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

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




      
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: