A 1-meter-diameter small transmitting loop for 7 MHz: part 6

Evaluating conductor loss in the total antenna context

This post provides more context to the conductor loss values computed in part 4 of this series (http://qrp-gaijin.blogspot.com/2022/02/a-1-meter-diameter-small-transmitting.html).

As a review of the simulation results in part 4, we determined that:

1. The radiation resistance of a 1-meter-diameter loop antenna at 7 MHz is 5.7 milliohms.
2. The conductor loss of an ideal round conductor (a hollow 10 cm round conductor with 200 micron shell) is 7.18 milliohms.
3. The conductor loss of a distorted conductor (a hollow 10 cm square conductor with disconnected and misaligned 100 micron shell) is 12.27 milliohms.
4. The effect of the conductor's shape distortion in (3) is therefore 12.27-7.18=5.09 milliohms.
5. Compared to the radiation resistance of 5.7 milliohms, the effect of the conductor's shape distortion was considered "significant", leading to investigation of an alternative configuration, the multi-turn loop, in part 5 of this series (http://qrp-gaijin.blogspot.com/2022/02/a-1-meter-diameter-small-transmitting_13.html).

However, this conclusion, that the 5.09 milliohms of loss due to conductor shape distortion was "significant", fails to consider the magnitude of other losses in the system. In particular, vacuum variable capacitor losses and ground losses may be large enough so that the relative effect of the conductor shape distortion is small.

A more complete analysis should:

1. Specify the entire system including all losses.
2. Define the target performance.
3. Evaluate the effect, on the whole system, of the additional 5.09 milliohms of loss due to conductor shape distortion.

As a starting point we can specify for example:

1. The lowest part of the antenna will be 1.5 meters above NEC-2 real ground, specified as having "Moderate" ground conditions. The capacitor will be assumed to have a Q of 10,000 at the operating frequency, which then introduces an Equivalent Series Resistance (ESR) of ESR=X/Q, where X is the reactance of the capacitor at resonance.
   
2. The antenna gain at 15 degrees above the horizon should be -7 dBi or higher.

Then, given this environment and target gain, we can evaluate the antenna gain (in a NEC-2 based simulator like 4nec2) without and with the 5.09 milliohms of added loss due to conductor shape distortion, and determine the effect on the antenna gain of these shape-distortion-related losses.

The preliminary result of such an analysis (a detailed example will be added later in another post) is that the added 5.09 milliohms causes less than 1 dB difference in antenna gain at 15 degrees above the horizon. The other system losses (ground loss and capacitor ESR) have already degraded the total system performance to such a degree that the added loss of conductor shape distortion is small -- less than 1 dB.

Having quantified the effect of the conductor shape distortion on overall system loss, the revised conclusion is that the loss may be acceptable.