Homebrew ultra low loss coax?

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Homebrew ultra low loss coax?

Post  G7VGG on Sun May 31, 2009 6:48 pm

The most important thing on 23cms is to use very good quality coax and N-type connectors or losses will be very high...In fact, its possible to lose all your power in the coax before it even gets to the antenna!
Westflex 103 is the cheapest coax you can get away with on 23cms (about £1.50 per metre) but even so runs should be kept as short as possible as at 23cms Westflex 103 leaks RF like a collander.
Andrews Heliax coax is highly recommended for 23cms use as it can have half the loss of Westflex 103 but it is much more expensive...Typically at least £5 per metre!
However, its possible to make your own rigid, ultra-low-loss coax for about 1/4th of the price of LDF-450 Heliax Coax and with considerably lower loss.
For straight runs simply use copper "half hard" plumbing tube as the outer "braid" and for bends use soft Copper tube that comes on a roll. 22mm or 28mm Copper tube will give much lower loss than 15mm tube and they dont really cost that much more. 16 SWG hard drawn Copper wire or 1/4" microbore copper tube can be used as the inner conductor. To keep the inner conductor and outer tube spaced correctly you simply need to glue some plastic discs along the centre conductor every inch or so, which need to be a sliding fit inside the 22mm or 28mm Copper tube so you can assemble you coax by sliding the inner conductor inside the outer tube. You can cut the disks into three pointed star shapes for even lower loss.
The ideal material for the disks or star shaped supports should be PTFE but at this is very expensive it can be substituted with foam uPVC, which can sometimes be found for free....Ask window installers for offcuts.
Suitably large diameter N-type plugs and inline sockets are available for 22m and 28mm tube but expect to pay at least £15 per plug!!!
Smaller N-type plugs are much cheaper ( about £2-3 each) and can be used instead. Some N-type plugs have a body diamater that is only about 1-2mm less than the inside diameter of 22mm Copper tube and these are perfect for the job but they are a bit small for use with 28mm copper tube unless you add some copper spacer bushing between the inside of the tube and the outer body of the plugs.
This spacer bushing can be made by machining some Copper bar on a lathe, but Copper bar is expensive to buy and you would lose a lot of copper as waste swarf, so its better to make your own Copper spacers by "slitting" some 28mm copper tube down one side and then rolling it around a former so that its a perfect fit inside the outer tube of your coax with no gaps along the longitudal slit/s...The slit/s should be then be sealed with solder. If necessary you can add more tubular copper spacers inside the first one to suit the outside diameter of your N-type plug body/s.
Then you simply solder the spacer bushes into the end of the 28mm Copper tube tube.
By sawing short four slots into the end of the 22mm or 28mm Copper tube Tube it can then be clamped onto the N-type plug bodies with Jubilee clips.
To ensure a weather resistant seal some self almalgamating tape or PVC insulating tape should then be wrapped around the end of the join of the plug with with coax tube.
Obviously rigid coax is'nt exactly convenient if you want to run a directional antenna that needs to be able to rotate via a rotator but at least it will give you very low losses from the shack to just under the antenna so that even if you then run a short length of flexible low loss coax from the end of your rigid coax to the antenna you should still have plenty of power left to play with and of course it will help with the RX side too.
73
Alf (G7VGG)
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