Draining Tower Static

We are all familiar with lightning, in which charge moves between ground and the atmosphere. There is less familiarity with the role that static plays on our towers and antennas. Charge doesn't move until the potential is large enough to bridge the distance from a location with a different charge. This static discharge can play havoc with reception. It is better to continuously bleed the charge so that its potential remains low. The best place to move that charge is to ground.

Let's begin with a story. Many years ago I visited a friend who was doing work on his modest suburban tower and antenna system. As is quite common during autumn in this climate there was light snow falling. As we were chatting we could hear a periodic snap sound coming from the doorway to the basement shack. We walked over to see what it might be.

Lying on the desk was the disconnected coax for his VHF yagi located about 15 meters up the tower. As we were looking around for the source of the sound a spark jumped across the coax connector. The snow was depositing a charge on the antenna which increased until the potential was enough to jump the gap and the charge would flow to ground via the coax shield. We could reproduce the spark at will with a screwdriver to reduce the gap with a wait of perhaps 15 seconds. At the time I was surprised this could occur with such a small antenna at a modest height.

Ordinarily the charge would be constantly bled through the receiver front end or a grounding style coax switch. When drained in this fashion the potential cannot rise high enough to cause any damage. But it would be unwise to plug the coax into the rig when there is a static charge present. Better to wait for the weather to settle.

A tower has a static charge on it due to its height despite being grounded. The ability to bleed the charge to ground can be difficult due to the potential gradient and the potential of the ground surrounding the ground rod. Although you cannot directly experience it there is a charge gradient along the tower, and indeed right up through the atmosphere. Just as there is no absolute protection from lightning the same is true of static charge, but the risk can be reduced. Our main concern is with reception. In extreme environments (high and very dry) transmission can be a problem but rarely at amateur power levels.

The Cushcraft XM240 40 meter yagi has a couple of inherent challenges with respects to static charge:

• Elements insulated from the boom
• Small diameter rods in the capacity hats

The former impedes the flow of charge to the boom and from there to the tower and ground. The latter increases the risk of corona and therefore more energetic discharges. Many hams connect the reflector element to the boom to permit charge flow. This has negligible effect on the resonance of the element. Although the driven element cannot be connected to the boom there is at least a long path to ground via the coax and into the shack.

I am motivated to do what I can since the static discharge noise on 40 meters can be dreadful when it is raining and sometimes when it is snowing. It is bad enough that the static discharges can be heard on nearby antennas for other bands. The XM240 is at particular risk of static charge since it is at the top of the mast. If it were completely bonded to the mast and tower to bleed the charge the QRN ought to be greatly reduced, protecting reception with it and antenna further down the tower.

Which brings us to the next difficulty: continuity between the mast and tower. If you've never thought of it before this may seem a strange question. Most would assume there is an electrical path between the mast and tower due to all those set screws and clamps holding everything together at the bearings and rotator. The problem is that electricity is being asked to conduct through bearings, which due to surface grease on the rolling components may not have reliable metal-to-metal contact.

The routine solution is a flexible wire between the tower and mast. Think of it as an insurance policy against poor continuity through the bearing surfaces. It might not help but it might help a great deal.

During a maintenance climb up the 150' tower several days ago one of my tasks was to install just such a wire. As you can see it's very simple. I cut a 2' length of heavy gauge, multi-strand speaker wire that was handy (though unlikely to be UV safe) and fitted ⅜" tinned lugs at both ends. One end connects to the tower via a top plate bolt and the other end conveniently fits onto a galvanized muffler clamp already on the mast. I was going to use the stainless hose clamp (visible in the picture) but I saw an opportunity and took it.

Will it help? I'll find out soon enough. I will need to see how it performs during rainfalls in the coming months. Unfortunately the insulated XM240 elements could muddy my observations. In any case the XM240 will come down soon and this time around I'll remember to ground the reflector to the boom. The "to be determined" antenna that will replace it at the top of the mast (46.5 meters high) will be bonded to the mast.

If the strap works I will do the same to my other towers. The fix is simple and justified even if the data of its performance is inconclusive.