Dear Group,
Rik and Jean Pierre were
asking
about
the new
aerial
here
.
Jean-Pierre sent yesterday
a series of captures
from
2208-0730
showing
much
variation
in the signal
levels
on
477.7 kHz.
They
were
quite
strong
between
0330 and 0500 though
there
were
other
peaks
and complete fades
during
the night
.
Many
thanks
Jean-Pierre.
Here
are the details about
the "Rotated
L" now
on
the air on
477.7 kHz.
Comments
and critiques
are most welcome
.
The current
was read
with
a clamp-on
ammeter
which
was then
calibrated
by
measuring
current
through
a 50 ohm load
. The
wattmeter power was 70 watts but
the scope was showing
200V
p/p
which
is 100 watts. Perhaps
the wattmeter is reading low.
It
had a nasty
accident with
a Decca
, was repaired
but
not
properly
recalibrated
. Assuming
the scope is the more accurate, the current
was I=P/V=100/71=1.4 A. Scaling
the aerial
current
accordingly
yields
1.6 amps
.
The RL extends
from
the feedpoint
up 27 metres
and slopes
down to
12 metres
over a 100m
span. The mean
height
is about
18 metres
.
It
is fed
with
~50 uH
of tuning
; no
R matching
was needed
.
A 30m
vertical
has Rrad
~1 ohm on
630 metres
. The ON7YD
page gives
an
_expression_
for
Rrad
for
a horizontal top load
, but
the RL is
sloping
this
will
reduce
Rrad
. Let's
guess
that
it's
2 ohms
So
... ERP=I^2R
= 2.6 R or
about
5 watts.
I'd
best crank the variac
down a few notches
to keep it
legal
!
73
Joe
VO1NA
