antenna-theory.com

[alrobnett]

I want to use a WL Toys v911 xmtr and rcvr to investigate near field properties. I am especially interested in the Coulomb field as seen directly off the end of the xmtr antenna (axial). To maximize that effect and minimize the normal EM radiation, I am tempted to surround a 1.25" monopole with a 2.5" radius cylinder, which is also 1.25" long. It seems to me that the electron surges in the monople and the cylinder will then be in phase, and that the wave reflected back to the monopole will be such as to provide positive feedback to the monopole. My chief concerns are:
1. Will the xmtr effectively be working into an extremely low impedance, likely to damage the output stage?
2. If the effective antenna impedance is changed, will it adversely affect the frequency of the xmtr?
3. If the idea would not be feasible for the xmtr, because of damage, could the same notion still be used on the rcvr to enhance sensitivity?
Is there some other forum that would be a better place to ask this?

[Mrcelloman99]

I'm no expert, but here's my take:

There are a couple things you should note here. The frequency you're working at is 2.4 GHz, meaning the wavelength is 12.5 cm or 4.9 in, as I'm sure you know. If you want the electric oscillations on the cylinder to be in phase with the monopole current distribution, the cylinder surface needs to be an integer multiple of full wavelengths away from the monopole, not half wavelengths. From image theory, if the source antenna is a half wavelength from the reflector plane, this will result in destructive interference at the plane and the source position.

But since the cylinder radius you have chosen cuts off the monopole's access to the ground plane in all directions (the monopole ground plane effectively has a radius of 2.5'' with the cylinder around it, but should be several wavelengths instead for ideal operation), the radiation pattern will not be outward, it'll be diagonally upward I believe. As a result, you will probably lose a bunch of radiation out of the top of the cylinder. Not only that, the exact image theory representation (circular plane around a source) of your configuration is nontrivial at my first glance, so the exact radiation pattern here is unpredictable, at least for me.

As for your questions:

1.) I don't think the impedance will be lowered significantly enough to result in damaging power reflections to the transmitter because of the lack of in-phase reflections, but I didn't build it so I don't know what it can handle exactly.

2.) Impedance does not affect radiation frequency; the current does. The feedback, whatever it serves to do, may result in current aberrations though and this could upset your frequency.

3.) The same problems will arise for a different antenna model I assume.

In order to get near field measurements, I think you can make measurements in the far field, then apply an inverse fourier transform to get the near field distribution using (x/(lambda*d), y/(lambda*d)) as the x and y plane transform frequencies. Look into this approach; it uses the property that a propagating E-field in the Fraunhoffer region is approximately the fourier transform of the field at the source. google around for it.

Hope this helps.

[alrobnett]

I am delighted by your reply, thank you so much. Let me explain my reasoning in choosing the radius of the reflecting cylinder to be half a wavelength. Given a changing current in a conductor, the current induced in another conductor, placed very close to the first, will be in the opposite direction (Lenz's Law). This 180 degree phase shift, added to the 180 degree shift during transit from the antenna to the cylinder would seem to put the electron motion in the cylinder in phase with that of the antenna.
Looking at it another way, when the charge distribution on the monopole antenna is at a maximum at the free end of the monopole, the resulting E-field must have a polarity which would push electrons in a similar adjacent conductor toward the opposite end of that adjacent conductor.
This is definitely not the sort of thing that I studied in antenna courses 60 years ago. Thanks again.

Allen

[Mrcelloman99]

You know what? I think you are correct about the current polarity; the boundary conditions check out such that the current direction on the cylinder surface will match the monopole current distribution, and since the surface is not an infinite planar ground plane, I'm not sure if my destructive interference claim holds or not.

Either way, it might be cool to try it and see what happens just to gain some intuition.

[alrobnett]

Thanks again, and I hope you are correct about my assessment. I've had other projects that have delayed my pursuing this. My biggest concern is finding a signal somewhere in the transmitter and the receivers that will enable me to discriminate time shifts on the order of 5 ns. I don't expect my 100 MHz scope to see the 2.4 Ghz signal, and I know nothing about the circuitry involved. You can find ALMOST everything on the Internet, but no everything.

[Mrcelloman99]

Yeah once frequencies get that high, instrumentation and components become harder and harder to find cheaply.

I wish you the best of luck though!