Saturday, June 16, 2007

40m vertical

This afternoon I built and tested the 40m vertical that our club will be using on Field day. It used the 10m DK9SQ 10m mast from Kanga to hold up the vertical wire and used two elevated verticals radials to complete the design. This is actually the first time I had used the mast in a ground-mounted configuration. The other times I had deployed an antenna with it we were able to find a roof or upper-story deck to mount it. In those cases, I would just bungie the mast to a railing or post and not guy it. Today, I used three pieces of dacron rope fixed to the mast about 10-feet from the bottom to hold it in place. The ropes were held to the mast by a single cable tie. The nice thing about that arrangement was breakdown of the system was a breeze. One snip and the guy ropes were freed.
The exercise was to build a 40m vertical antenna for the local club's Field Day outing next weekend, but I also wanted to make something that could be taken to St. Kitts this fall. So, rather than use heavy or stiff wire, I used some of my magic #534 from The Wire Man which is claimed to weigh less than one pound per 1000 feet. It is light!
Here are the details of the construction: The mast is 10m long. I needed 10m lengths of wire to make the 40m quarter-wave lengths. So, I used the mast as my ruler to cut the three pieces of wire to the correct length (adding a generous amount of wire in each segment as it is easier to make wires shorter than "cut them longer"). I then cut a 2-foot length of 3/32 Dacron rope and tied the end of one of those lengths of wires to this rope. Some masts have enough carbon in them that they are conductive and could couple with our vertical wire element. The Dacron rope allows us to attach to the top of the mast without creating any coupling complications.
The Dacron rope is attached to the top of the mast by wrapping it around the very thin top section six or eight times, then taping it with regular electrical tape. It is a fine way to do it if your antenna needs to only last a day or a week. I'm sure it would not last a year that way. But, luckily, we 100 Pound DXpeditioners can take a few shortcuts!
The vertical element, now tied to the Dacron rope (which is in turn afixed to the top section of mast) should be run to a point well away from the bottom of the mast. When we erect the mast we will run the wire down at an angle to ensure it will not couple with the mast.
The vertical element and two radials terminate in a small, inexpensive center insulator normally used for dipole construction. These have an SO-239 connector and two wires coming out: one which connects to the center pin, and the other which connects to the shield. Use your multimeter to determine which side is the "hot" and which is the shield. Connect the vertical wire element to the "hot" wire of the insulator. I like to make a good mechanical connection by crimping on a butt splice or some other physical connector Soldering is OK, too, but this seems easier in the field. Since the 26 AWG antenna wire is so small, I just wrapped it around the bigger wire from the center insulator and honked on one of these splicers. The radials were attached the same way to the other side of the insulator.
The beauty of this antenna system is (1) it is very light, (2) it can be erected by one person, (3) if you already have the wires cut, ropes cut, and all the connections made, it can be up-and-running in 15 minutes. We'll try to make that time on the Saturday morning of Field Day as this is one of my Toolbox Talks!

I am off to Florida in the morning. I will be blogging on the trip assuming there is good Internet access in the hotel. Though I have plenty to do, I am taking the KX1 with me on the trip along with a Buddistick. I might try to make a couple of contacts from the hotel room... assuming I can get the window open!


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