Saturday, June 8, 2013

Horizontal and Vertical Doublets (Dipoles)

Horizontal and Vertical Doublets (Dipoles)

[ Austin, 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

[ Marquette, 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 hoizontal portion.  3 or 4 elevated radials are required for each band for an efficient antenna.  (I only use 2 for 80 and 1 for 160 meters.)
To keep feed-lines to a minimum, both inverted-L antennas are fed with the same line.  The 50 ohm, 8 foot feed-line runs from the tuner in the shack to a 1:1 choke at the base of my 36 foot tower.  A 3 foot piece of open-wire feed-line runs from the choke to the opposite side of the fence that surrounds the base of the tower.  The photograph to the right shows how I tied the two vertical sections of the inverted-L antennas together.  You can also see how I designed the spring tension system.  This is required, because trees and their branches do move in the wind.
Each vertical leg runs up the side of the tower to the 32 foot level.  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, but it's slanted downward and connects to the tree about 10 feet off the ground.
Note the attachment point of the three above-ground radials at the base of the inverted-L.  (Left insulator.)  One radial is used for 160 and two are used for 80.  This is certainly not optimum, but works OK.  Look at the pictures to see how 2 of the radials (one-160 and one-80) are run along the property fence line.  The other 80 meter radial runs out the side yard to a tree, bent 90 degrees, ending at another tree.  Radials are 7 to 10 feet off the ground.  Keep the elevated radials high if foot traffic can cross under the lines.

73 and have fun with your construction projects...
Joe (AJ8MH)

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