2
$\begingroup$

So I'm trying to understand how radiation pattern are calculated and it's going well (so far!). I've come across a bit of an issue when it comes to the following situation:

If we have a omnidirectional antenna and then place some reflective material collinear with the direction of the radiation (so if the antenna was a flag pole, the material could be like a flag) does the reflective material interfere with the radiated signal?

Now intuitively I would say no, since the material is collinear with the radiation then it can not reflect the radiation in any way, I understand that the material couldn't be infinitely thin so there would be a small section of reflected/absorbed radiation, but that would be pretty tiny.

So where am I going wrong in my understanding here? I've found another forum where someone says this isn't how it works, but not why.

Thanks!

$\endgroup$
3
$\begingroup$

If the only thing the material did was "reflect", and it was like a flag on a vertical antenna, then I guess it wouldn't have any effect on the radiation pattern.

The problem is that no such material exists.

One problem is that for a material to "reflect", it has to be many (at least 10, say) wavelengths long. Any less than this, and effects like diffraction will be significant, and so the material won't behave very much like an ideal reflector. There are "reflectors" that are less than many wavelengths long, such as the elements of a Yagi antenna, but these work by being tuned to very specific sizes, and that's probably not what you had in mind.

Another problem is that any material that you might consider for this purpose (like metal), will have conductivity and permeability significantly different than that of air. The electromagnetic radiation an antenna produces is the result of mutually perpendicular electric ($\vec E$) and magnetic ($\vec B$) fields. In the case of a vertical antenna, the electric field is vertical, and the magnetic field horizontal, concentric with the antenna:

Felder um Dipol.jpg
"Felder um Dipol" by Averse - http://de.wikipedia.org/wiki/Datei:Felder_um_Dipol.jpg. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Were you to put a metal "flag" on this antenna, it would be coplanar with the electric field. Metals are very conductive (that's what gives them their shiny appearance), and so they will have a very significant effect on that electric field, which in turn affects the radiation pattern.

Put more generally, your line of thought seems to be based on reasoning that works with mirrors and visible light. But even though visible light and radio waves are electromagnetic radiation which differ only in wavelength, this turns out to be very significant. Visible light has a wavelength on the order of 500 nm, whereas the radiation from a 2.4 GHz Wi-Fi has a wavelength of approximately 125,000,000 nm, or approximately 250 thousand times bigger. If you could make your bathroom mirror 250 thousand times smaller you'd find intuition breaks down, and you will start seeing effects not usually seen in everyday life simply because they are so small.

$\endgroup$
  • $\begingroup$ Hi Phil, Thanks for the very complete answer! $\endgroup$ – FraserOfSmeg Jul 2 '15 at 0:26
2
$\begingroup$

If the transmitted signal induces any currents in the "sheet", the sheet will affect the radiation pattern. One simple argument: if you run a wire parallel to the "flag pole", there will be induced currents. They may be small (if the wire is not resonant) but there will be some current. (Think of how a yagi antenna works with its "parasitic" elements.) The sheet can be approximated as a superposition of many wires.

Non-conducting (dielectric) obstacles also will change the pattern.

$\endgroup$
1
$\begingroup$

Much depends on the relative sizes. A metal flag on an HF vertical antenna is not likely to change the pattern in any way you'd notice. A gum wrapper near a 2.4GHz antenna definitely will change the pattern.

I once did a design for a 2.4Ghz consumer product and ended up sculpting the antenna pattern with a block of nylon adjacent to the antenna, acting as a lens by diffracting.

You might gain some insight by looking at Remcom's web site and some of their demos for XFDTD.

$\endgroup$
  • $\begingroup$ It would be helpful if you could edit your answer to include links to that site, and a brief summary of that info. Thanks. $\endgroup$ – Mike Waters Oct 19 '17 at 17:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.