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let's consider a common wifi antenna (used in routers) like this: enter image description here

I don't understand some its basic features:

1) Is it a dipole or monopole antenna? I see only one conductor, so I'd say it's a monopole. But, where is the ground plane? As far as I'm concerned, a monopole antenna needs a second metal object in order to radiate.

enter image description here

Picture taken from here.

The antenna may be supplied with a coaxial RPSMA cable: where does its external conductor go? It should go in the ground plane, but I don't see it.

2) I don't see even the metal conductor, but only a plastic monopole. So, I'd think that the inner part is a conductive monopole. But why is there such a plastic external shield?

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  • \$\begingroup\$ Durability, cost and looks would be three acceptable reasons to use plastic in the design, and that's without finding any special use for the plastic. \$\endgroup\$
    – K H
    Commented Jan 23, 2021 at 8:13

3 Answers 3

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  1. Is it a dipole or monopole antenna? I see only one conductor, so I'd say it's a monopole. But, where is the ground plane? As far as I'm concerned, a monopole antenna needs a second metal object in order to radiate.

The first image that you've attached is a rubber-ducky monopole antenna. An open-ended transmission wire (line) when excited/driven by an AC voltage source does act as a radiator but just not efficient. When you bend the two open-ended wires to a certain length, it performs better. The oscillatory voltages and currents produce oscillatory electric and magnetic fields. When the effective bending length is a half-wavelength, it performs still better and there is maximum potential difference at the edges. Current is maximum at the center.

Coming back to the Wi-Fi antenna: consider a quarter wavelength monopole antenna. One of the open ends is grounded and the other end is a quarter wavelength long. Excitation is the same, an AC voltage source. But in this structure, the maximum potential difference at the edges is half of that in the case of a half-wavelength 'dipole'. Consequently, there is not strong enough E-field as in a dipole but this is sufficient.

The first image is of a monopole antenna and its radiation pattern is shown in the following figure. It has an omnidirectional radiation pattern, resembling the figure of the number 'infinity or a dumb-bell'. The mobile phone Wi-Fi antennas are low profile patch antennas embedded onto a substrate and fed internally. monopole antenna

radiation pattern

  1. I don't see even the metal conductor, but only a plastic monopole. So, I'd think that the inner part is a conductive monopole. But why is there such a plastic external shield?

Yes. The outer shielding may be for protection against damages. This may be thought of as equivalent to a 'radome' present with parabolic reflecting structures.

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I don't agree with the other answers that describe this as a monopole antenna. I believe it's actually a coaxial dipole antenna, first described in a 1939 Bell Labs patent. Just for fun, I'm embedding one of the illustrations from that patent here.

illustration from Patent

How do I come to this conclusion? First, it's much longer than 1/4 wavelength at wi-fi frequencies (2.4 GHz = 12 cm wavelength). Second, the larger-diameter base section and thinner-diameter tip are unnecessary bulk for a simple monopole whip.

The advantage of the coaxial dipole is that it does not require a ground plane. The purpose of the thicker base section is to form a quarter-wave "choke" that isolates the lower tip of the dipole from the coaxial feedline.

This type of antenna is technically "ground independent" meaning a ground plane is not essential to its operation. The radiation pattern will still be affected by the router chassis in proximity, but not as strongly as if it were a quarter-wave monopole.

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Is it a dipole or monopole antenna? I see only one conductor, so I'd say it's a monopole. But, where is the ground plane? As far as I'm concerned, a monopole antenna needs a second metal object in order to radiate.

It's a monopole and the body of the router will act as a ground plane; the PCB inside the router will have a ground plane and that will have enough capacitance to "real true" earth that at 2.5 GHz, it looks like a pretty good connection. Plus, the wires going into the router will have capacitance to earth etc..

If the router is capacitively coupled by 10 pF to earth, then at 2.5 GHz, that's a reactance of 6.4 Ω.

I don't see even the metal conductor, but only a plastic monopole. So, I'd think that the inner part is a conductive monopole. But why is there such a plastic external shield?

The inner part will be a wire and the plastic outer supports that "thin" wire and protects it. There are also dielectric properties in the plastic that mean it will be slightly detuned from the theoretical monopole length but, that will be accounted for in the overall length of the design.

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  • \$\begingroup\$ can you tell me why is the capacitance to earth important? Why shouldn't the monopole be floating with respect to earth? \$\endgroup\$
    – Kinka-Byo
    Commented Jan 23, 2021 at 14:46
  • \$\begingroup\$ A monopole needs a ground plane/earth plane @Kinka-Byo and although it's capacitive, it has a reasonably low impedance that it makes the PCB ground plane in the router look like real earth. \$\endgroup\$
    – Andy aka
    Commented Jan 23, 2021 at 15:04
  • \$\begingroup\$ @Andyaka your knowledge on this board blows me away, but I don't agree here. See my answer for why I think it's not a monopole. \$\endgroup\$ Commented Jan 23, 2021 at 16:10
  • \$\begingroup\$ You are fully entitled to not agree. \$\endgroup\$
    – Andy aka
    Commented Jan 23, 2021 at 16:13

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