Antenna theory

Discussion in 'Wireless & RF Design' started by Georacer, Oct 4, 2010.

  1. Georacer

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    Radio is another story. Antenna theory is based on anlternating polarization of the emmiter and the receiver antennas, in order to produce an electromagnetic wave in space. In antennas we do exactly that, applying a voltage on a wire without grounding the other end.

    This is totally different from all the other circuits where you want electrons to actually flow in order to produce work.
     
  2. t_n_k

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    You might want to check or perhaps qualify that statement.
     
    Last edited: Oct 4, 2010
  3. Georacer

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    I have sure tested it on a small scale. At the last semester's lab I transmitted a AM modulated sine by attatching a wire a the + end of the signal generator and I received it and heard it on a hand made radio.

    It makes sense to me since if you attach only one end of the source on the antenna, it will apply voltage on it but electrons won't be able to move freely. The result will be that electrons will be forced to the farthest end of the antenna when a negative voltage is applied on it and towards the source when a positive voltage is applied on it. This will create areas of lack and surplus of electrons on the antenna. Sounds familiar? It's just like on two ends of a capacitor. In between them an electric field is produced. Now, if this field is alternating, due to the sources frequency, the electric field will become electromagnetic and travel through space.

    I have done a bit of research on wikipedia and all simple types of antennas work that way. I cannot swear the same for the more complex types (like parabolics and such) but I guess there's no need at this point.

    Just think about your kitchen table radio and you cellphone's handsfree. Their antennas constist only of a single piece of wire, right?

    P.S. @ t_n_k: I sent you a friend request some days ago, but it seems you don't look at you inbox. Accept or reject it at your own discretion, just don't leave it hangin. Thanks!
     
  4. t_n_k

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    Hi Georacer,

    Yes I'm rather slack about the friends request thing. I've taken your timely reprimand on board.

    On the antenna grounding thing, I'm pretty confident you are strictly correct about not connecting one part of the antenna to "ground" and the other to an active source. Although, a folded dipole might notionally fall into that category - albeit with the qualification that such an antenna is typically fed in a balanced mode. An unbalanced feed antenna such as the 'J' pole is another case to be considered. And so on ....

    I also had in mind those situations where, in particular, a ground plane is an essential consideration of the antenna design. Such as the "grounded" dipole.

    In any event we have wandered rather off topic. Perhaps we should start a new thread on which members could comment.
     
    Last edited: Oct 4, 2010
  5. beenthere

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  6. Ghar

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  7. bertus

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  8. Georacer

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    Aaaaargh!... So... much... info...

    But seriously, let me recap and see if I get it right: In order to produce an EMP you will need two planes on which the signal voltage will be applied, one for the "plus" and one for the Ground terminal, and in this way create a "radiating capacitor" of some sort. This is clearly depicted in a bipole antenna. But even in the monopole antenna, the hot terminal will couple with a large metallic object which will act as a ground.

    I said what I said about my efforts of transmitting AM for testing purposes before I read and learned this stuff - no one taught me any better at the lab. Pretty interesting stuff.

    But I wonder: The signal generator that I used was grounded for sure and therefore its Ground terminal had the same voltage as the casing all the equipment of the lab - the large metallic object. Would the transmition be as effective if the signal generator had a floating voltage difference?
     
  9. bertus

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  10. Georacer

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    To answer my own question: From what I understand from the information I have read, the large metallic object that will serve an the ground terminal for a monopole antenna must be connecter to the ground terminal of the signal voltage source. Floating voltage will not do.
     
  11. Ghar

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    This is why the word ground is so misused and confusing... really, the requirement is that you have a voltage between two conductors.
    There is nothing special about Earth ground and it only really plays a role when the source is somehow related to the Earth, like a lightning strike or the power grid which gets physically connected to Earth.
     
  12. Wendy

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    Dipole is the most basic of antenna's. A monopole uses the ground plane (earth) as a mirror, to create a virtual dipole. With the ground mirror reflection is half of the dipole.

    Antenna's are similar to tank circuits, they have resonance. At resonance they have gain. It isn't active gain, but it does present a RF signal at higher levels than a plain piece of wire.

    With a piece of wire cut to resonant length the signal bounces between the open ends on the wire. This is real signal in the wire, and at this point is electricity. Again, if it is at resonance the signal will be converted to EMR at maximum efficiecy.
     
  13. sceadwian

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    I think I would say a whip is more basic than a dipole Bill, but only because whips are so much more common. Dipoles are even more prone to interferences with materials immediately near them than whips are and whips are bad. Just try to find a dipole on a consumer FM radio ;)
     
  14. t_n_k

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    This is an interesting quote from an Australian Govt paper on AM Radio Issues.

    PROPAGATION.

    AM suited to distance propagation, along the ground.

    AM radio has a technical advantage over FM radio in one key respect. The propagation characteristics of AM radio are such that it is better suited than FM to wide area transmission in sparsely settled areas. The potential coverage of an AM service in remote or rural areas can be superior to an FM service, particularly in areas where there is no suitably high mountain on which to site an FM transmitter.
    The surface of the earth plays an important role in AM broadcasting services, as ‘ground’ waves are essentially responsible for the propagation of electromagnetic radiation in the MF radiofrequency band. Therefore, AM broadcasting stations are sited at relatively flat locations with high ground conductivity. High ground conductivity is usually found in wet areas, like swamps. To improve local ground conductivity ‘earth mats’, consisting of radial wires, extend from the base of the tower.
    AM broadcast antennas are generally large, ranging between 30 m and 200 m tall. As they are so large, the mast or tower also doubles as the antenna. The whole structure therefore actively radiates the AM signal.


    http://www.acma.gov.au/webwr/_assets/main/lib100068/amradio_issues.pdf
     
  15. sceadwian

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    http://en.wikipedia.org/wiki/Near_and_far_field might be a good read for further understanding of the technical nature of things.

    http://en.wikipedia.org/wiki/Ground_wave would be another one.

    http://en.wikipedia.org/wiki/RF_radiation

    Would be a third.

    Nearfield is most easily understood as magnetic coupling as the wavelength is actually pretty large compared to the space between the antenna, all mains transformers technically use the nearfield effects at low frequencies for power transfer. I guess there is some portion of radiated nearfield which makes nearfield antennas a very special case for RF transmission. It's activly being researched for non contact power transfer for cell phones and other mobile type devices where radiated power can be kept low because the frequency is relatively low.

    Ground Wave is just that, the EM field oscillated close through the surface of the earth with the earth as a portion of the conductor, the worlds largest coax cable (pun indended)

    Then there's free space transmition which is the primary use in all modern telecommunications as above a few hundred mhz such as in the 2.4ghz+ the nearfield only exists within a few wavelength (feet) of the antenna and the ground wave can be said to essentially not exist it's a dead short, This is why low frequencies can be used for in building transmition where something in the range of 2.4ghz the walls themselves create an effective local ground and eat some of the signal.

    All of these methods of transmition exist at the same time so multi-mode reflections cause all sorts of weirdness you'll get listening to AM (or expecially FM in a city)
     
  16. Ghar

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    I should've been more careful (especially since this is a thread about antennas...) but I didn't mean Earth has no role.
    Like anything decently conductive the Earth will be influencing wave propagation and it's definitely a key component of many kinds of transmitters.

    My intent was to just get rid of the idea that the Earth is somehow a requirement for transmission.
     
  17. nsaspook

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    How about classic TV antennas. Some of these things were works of art.

    http://www.godarusa.com/id72.html

    [​IMG]
     
  18. Wendy

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    Remember what I said about the earth mirror? A whip is a prime example of a dipole where half of the dipole is in earth or car ground (the body of the car exhibits the same characteristics as earth in many cases).

    The most efficient length for dipole is ½ wavelength. The most efficient length for a whip is ¼ wavelength, the reflection accounts for the remaining ¼ wavelength. Like all generalizations, there are major exceptions everywhere. It is possible to tune an antenna with a cap or a coil, and if an antenna is off length it looks like a cap or a coil to the RF generator. This can be tuned with the right amount of counter reactance.

    Ever notice FM radios occasionally get better reception with you holding it? You have become an RF mirror.

    While I've forgotten a lot of my classes, this was my major in college. I have build quite a few antenna's in my day, of every type. Yagi antennas tended to be the standard though for my early projects. I've also built (kits) for 2.4Ghz receivers using 1 pound coffee cans and washer yagi antennas (they can be intimidating looking, much like a Buck Roger's ray gun).
     
  19. sceadwian

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    That's a really nice one NSAspook.

    Yes I remember what you said earlier Bill, it doesn't change anything that I was saying though. A whip's construction is far simpler than a dipole, and more immune to interference from local objects because it's hard to interfere with the earth ground directly, at least a far as household FM radio's go, a dipole for an FM radio would be twice as long because the ground as a reflector wouldn't exist anymore, and ideally it would need to be at least a wavelength away from any material regardless of it being a conductor or not, you'd get great reception on it though.

    I wouldn't use the word efficient with those wavelength listings either, 1/4 and 1/2 wavelength is the resonant frequency of a whip/dipole, the word efficient carries too many connotations with it which are highly specific to the application. With antenna's there are no rules, only exceptions =)

    It's taking me quiet a while to get down to brass tacks as far as antennas go through because if you say antenna 50 RF freaks will spend a week arguing about what's best =) There is no best, there is what works and what doesn't.
     
  20. Wendy

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    Back when I was building crystal radios my standard antenna was a long length of wire running out my bedroom window to various trees. I'm not sure how you would classify that one, but it worked. Most times my best ground was my bed springs under the mattress.

    I'll disagree a little on there not being a best, many ideas simply don't work, or work so poorly they are unusable. Of the ones that do work there is better and best, measurable and quantifiable with numbers. The problems come in when managers and other non technical types try to change the specs from under you. I think this is what happened to Apples latest phone and the gripe of death. That or you try to do too much with too few parts.
     
    Last edited: Oct 8, 2010
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