[SI-LIST] loop antenna (mis)behavior
- From: Gene theBean <mosfet_the_gr8@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 28 May 2008 14:53:07 -0700 (PDT)
I need some help from anyone who knows about fields and/or rf propagation.
Here's my issue:
I have a system with a transmitter sending a continuous 2 MHz signal out using
a loop antenna. The loop is electrically small, maybe 5" in diameter and
around 7 turns. The receiving antennae (there's 2 of them) are likewise the
same size and number of loops, and is around a couple of feet away from the
transmitter. The RX antennae are just a couple of inches away from each other.
They all share a common centerline. Also, the antenna positions are fixed on a
common, non-magnetic, core.
Here's the problem: there is absolutely no phase change in either RX antenna.
That is, they are at 0 degrees phase, relative to one another. Changing
polarity on either RX antenna does not affect the relative phase, where the
expected phase shift would then be 180 degrees.
In an experiment, using duplicate RX antenna sections, we discovered that the
very near field behavior is quite different than the far. And, this 'near'
behavior only is true withing an inch or 2 of the transmitters. There, the RX
phase can changed by swapping the polarity of the antenna
leads. Fine. Moving these 'floating' sections in the vicinity of the
TX reveals phase inversions depending upon the position. But, again, moving
away from this near field area, the phasing is always zero degrees regardless
of the antenna polarity.
Anyone know what may be going on?
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