Topband: RX Loops

[Michael Tope]

----- Original Message -----
From: "Tom Rauch" <w8ji at contesting.com>

> In the farfield every antenna looks exactly the same so far
> as field impedance goes. The response is only to the EM
> field, not to a "magnetic" or "electric" field. The ratio of
> electric to magnetic component, or field impedance of the
> antenna, is identical regardless of the source.
>
> It's also well known and established taking the time-varying
> electric field to zero causes the time-varying magnetic
> field to go to zero right with it.
>
> Anyone doubting this only needs to borrow a standard BC
> field strength meter and have a look at the scale. Such
> meters are calibrated in (milli- or micro-) volts per meter,
> an electric field term. The antenna in the meter is a small
> loop. It is the very fact that the electric and magnetic
> field ratio of radiation is ALWAYS in a proportion
> established by the impedance of the media propagated through
> (freespace is ~377 ohms) that allows a small "magnetic loop"
> to accurately measure electric field intensity. BC field
> strength meters that read in V/m are actually calibrated in
> volts per meter with a current carrying conductor placed
> next to the loop!
>
> 73 Tom


Tom,

While agree with the gist of what you are saying, there is
one point where I am a little fuzzy. It has to do with
reciprocity. The plot on your website (I think you attribute
it to W7EL), shows the field Z of a magnetic loop antenna
as a function of distance from the antenna. As expected,
the field Z asymptotically approaches 377 ohms as the
distance from the antenna increases. So far, so good.

Thus I would expect that if I excite the magnetic loop
as a transmit antenna, the plot would accurately
predict the ratio of the E to the H field that I would
measure with small calibrated E and H probes at
some point distance, D from the antenna. Again, so far,
so good.

Where I am unclear, is when you invoke reciprocity
to talk about how the magnetic loop will respond to a
signal source that is a distance, D away from the magnetic
loop antenna. In this case where the magnetic loop is
the receive antenna, shouldn't we be talking about
the field Z plot of the source antenna, so that we can
figure out the relative strength of the E and H fields
from the source when they impinge on the magnetic loop?
In other words, if the source has a different field Z profile
than the magnetic loop, then the field Z plot of the
magnetic loop antenna won't necessarily predict
accurately the relative strength of the E and H
components emanating from the source antenna and
incident on the magnetic loop. So again, shouldn't
we look at the field Z plot of the source antenna to figure
out whether it will be producing a strong E field or a
strong H field at the physical location of the magnetic
loop?

Clearly it doesn't matter if you are in the far field of
both antennas. At that point both plots converge to the
same answer,  377 ohms and it is a no brainer. But in the
near-field, I am not convinced that it makes sense to talk
about how a particular antenna responds to a source at
some distance D based on the fields which that same
antenna produces as some distance D when it is used
as a transmitting antenna. Or maybe it does and I am just
too thickheaded and dumb to see it.

73 de Mike, W4EF...........................