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To All from PA0SE
There has been some discussion on the
reflector about the use of a counterpoise. I therefore did some modelling using
Brian Beezley's (K6STI) computer programs NEC/WIRES and ANTENNA
OPTIMIZER.
There are an almost infinite number of antenna and
counterpoise configurations possible so I had to make choises. I believe them to
be fairly typical but it is up to you to decide whether this is
true.
I made the following assumptions.
1. The antenna is an inverted-L with a vertical leg of 10
metres and a horizontal top wire of 20 metres.
2. As a reference antenna I used the one under 1, fed
against earth; the earth resistance assumed to be 50 ohm. (If you know the
earth resistance in your own situation you can use that value and correct the
total resistance in the table for the difference with 50 ohm.)
3. The counterpoise is a horizontal wire of 20 meters in
the same direction as the top of the antenna and at a height of 3 metres, so you
won't trip over or run into it.
4. The loading coil has a Q of
300.
5. Antenna and counterpoise are made of 1.5 mm
copper wire (16 SWG)
6. The loading coil is
connected between the lower end of the vertical antenna leg and the
counterpoise. There is no connection to earth so the whole antenna system is
floating.
7. For ground constants I used
terms and figures from CCIR Recommendation 368-7: GROUND-WAVE
PROPAGATION CURVES FOR FREQUENCIES BETWEEN 10 kHz AND 30 MHz.
A problem with NEC/WIRES is that it produces the resistive
part of the impedance in the feedpoint as the result of losses in the wires +
earth losses + radiation resistance, but not the radiation resistance
separately. To get around this problem I modelled two cases in which there is no
ground resistance and the radiation resistance could be derived from the
modelling:
A. Antenna plus counterpoise in free space: radiation
resistance 17.8 milli-ohm
B. Antenna plus counterpoise over perfect ground:
radiation resistance 30 milli-ohm.
For real ground, going from very good to very poor, I
used (un?)educated guesses between these two values.
Case Ground
Conductivity Diel. Const. Rad
Cant Lcoil Rtotal
Efficiency
S/m
ohm pF
mH
ohm
%
Reference antenna, no counterpoise, fed against
earth
1
-
50 ohms used instead
0.022 176 7.69
50.22 0.030
Antenna with counterpoise
2
ideal
-
-
0.030 74 18.2
52.8 0.057
3
land
30.0
40
0.027 74 18.2
53.0 0.053
4 wet
ground
10.0
30
0.023 74 18.2
53.3 0.043
5
land
3.0
22
0.020 74 18.2
54.4 0.037
6 dry
ground
0.3
7
0.019 74 18.2
70.100 0.027
7 very dry
ground
0.1
3
0.018 74 18.2
111
0.018
In case of the counterpoise the snag is obviously the low
antenna capacitance and as a result the big loading coil, a real
monster.
This can be improved by increasing the "capture
area" of the counterpoise by increasing the number and length of its wires,
so increasing its capacitance to aerial and
earth and as a bonus less loss in the coil. Increasing the top loading also
helps of course
The increase of total resistance when going from good to
poorer ground is caused by increasing earth losses. This shows that the electric
field lines run not only between the antenna and the counterpoise but for a part
also between antenna and earth and then back via the capacitance from earth to
counterpoise. To check this I lowered the whole antenna system for case 5 so
that the counterpoise was only 0.5 m instead of 3 m high. This increased the
antenna capacitance from 74 to 79 pF. As the capacitance between antenna and
counterpoise can not have changed it must be the capacitance to earth that went
up. Also earth loss resistance increased by 5.4 ohm. This is a undesirable
effect so it pays to keep the counterpoise well off the ground.
It is not absolutely necessary to have the counterpoise under
the top of the antenna. I also modelled case 5 but with the counterpoise wire in
the opposite direction to the antenna top wire. Assuming Rrad = 0.020 ohm again
the result is a capacitance of 73 pF, a total resistance of 56.6 ohm and an
efficiency of 0.035%, so hardly different.
Concluding we can say that under the above assumptions it
depends on the earth resistance in case of feeding against earth (here assumed
to be 50 ohms) whether a counterpoise is a useful
substitution.
As the loading coil is not connected to earth
you cannot connect the transmitter to a tap on the coil. Link coupling has to be
used instead. I would first excite the system lightly via a one turn link and
with a neon lamp or fluorescent tube find the "cold" point on the
coil. The final link can then be positioned at that point of minimum voltage.
As a
final remark it is interesting to note that in the early days of radio, for
instance in the ARRL Transatlantic Test of the early twenties, stations
invariably used antennas with counterpoises, oft in the form of wire cages.
Frequencies in those tests were between 1.5 and 2 MHz.
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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