Monday, 20 December 2010
Slim Jim Antenna for the 4m Band.
This 4m Slim Jim is cheap and easy to build yet it greatly out performs the more usual dipole due to its low angle of radiation. An SWR of 1:1 is obtainable across the 4m FM band with simple adjustment. The photograph and PDF diagram show the construction, however a brief description is also given. The spreaders are made from plastic knitting needles; two small holes are drilled about 10mm in from each end of the spreader to accommodate the aerial wire. Suitable sized holes are then drilled in the fishing pole to mount the spreaders. The spreaders are then glued in position using a two part epoxy. The telescopic fishing pole sections are also glued to each other using epoxy. Once the glue is set the antenna wire is then threaded through the spreaders according to the diagram. RG-58 is connected to the matching section but not soldered yet, just wrap the braid and inner around the aerial wire. Raise the antenna at least 3m above ground and clear of any objects (walls etc), Apply a few Watts of RF and adjust the feed point to give a 1:1 SWR (slide the RG58 up and down the matching section). When the correct feed point has been found, solder the feeder connections. I dabbed some yacht varnish over the connections and the first couple of inches of the coax to stop water ingress down the feeder. Mount the aerial as high as possible; due to the construction materials the wind resistance is very low so it should survive the worst of any storms. N.B. I did try making a 4m slim Jim out of 450 ohm ladder feeder from a diagram posted on the web, however i could not get it to work correctly (maybe the two conductors in the ladder line are too close for 4m use, I also made one for 2m using the same ladder line from a design posted on the web and this one works great!) construction diagram pdf format
Barry Zarucki M0DGQ