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Near field antenna 1

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antenna21

Electrical
Jan 25, 2006
18
US
Hello guys,

I am looking for UHF (915 MHZ) near field antenna. Can anybody help me regarding what are the possible types like one possiblity is small dipole antenna. Can we design a microstrip patch antenna for near field response.

It will really help me if you guys can give links to some material about near field antenna design.

thanks alot
 
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Please guys show me some light, Iam totally confused about it.
 
All antennas work at near and far field.
VSWR changes greatly only when dipoles get about 1/4-1/2 wavelength from metal, which is pretty close.

Explain the application please. Heating? probing, reduction in snoring?
kch

PS re: snoring, (not a joke, latest Microwave magazine shows snoring is stopped by nearfield microwave heating to form a deep scar in tissue stopping nighttime vibrations)
 
Hi,
thanks for the reply.

I am trying to design UHF RFID near field reader antenna, from what I understand, when you make dipole with much less lenth than required half wavelenth and able to match the impedance, that dipole antenna can acts as a good near field antenna than the far field antenna. May be iam wrong in my opinion. For the reader should I use microstrip patch antenna or dipole. Which one gives the better performance in the near field.

thanks
 
Short antennas aren't efficient that's for certain.
What are your size restrictions? i.e. what's your frequency and what physical size is your maximum for this antenna?

Patch antennas are more easily shrunk than dipoles.

kch
 
Hi

I dont have any restriction on antenna size, it can be any size but I need a good antenna for near field reading.

thanks again
 
IF you haven't tried any yet, then maybe a simple monopole will work. Take some cable and strip away the outer conductor (about 1/4 wavelength) at the end. If you have a Network Analyzer, trim the length to get best VSWR at your frequency.

Just about any antenna that's 1/4 wavelength having good VSWR will be efficient. You will not see much variation in results unless you have some metal environment, then placement of the antenna will change the results. If you compare the antenna I described to a perfect dipole or patch, they will probably be very similar.

kch
 
You can use any antenna you like in its near field and as Higgler said, the VSWR will change, so will the beam pattern, but does this matter. You have left a lot unsaid, in particular the signal strength requirements, but

The 0.25 wavelength dipole he suggested would work OK or you could bend it into a loop, soldering the center conductor back to the braid. The loop will have a poorer VSWR but a more localised reception pattern and might be better in the near field.
 
Hello

Really thanks for the reply.

I am trying to design near field antenna for UHF RFID reader. We need some kind of coupling in near field for RFID system to work. HF RFID systems use magnetic coupling with the help of two electrically small loop antennas ( one in reader and one in tag). But for UHF RFID system we already have elctrically small dipole antenna for tag, so I am looking for reader antenna, which I can use for communication between reader antenna and tag antenna. I think, if I use loop antenna with an dipole antenna on the other end do you think is there any kind coupling between them in the near field? can we use dipole antennas for capacitive coupling?
 
In the near field there are three fields, a radial electric field, a tangential electric field and a magnetic field. The radial field is stronger than the tangential field in close to the antenna but attenuates much faster than the tangential and magnetic fields with distance and after a few wavelengths is very weak.

For this reason it is difficult to calculate what the near field coupling will be and I would take a practical approach to this and try it on a spectrum analyser. Measure the signal from each antenna with the ID tag held nearby in all sorts of attitudes. You need signal strength - and capacitive coupling will be effective very close in.


 
As an example of near field usage, medical "hyperthermia" has alot of antenna variants used to heat body tissue. Dipoles radiate or heat tissue in three hot spots, center and both tips. Far field dipole radiation is typically zero at the tips and peak in the center (half wave dipole center fed of course).

kch
 
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