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Re: [TowerTalk] Station Ground

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Station Ground
From: Jim Lux <jimlux@earthlink.net>
Date: Sun, 19 Oct 2014 11:16:55 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 10/19/14, 10:23 AM, Gary J - N5BAA wrote:
1 to 1 1/2 inch tinned copper braid or copper foil from your grounding
plate located at operating position to the ground rod.  All other
equipment including the radio grounded to the grounding plate.  I use
1/4 inch braid from each piece of equipment to the grounding plate.  At
the grounding rod each coax running through an Alpha Delta TT3G50 surge
suppressor attached to the ground rod on the same copper plate that the
station ground attaches to.



Unless it's convenient mechanically, there's no advantage in strap or braid over round wire. Braid is notorious for partially oxidizing and forming dielectric junctions causing intermods, but that's usually a VHF/UHF repeater kind issue, not a HF.

There's a lot of confusion about the use of braid/strap/wide bar vs round conductors. The wide and flat stuff has lower AC resistance than round conductors, but about the same inductance. So you need to look at whether the problem you are solving with your ground wires is one that inductance dominates (transient voltages) or where resistance dominates (AC loss).

The connection from rig and box to your entry panel is hopefully not carrying any RF current during normal operation, so low RF resistance isn't needed (the usual reason for using strap). Strap is good for vertical antenna grounding systems (you want low AC resistance).

Strap is also convenient for "wrap and bolt" kinds of installations: it's a lot easier to drill a hole through a flat bar or clamp some strap to a round antenna member.

From a transient suppression standpoint, you've got a tough decision.. most likely, your gear has a 3 prong plug, so the chassis is connected to the "house ground" via that means. If you run a parallel ground via your panel (or the coax shield), and it doesn't happen to follow the AC wiring, then you're forming a big loop in the ground system, which can pick up transient fields (from lightning), or RF.

In a perfect world, you'd bring the AC service in at the same place as the antennas, solving this problem, but most people don't have that luxury, so you kind of have to live with whatever you get.


For transients, inductance effects dominate over AC resistance and inductance doesn't depend very much on shape of the conductor, more about the length, so the physical size of the wire should be sufficient to carry the fault current without melting (that generally means "as big than the AC supply wire). There's not much to be gained by using a giant conductor for chassis grounding.

(lightning rise times are in the 10kA/1 us range, and 1 meter of just about any conductor will have 1uH inductance, so 10kV voltage drop. Even a small AWG20 wire only has 30 milliohms resistance/meter, so for the same 10kA, the voltage drop from the resistance is 300V.. clearly, inductance is what you worry about).

(handy rule of thumb: AWG 10 is 1 milliohm/foot, and every 10 gauges increases it by a factor of 10: so AWG 20 is 10 mOhm/ft, AWG 30 is 0.1 ohm/ft)



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