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This question pertains to the EM field of a Pulse W5017 dipole antenna. The data sheet of this antenna can be found here (which contains information regarding the radiation patterns).

There are TWO parts to this question. Edited to remove the second part. Let's see if anyone can answer the first part.

The radiation patterns of the antennas are as shown below (from the datasheet):

Radiation Pattern

Can the 3D view of the radiation pattern (combining information from both horizontal and vertical planes) be visualized as below? (Ignore the horizontal antenna in the picture, just the two lobes are relevant; in fact my antenna is positioned vertically).

What happens to this lobe with distance?

Radiation Pattern in 3D

The answer will only be accepted (and the bounty awarded) if a good intuition is provided in the three dimensions. I have removed the second part already since it requires more research.

An insight into the following parameters would be a bonus!

Frequency [MHz]: 868 - 928
Nominal Impedance: [Ω] 50
Return Loss (dB): -8
Gain (dBi): 2
Polarization: Omnidirectional

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  • $\begingroup$ Is there no alternative to physically measuring the fields? In the case of a simple dipole antenna, the fields produced are shown by the diagrams above. I even included a 3D model to visualize horizontal and vertical radiation pattern in one picture. Even if I do measure fields, and find out that the intensity around it changes with height above the floor, I still need a reason for that. $\endgroup$
    – user123472
    Commented Jul 15, 2016 at 18:57
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    $\begingroup$ You can waste your time on endless FDTD simulations, if you like, but personally that wouldn't be my choice. Get yourself a suitable calibrated measurement setup (RSSI is not useful to begin with) and do it right if you actually care to know what is happening in reality rather than in the numerics of somebody else's electromagnetic code. The "reason" for the field configuration are Maxwell's equations. I don't know what that "insight" buys you, though? I suspect this is a multiple scattering problem, so the solution will be ugly, either way. Have you done any control experiments? $\endgroup$
    – CuriousOne
    Commented Jul 15, 2016 at 19:02
  • $\begingroup$ Yes. When placed on floor (and no breach), there is no dip like that in the first picture. With breaches, there are dips, at the correct times. When placed on bench, the output is crap. Repeated both of these to confirm. The observation is correct and I described my guess for it being the way so is described above (the intruder cutting a greater section of the EM field). $\endgroup$
    – user123472
    Commented Jul 15, 2016 at 19:06
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    $\begingroup$ The floor may be behaving like a virtual ground plane depending on its material composition. You may consider using image theory to find the overall radiation pattern when the antenna is placed at uniform distances from the ground, i.e., at $\lambda_0/1,2,4$. You can find the correct polarization for the image fields in any good engineering E&M book such as Balanis' EM book or Balanis' antenna book. The distance of the antenna from the floor will certainly affect the overall radiation pattern. $\endgroup$
    – Captainj2001
    Commented Jul 16, 2016 at 19:36
  • $\begingroup$ @Captainj2001, that is a good research direction. Let's see if someone has a solid answer to this. $\endgroup$
    – user123472
    Commented Jul 18, 2016 at 5:11

1 Answer 1

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The radiation pattern of any dipole antenna looks similar to what you are showing in the 2D plots - but in your interpretation of the 3D pattern you have the axes wrong.

A dipole antenna with the main axis vertical will transmit power in the horizontal plane, with less and less power as you go further away (inverse square law). If you measure the power as a function of angle $\theta$ to the horizontal, you will get a $\cos^2\theta$ relationship. The 3D pattern can be found on many sites - it is shown on http://www.antenna-theory.com/antennas/dipole.php as

enter image description here

Your datasheet is a bit confusing: when it shows the gain "in the vertical plane", 0 degrees corresponds to the horizontal direction. It would have been so much better if they had rotated the graph 90 degrees...

Why does it have this shape? Well, it follows directly from the fact that electrons in the antenna are accelerating in the vertical direction, and that the power depends on the direction of the acceleration. A derivation for this is given in the wikipedia article on the Larmor Equation. Intuitively, you need to "see" the electron moving in order to see its radiation - when it is moving straight towards you, you don't see the lateral motion. And since EM waves require a lateral (transverse) E vector, there can be no EM radiation in the direction that the antenna is pointing.

As for the gain of the antenna: a dipole can have a gain of 3 dB in the horizontal plane. This follows from the fact that it is "radiating only to half of space" - with twice as much power being available at the peak (this is a hand waving argument but you were asking about "intuition".) Antennas that are more directional (restrict the power to a narrower angle) can have greater gains. The datasheet says this antenna has a gain of 2 dB: a bit less than 3 because of losses.

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  • $\begingroup$ I've spent some time modeling this sort of thing using XFDTD at 2.4 GHz. In free space, the pattern carries out to infinity. In real space, everything affects the propagation. For one project with a transmitter powered by a wall outlet, I modeled the antenna, and the entire PCB it was mounted on, the plastic enclosure, power supply, the entire wall outlet assembly and box, 2x4 stud and both layers of drywall. I could watch the signal refract through all the dielectrics, and reflect off the conductors, and see the distorted far field patterns. Plastics act to RF as glass does to light. $\endgroup$
    – user103218
    Commented Feb 23, 2019 at 16:05

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