What kind of antenna should I chose?
I need an antenna. It should be compact, preferably flat (maybe micro-strip?), directed as much as possible and support a single frequency of AM or FM or in low GHz.
The application is non-ferrous metal directed sensor with a minimal penetration in metal-free environment.
Please suggest.
Thank you in advance.
Your requirements are a little un-focused.
Generally speaking, the "as much as possible" aspect of your "directionality" and also your desire for compactness are rather at-odds with one another.
How much is enough directionality?
Does it have to be flat (PCB-based)? What surface area can you dedicate?
Also, AM and FM are modulation schemes and have nothing to do with frequency.
What's the frequency of operation?
I think about a single frequency operation anywhere in medium frequency radio band, ~500KHz..~1,600KHz
It wold be nice if the antenna were flat, but a slightly curved single surface will do as well
I can dedicate up to 2 X 2 inch square area and I'd like to reliably cover area of not less than 3 X 3 inch but no more than 6 X 6 inch, depth of earth material of no more than 2 inch.
Thank you.
I see that "AM" was intended as synonym of medium wave band. A 2x2 inch structure can't work as electric antenna in the MHz range (too small related to wavelength). You can make a magnetic antenna respectively a coupler coil.
The purpose of the "antenna" isn't quite clear. Is it an inductive proximity sensor or for communication purposes?
A capacitive proximity sensor working in the MHz range could be made as well, but it won't be specific to metallic objects.
From all said above , what you need is not an antenna but a sensing coil or a set of coils like used in metal detectors. Your sensor can be a simple oscillator which will change frequency when a metal object gets lose to the sensing coil.
Try to test also a metal stud detector for nails under walls, maybe it can solve your problem.
The purpose is of non-ferrous metal proximity sensor over a moving line of earth/clay/plant matter mix.
I'd really like to keep the antenna/sensor small, up to 2.5 X 2.5 inches, I do not mind using higher frequency.
I can figure out details of electronics by my own, however, I need guidance about a compact antenna/sensor - I need it to detect the metal only when it passed in front of it. Micro-strip seems to be a nice choice, but designs that I could look up are uniform, not directed like I need.
Thank you.
Directed antennas are only small at high frequencies. Patch antenna at some GHz might be suitable for your needs. Maybe 5.8GHz, check the ISM bands for your country.