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#1 2016-02-03 00:10:52
- mbjvfx
- Member
- Registered: 2016-02-03
- Posts: 2
Inductor used on LF PCB antenna
Hi There,
I have had the J Matheus's LF PCB antenna using his provided PCB source files.
I cannot find any BOM or recommended component values(resistor and inductor?) that are used to tune the antenna. I had a ryscorp stock pcb antenna and tried to measure the component values. There is a zero ohm resistor on one side and what appears to be an inductor on the other.
The inductor was measuring about 21 ohms with an ohm meter. I unsoldered the inductor form the PCB and solder some wire leads to connect it to an LCR meter. But, the wire was too strong and broke the inductor before I could connect to the LCR. I am sure as heck hoping to not have to pay the overpriced $50 to get a new one to test 
.
Can anyone provide the inductor value or a good starting value to use?
Thanks!
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#2 2016-02-08 22:58:15
- mbjvfx
- Member
- Registered: 2016-02-03
- Posts: 2
Re: Inductor used on LF PCB antenna
Well, did not hear anything for you folks, so I ended up having to order a new LF antenna. I did some measuring with an LCR meter and the inductor value is ~693uH @ 10Khz with DSR of 120 ohms. The package appears to be 0805. I ahve searched the mainstream distributors (digikey, mouser) and not been able to find anything this big up in this package.
The only option that appears will work is this: CB2518T681K @$.19 1x qty, 680uH in pakcage size1007 and DCR of 20ohms and 90mA. The Q factor will be changed based upon the ESR change and I hope the current rating is sufficient, I would guess so without testing.
I have some on order and will do some testing.
For anyone interested, I did ping J Westhus on this and the legend got back to me (stud). I hope it will be helfpul to others in case you want to tune the antenna with alternative values.
If I remember correctly, that antenna's inductance is a little smaller
than you want for resonance against the 1 nF cap on the proxmark3. Your
options are to (a) increase the cap on the proxmark3, (b) populate a series
inductor, which as you note is impractical, or (c) populate a shunt
capacitor. Of those, option (a) is easiest to analyze. Option (c) avoids
rework on the proxmark3. It introduces an additional parallel resonance
(sim in SPICE if not obvious), which is a slight complication but mostly
harmless.
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