Directional Antenna with 90deg field of view - Page 1
Forum section: electronics / projects

author: Sypher
author_rank: Contributor
timestamp: January 25, 2014, 05:09:48 am
content: I'm looking to create a directional antenna, and I'd like some feed back on my design.All I'm trying to do is have a rough estimate of the power received and a 90deg estimated angle of arrival of a completely arbitrary signal. This is purely an experiment to verify a simulated algorithm. Keep in mind, the antenna has no real purpose other than sensing an RF signal and giving an estimated signal strength and angle of arrival.Requirements:90 degre "field of view", -70db outside of the FOVSmall, approx 2"x4"x2" or 50mm x 100mm x 50mmOnly need 1 freq in the range 50MHz-2.2GHzVery cheap.Don't really care about much else.Plan:Start with a cheap omni directinal antenna:http://goo.gl/5GKH9REnclose it in a shield angled at 90 degree, so the antenna is in the corner. Shield will most likely be copper.http://goo.gl/g28hbOCover the inside of the shield with a combination of "lossy" foam and microwave absorbent material material:http://goo.gl/pq87p5Thought process is that any signal coming from outside the shield will obviously be reflected back and any signal coming from the front of the antenna, but slightly outside the 90 deg will be absorbed (not sensed). That is, if it's in front of the antenna but has an angle to bounce into the antenna by reflecting off the inner sides of the shield, it should be absorbed by the material and not sensed.How effective do you think this will be?The main thing is that I need to read "no signal" outside the 90 degree field of view. I don't want to use say a yagi antenna, that still has -30db from the side-- as that could easily be miss-interpreted as a signal dead on that is much, much farther away.

author: AG6QR
author_rank: Frequent Contributor
timestamp: January 25, 2014, 05:48:04 am
content: Quote from: Sypher on January 25, 2014, 05:09:48 amIt looks to me like your requirements are impossible to meet, due to diffraction limits, and the wave nature of radio.A 50MHz signal has a wavelength of about 6 meters, or around 20 feet.Your requirement that the antenna be 4" in its longest dimension means your aperture is less than 2% of a wavelength at the low frequency limit.The Rayleigh criterion for angular resolution says that if you take the wavelength divided by the aperture, that will give you approximately the best possible angular resolution in radians. In this case, we're talking about 50 radians, way more than a full circle. We're not anywhere close to getting any reasonable resolution at 50MHz.Even at the max frequency of 2.2GHz, your antenna size is comparable to a wavelength.To get reasonable resolution, you'll need an antenna with an aperture of multiple wavelengths, at least.

author: hagster
author_rank: Frequent Contributor
timestamp: January 25, 2014, 07:22:23 am
content: Download the free 4nec2 and do some simulations. I'm pretty sure there is a corner reflector in the example models that comes with it. You will never get close to your requirement though for all the reasons AG6QR says and more.

author: Mr Simpleton
author_rank: Unknown
timestamp: January 25, 2014, 11:41:24 am
content: Good Luck on the -70 dB F/BNot sure I have ever seen such a device, ever!Except for computer simulationsEven half that number is not an easy task in real life...

author: KJDS
author_rank: Super Contributor
timestamp: January 25, 2014, 12:36:23 pm
content: Quote from: Mr Simpleton on January 25, 2014, 11:41:24 amIt's possible with a solid dish. A solid dish isn't really possible at 50MHz unless someone knows how to make one that would cover about eight football pitches.

author: Co6aka
author_rank: Unknown
timestamp: January 25, 2014, 07:10:21 pm
content: Quote from: Mr Simpleton on January 25, 2014, 11:41:24 amYeah, and when the prototype doesn't work it's reality that's broken, not the computer model...Anyway... It'd prolly be more practical to build an efficient rubber duckie for 136kHz.

author: Co6aka
author_rank: Unknown
timestamp: January 25, 2014, 07:22:38 pm
content: Quote from: Sypher on January 25, 2014, 05:09:48 amYears ago I was experimenting with DF (direction finding) antenna ideas... Basically, over a ground plane, build (what are essentially) directional couplers radiating outward from the center; feed points are at the center, terminations are at the circumference. Lots more to it of course, but the silly thing actually seemed to work.