Talk:Dipole antenna

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Dipole antenna was a good article, but it has been removed from the list. There are suggestions below for improving the article to meet the good article criteria. Once these are addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake.

Delisted version: January 6, 2006

Contents

[edit] impedance at wavelength?

"Impedance is not real but it does becomes real for a length of about .46 λ"; I take it to mean this is because the "Im" curve of the graph shown crosses the x-axis at about .46. But the impedance always has a real part and an imaginary part; the imaginary part is just positive past .46 λ, right? Could somebody with a little more savvy check me on this?

74.193.185.169 17:47, 18 March 2007 (UTC)

An impedance is normally considered real if the imaginary part is zero. An impedance with both a real and imaginary part is normally considered complex. At some length a little under a half-wave, the imaginary part of the dipole's impedance will be zero, so the impedance will be considered real. (The exact length the antenna becomes resonate, and so have zero imaginary part, depends on the diameter to some extent too). So for example, 40 + j 0 Ohms is real, 0 + j 23 Ohms is imaginary and 34 + j 89 Ohms is complex. Drkirkby (talk) 10:04, 30 March 2008 (UTC) Drkirkby (talk) 10:05, 30 March 2008 (UTC)

[edit] improvements needed

This page needs VERY desperate attention. This is probably THE most important antenna type, since most antennas are built using arrays of dipoles. Main problem, too much fluff, not enough actual info.

TODO:

  • inventor and first use
    • The dipole is so simple, I would think that it would have had to be invented simulatenously with the radio?
    • Credit is given to Hertz for this one.Kgrr 11:21, 15 January 2006 (UTC)
  • (radiation efficiency vs length)
    • What do you mean by "radiation efficiency"? May be the good term is "gain"? Radiation efficiency is almost always near 100%: all power fed to antenna is radiated. Loses in wire resistance are negligible.LPFR 12:18, 27 July 2006 (UTC)
  • (frequency tuning)
      • done Kgrr 12:02, 15 January 2006 (UTC)
  • theoretical impedance with infinitely thin (and change due to having thickness)
    • Theoretical resistive part of the impedance can be calculated "easily" with a bit of numerical integration. The reactive part is another history. I put a diagram of measured real and imaginary parts of the impedance of a dipole from \scriptstyle{0.4\lambda} to \scriptstyle{0.6\lambda}. Influence of thickness is known (the antenna becomes more capacitive) but I do not think it can be theoretically calculated. There are thing that are far easy to measure than to calculate... antennas for one! LPFR 12:18, 27 July 2006 (UTC)
      • The reactive part of the input impedance CAN be calculated. There are several methods that this can be done - some give a pure analytical result, others need numerical integration and so are more suited for use by computers. See for example Antenna Theory - Analysis and Design by Constantine Balanis, 3rd edition, 2005, page 258 (ISBN 0-471-66782-X). Antennas by Kraus (1988) also has a formula for this, based on the work of Brown and King, which was published in Proc. IRE, volume 22, pages 457-480, (April 1934). Drkirkby (talk) 11:14, 30 March 2008 (UTC)
  • theoretical gain over isotropic (2.4 dBi?)
    • done. LPFR 12:18, 27 July 2006 (UTC)
      • done Kgrr 12:15, 15 January 2006 (UTC)
  • theoretical radiation pattern
    • A graph would be nice --ssd 09:10, 8 January 2006 (UTC)
      • done Kgrr 11:21, 15 January 2006 (UTC)
      • The diagram of radiation produced by Kgrr has a logarithmic scale. It should be indicated. Diagrams in the sections "short dipole" and "half wave dipole" are in linear scale. LPFR 12:18, 27 July 2006 (UTC)
  • that it requires a balanced feed (and reference to balanced and how monopole is unbalanced)
      • done Kgrr 12:21, 15 January 2006 (UTC)
    • I have added a remark about the correct way of feeding the dipole from coax (using a BALUN)Cadmium 12:21, 13 December 2005 (UTC)
      • yes, ideally but not required; done Kgrr 11:21, 15 January 2006 (UTC)
  • change in radiation pattern vs height above ground plane
    • I have posted "Effect of ground" in antenna as the effect of ground is the same for all antennas. LPFR 11:10, 17 August 2006 (UTC)


  • fact that monopole over groundplane IS same as dipole in free space
    • Last two points: I think that an entry "ground effect in radiation patterns" is missing. It should settle some issues as "dipole vs. monopole" or "Influence of height of the antenna. In a near future, I will fill this gap. LPFR 12:18, 27 July 2006 (UTC). Done.LPFR 11:10, 17 August 2006 (UTC)
      • The two are not the same. The most obvious difference is that a monopole over a groundplane will radiate only above the groundplane and not below it, whereas a vertically polarised dipole will have a symmetrical pattern above and below the centre. Drkirkby (talk) 10:16, 30 March 2008 (UTC)
  • reference to electromagnetic waves
    • At least partially done in "elementary" to "half wave dipole". LPFR 12:18, 27 July 2006 (UTC)

Notes: From Warren L. Stutzman and Gary A. Thiele, "Antenna Theory and Design" and assuming a 100% efficient ideal dipole the theoretical gain would be 1.76dBi ( 10*log(3/2) ).

(Note: this critique was contributed 9 July 2004 by Brandon.irwin. This article has been editted over 20 times since.)

  • Possibly the variations of the dipole should be organized and listed together, rather than being spread through the article. --ssd 09:09, 8 January 2006 (UTC)

[edit] folded dipole

I don't think it is correct that a folded dipole is a loop. Loops and dipoles are very different. I'll have to double check, but I don't remember seeing the ends of any folded dipoles being connected. --ssd 12:43, 27 December 2005 (UTC)

At a glance, the folded dipole might look like a loop, as the ends of the dipole are joined. However, this is not a loop; it acts like a dipole with an impedance transformer, with the radiation pattern of a dipole, not that of a loop. The best reference for this I could find was W4RNL's folded dipole article. (The Antenna Book mentions the folded dipole, but the information is scattered through chapter 6, with little theory even mentioned.) --ssd 14:40, 28 December 2005 (UTC)

John D Heys (G3BDQ) in his book Practical Wire Antennas (ISBN 0900612878) lists the folded dipole under loops, and he regards the folded dipole as a loop. It is possible by adding a third wire to the folded dipole in increase the radiation resistance yet further. In this book the theroy of the folded dipole is explained. Cadmium 20:49, 29 December 2005 (UTC)

I'll have to look for that text. Does it discuss radiation pattern of a folded dipole? --ssd 00:22, 30 December 2005 (UTC)
the book states on page 37 that the radiation pattern of a folded dipole is the same as that of a normal dipole.Cadmium 10:30, 30 December 2005 (UTC)
That's what I thought. That would mean it isn't a loop, despite appearances. Thanks! --ssd 15:20, 31 December 2005 (UTC)
Folded dipoles and loops behave very differently. A loop is more similar to a quad in its operation and radiation pattern.Kgrr 06:12, 13 January 2006 (UTC)
A loop is to a quad what a dipole is to a yagi. --ssd 06:47, 13 January 2006 (UTC)

Why does the text say a folded dipole is half a wavelength while the image says it's one wavelength? —Preceding unsigned comment added by 99.231.124.188 (talk) 05:00, 13 June 2008 (UTC)

[edit] merge with Half wave radiator

I think Half wave radiator needs to be merged into this article. I think that's really just a huge dipole, right? --ssd 13:11, 27 December 2005 (UTC)

A half-wave radiator is not a dipole. It's actually more like a fullwave antenna. The ground plane acts like a mirror, providing the other half of the dipole. It's electrical impedance and coverage pattern is very different from a vertical dipole.

A quarter-wave vertical antenna is electrically similar to a vertical dipole because the ground plane forms the other half of the radiating elements. However, still, they are different antennas. Kgrr 06:01, 13 January 2006 (UTC)

This statement makes no sense to me, and contradicts what is currently in the article. A quarter wave whip is certainly a different antenna than a half wave radiator, but they are still both dipoles. The quarter wave whip uses a groundplane as the image, as you say, but the half wave radiator is more like an end fed (rather than center fed) dipole. --ssd 06:41, 13 February 2006 (UTC)
I do agree with Kgrr. A quarter-wave antenna is just one-half of a half wave dipole, as is also its impedance (the impedance of a quarter whip is 36+21j ohms). The misunderstanding comes from the fact that a quarter-wave antenna has always a reflective surface (a real or an artificial ground). If this surface does not exist, you do not have a quarter-wave antenna but an end–feed antenna with a bad length. LPFR 12:34, 27 July 2006 (UTC)

[edit] Delisted Ga

No references! Being that references are a requirement to reach GA status, the article is being removed from the GA list. If references are properly added, this article should be able to make GA status again. AndyZ 23:27, 6 January 2006 (UTC)

  • I would suggest improving the lead and the extremely short sections. AndyZ 04:42, 15 January 2006 (UTC)

[edit] baluns and stuff

The balun section is beginning to make me uncomfortable. The original wording in the article is somewhat confused. (Not exactly wrong, but not right either.) I think the details are too much of a tangental issue to all go into the article, but some of this should. Let me try to clarify some things here:

  1. Balance and match are different. A mismatched feedline has the wrong impedance. Impedance mismatches cause reflections at the impedance boundary, which adds up and is measured as SWR. (See Antenna (radio)#impedance for a longer explanation.) In a transmitter SWR multiplies normal losses in the feed line and increases voltage spikes and tranceiver heating, nasty stuff. In a receiver, it is still bad; the impedance mismatch will cause ghosts in a video signal, for instance.
  2. An unbalanced feed to a balanced antenna causes two separate problems with a related cause.
    common mode current
    Normally, the current in the shield and the current in the center conductor are 180 degrees out of phase, so that their E-M fields cancel, and the power is transmitted through the transmission line. Common mode current happens because when the current traveling in the shield hits the antenna, part of it goes into the shield half of the dipole, and part goes back down the shield. The current traveling back down the shield is in phase (same mode) with the center conductor, so the fields do not cancel, so the feedline radiates, thus becoming part of the antenna, rather than just a feed line.
    distorted radiation pattern
    The current in the center conductor all goes into its dipole half, but the shield current is split. Thus, the center conductor's half of the dipole's pattern will be stronger than the other, and the shield will add a smaller omnidirectional pattern as it radiates. Of course, when you add them, you don't get a circle added to the nice 8 pattern a dipole would normally get, because some of the phases cancel. Instead, you get a distorted cartoid. (There's a nice direction finding antenna that balances these two patterns to cancel completely in one direction and get a single null, but I digress.) Usually the common mode radiation is weak enough to be ignored in the far field, so you just get a distorted 8 pattern. (Imagine the lobes in the nice symmetrical picture on the page drooping a bit.)

At high power, the common mode current will cause your feedline to radiate into all the surrounding equipment, causing all kinds of problems, and maybe even RF burns. At lower power, that can be ignored, and the distortion sometimes is not of great concern either. However, since the coax is now part of the antenna, you've just added a big long inductive piece to the antenna, and the antenna will have a nasty complex impedance with a non-zero imaginary part, which leads to higher SWR, and much higher loss in the feed line. (And if you are low power, you can't afford to lose much power this way.)

Now, here's where the confusion lies:

  1. Baluns are typically transformers. If it is only a balun, it is a 1:1 transformer. The balun blocks unwanted common mode current by forcing the current in each half of the transmission line to be the same.
  2. Impedance matching can be fixed with a transformer too. For example, a 4:1 balun/transformer can transform a 75 ohm unbalanced coax impedance into a 300 ohm balanced twin lead impedance.

Essentially, one device does two jobs that are not clearly related. (This should be rewritten, and half put in the balun article in a theory section, and half put in the dipole article.) --ssd 08:14, 12 January 2006 (UTC)

Baluns are not essential to the operation of a dipole. Perhaps the balun treatise should be handled under baluns rather than dipole.Kgrr 06:07, 13 January 2006 (UTC)

No argument here. But someone wanted baluns mentioned, and if we're gonna do it, it should be done right, not semi-factually. That's why I put this here, not in the article. Is there too much in the article now? Perhaps not; it is showing the correct way to feed a dipole, at least. --ssd 06:51, 13 January 2006 (UTC)

Another tangentially related item... At low power, the common mode current in the feed line can be lived with by making the feed line a multiple of a quarter wavelength long (adjusted for velocity factor of course). You'll still get radiation from the feed line, but at least the reactance will be zero. Essentially, the feedline becomes part of the antenna, and this tunes it. If it is feeding a vertial dipole anyway, this might actually increase your gain by making it a much larger antenna. RF burns at higher powers are still a danger, though. --ssd 06:51, 13 February 2006 (UTC)

[edit] Speed of signal on a wire

I erased the assertion that "the velocity of propagation of electromagnetic waves in wire is slower than that in free space". The propagation of electromagnetic waves in a wire or in air-filled coaxial line, or in a bifilar line is the same as in free space. The reason for the resonance of a dipole at a length a little shorter than half wave is due maybe to a non-sinusoidal current distribution in the dipole (but I am not certain of this). LPFR 14:18, 16 August 2006 (UTC)

Actually, the speed of an electromagnietic wave in a (pair of) wire(s) IS less than that of free space. This is due to the capacitance and inductance between the two wires (the delivery and the return wire) making the transmission line less than ideal. --DrBob127 08:02, 23 November 2006 (UTC)

[edit] Speed of signal-propagation-down a wire

  • I'm not sure why you erased this reference as the propagation of energy is slower down wire/coaxial cable and anything else except free space-you are correct as far as I know.
The propagation speed of signals down a coaxial cable, with ordinary (non-magnetic) isolation is \scriptstyle{v={1\over \sqrt{\mu \varepsilon}}={c\over n}} where \scriptstyle{n =\sqrt{\varepsilon_r}} is the refractive index and \scriptstyle{\varepsilon_r} is the dielectric constant of the isolation filling the gap between the inner and outer conductors. If the coaxial cable is air-filled, the speed will be the same as the speed of electromagnetic waves in free air (almost the same speed as in vacuum). A single wire in air, as is each branch of a dipole, can be seen as a coaxial cable whose outer conductor is very far. As this "coax" is filled with air, the speed will be the same as in the air, and not 5% smaller as stated the phrase that I erased. -- LPFR 11:42, 27 October 2006 (UTC)

Hi LPFR,

This may be so, however there is also the effect of end capacitance /support insulators that tend to introduce some difference in the physical length of a dipole versus the electrical length. the effect is in the order of from 2% to 5%. --Read-write-services 00:27, 28 October 2006 (UTC)

[edit] Merging with "antenna (radio)"

  • I was thinking that perhaps this article needs to merge with or, at least link to [[1]]?


Wikipedian self-named Read-write-services is asked to respect wikipedia usages. This point is not an item of "Speed of signal-propagation-down a wire". LPFR 11:57, 27 October 2006 (UTC)

I apologise, not all of us are experts. --Read-write-services 00:29, 28 October 2006 (UTC)

As for the asked merging with antenna, I do not think that it is a good thing. Antenna and dipole articles are too long to be merged. In fact, I plan to split antenna article (see talk:antenna). As for the link to antenna, it is done in the second line. LPFR 11:57, 27 October 2006 (UTC)


[edit] Included diagrams are of unreasonable size

The diagrams included in the baluns sections are of a ridiculous dimension, despite being very company file size wise. My browser, literally, had to hit swap for 4 minutes before all the PNG files were loaded into memory and stabilized to a point that I could scroll. It also does not appear to be enough detail in the images to justify the several million pixels used, per diagram. Please resize said images to a more reasonable and readable resolution. 24.80.114.123 19:20, 11 December 2006 (UTC)

Yep, my computer crashed several times while trying to view this page. Sewebster 03:56, 3 February 2007 (UTC)
I switched the balun images to the linkimage template so that the thumbs won't be displayed. Someone needs to resize all of them, reupload, and switch back to the normal image format. I might do it, but I'm not an image upload expert (maintain proper authorship etc etc.) Sewebster 04:09, 3 February 2007 (UTC)
I've taken care of it (hopefully). - mako 05:00, 3 February 2007 (UTC)