Hello Roger,
assuming that the underground
antenna behaves like a loop:
For a classic loop area of the loop
and
the current flow are well defined
(the current goes through
the wire = circumference of
the loop).
This is not the
case for the underground
antenna, there will be
a current flow through an area or
even a volume.
But one could define the virtual area
of the underground antenna as the area of a classic
loop that procudes
the same ERP (with
the same antenna current) as the underground
antenna.
But how
to determine this virtual
loop area?
Suggestion:
You can measure
the relative ERP of your underground
loop (at a given antenna
current) using daytime WSPR
and some local RX
station (daytime
and local will rule
out any propagation
changes).
Let's define
the virtual area of the underground
antenna as A1 (m^2) and the relative ERP
as P1 (=
dB values from
WSPR).
Next you could raise
the wire, increasing the
loop area by
a know value
A2). Again measure
the relative ERP P2 (as
before, using
the same WSPR RX
stations at daytime).
Now the total
loop area is A1+A2, where
we know A2
(the above ground
part) but not A1
(the virtual underground part).
A1 can be
calculated:
At a constant
antenna current the
ERP will be proportional
to the square of the loop area.
So P1
is proportional to A1^2
and P2 is proportional to
(A1+A2)^2.
Or: P2/P1 =
(A1+A2)^2/A1^2 =
(A1^2+2*A1*A2+A2^2)/A1^2
And after some more
math:
A1 =
((1+sqrt(P2/P1))/((P2/P1)-1))*A2.
So based on
the ERP increase (P2/P1) and
the above ground area
(A2) of the loop the virtual area
(A1) of the "submerged"
part can be
calculated.
If this
is done for some
different heights of the wire (=
different values of
A2) one would
have an indication if this theory
is valid (the outcome of
the virual area should be
more or less the
same).
73, Rik ON7YD
- OR7T
Ok, so as the final instalment of this test I have now got the wires from
the earth rods coming to the transverter just resting on the wet grass (what
was elevated by 1.5m is now zero). It seems to work best just feeding the
transverter straight into the rods without matching suggesting low reactance
and near 50 ohms.
I'm still getting decent reports! So, it confirms
the "loop in the ground" theory, or it is acting as some bizarre E field
antenna.
Will again leave it running overnight to get a decent number
of spots but already I think we have an answer.
What I can try next -
is this ever going to end!? - is elevate the loop part "in the air" to much
greater height (about 9m to match the top of the Marconi I just took down) and
see how much improvement I get. More in the air implies greater loop area so
more signal. Just how much will indicate how much of the loop's effective area
is in the soil/rock.
WSPR spots please folks.
73s
Roger
G3XBM
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