Thursday, March 01, 2012

Handiham World repost for 01 March 2012

Welcome to Handiham World.

Ice!  Are you ready?
Ice on center insulator and dipole antenna
Photo:  Ice and snow cling to the dipole at the WA0TDA station in Minnesota.  The 450 Ohm feedline and the antenna wire are carrying a coating of heavy ice, as are the nearby tree branches.

icy branches pulling on antenna wire
Photo: Iced birch tree branches pull the antenna wire down.  

ice on 450 ohm feedline
Photo:  Heavy ice coats the 450 Ohm ladder line in this close-up.

Here it comes: The annual Spring severe weather season is here in North America.  Tornadic winds hit in the southern Midwest states of Missouri and Kansas last night, while the same huge weather system brought Minnesota freezing rain and snow.  The transition from winter to summer often means that we will be visited by bad weather that can take down antennas and put stations off the air at the very time their communications capability may be needed.  This storm was well-forecast because it was being watched even as it approached the west coast from the Pacific.  Computer modeling lends a new degree of confidence in such forecasts, so it is perhaps a bit easier than ever to be ready.  

The problem for any given amateur radio operator is that forecasts cannot predict exact weather circumstances in a small geographic area. In this particular storm, heavy snow fell north and west of my location but we only got about 3 inches worth.  Our snow was preceded by rain - freezing rain - which coalesced around antenna wires and tree branches.  When the snow came, it added to the mass already collecting on the branches and wires.  This was a prescription for power outages because tree branches would inevitably begin to break under the weight of the ice and fall across power lines. The power lines themselves, if in the clear, seldom collect enough ice to fall on their own. Sure enough, this morning almost 15,000 customers were without power here in the Twin Cities. Since the storm was more severe in the northwest part of the urban area, that was the place with the most power outages. Even so, in my town there were over 400 customers without power. Our power never failed or even flickered, probably partly because of just plain luck and good switching at the power company to keep failed power lines from bringing down the entire system. One thing I looked for specifically when purchasing my property was underground power lines. I have lived in too many neighborhoods where tree branches fell across lines and cut the power in almost every severe storm.

So what can you do to keep your own antenna systems from failing under the weight of snow and ice?

Wire antennas should be installed so that they have some "give" to them. That means that if the wire should be stressed by the extra weight of ice, the antenna will be able to bend with the weight enough to avoid outright failure. There are various methods of making a wire antenna a bit more flexible. The obvious one is to make sure that when the antenna is installed that the wire is not pulled up tight. Sometimes ingenious methods can be designed to allow an antenna anchored in a tree to move freely as the tree moves in the wind. Usually unless the tree is exceptionally flexible it will be enough to simply allow enough slack in the antenna wire to make for reasonable movement.

Rigid metal antennas are another story. Most amateur radio beam antennas are made of aluminum tubing. Some types of aluminum tubing are "aircraft grade" and may flex more than standard tubing before breaking. No matter what kind of aluminum tubing is used, it is not immune to severe damage from ice loading. If the weight of the ice itself bending the aluminum doesn't break it directly, wind that comes up after the ice is coated onto the elements may very well finish the job and bring the entire structure down in pieces. I am not sure that there is any practical way to prevent this kind of damage in a beam antenna system, but perhaps someone with experience can weigh in on the matter and let us know. Few amateur radio operators have tilt over towers that can perhaps be used to bring the whole antenna down close to the ground with the elements 90° to the surface of the earth so that water will run off of them. But what happens to the horizontal portion of the tower that will then be collecting ice? It's hard to figure out how to prevent ice damage on a beam antenna system, so keep your insurance paid up.
An antenna that is coated with ice and snow will not necessarily tune correctly. When I tried using the LDG auto tuner this morning to tune my 200 foot wire antenna on a frequency that had been previously "memorized" by the tuner, it behaved exactly as if it were visiting that 75 m frequency for the very first time. The tuner cranked away for a while before finally settling on what had to be a very different combination of capacitance and inductance to allow for a reasonable standing wave ratio. Once the ice melts off the wire, the auto tuner will have to search again for a new combination as things return to normal. One thing to consider is that not all automatic tuners will be able to match an antenna that is heavily loaded with ice and snow. The operator must be aware of this and be careful not to operate with a high standing wave ratio.
The antenna wire itself is not the only thing affected by ice and snow. If you are using open wire feed line as I am, you can expect ice loading on the feed line to contribute to changes in how the antenna behaves on the air. If you use coaxial cable, your only real concern is weight of the ice on the cable itself. Any place feed line comes into the house it should have a "drip loop" so that water can drip off the bottom of the loop of feed line as the ice melts. This prevents the water from following the cable through the wall of the house and into the ham shack.

Your antenna system will be more robust if you use good quality materials to construct it in the first place. Good antenna wire may be more expensive initially, but it will be more likely to stay up under ice loading than some bargain wire. As the old saying goes, "a chain is only as strong as its weakest link". In terms of a wire antenna system, this means that a cheap insulator could easily be a failure point no matter what kind of expensive wire and feed line you use. Needless to say, you should always take the time to secure wires properly to center and end insulators so that it will not work loose under pressure as ice pulls on the wire.

Following a weather event such as high wind or icing, you should plan to inspect your antenna systems for any possible damage or tree limbs that might've fallen against the antenna wire. Any kind of antenna system should always be located well away from power lines so that a failure in either the power line or the antenna will not make one of them come in contact with the other.
Tomorrow it will be March, and that is the month that I usually think of as being the start of this severe weather transition season. Maybe it's time to take a look at that go-kit and make sure that you are ready.

For Handiham World, I'm...
Patrick Tice,
Handiham Manager