[SI-LIST] Re: loop antenna (mis)behavior
- From: Gene theBean <mosfet_the_gr8@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx, Andrew Ingraham <a.ingraham@xxxxxxxx>
- Date: Wed, 28 May 2008 20:35:59 -0700 (PDT)
Hi all, and thanks for the replies. I'll try to answer most of them in one
message, so here goes:
Rick: No, not school project. This is an actual device. I've been out of
school since '86! Unfortunately, my field and wave skills suck :(
Tom: I am measuring phase on an oscilloscope. It's not the phase relative to
the TX that I'm looking at but the phase of the 2 RX signals relative to each
other Yes, it may be that the loop is NOT or maybe not the only antenna. I
think someone else replied along that same theory as well.
Andrew: I'm not sure what you mean by electrostatic coupling. Can you
elaborate? I'm looking over my old fields and waves text
book, and electrostatic seems to literally mean 'static' charges - e.g.
a charged capacitor. This system uses ac at 2 MHz. Although I need to
recheck, but am fairly sure that shorting out the RX loop kills the signal.
Regarding balance - I'd say no, they are not balanced. The RX circuit contains
the antenna with a cap across it. The cap tunes out the inductive portion of
the loop, hopefully and seemingly so, leaving only the resistive portion
(measured to be around 2.5 ohms). One end of the loop connects to ground. The
other end, connects to the non-inverting input of an op-amp with some fixed
gain.
Interestingly, just before leaving today, I tested a different configuration on
the floating test sections. It looked good in SPICE simulation, so I gave it a
go. Here, the loop leads, call them loop+ and loop-, I connected loop+ to
100-ohm resistor to ground. Then, loop- to another 100-ohm to ground. The
same tuning cap across the loop. Then, using an opamp in differential mode,
connected the loop- and loop+ leads to it. I believe the gain is set to around
100. In very close proximity to the TX, the phase (that is, between the
2 antenna) changes by position, flipping from 0 to 180 degrees. In a static
location near the TX, swapping the leads does in fact cause a reversal of
phase. Moving away from the TX, once again the phase is 0 degrees regardless
of wiring.
Brent: I think you meant you would not expect much phase difference at 2" path
difference. If so I agree with you. I was hoping to see zero phase shift
between them, or, if one antenna is reversed, then 180.
Are you saying that e-field coupling will not show the phasing? Can you
explain that?
Jerry: Yes, very possible that some other path is the culprit - just not sure
where to look or even how to look :)~ On the system, there is no common
mode filtering. In fact there isn't any filtering other than decoupling
caps. On my test sections/breadboard, the power is filtered with some hefty
inductors and capacitors But if your theory is correct then the problem may
be on the TX and associated wiring itself. Hmm. What if I try to power the TX
circuits with a battery. That would isolate it from any other wires - maybe.
Of course, if there's some coupling between the card
and the system wiring, then this may still not work.
Orin: The loops are contained within slotted non-magnetic metal.
regards,
gene
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