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Netherlands,
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The first question to come up is usually the one of how big the loop should be. This really depends on the frequency range you would like the loop to operate on, the available space and an as high as possible loop efficiency. I came across a loop calculator program written by KI6GD and used this to play around with some of the variables. Given the space available and the frequency range of interest, I came up with the octagon design (see images above) where each side of the octagon has a length of 50cm. This effectively gives a loop circumference of 4m (the resulting diameter is approximately 1.2m). Made out of 22mm copper pipe and using say 100W of power from the transceiver, the calculator shows the following values :
As you can see from the table above, this loop size is too big to operate on 28MHz. The capacitor value even went negative ... While this size loop still works reasonably well on 7MHz (where the efficiency already dropped to 25.5%), there is not much left on 3.5MHz. That tells me to make 2 loops next time : A bigger one for 80m and 40m, and a loop smaller then this one for the higher bands including 10m. For now, I will continue to experiment on the current version. Construction details As stated before, this loop is constructed using 22mm copper pipe, easily obtainable from just about any DIY store. To get the octagon shape, I used 22mm copper elbows. For starters - using a pipe cutter - I cut 8 pieces of copper pipe each 50cm long and then started soldering the left and right loop halves together using a blowtorch. For the feedpoint at the bottom I used a brass press-fit T-connector, the top ends of the loop (where the capacitor resides) are tied together with a PVC coupler. Doing it this way also allows me to take the loop apart into 2 halves for easy transportation. In the bottom outlet of the T-piece, I mounted an N-type connector. The middle pin is brought up through a hole drilled in the top of the T-piece and I used a piece of white PVC as an insulator. This is where the Gamma match is connected, which is made out of 6mm2 wire, usually used for grounding purposes. A simple clamp allows me to find the point of lowest SWR on the main loop. The whole loop is placed on a stand made out of 40mm PVC pipe - again obtained from the DIY store. 2 PVC cutting boards came in handy to mount the loop on the stand using standard antenna brackets. Future plans At the moment, I am using an air variable capacitor that is not really rated to handle 100W of tranceiver power. I managed to obtain a vacuum capacitor which is much better suited for the job and will put that into action shortly. First though, I am working on a remote controlled motor drive to make tuning the loop from the shack a lot easier. More on that here |
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© 2006-2007 combotec.com - PA3CNJ
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