[SI-LIST] Re: loop antenna (mis)behavior

Hi all -  again, thanks for the help.
I've come up with, what I think is the solution.  Check out this circuit:
 
http://www.geocities.com/fazool1_2000/si_list/si_list_loop_problems.pdf

L1+R1 represent the antenna loop at 2MHz, as measured by an LCR meter.  C1 is a 
tuning cap that negates the inductive reactance of the loop.  R3 and R4 give 
both ends of the antenna a ground reference.  In principle, the induced signal 
on the antenna produces a +V/2 on one side, and -V/2 on the other.  SPICE 
simulation concurs.  The opamp is setup in differential mode with some gain 
that is sufficient to make the signal visible on the scope.  Although the 
design shows 10X gain on first stage, and 10X on second stage, the gain 
bandwidth of this opamp degrades that somewhat.

In the lab, the +V/2 and -V/2 signals are clearly visible, 180 degrees out of 
phase, and both have zero volts common mode.  The range of reception works for 
around 6 feet or so, in any direction away from the TX.  In the immediate 
vicinity of the TX antenna, the system behaves in a fashion that looks like 
mutual coupling between the loops. As I wrote earlier, the phase within an area 
surrounding the TX changes drastically depending upon position.  But once 
outside this zone, then the phase between 2 RX antenna remain constant. 
 Swapping lead polarity on an antenna changes phase by 180 degrees.  All cool!

The major difference between this setup and the one that fails, is the way in 
which the antenna is connected.  It's more of a single ended design.  I think 
the reason it misbehaves is that there's some common mode signal on the antenna 
(e-field) that the opamp picks up on simply due to single-ended construction of 
circuit.  This alternate design, using a balanced design, exploits the cmrr of 
the opamp nulling out any such affects.  Sound logical?

BTW, the tin foil test (on this balanced design) revealed that the signal is 
indeed magnetic, not e-field.  Thanks, Andy!

regards,

gene



--- On Thu, 5/29/08, Eric Bogatin <eric@xxxxxxxxxxxxxxx> wrote:
From: Eric Bogatin <eric@xxxxxxxxxxxxxxx>
Subject: [SI-LIST] Re: loop antenna (mis)behavior
To: "'Andrew Ingraham'" <a.ingraham@xxxxxxxx>, si-list@xxxxxxxxxxxxx
Date: Thursday, May 29, 2008, 5:18 PM

Gene-  Andy has a good idea, especially if you are measuring the voltage across 
the coils directly with a scope probe and the input to the scope is set to 1 
Meg Ohms. In this case you probably are sensitive to capacitive coupling. Maybe 
put a low resistor (like 100 Ohms) across the ends of the coils and measure the 
voltage across the resistor.   Keep in mind that at 2 MHz, the wavelength is 
500 feet, so in the lab you are always in the near field coupling.   If you are 
using an rf pre-amp, then try to find one with a low input impedance.  --eric  
************************************** Dr. Eric Bogatin,  Signal Integrity 
Evangelist Bogatin Enterprises, LLC Setting the Standard for Signal Integrity 
Training 26235 w 110th terr Olathe, KS 66061 v: 913-393-1305 f: 913-393-0929 
c:913-424-4333 e:eric@xxxxxxxxxxxxxxx www.BeTheSignal.com   Upcoming Signal 
Integrity Classes San Diego: EPSI, BBDP, July 28-31, 2008 San Jose, SICT, Aug 
12-13 San Jose, EPSI, BBDP,
 Sept 29-Oct 2 ****************************************    -----Original 
Message----- From: si-list-bounce@xxxxxxxxxxxxx 
[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Andrew Ingraham Sent: 
Thursday, May 29, 2008 8:44 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: 
loop antenna (mis)behavior  Gene,  I may not be correct in my use of 
"electrostatic," and yes, the prefix does imply static conditions; but I 
believe that's not always the case..  What I was referring to was plain old 
capacitive effects, not an RF field.  (Which admittedly is somewhat 
questionable this close, relative to a wavelength, between antennas.)  What I 
was trying to imply, is that you have two pieces of metal which are near one 
another.  The fact that they are formed into loops doesn't matter. You've 
excited one of them (the TX one) with an overall voltage relative to ground, by 
driving it unbalanced.  You take the other one and measure its voltage relative 
to ground, because
 it also isn't balanced.  And you detect a signal.  If you flip the polarity of 
the RX "loop" by reversing the grounded and sense ends, you're still going to 
detect the same exact signal, because it is being picked up by the capacitive 
coupling between the two pieces of metal that you call antennas.  Or, perhaps, 
by the (capacitive) coupling into the scope probe/leads, or the leads to your 
RF preamp.  Andy  
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