Pirate Nation Radio - Horizontal Loop Dipole
Description: Hello home builders, this time we build a Horizontal Loop Dipole Antenna.
Most dipole antennas have straight dipole elements, the loop dipole has its elements bent into
a circle, loop or box shape. Almost any shape will do, as long as the both sides are the same and
the ends do not meet closer then 3-4" apart. This antenna has 0 db gain, but stacking two
antennas will give you 3 db gain and a stack of four antennas will give you 6 db gain over a
single antenna.
Horizontal Loop Dipole feed and mounting Construction: The Horizontal Loop Dipole elements are made from 1/4" soft copper tubing, available at almost any hardware store. Each element is cut for the operating frequency plus 6 inches, our example is for 89 Mhz, so each element was 38" long. Carefully straighten each element out of a flat surface then use tape or cable ties to strap them together. This makes it easier to bend both sides alike. The first bend must be at 6", as it forms the hairpin match and mounting point. Make each bend slowly, trying not to crimp or kink the copper pipe. You can use a piece of pipe or pvc to help you form the bends too. As stated before, the first bend is at 6" and the shape after that is up to you. We use a couple of pieces of aluminum L stock, as a clamp to hold elements to the mast mounting bracket. A stainless steel muffler clamp is used to hold the assembly to the mast. Please check out the photo "Horizontal Loop Dipole feed and mounting" above and the photo "Horizontal Loop Dipole mounting assembly" below for clues. The distance between the 2 elements, from the first bend to the mount is 1" apart. The distance from the first bend to the mount is 5", with 1" of the element ends under the mount's clamp. Aluminum rod could also be used for a more sturdy antenna but for the purpose of this project, copper tubing was easier and cheaper. 
Elements strapped together before making bends Tune up and use: The coax cable solders directly to the copper elements at the first bend. With the antenna cut for our operating frequency of 89 Mhz, the length of the elements from this point to the ends will be close to 32" on each side. To fine tune the swr, move the coax leads slightly, 1/8" at a time, back and forth for the best match. With very little effort, I was able to get a swr of 1.2 to 1 or less. If you find that you need to move the coax much beyond the first bend, to lower the swr, you will need to shorten the length of each element. Do not remove more then 1/4" of element at a time, resolder the feed at the bend and repeat the process. Also you will need to seal the coax at the feed with tape or heat shrink. 
Horizontal Loop Dipole mounting assembly Performance: I compared the performance of the Horizontal Loop Dipole to a 1/2 wave vertical dipole. Both antennas were mounted, one at a same, using the same mast and feed line. With my FM Walkman and note pad in hand, I headed out the door for my daily walk. I noted where the signal went sour on my hand drawn map. Then I swapped antennas and repeated the process several times over 2 weeks. I also made several test using my family's cars, although not as detailed with the results. Here's the scoop ! The Horizontal Loop Dipole was as good as or better then the vertical dipole most of the time. The only exception was when I drove momma's car, with its vertical antenna, the reception was typically better from the vertical dipole. My car, with it's antenna in the windshield, the 2 transmitting antennas performed about the same. So there you go! One Important Note: This antenna must be in the clear, keeping all other stuff away at least 36", except the mast. 
Stacked Horizontal Loop Dipoles Final Thoughts: By using 1/4" soft copper tubing to build a Horizontal Loop Dipole, the antenna will not be very sturdy for outdoor use, as weather and birds will tweak the elements. For attic or portable use, this would be a great design. For a permanent outdoor installation use 1/2" soft copper tubing instead. Only minor changes at the mount would be needed, mainly drilling holes in the elements ends and using 4 screws instead of a clamp. - DJ Pirate Joe |
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