I need to locate the antenna in a separate location from the unit and have the datasheet which seems to have a good writeup on winding and testing your own antennas, however I'm not having success and I'm wondering if anyone out there can help me to interpret my findings.
I've wound and secured a couple of loops of enameled wire at different diameters and none of them seem to do anything.
There are two antenna connectors on the RFID-20 if I probe one pin I get a 5V square wave (slight peak on rising edge). When I probe the other I get a very clean 100V sine wave as 125KHz which seems right based on the fact that this is a 125KHz tagging system.
Now, when I connect the two pins through the antenna loop I get (understandably) the same output on both pins: a sine wave at 125KHz with a step in it that matches the square wave. However the amplitude of that wave is now only 10V instead of 100V...
I've done the aluminum plate test in the datasheet and my amplitude decreases substantially when I bring my aluminum coffee pot (biggest chunk I've got nearby) close to the loop.
HELP! What am I missing here, probably something simple? I know I can add a tuning capacitor, but the severe change in amplitude (voltage) with the antenna attached has me fuddled.
Anybody have a great diagram to demonstrate how the inductance of the loop changes the other electrical characteristics of the system? I'm a visual learner
The voltage drop when you attach the coil is due to the impedance of the coil. If you add a series or parallel capacitor so the impedance matches and the voltage you see will rise.
Anyway it is not the voltage across the coil that matters but the magnetic field it generates. This is a function of the current through the coil and the number of turns. How many turns do you have on the coil?
Is that why RFID readers drain so much battery? I've always wondered about that and what could be done to decrease it... I guess the only answer is turn off the antenna? Well, that kind of stinks.
Thank you so much for your helpful answers so far!
My antennas is approximately 63 turns on 120mm diameter.
What I'm gathering is that the capacitor does more than "fine tuning."Not being an RF engineer my theory is that it's not working because the impedance mismatch is causing reflection. How far off am I?
Is there a formula I can use to relate the impedance of the coil to the capacitance I need? The datasheet suggests a 1.5nF (1n5) capacitor, how did they arrive at this?
I hadn't ever worked with an LC circuit before and hadn't grasped the concept of the purpose of the cap when working on this!
Wiki-Pedia filled me in on how exactly the LC circuit works and I'll post the link here for posterity: LC circuit - Wikipedia
To wrap this thread up I connected a metal film capacitor in the circuit as described in the datasheet and my antenna will now read tags! The range is very poor but I only had one capacitor and will get a few more (polyester this time) to fine tune.
Glad to here it worked. With a few more values you should get the range.
Secret hint - you get better range when the coil is just off resonance not spot on.
I'm a visual learner