[AAVSO-SID] Re: Minimum interference reception

[Peter Wilhelm Schnoor]

Hi David,

First: "Minimum interference reception" of course ist an
unprecise expression on this case. What has been meant 
is "minimum ground wave reception".

David Saum wrote:
> 
> Is it possible to adjust the loop antennas in the Cap simple
> receiver type SID receiver for "minimum interference reception"?

I don't know since I do not know a "Cap simple receiver".
 
> I have not had any luck with this, but maybe I do not understand
> what it going on.
> 
> 1. I get max signal strength when the plane of my loop is vertical and
> pointed toward the Maine 24 kHz transmitter (approx. NE from
> my VA location)

This is O.K. You are receiving the vertical polarized vector
*mix* of both ground wave and sky wave. Ground waves are 
coming in at elevation angles near zero. Sky waves, what we 
are catching for, in contrast are coming in from above at 
various angles.

Magnetic loop antennas are bidirectional antennas with two
more or less sharp "nulls". In case 1. these nulls are
pointing orthogonal to the incoming ground waves from Maine.
So you will get maximum signal strength..

> 2.  I get minimum signal when I rotate the plane of my loop
> parallel to the ground, or the axis of my loop is pointed toward
> Maine.

O.K. too. You are trying to receive the horizontal vector of
the incoming signal. Raising this antenna up to some hundred
meters (!) should work better if the transmitter signals
include some horizontal polarized waves. But for ground
waves at vlf this is not likely.
Receiving of horizontal polarized waves at very low antenna
heights compared to wave length does not work well due to
ground effects. For the second case here more below.
 
> 3.  Any other position of my loop gives some intermediate
> value of the signal strength.

This is what I expect. Rotation of your configuration "1."
of 90° around the vertical axis should show a more or less
unsharp minimum. This is the second case in 2.

The residual reception signal is coming from sky waves
depending on your distance to the transmitter and
propagation conditions  (daytime, solar flux). The longer
the distance to the transmitter the more unsharp should be
this minimum. This is the main reason why navigation
officers are avoiding long distance propagation paths for
direction finding.

The higher the elevation angle of the incoming sky wave the
stronger the residual signal will be received. I'm leaving
out phase effects (elliptical phase rotation) for clearness.

In other words: cancel out the vertical polarized incoming
ground wave at elevation angles near zero and the residual
is what we are catching for, the sky wave. Pointing your
loop axis toward Maine will do this. The problem in this 
case is that you need somewhat more amplification. At 
least 10-20 dB at the same SNR of your first stage.

> 4.  I do not see any difference when I reorient my loop at
> local noon or any other time.

So the ground waves must be very strong at your location.
I'm using HBG here because it's most far away and stable.
Military stations in Europe show more down times and they
are playing around with directive arrays and/or power. The
second reason for this choice is that the propagation path
is running approx. South<->North. I observed more "clean"
effects on SIDs compared to West<->East pathes (e.g. MSF in
GB, 60KHz).
 
> What stupid mistake am I making now?  I really would like to
> get "minimum interference reception".

No mistake. Only some misunderstandings due to my bad
English I guess...
 
Very best regards,
Peter
54°16'N - 10°04'E