Friday, October 21, 2022

The Single-Point Ground (SPG)

SPG all hooked up to coax
and control lines.
As radio amateurs, taking proper precautions against lightning and electrical surges can be a difficult and expensive proposition. We often neglect them and rely on luck.

After reading Bonding and Grounding for the Radio Amateur,  I decided it was worth the investment. I already had all ground connections - house, tower, station - bonded. What was missing was bonding the antenna connections to the ground as they entered the house. I needed a Single-Point Ground panel.

A Single-Point Ground is a metal panel or box providing a common bonding point for all cables as they enter a building. This metal is connected to the external ground rods, which are all bonded together. The idea is to bond the cables connected together, so that lightning or surge potentials all rise and fall together. Without a potential difference, damaging currents cannot flow. 

SPG panel ready to mount
on basement wall
I cut a 0.090" Aluminum panel 12" by 16" and mounted the KK1L antenna switch to the bottom edge. I added lightning arrestors for the antenna coaxial cables, plus a barrier strip holding gas discharge tubes for the rotator and shunt antenna control cables.

I chose MFJ-272 lightning arrestors. I needed six of these, and the MFJ units were about $40 each. The Alpha Delta and Polyphaser units were more expensive. The arrestors are mounted to the panel with a single large screw, supplied with the arrestor.

Short coaxial jumpers connect the arresters to the KK1L switch. I made these out of 18 inches of RG-400 coax using new Amphenol UHF connectors and a UG-175 adapter. RG-400 is doubly-shielded, eliminating the potential for any coupling between the jumpers. (Notice I didn't quite have enough RG-400 for all six jumpers, so there are only five at the moment. I'll have to purchase more)

The panel mounts on the basement wall where the cables enter the house through a 4 inch PVC pipe. A grounding block connects a short piece of copper wire to an eight foot ground rod on the outside. The ground connection is also bonded to the perimeter ground wire between the house, tower and station grounds using a split bolt. 

I had a bit of trouble with the coax running from the A and B ports of the KK1L to the operating desk. Since the coax to the antennas ended at the basement wall, I trimmed some coax from the antenna feed lines. To get the right length, I pulled the feed lines outside, then trimmed them from the far end. Simple, eh? Except the first one looped around and got stuck, and the cut piece ended up being about five feet too short.

The next feed line was a newer run of Davis Buryflex. I ran it to the A3S/A743, and then cut the right length. For some reason, I had problems with the connectors on both ends, but once they were redone, the cable worked as expected. For a short while, I thought the cable itself might be defective.

To do these tests, I made good use of my RigExpert AA-55 Zoom. Which of two pieces of old coax is any good? The cable loss test is quick and give one real numbers to compare. You just need to test the coax both open and shorted. 

With the SPG installed, the next step is to automate the switching of the KK1L.

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