Notebook Series - Horizontal and Vertical Doublets (Dipoles)
TX ] Here are two antenna projects that show the simplicity of homebrew antennas that perform
remarkably well. So, before you spend big $$ on commercial antennas, take a look at these.
While living in Texas and surfing the WEB, I ran across a site hosted by W4RNL.* One of his many pages described a doublet that would fit in a limited city lot. I've played with a couple variations of his 44 ft and 88 ft designs, and they all worked great.
I've used open-wire feed, 300 ohm TV ribbon cable, 300 ohm ladder-line, and 450 ohm ladder-line with good results. Of course, a balanced antenna tuner should be used with this type of antenna, and they can be easily built, too. All my tuners use choke baluns. (Note: I haven't had any tuner heating issues tuning these antennas.)
Another interesting and surprising antenna is the vertical doublet. I used one for many years in Texas (without radials) mounted just a couple feet off the ground.
When compared to a horizontal doublet at 19 feet above ground (neighborhood constraints), the vertical was superior for DX on 20 meters. This antenna was a surprising performer. It even loaded well on 40 meters with a balanced tuner, and did a respectable job. I've been thinking about linear-loading this type antenna, but haven't followed-through.
Note: I finally got around to linear-loading this vertical doublet after moving to Michigan, and details are included in my Notebook Series - Linear-Loaded Vertical Doublet (Dipole)
Construction details can been seen to the left and below.
Notebook Series - The Inverted L
MI ] Don't have room for a full size 160 or 80 meter antenna? Want to capture a little DX
on these bands? How about an inverted-L? It's another simple antenna that does a good job, but
keep in mind that my installation is not optimum. It's a compromise, built to take advantage of what I
had available and the constraints of my location.
The basics: Each "L" and each elevated radial are a quarterwave. That's 130 feet for 160 meters and 67 feet for 80 meters. Cut the wire long and trim as necessary or use a tuner like I do. The vertical portion of the "L" should be as high as possible. Use the remaining length for the horizontal portion.
Since I removed my antennas for 40 and 30 meters, I also use the 80 inverted-L on 30 and the 160 on 40 with a 40 meter trap made by Unadilla.
For an efficient antenna, 3 or 4 elevated radials are required for each band. Because of yard constraints, I only use 1 radial for 160, 2 for 80, 2 for 40 and none for 30 meters. (Note that the radials and feed points are off the ground 7 to 10 feet to minimize ground loss.)
At one time, I used one feed-line for both inverted-L's, but decided on separate lines just to minimize interaction between antennas. The 50 ohm, 8 foot feed-lines run from an auto-tuner in the shack to 1:1 chokes at the base of my 36 foot tower. 3 foot pieces of 300 ohm ladder-line run from each choke to the opposite side of the fence that surrounds the base of the tower. (These would not be needed, but my coax was short since these line were used with another antenna project.) The photograph to the right shows how I tied the two vertical sections of each inverted-L to their feed-lines.
You can also see how I designed the spring tension system. This is required, because trees and their branches do move in the wind. I use pieces of wire braid to jumper the feed around each spring. (I placed each spring inside a PVC tube, so the flexible feed jumpers don't rub against the spring.)
Each vertical leg runs up the side of the tower to the 32 foot level and separated from each other as much as possible. Each wire passes through an insulator that is tied off to the tower. This keeps each line a little over 2 feet away from the tower. The 80 meter wire continues horizontally to a tree. The 160 meter wire continues to another tree. Each end is about 30 feet off the ground.
Note the attachment point of the above-ground radials at the base of the inverted-L. This is certainly not optimum, but works OK. Look at the pictures to see how the radials are run along the property fence line. Radials also run out the side yard to a couple trees. One 80 meter radial is bent 90 degrees. Again, radials are 7 to 10 feet off the ground. Keep the elevated radials high so foot traffic can cross under the lines.
ex: WPE8EUM, WN8AQL, WB5FCO and WJ5MH
*L. B. Cebik, W4RNL ~ 1939 - 2008 ~ SK as of April 2008