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How RF Antennas Work

  1. Nov 9, 2015 #1
    Okay, sorry for my frustration at this fundamental subject, but I am going completely insane not understanding it! All this near-field / far-field stuff is messing with my head. I simply want to know how RF works. Let's start simple. HOW IN THE WORLD IS AN RF SINGAL MADE!?
    Let's say I have some AC current. It's flip flopping nice and awesome. All since wave and stuff. Where the heck is the RF generated? Are the two wires connected together, and then an antenna is sticking off of that loop? That can't be right cuz that's a short and it'd burn out right away. And it can't be a coil, right? Cuz that'd just make a stupid magnet. And it can't be an open circuit cuz, well, then no current would flow and that doesn't make any sense. Some, please. Save me. I can't find anything anywhere that explains it.
     
  2. jcsd
  3. Nov 9, 2015 #2

    davenn

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    simply, the RF aka electromagnetic energy and field is generated by the moving electrons of the AC current
    Whenever there is moving electrons there is an EM generated and radiated

    An antenna is lengths of tube, wire etc ( may be straight lengths or even loops) that is cut to a specific length so that it is resonant at the frequency
    you want to radiate a signal at. When the antenna is at a resonant length, it works at maximum efficiency at that frequency

    when dealing with RF currents ( AC) circuits that would appear to be a short circuit at DC are not with AC
    This can become some involved learning to understand the difference between AC impedance and DC resistance and other things

    there's a start for you

    Dave
     
  4. Nov 9, 2015 #3

    davenn

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    have a look at this antenna for 1296MHz ( bottom end of the microwave band)

    KQ006sm.jpg

    The radiating element ( known as the driven element) is a loop. It would be a short circuit at DC


    Dave
     
  5. Nov 9, 2015 #4
    Thanks for the response! I tried looking up your direction, but still got very lost. It makes sense what you're saying, but I can't find much on it. Like, if I were to short the neutral and hot wire on 120v mains, I know there'd be an explosion. If it's oscillating at a higher frequency, does it no longer pose a threat of bursting into flames...?
     
  6. Nov 10, 2015 #5

    davenn

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    yes it will ... don't try it

    not quite ... its still a matter of x amount of power ( voltage and current) into X amount of load

    consider some of the loads put across your mains 120VAC supply

    A 100w light globe -- glows extremely bright white -- low resistance

    A 1000W bar heater -- glows red/orange -- low resistance but a bit higher than the light globe

    now for you to work out....
    you know your voltage supply, I have given you a couple of Watt ratings of several items
    tell me the current flowing for each
    then knowing the current, you can work out the resistance of each of those items


    Back to RF
    Now that antenna I showed you above... could hand around 100 W max. BUT, you are not feeding it with 120 V mains
    instead that 100W from the transmitter could be around 12 V @ 8 Amps flowing in the transmission line and antenna)
    Now if I tried feeding 500W or more into it, the results would be bad for the antenna, it is likely to start
    glowing like a heater element

    there's a little more to it than that ... without going too deep ....
    to generate that 100W of RF power out actually requires a lot more power being used by the transmitter ....
    No transmitting device ( transistors, FET's, Old style valves (tubes) are 100% efficient, they all have various inefficiencies
    depending on their type.

    My 100W transistorised transmitter requires 12 V at around 20 Amps that equals 240 W !!

    WOW you say, what is happening to the other 140 Watts ?
    well it's mainly being dissipated as heat by all the electronics in the transmitter
    so you can see that the efficiency is relatively low .... ~ 40%


    OK will stop there for this post
    Dave
     
  7. Nov 10, 2015 #6

    meBigGuy

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  8. Nov 12, 2015 #7
    I don't think a short circuit is the case. The feed to the antenna is split. The ground connection and the other bit sticking up in the air.

    There is no contact between the two or it would just be a series circuit straight into the dirt.


    Have any two wires not touching at one end and the other ends to the terminals of a really high frequency source and you have a radio station.....a really bad one.


    Pretty much every conductor on the planet is an antenna, just cut it in half and stick one end to ground it it will receive something.
     
  9. Nov 12, 2015 #8

    davenn

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    depends on the type of radiating element ... see the example in my photo above


    any RF source will do


    and you don't even need to do that


    cheers
    Dave
     
  10. Nov 12, 2015 #9

    sophiecentaur

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    In every home, there are transformers that consist of two or more multi-turn coils. The mains input is connected across the Primary Coil, which is 'just' a length of wire that happens to be wrapped around an iron core. If you connected 240V DC across it, the wire would be very nearly a short circuit (a few metres of wire with a very low resistance). It's the Inductance at 50Hz that limits the AC current that can flow.
    Many antennae take the form of loops which have a low Inductance but, at the RF frequency, the Reactance means they're not a short circuit.
     
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