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Talk:Monopole antenna

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Wrong name

[edit]

The term "Monopole" as used here is completely wrong.

A monopole antenna is impossible without its reflected image in the ground-plane. A "monopole" with groundplane, is just a somewhat rearranged half-wave dipole, eg similar to a Coaxial (or Sleeve) Dipole.

A true monopole antenna would be logically equivalent to a magnet with only a North or South pole, or a Positive electric field without the Negative counterpart (or vica versa).

In reality a 1/4 wave "Monopole" always has the missing 1/4 wave reflected in the ground-plane. So in effect it is always a full half-wave.

If you removed the ground-plane and tried to feed the end of a 1/4 antenna (as a Voltage node) it would be utterly useless as an antenna.

The image used clearly illustrates this point.

Comment: This is just one of many cases of incorrect use of terms by the electrical engineering community. Physically, a monopole could not radiate (there is no transverse field component, and EM waves are transverse); moreover, the idea of an oscillating monopole is fundamentally incompatible with Maxwell's equations (continuity equation for charge). Actual radiation of a "monopole" antenna is due to its dipole moment (and higher multipole moments). But wrong use of terminology is firmly entrenched among (not just electrical) engineers.

The image used here is not an antenna, but a tower. the antenna is on the top of the tower but is not visible here. Glover (talk) 13:49, 18 November 2008 (UTC)[reply]

I don't think so. That station has a frequency of 1250kHz. The quarter wavelength for that is 60 m. This whole tower is about that length judged from the photo, so the whole tower is the antenna itself. Also, monopole antennae need the ground at their base. So when they are put on top of towers, you can see at least 3-4 wires (of about quarter-third wavelength) extending horizontally (or slightly downwards) outwards. Some radio towers were 3-4 times the size of this one :-) Hoemaco (talk) 09:15, 2 March 2009 (UTC)[reply]

It says "However, a monopole will have a directive gain of 3 dBi, and a lower input resistance, resulting in overall lower efficiency." First, if it had a 3dBi gain, it would be better than a dipole (having 2.15dBi), so that wouldn't mean a lower efficiency! Second: to my knowledge, the quarter-wave monopole has the same gain as the dipole(from the mirroring theory). Unfortunately some sources on the net also list 3.15, which would mean that nobody would use dipoles... I'll correct this when I find good sources. Hoemaco (talk) 19:15, 2 March 2009 (UTC)[reply]

You're on the right track. Corrected the earlier entry about lower efficiency. Lower resistance doesn't mean lower efficiency (assuming things are designed to match at the operating impedance). "Gain" is another whole topic; antennas have gain in some direction at the expense of loss in others. Someone had put in the right numbers of 5.19 (directive or isotropic) and 3 dB (vs a dipole) of gain so I left these in place. Altaphon (talk) 19:15, 18 November 2009 (UTC)[reply]

Doesn't this describe a 1/4 wave ground plane antanna? Rsduhamel (talk) 18:52, 3 June 2009 (UTC)[reply]

Not necessarily 1/4 wave -- can be longer or shorter and tuned appropriately. 1/2 and 5/8 wave are common.Altaphon (talk) 19:15, 18 November 2009 (UTC)[reply]

He is absolutely correct: The "missing" mirror image is always present in the ground plane. A true "monopole" antenna is impossible. — Preceding unsigned comment added by 119.18.11.19 (talk) 09:12, 1 November 2017 (UTC)[reply]

Isn't there an error in the impedance graph? The graph is for a half-wave dipole. Isn't the impedance of a monopole half that of a dipole? --Chetvorno^TALK 23:03, 17 December 2010 (UTC)[reply]

No, the impedance at the centre of a half-wave dipole is essentially the same as the impedance at the base of a 1/4 wave ground-plane. It's the name which is wrong. There is no such thing as a true "Monopole". There is always a mirror-image in the ground-plane.

[File:S11 of monopole, folded monopole, inverted-f antennas..png|thumb]] I just redirected Shorted monopole antennatoInverted-F antenna as they are the same subject. The former article had this comparison of S11 responses for three types of monopole. Posting here in case it is useful for this article and so it doesn't disappear altogether. Some text and other diagrams that went with it can be found on this version of the page. SpinningSpark 16:18, 30 October 2015 (UTC)[reply]

The grounding structures appearing with some monopole antennas is nonplanar and is called a “counterpoise.” This can be seen in the provided example photo of a VHF monopole antenna. 72.15.92.76 (talk) 21:49, 21 November 2022 (UTC)[reply]