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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

E-mail: d.w.rollema@gironet.nl or pa0se@amsat.org