit and the audio output of that amplifier is applied to the crystal oscillator. This causes the the frequency to vary slightly in sympathy with the changing audio level. presto, you have a FM transmitter on 48MHz. That 48MHz freq may then be doubled in a following circuit to 96MHz in the FM broadcast band, then amplified in the next stage before going to the antenna.

    A real simple FM receiver for the above transmitter ...
    the received 96 MHz gets amplified by a sensitive transistor often a FET (Field Effect Transistor). FET's are used over bi-polar transistors cuz they usually have a much lower noise figure ( thats means they dont generate much of their own noise to get added to the received signal). After the receive RF amplifier the signal goes into a mixer often one gate of a dual gate FET. onto the other gate is applied a LO (Local Oscillator) signal from a crystal oscillator. Our aim is to produce a lower and more useable frequency from which to recover the audio from. This lower freq is called the IF frequency (Intermediate Frequency) and commonly for VHF and up receivers can be 10.7 MHz
    (in AM broadcast receivers on 540 - 1400Khz band a 455kHz IF freq is the most common)
    That 10.7MHz is called the 1st IF. From here a bit of black magic happens we can apply that 10.7MHz to an IF intergrated circuit like a MC3359 (go look it up and get a data sheet and find out how it works) the 10.7 goes into another mixer and another LO is applied 10.245MHz this results in a 2nd IF of 455kHz which goes into a demodulator section of the MC3359 and the audio is recovered.


    The transmitted signal is produced in the same way as above for an FM video/audio transmitter (say those video senders and receivers you can use at home for sending a signal around the house) The composite video signal modulates a FM transmitter.

    For commercial analog TV broadcast (non digital which is basically a FM signal) AM (Audio Modulation) is used for the video and FM is used for the sound. The sound signal modulates a 5.5 to 6.5MHz (depending on country and TV system used) oscillator and that signal is superimposed on the AM video transmission. We call it a sub-carrier and if you look at a TV transmission with a spectrum analyser you will see a big spike of the video transmitter on say 50.75MHz (our channel 0) and 5.5 MHz below that on 45.25MHz you will see a smaller spike of the audio signal.

    I wont go into the receive demodulation side on here it gets a bit more complex compared to a standard AM or FM receiver. Tho it starts the same way. go have a read :)

    cheers
    Dave
    VK2TDN
     
  7. Militoy

    Militoy

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    Aug 24, 2010
    I'll have to pull out my propagation theory handbooks or snoop around online a bit, to compare the typical pattern of a "co-linear" with a dipole - since I don't have any design experience with that type antenna. Most of the designs I worked with in the past were dishes, Yagis or cubical quads - and the military antennas on our robot are simple loaded 1/4 wave whips, stripline dipoles, UHF "sharkfins" or radar waveguides. If it's a vertical-oriented configuration though - I would suspect that it trades energy wasted into space for horizontal gain. From your experience - would a co-linear design be as effective at propagating at a station located directly above the antenna, as a dipole with the same phase orientation?
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    hey Militoy :)

    there's nothing off the end tip of a dipole its a null point :) and the same with a colinear

    Colinears are usually a vertical antenna. It can be a a phased set of 1/4, 1/2 or even 5/8
    elements or even a mix of the above. That tri-band mobile I spoke of in the other comment
    is a 5/8 element on 144MHz, 2 x 5/8 elements on 440MHz and 5 x 5/8 elements on 1296MHz
    Colinear arrays are often used for TV transmitters, FM broadcast, land mobile repeater systems (ham radio/commercial) etc. where you need omni directional coverage

    Have a look in the ARRL Antenna Handbook and you will see some examples
    below is an example drawing of a coaxial colinear I made some years back for 1296MHz
    also a rough drawing of radiation from a dipole

    wonder if we will get a response from Zander ?? ;)

    Dave
     

    Attached Files:

    Last edited: Sep 17, 2010
  9. Militoy

    Militoy

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    Aug 24, 2010
    Hi Dave –

    I wasn’t trying to address the practical aspects of antenna gain (nor to detract from your answer) – I was attempting to point out the physics behind gain in general terms (“conservation of energy”). I’m traveling this week – and a little pressed for time, so I wasn’t able to dig into my books – but I was able to briefly look into some co-linear designs and their propagation on the net. From the info in your diagram and several sites (Nova-Star, Net Pro, Wikipedia), I gather that co-linear (or collinear) antennas are a type of phased array antenna made by stacking or interleaving multiple dipole elements to increase gain. They are typically vertically polarized – and are designed to increase horizontal gain by compressing and flattening the toroidal, vertical plane pattern. In other words – by diminishing vertical radiation (usually useless for earth communications) – they increase useful horizontal radiation. I’m actually very intrigued by the idea of an omni antenna with so much gain. I plan to play around with one when (if) I get time.

    Maybe Zander will jump back in – or perhaps he was just a “seagull” flapping through(???)

    Best regards -
    Ralph <Militoy>
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    would seem that I probably wasted my breath :(

    Hopefully some one somewhere got something out of my ramblings ;)

    Dave
     
  11. Militoy

    Militoy

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    Aug 24, 2010
    Well - I'd say I picked up some useful knowledge. I've designed a few antennas - but was a complete bonehead as far as co-linears are concerned. You have added to my list of "stuff to check out". Thanks!
     
  12. davenn

    davenn Moderator

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    your're welcome :) and congrats on reaching VIP member status :)

    Dave
     
  13. Militoy

    Militoy

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    Aug 24, 2010
    Thanks - Not quite sure how that happened, being as I'm kind of a newbie here - but I'm honored to join the ranks of the "VIPs" - as the most junior member!:)
     
  14. davenn

    davenn Moderator

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    Sep 5, 2009
    it happened cuz you hit the 100 posts mark :)

    Dave
     
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