[600MRG] Loop on the ground

[Rudy Severns]

Dave,

 

The answers to RX antenna questions will depend strongly on the particular
situation at a given QTH.  I spent the last year frantically trying many
different RX antennas but ended up learning about reality.

 

First, how close, in wavelengths, is the RX antenna to the noise source(s).
Anything less than 1/4 wave means you're in the inductive near-field.  A
quarterwave on 630m is about 500' and 3600' on 2200m.  When you model RX
antennas within that distance you have to look at the near-field pattern
which is often very different from the far-field pattern.

 

Second, power lines are a combination of horizontal wires with occasional
(or frequent) service transformers and drop lines which have a vertical wire
from each transformer to a ground stake.  This means you have both vertical
and horizontal RFI currents.  

 

The magnetic field from the horizontal lines will couple very nicely into a
horizontal loop, in the air or on the ground.  They also couple nicely with
vertical loops parallel to the lines.  The vertical ground wires couple
nicely into vertical loops perpendicular to the lines.   In my case where
the lines are perpendicular no matter what direction I point a vertical loop
I get really great reception of the line RFI.  I have a loop mounted on a
rotor which shows a very sharp null for sky-wave signals but no help at all
for the line RFI other than to move the bars on the waterfall display
around.

 

I have the worst of all: power lines, all well within the near-field, in the
form of a T.  My greatest distance from a line is 400'.  On my south
boundary I have a polyphase industrial service running E-W forever.  On my
west boundary I have a branch line, also polyphase, running N-S.  In most
urban areas (I'm out in farmland BTW) this  would be a typical geometry.
You are likely to be surrounded with lines.  Erecting a vertical loop is the
same as adding a secondary winding to a transformer, i.e. the power lines
are the primary.  Forget radiation, this is just plain old inductive
coupling. Horizontal loops, BOG's, Beverages, etc, same problem.  Laying
down a 700' BOG greatly improved my reception of the power line RFI.  

 

In my situation at least, the noise is mostly horizontal from the lines and
to a much lesser extent vertical from the vertical grounding wires.  The
near-field usually falls roughly as the inverse of the cube of distance.  My
best antenna is a simple vertical located in the far N-E corner of my
property.  As discouraging as my situation is all is not lost, I still
decode VK4YB WSPR on 2200m regularly. 

 

73, Rudy N6LF

 

 

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