The return of some sunspot activity got me thinking about building antennas again (one of my favorite ham radio activities). During the last couple of years of low activity I've been using an L-shaped 130 foot dipole fed about 30 feet from one end. It is only about 20 feet above ground at the feed point (the chimney of our house) and the long end runs 100 feet over to the neighbor's tower (payment for climbing it and installing his antennas).
There is a remote automatic antenna tuner (LDG AT200-PC with a second antenna connector which was begging to be used) in a weather proof box within about 5 feet of the feed point of the dipole. It has no problem getting a match on most bands 80 thru 10 meters thanks to the careful choice of feed point offset position. The station feedline and tuner control and power lines run down through the chimney into the living room fireplace (which houses a TV) and then over to a table where I have my TS-480 set up.
On a side note, the TS-480 makes a wonderful living room ham rig. Only the tiny control head and Swedish pump straight key sit on the table while the RF box is tucked inside the hi-fi cabinet. And I must mention the TS-480 has one of the best QSK implementations I have ever used.
While the low 130-foot multi-band dipole has worked well for casual contacts, I wanted to experiment with something more consistently omni-directional and with a lower radiation angle on the higher bands for a shot at some better DX during this next cycle. So I started thinking about a vertical but I really don't have any space for radials (and even if I did the WAF of radials is really really low). Reading about the GAP antennas got me to pondering construction of a full-size 15 meter vertical dipole using left over pieces of an old vertical that have been taking up space out in the garage. Some spring-like weather in the middle of February provided the final motivation I needed.
The base of the old vertical could extend to about 12 feet (with the help of a 3 foot stainless steel whip at the end) and I had an 11 foot mast pole my neighbor gave me that was only a little bit bent up. So I put the two together for my 1/2 wave dipole. I also had a coiled coax common mode choke I had used with a 17 meter delta loop during the last sun spot cycle so I recommissioned it to help balance the feed.
I did a little antenna modeling (using EZNEC demo, 4nec2, and xnec2c). I toyed around with adding some extra elements (think vertical fan dipole) in an attempt to cover 20 and 17 meters as well, but they tended to mess up the radiation pattern and were really tweaky so in the end I decided to keep it simple for this first attempt.
We have a wooden fence in the back yard with well planted 4x4 posts rising about 7 feet out of the ground. All I needed was a way to attach the bottom pole while keeping it insulated. That old nylon cutting board in the kitchen was needing replacement anyway ...
Bisecting the cutting board gave me the two mounts I needed.
Using an antenna analyzer, I adjusted the length of the top section with the assembled dipole laying horizontally only a couple of feet off the ground on a wooden table. Surprisingly, that length adjustment remained correct in the finished upright position with the feed line coming away from the antenna at a right angle.
That is the chimney over there on the right where the antenna tuner is mounted and the 130-foot dipole feed point is near the top of the mast. For the new vertical dipole I'm using a 50 foot length of RG-8 coax with the last 10 feet coiled up at the tuner. The SWR is very low over the entire 15 meter band with the tuner in bypass mode. (The analyzer was showing about 80 ohms when attached in place of the tuner.)
My first contacts during the DX contest on 2/20/2010 yielded very satisfying results. Running 60 watts I made many contacts with stations in Japan as well as contacts in Russia, New Zealand, and Brazil from here in the Seattle area.
The antenna tuner is able to obtain a match on 20, 17, and 12 meters but I have not attempted any contacts yet. I'll post again with some additional results after I have used this antenna for a while.
73 and may you have good DX this sunspot cycle.
Harry, AB7TB
