A 1-meter-diameter small transmitting loop for 7 MHz: part 8

The effect of thin, crumpled, separate copper strips on antenna gain

A small loop antenna with the following parameters was simulated in 4nec2:

1. Dimensions of 1 meter square
2. Conductor diameter of 10 cm (radius of 5 cm)
   
3. 1.5 meters above real ground with "Moderate" conditions
4. Resonated with 276 pF capacitance for zero reactance at 7 MHz
5. 8.2 milliohms of capacitor loss, corresponding to a Q of 10,000 
   

Antenna gain when using a perfectly-shaped round copper conductor

If the conductor loss resistance is computed by 4nec2, the gain at 15 degrees above the horizon is 6.4 dB below that of an isotropic radiator. The same gain value is obtained if the round conductor's loss resistance is analytically computed and specified as 8.72 milliohms (400 cm conductor length / pi*10 cm conductor width = 12.73 copper squares, with 0.685 milliohms RF resistance at 7 MHz for a copper square of 5 skin depths thickness, giving 8.72 milliohms).

Antenna gain when using thin, crumpled, separate strips to form a distorted approximation of a square-cross-section conductor

If the conductor loss resistance is explicitly specified as 21.4 milliohms (located at the highest-current segment of the antenna), to correspond to the values computed in the last article for thin, crumpled, separate strips of copper tape, then the gain at 15 degrees above the horizon reduces to 7.45 dB below that of an isotropic radiator. 

Conclusion

The effect of using thin, crumpled, separate strips of copper tape is to reduce the antenna gain, at 15 degrees above the horizon, by 1.05 dB compared to that of a perfectly round and smooth conductor, giving a gain of -7.45 dBi.

The conductor's RF resistance increases above that of a smooth, round conductor (8.72 milliohms) by 12.68 milliohms, to give 21.4 milliohms RF resistance for the thin, crumpled, separate strips of copper tape formed into an approximately-shaped square-cross-section conductor.

Other antenna simulations were performed in 4nec2 for similarly-sized loop antennas with different configurations, such as multiple turns wound solenoidally, or multiple turns would in a concentric, spiral configuration. When including ground loss, capacitor loss, and the conductor loss as predicted by FEMM, all simulations yielded roughly the same result: around -7 dBi at 15 degrees above the horizon. With the constraint of 1-meter diameter, no multiple-turn configuration of the conductor could be found that significantly exceeds this performance.

The conclusion is that using readily-available copper tape (50 mm wide, 0.030 mm thick), applied to a reasonably but not perfectly-shaped square-cross-section plastic tube, is a feasible method -- and probably the easiest method -- for constructing a low-loss conductor at 7 MHz for use in this application.