# 125kHz RFID Reader and Huge Field Antenna

#### Hordeus

Jul 30, 2019
6
Hello everyone
I'm working this circuit for a weeks.
But i can't seem to understand full details and it's working since i'm almost newbie to Radio Communications.
So i have few questions on this circuit.
2.Is it possible to attach 1.5m diameter antenna?
3.And i can't seem to find equation's related to calculations of coil turn?

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#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,476
Welcome to EP.

An explanation is given on the website you linked. Have you asked the original author for details?
.Is it possible to attach 1.5m diameter antenna?
Possible: yes, useful: no. Why would you want to? L1 is the receiving antenna tuned with C2 for the correct frequency. there is no need for an additional antenna.
And i can't seem to find equation's related to calculations of coil turn?
Have you tried e.g. to Google "air coil inductance calculator"

#### Hordeus

Jul 30, 2019
6
Welcome to EP.

An explanation is given on the website you linked. Have you asked the original author for details?

Possible: yes, useful: no. Why would you want to? L1 is the receiving antenna tuned with C2 for the correct frequency. there is no need for an additional antenna.

Have you tried e.g. to Google "air coil inductance calculator"
Author haven't responded in weeks.
No not external antenna i wan't change antenna diameter to 1.5 meters.
And i'm newbie in signal processing so i can't understand what is happening behind those.So i really need deep understanding of this circuit.

#### Hordeus

Jul 30, 2019
6
Hello everyone
I'm trying to make a RFID reader with diameter of 1.5 meters.
So far i managed to work 125kHz RFID reader with 12 cm diameters.
But yesterday i tried coil antenna with 35cm diameter and amplitude of coil antenna's signal didn't dropped much almost 200mV.

And there wasn't anything on the output.
So i think the signal strength is the problem.
Thank you

Last edited by a moderator:

#### Nanren888

Nov 8, 2015
622
Sorry, a little lost. Why exactly do you want to change the coil diameter to 1.5m?
Are you trying to increase range? To increase the area over which it works?

#### Hordeus

Jul 30, 2019
6
Sorry, a little lost. Why exactly do you want to change the coil diameter to 1.5m?
Are you trying to increase range? To increase the area over which it works?
Range of 1-2 cm is fine with me since i don't feed to read it from much range.
My goal is to read tags in 1.5 m round table.

#### Nanren888

Nov 8, 2015
622
This is not really my area, but maybe I can have a try at helping and people more knowledgible can correct.
Appologies if I lead you the wrong direction, or just plain guess wrong.
.
Seems a lot of RFID these days uses passive tags. That is, the tags are powered from the reader while nearby by magnetic coupling.
Maybe you have a way to tell whether a tag is being energised, so you can tell whether your problem is energising, waking the tags or signal strength?
.
I guess the 125kHz source and FET in your diagram might be creating a field in the coil that enegrgises the RFID electronics by some cunning means.
I don't have a good mental image of how coils and magnetic fields work, but I sort of picture a feild strength as a number or density of magnetic flux lines passing through the area of the coil. The RFID coil intersects a proportion of these flux lines and reciprocity gets it a current to its electronics and viola power. If the old system was 350mm diameter, I guess this created a flux density that was enough to energise an RFID reciever from its coil.
Now, you want to make a coil covering 1.5m. I don't have a good feel for the effect of a small coil on a large coil, but if we assume the small RFID gets a proportion of the field lines related to the area, and area is pi * diameter, seems as a first approximation, you might need 1.5/0.35 = a little over four times the flux lines, or input drive, to get the same number of lines intersecting any one RFID in the area.
So, more power into the coil, by a factor of four, just to wake one up.
.
From memory coils producing fields are moviated by current and turns. If you inductance is way up from the diameter increase, you;ll have to change things to keep resonance if that's what you need and getting more current through it is also going to take some work.
.
Not sure about how the return path works. Could be the coil area affects the signal levels back too.
Hope this helps in some way.
Those of more experience; please feel free to correct.
.

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