portable 3/4 co-linear


When going out walking with your portable Amateur radio, you may find yourself in an area of poor reception, but you don't want to be carrying a large antenna for such an occasion, you need something that will pack away neatly, but is easy to set up.

So I was looking through my "Two-Metre Antenna Handbook" written by Fred Judd (G2BCX), and came across a mobile co-linear, and thought it would be good if the sections were telescopic, I had a few old antennas around that were in the region of the right lengths, and sturdy enough for the job.
Here is the link to a scan of the original diagram, the 'car top' is where i mounted the ground plane
After measuring the verticals, I had to find a way to join the sections as there is a matching stub in the middle, a 4mm brass spacer soldered into the end of the bottom 1/4 wave section was the way to go, this section was also soldered to the pin of a panel mount SO239.
I decided to rigidly fix the top 1/2 wave section to the top of the stub, and appropriate holes were drilled for this and the 4mm screw in the bottom, I passed the wires through the insulator and soldered them on the top section and the screw before fitting, then wound the stub around the insulator, covering with insulating tape to hold in place

As the original antenna was designed for mounting on a car, we need a ground plane, at the local supermarket i found some telescopic magnetic pick up tools, basically a small telescopic antenna with a magnet on top, they were cheap enough, so were pressed into use once cut to the 1/4 wave, but how to mount them. A hosepipe adaptor for a tap had just the right thing, a brass ring which could be drilled to accept some screws, these were soldered into place so the antennas could be screwed on easily, and the hole in the middle was just the right size for the connector.

Now we come to mounting the whole thing, I wanted to give myself several options, yet another section of old antenna was used (offcut from the 1/4 wave part), a bit of adapting and I got this section fitted to a PL259, threading the inner from some low loss coax down the middle, a bit more fiddling and I had the BNC on the other end, covered with the insulation from the coax, and a plastic tube added to build up the diameter in one area.
Here's the kit of parts including a piece of fishing rod case, a tube that fits inside the case with enough room to wind about 75 feet (22m) of rg58 cable around it, a couple of pairs of 15mm pipe clips, and a reel with string and a weight.

And a photo of the base clipped to a pipe as one method of mounting

The antenna could be hand held, for instance just collapse the telescopic sections wile you walk to another place, and pull it all out when you stop.
If you set up camp somewhere, you can use the string and weight to throw a line over a branch to suspend the antenna.
I'm sure there are other ways this system could be used, being contained in a tube it should be good for a backpacking holiday for instance.








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