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To All from PA0SE
A question by Jaap, PA3GUC, prompted me to investigate
the directivity of an Inverted L-antenna.
I modeled by computer an antenna with 10 m vertical and
a horizontal part of different lengths; made of 1.5 mm diameter (16 SWG) copper
wire. .
"Front" in F/B ratio is along the top wire in
the direction of the open end.
The impedance at the feed point is calculated over perfect earth. For
gain, F/B ratio and take-off angle real ground is used.
Gain is expressed in dBd; gain over a halfwave lossless dipole in free
space.
Rrad is the radiation resistance; Rloss the resistance of the wire; X
the reactance at the feedpoint. The minus sign indicates capacitive
reactance.
Zant = Rrad + Rloss + jX.
Length of
Rrad Rloss
Xant F/B Sky wave F/B surface
wave Max. gain Take-off
angle
top
(m)
Ohm Ohm
Ohm
dB
dB
dBd for max.
gain
degrees
10
0.0176 0.130
-9659
0,17
0
-7.76
18
20
0.0221 0.198
-6609 0
.41
0
-8.39
18
50
0.0276 0.403
-3386
1.16
0.02
-10.00
18
100
0.0297 0.752
-1835
2.34
0.05
-11.49
18
200
0.0337 1.496
-885
4.14
0.11
-12.74
20
400
0.0383 3.44
-256
4.40
0.23
-13.21
30
537.57
0.051
5.63
0
1.99
0.29
-12.92
58
During daylight contacts over a few hundred kilometres are made by the
surface wave and the Inverted
L-antenna shows no useful directivity.
After dark the skywave appears and some directivity can be obtained, but
only by long top wires.
The decreasing maximum gain with longer top wires is caused by a widening
vertical arc within which radiation takes place.
In the case of a resonant antenna (last line) the gain is
constant within 1 dB over elevations from
about 15 to 100 degrees!
73, Dick, PA0SE
JO22GD
D.W. Rollema
V.d. Marckstraat 5
2352 RA Leiderdorp
The Netherlands
Tel. +31 71 589 27 34
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