Friday, November 17, 2006

Thinking vertically

My plans to try out the new low band coil for the Buddipole this weekend might need to be put on hold. Some pressing business at work will likely keep me out of the ARRL November Sweepstakes as well. So, I will have to content myself with just doing a little reading and researching over the next couple of days when I'm not in the office.
I only purchased one of the low band coils. With this single coil I should be able to make a shorted vertical antenna for 80 meters. How shortened? Consider the parts we'll be using:
  • 2 "arms" -- The Buddipole uses 22-inch aluminum rods that are about an inch in diameter. Two arms come with a standard Buddipole package. That gives us four feet.
  • 7 section shock-cord whip -- From the Buddipole accessories section you can purchase 3, 4, 5, 6, or even 7 section shock-cord whips. In my big Buddipole system I use the 5 section whips when operating in the dipole configuration. When constructing a vertical, I typically use my single 7 section whip which is over 12 feet in length.

A full sized vertical would be
234 / 3.5 = 66.8 feet high
We only have 16 feet of radiator between the arms and the whip. The rest will need to be made up with the coil. That might sound like a bad deal but keep in mind that this whole package weighs only a few pounds, breaks down into a small bundle, and can travel with you to faraway places. Plus, there is a lot of gray area between a dummy load and a near 100% efficient antenna. On these 100 pound DXpedition you need to give these compromised antennas a chance to surprise you.
The other problem you're going to have is, well, the other half of the antenna: the radial system. This is a little easier since the radials for the vertical can be made with very, very lightweight wire such as The Wire Man model #534 copper-clad steel which the vendor claims weighs less than one pound per 1000 feet!
Finally, you want these little wires to be the radials, not the outside of your coax. The Triple Ratio Switch Balun plays an important role here. This is from a note from the Buddipole User's Group on Yahoo! "The TRSB is 50:50, 50:25 and 50:12 ohms (nominal). It is designed specifically to match low impedance antennas, and to provide very high isolation (equivalent to approximately 80 beads)." I believe this little accessory is very helpful for use on the higher bands (40-10), but it is essential on 80 meters.
For now, though, the parts sit in the box awaiting their first experiements. Perhaps I'll get a little time over the Thansgiving holiday.


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