[SI-LIST] Re: loop antenna (mis)behavior
- From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
- To: "'Andrew Ingraham'" <a.ingraham@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 29 May 2008 12:18:34 -0500
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
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-----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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