A quick search on the net I came across two ways of doing this one based on using coax and the other using aluminium box section with a 15mm copper pipe forming a square piece of coax.
You can buy a ready made coupler but I so happened to have a piece of aluminium box section of the correct length as well as a piece of 15mm copper pipe and three N-Type laying around so I though I'd see if I could save myself £70 and build one myself.
The way to work the dimensions you use a piece of software called appcad this will allow you to calculate the length of the inner copper pipe which turns out to be 520mm for my size of aluminium box I followed the design for the two port coupler from DG7YBN's website.
What I found was the length isn't particularly critical but the distance between the copper tube running thought the box section is my ali box has an inner dimension of 24mm and as mentioned before the copper pipe is 15mm which requires a spacing of 4.5mm to obtain this I wound insulation tape around a screwdriver until it conformed to the 4.5mm and then used this to centralise the tube in the section.
An afternoon with the hacksaw and drill press I had the basic coupler built and I ten minutes with a 100W soldering iron had the copper tube soldered to the N-Type sockets.
I tested the coupler on the VNA and got a return loss of around 25dB and a VSWR of 1.1:1 which is near enough so I'm a happy bunny.
I'm still awaiting the materials to build the powabeam yagi's but I'll post again with the build and how they went together.
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| 2m Quarter Wave Coupler Results @ 144.3MHz |
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