single vs double ended antenna

Discussion in 'The Projects Forum' started by ScottSmith, Jun 13, 2013.

  1. ScottSmith

    Thread Starter New Member

    Jun 13, 2013
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    I'm building a circuit using a SI4362 receiver, which has a differential input. There is an application note from SiLabs AN643 which describes both how to match the impedance of the antenna to the LNA, AND how to provide a single ended to differential input function.

    Unfortunately I barely understand RF (which probably means I shouldn't be undertaking this project, but hey, that's what hobbies are for).

    For an antenna, I was thinking of using either the ANT-315-HETH or ANT-433-HETH, because I want to easily be able to switch my board from one frequency to another (both antennas are the exact same size).

    Here's where I get confused: I believe the antenna is double ended, so I think there is no need to ground one end of it. In AN643, Fig 1 and 2 show how to build a circuit for a grounded antenna. I suppose I could just ground the antenna in my circuit, but if it's unnecessary, I'd rather not -- it means tying a potentially noisy ground to the antenna path, or routing a separate ground.

    So my questions:
    1. Can I follow Fig 1 or 2 of AN643, but instead of using ground in the antenna path, just connect all those points together?

    2. If the answer to #1 is yes, then is it still true that the 4 element match network (figure 2) is better than the 3 element match network? It seems to me that a symmetric circuit would be better.

    3. Again, if the answer to #1 is yes, then should the values of CR1 and CR2 be different (from fig 1)? On pages 9-10 they list the equations for assigning Ctotal between CR1 and CR2, but again my intuition says a symmetric circuit (one where CR1=CR2) would work better. I'm wondering if the asymmetric assignment only makes sense when tying one side to ground, or if it is always the correct design?

    4. Or... maybe I'll use a splatch antenna (ANT-315-SP/ANT-433-SP). How do I decide which antenna is better for my purpose? I'm making a TPMS receiver; distance isn't far (maybe 10ft) but there are a lot of metal obstacles. Need to receive both forward and back (though more in the forward direction), and side to side.

    Thanks for any help!
     
  2. tracecom

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    Apr 16, 2010
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    I can't answer all your questions, but I do have some experience with antennas.

    Helical antennas are not grounded, AFAIK. They are fed at one end and the other end is unconnected. The antennas you cited look as if they are made to be soldered on both ends to a PCB, however, I think that one end is the feed point and the other end is soldered to an unconnected pad. That is when the antenna is mounted parallel to the ground plane. When it is mounted perpendicular to the ground plane, one end is left unconnected.

    In the application note, I think the representation for the antenna assumes a simple 1/4 wave vertical. In that arrangement, the actual element is connected at one end, and the ground connection in the diagram references a ground plane.

    At least, that's the way I interpret it.
     
  3. ScottSmith

    Thread Starter New Member

    Jun 13, 2013
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    Oh interesting. I guess that answers most of my questions then, looks like I started with a faulty assumption.

    So I guess that just leaves me with deciding between a helical antenna and a "splatch" antenna... which is further complicated by the fact I can get a smaller splatch antenna in 433MHz but not 315.... size constraints may make my decision for me.

    Thanks!
     
  4. wayneh

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    Sep 9, 2010
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    I don't think any antenna can actually be grounded and still work, right? A low impedance path to ground should pretty much ensure no ability to detect a high impedance fluctuation in the air. On a transmitter, there'd be no way to get the antenna voltage elevated above ground if it's grounded already. I suppose you can broadcast a magnetic field by passing current through a conductor but I don't think antennae work that way, do they?
     
  5. ScottSmith

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    Jun 13, 2013
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  6. tracecom

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    Dipole antennas are center fed, and one side is connected to the shield of the coax, which is in turn, connected to ground. Technically, I suppose the "antenna" is only the half of the dipole that is connected to the coax center, and the half that is connected to the shield is the "ground plane."
     
  7. Papabravo

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    Feb 24, 2006
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    Guys...
    Any antenna with RF voltage and current needs a ground to work against.
    The driven half of a diople works against the the other half connected to ground. The dipole in parallel with free space yields a match to the transmission line of 50 Ohms.
    My 160M Helical Antenna has four quarter length ground radials that it works against. What looks like a single ended antenna is actually working against a ground somewhere. It may be a poor ground, but the RF gotta go somewhere.
     
  8. t_n_k

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    Mar 6, 2009
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    I'm curious about this statement. Is this a center fed dipole in free space? What conditions apply?
     
  9. vk6zgo

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    Jul 21, 2012
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    Sorry, Pb,it doesn't work that way!

    Traditionally,dipoles were fed with balanced feeders,with no connection to earth.

    A half wavelength dipole in free space,has a feed impedance at its resonant frequency,of approximately 75Ω,which is the impedance which was commonly used for twinline.

    Now that Amateurs mainly,for convenience,use coax,we could use RG6 or similar,but mostly we use 50Ω,which isn't that bad a mismatch.

    In fact,dipoles at normal heights look more like 50Ω,anyway.

    We really should always use baluns at the antenna end if we use coax,but in Amateur use,we often "get away with it".

    If I had the technology,and the capital,I could launch a 3.5MHz beacon into space,using no feeder & a miniaturised crystal oscillator,many times smaller than a wavelength, fitted between the feedpoints,

    This dipole would happily radiate,although it was many hundreds of kilometres from Earth,& a remote controlled field strength meter on another small satellite could fly around it recording the familiar dipole radiation pattern shown in all the books.

    A 1/4λ groundplane antenna has a feedpoint of approximately 37.5Ω,which is half that of a 1/2λ antenna.
    The earth,or groundplane radials replace the "missing half" of the dipole.
    OK,it's a lot more messy than that,but it gives a good idea of what is happening.
     
  10. wayneh

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    Sep 9, 2010
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    I've always been confused by antennae, so excuse a stupid question. If I have a long wire stuck in the ground on one end and on the other I apply an RF signal, driving a few amps through the wire into the ground (low wire voltage but high current), is that an antenna? Or does the antenna need to be held in the air, not grounded, so that it sees large RF voltage swings (with little current flow)?

    I was earlier suggesting that the grounded antenna wouldn't radiate much, but again, I find it all confusing.
     
  11. Papabravo

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    Feb 24, 2006
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    Perhaps you thought I was implying that half the antenna remains at ground. What is at ground potential is outer shell of the connector(PL-259/SO-238) on the back of the transceiver. There will be RF current in both legs of the dipole and on the transmission line. Where there are currents there are voltages.

    Taking a random length of wire and putting an RF signal on one end will result in both radiation and reflection. How much of each depends primarily on the length of the wire and the frequency of the signal.

    I'm not sure about a wire into the ground with an RF source on the other end. I think there is such a thing as a beverage antenna used for receiving that has a resistor to ground.
     
  12. vk6zgo

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    Jul 21, 2012
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    Some transmitters have balanced outputs with no connection to earth,& use balanced feeders.
    This was in fact,common with "Ham" transmitters in the past.

    In my example of an orbiting HF beacon,there is not,& cannot be,any connection to earth,so your original contention,

    "Guys...
    Any antenna with RF voltage and current needs a ground to work against.
    The driven half of a diople works against the the other half connected to ground"

    is seen to be incorrect.
     
  13. nsaspook

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    Aug 27, 2009
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    I wouldn't say it's incorrect just incomplete (and messy) as the ground (earth or any ground plane conductor of the correct size ) acts to form a virtual antenna element.
    http://en.wikipedia.org/wiki/Image_antenna
     
  14. vk6zgo

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    Jul 21, 2012
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    Sorry,but that is a bit of a "red herring".

    Yes,the earth,or any large conductive object reasonably close to an antenna will affect its radiation pattern,so that a 1/2λ dipole mounted close to ground level will have a higher angle of radiation compared to the same antenna in free space.

    This is not necessary for the operation of a balanced antenna,however,& it will operate equally well in Earth orbit.or Interstellar space if it comes to that.

    Thinking for a moment about an elevated 1/4λ vertical ground plane antenna.mounted on an insulated pole & fed with coax :

    It will operate just as happily as if its ground plane was a perfectly conducting Earths surface,albeit with a slight mismatch due to being 37.5Ω instead of 50Ω at the feedpoint.

    If I now fit 4:1 baluns to the transmitter & antenna,& replace the coax with a 200Ω balanced line,neither the transmitter or antenna will see any difference,& the antenna will merrily radiate as before.

    I now get a bit fussy about matching,& I know from experience & antenna books,that if I slope the ground plane elements down at an angle from the horizontal,the feed impedance will increase.

    I set them so the feed impedance is 50 Ω,& I have really good matching.

    At this point,I wonder what will happen if I slope them further.

    55Ω ,60Ω ,65Ω ,70Ω,,,,,,
    By this time they are almost in line with the radiator.

    Imagine if I could make them very thin & combine them,so they are in line with the original radiator,& the feed impedance is 75Ω .

    "Hey guys! I've got a 1/2λ dipole!!"
     
  15. t_n_k

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    I would think maybe something like this ...

    The driven top end of the wire would need a reference of some sort - most probably a virtual ground plane. Suppose you start with your properly tuned (matched?) & driven wire antenna with the aforesaid virtual ground plane but held a substantial distance above real ground. It will radiate RF power. The better the match with the driving source the better the radiation properties. As you progressively bring the bottom end of the antenna towards the real ground its impedance will begin to change (probably markedly increase) and the radiation power & pattern will degenerate with increasing mismatch with the driving source & interaction with the real ground. At the point at which it just touches the ground the degree of mismatch would be overwhelming and the radiated power would be very small in comparison to the starting condition with the antenna placed well above ground.
     
  16. wayneh

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    Sep 9, 2010
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    So which is a better way to broadcast a signal using a 1 meter piece of wire: 1) Drive 10A at 1v rms of RF current through the wire into ground to give 10W of power, or 2) Drive 10W as 100mA at 100v rms through the same wire, not grounded.

    I'm not sure the question even makes sense but the 2nd scenario is the more likely answer in my mind. You want to propagate a voltage swing, not a magnetic field even though they are fundamentally the same thing. :confused:
     
  17. nsaspook

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    It really makes little difference a few wavelengths away from a (equally matched and efficient) transmit antenna. The reactive near EM fields from currents and sources change into far field self-propagating waves with E/H field strength ratios set by the impedance of free space instead of the feed-line impedance of the antenna.

    [​IMG]
     
  18. wayneh

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    Interesting, thanks. I guess that's why I could never reconcile which way it was - it's both. Now if I can just sort out that wave-particle thingy. :p
     
  19. t_n_k

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    Surely the operating conditions depend upon the antenna impedance at the particular frequency & physical configuration. A dipole will probably have a different impedance to another antenna type such as a multi element Yagi or a rhombic - and so forth.
     
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