Towertalk
[Top] [All Lists]

## [TowerTalk] Ground wire impedance

 To: towertalk@contesting.com [TowerTalk] Ground wire impedance K4SAV Wed, 12 Jul 2006 16:40:25 -0500
 ```All this grounding talk has got me thinking again. Most of us know that the impedance of a wire is increased by its skin resistance, and that a wire with more skin area (such as a strap) will provide a lower impedance. But how much lower? I decided to break out my spread sheet I made for calculating these things and take a look. The data is tabulated below. Some interesting conclusions can be drawn from this data. For example, at 330 HZ the impedance of a #4 wire due to its inductance is the same as its skin resistance, but the DC resistance still dominates. At 1660 Hz the impedance due to inductance is equal to the total DC resistance (skin plus DC resistance). That means for any frequency above 1660 Hz, the impedance of the wire is due primarily to its inductance, not its DC resistance or skin resistance. As a matter of fact, above about 20 KHz, the DC resistance and skin resistances are insignificant for this wire, even though they are continually increasing. Since all these parameters are linear with length, this conclusion is the same regardless of the length of the wire. For a #2 wire, this same point happens at 1100 HZ instead of 1660 Hz. Note that when applying these conclusions to other things besides ground wires, there is a difference between impedance due to inductance and impedance due to resistance. --------------------- Calculated data: #4 wire, 10 ft length, L = 0.43 uH (straight wire in free space) Note: A wire in the ground will appear as a higher inductance than shown here, because of the decreased velocity factor of the medium. Z(L) represents impedance calculated from inductance only. Freq Z(L) DC res Skin res 330 Hz 8.86e-4 2.49e-3 8.86e-4 920 Hz 2.47e-3 2.49e-3 1.48e-3 1660 Hz 4.46e-3 2.49e-3 1.99e-3 10 kHz 2.69e-2 2.49e-3 4.88e-3 100 kHz 0.269 2.49e-3 1.54e-2 1 MHz 2.69 2.49e-3 4.88e-2 I didn't have a spreadsheet already made up to calculate the skin resistance of a strap, but I do have one to calculate its inductance. Since the inductance is the predominate parameter, it's probable all you will need anyway. The calculations are for a strap thickness of 0.05 inches, and a length of 10 ft. Since the thickness doesn't effect the inductance very much, it wasn't included as a variable parameter. Compare these numbers to a #4 wire, same length, which was 0.43 uH. Strap width Inductance uH 0.5 in 0.40 1 in 0.36 2 in 0.32 3 in 0.30 4 in 0.28 5 in 0.27 6 in 0.26 The formula for the wire inductance and strap inductance came from the Polyphaser book, Grounds for Lightning & EMP Protection. One other note of significance: None of these calculations include resonant effects. For wires that are long compared to the frequencies being considered, resonance effects can increase the impedance by a huge amount compared to an impedance value calculated from wire inductance. Jerry, K4SAV _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread [TowerTalk] Ground wire impedance, K4SAV <= Re: [TowerTalk] Ground wire impedance, Jim Lux Re: [TowerTalk] Ground wire impedance, K4SAV Re: [TowerTalk] Ground wire impedance, Gary Schafer Re: [TowerTalk] Ground wire impedance, K4SAV