One Screw Defeats Two Antennas

When we left the 30 meter delta loop I mentioned that it had to be disconnected for the time being. The reason is that it did not have a feed line. Instead I used the one for the 160 meter antenna (shunt fed tower). To conserve Heliax my intent was to have them share the same transmission line. That required a way to switch between antennas at the tower base. It's an economical arrangement since those antennas are never used at the same time; 30 meters is not a contest band.

Switching up to a kilowatt in a low impedance (50 Ω) system is conveniently done with an 8 to 10 amp modular relay. I keep them in stock for applications such as this. I selected a 12 amp SPDT relay, the same device I use in many of my home built antenna switches. "Dead bug" style construction is ugly but it works fine in this application. No one will see it with the enclosure cover in place.

The enclosure at the tower base was previously drilled for 2 auxiliary antenna ports (UHF chassis connectors), so I removed the tape from one of them and installed the connector, relay and flyback diode (1N4007).

The default path is the 160 meter antenna. There is no external connector for it since the enclosure includes the gamma capacitor and connections to the gamma rod and ground. When the relay is energized the RF path switches to the 30 meter delta loop. 

When (if) I add another antenna port, a second SPDT relay will be added. The relays would be chained by taking the wire to the 30 meter connector to the second relay, which would switch between the two auxiliary ports. That is, to select 30 meters the first relay is energized and to select the other auxiliary port, both relays are energized. There is negligible impedance "bump" at HF from this circuit.

The configuration of the antenna selection software was changed to select the delta loop on 30 meters and manage contention in case two radios (SO2R or multi-op) attempt to select more than one of the antennas on the transmission line.

If you've read this far you may be thinking that this is routine and hardly worth writing about. What's interesting about relays switching RF? It's done all the time. Despite hinting that I would write an article about the 30 meter antenna switching I had no real intent of doing so. What changed my mind was what happened a couple of weeks later.

The antennas and switch had been working well. Then one morning they weren't. Both antennas showed high SWR and poor receive signal strength. On a warm and sunny weekday morning I grabbed a few tools and my antenna analyzer and walked across the hay stubble to discover what was going on.

There were no obvious problems. The antenna analyzer confirmed that the transmission line was okay and that the trouble lay inside the enclosure. There were no visible faults when I opened it. The DMM confirmed that the relay was operating and the RF paths were fine. This had become very puzzling indeed.

Since the enclosure is non-conducting I moved on to test the ground wiring.  Unlike in a metal enclosure those wires are necessary. 

When I measured the resistance from the ground lug (bottom centre on the right panel above) to the transmission line connector flange there was an open circuit. How was that possible? The trouble spot is pointed out by the blue arrows.

The picture above shows what it looked like after the repair. At right is the #4 screw that I replaced.

There is rust on the screw head, top and bottom, and the first couple of threads. The rust acted as an insulating layer between the screw and connector flange, both it's outer surface and the screw hole interior walls. I used plated screws since they were handy at the time. But I did not replace them with 304 stainless screws that I bought this summer for this very purpose. I forgot.

As already mention, for a plastic enclosure there is no automatic ground path from the connector flange. It must be explicitly wired. The flange screws are critical since the solder lug, which is wired to the earth (and tower) grounding lug, must have a solid electrical connection to the flange. 

I opened the box of stainless screws and replaced the rusty one. That fixed the fault. I'll have to do the rest, just in case. Although not all are used for ground connections, it's sensible to replace them all. Stainless should not be used everywhere but when you need it, you need it. The corrosion deposited on the silver plated N-connector flange was removed by light buffing with steel wool. 

Even the biggest antennas can be defeated by a tiny screw. Don't overlook their importance.