Sunday, 25 May 2014

Getting Sectioned

A few people over the years have suggested I should be sectioned! But its actually some coax thats being sectioned here!

Ive been tasked at work with producing and teaching a Basic Broadcasting course, a sort of Radio 101, and I like to have hands on demos of things, rather than just death by powerpoint. My course is to explain in basic terms essentially how radio works and how we broadcast with it! And part of that means a bit about feeders.

So, I decided I needed some coax that I could bare back and use to demonstrate how it works. I could use a bit of RG-214, but its not easy to see at the back of a classroom, so I found a 5m patch-lead (for want of a better term!) of this stuff -

This is 3.8 inch RFS Heliflex. Its a semi-rigid cable similar to Cellflex or Heliax, only bigger! Ive done a comparison photo below with a bit of RG-174!


I also have this bit of 4" Rigid line. Now this was part of a combiner system some time in the past, and this particular section was modified with an inter-digital filter and two BNC sampling points. It also has an N type adapter on one end!

I am slot sectioning the rigid line, which means essentially cutting a slot down one side so the innards can be seen. Ive removed the sampling points and the filter plungers, and my nibblers made swift work of roughing out the slot, helped by the outer 'braid' of this coax being aluminium tube!
The Heliflex is a bit harder. This needs a staggered sectioning of the open end, plus a slot section down about half its length. Having cut both ends off, I have two pieces with connectors on, each about three feet long. Those connectors are massive, and silver plated!

The picture below shows one section with the end carefully and neatly cut, and the outer sleeve cut back where the slot will go. The next task is to remove the copper outer conductor from the slot area. This won't be easy, as the copper is thick and corrugated.

 Looking into the open end, you can see how this is made up, and also appreciate why we have to use dehydrators to fill them with dry air! The yanks use nitrogen for this. By pressurizing the feeder systems, it prevents moisture getting into the feeders.

I have about four foot of this spare, which I will chop up and donate to local radio clubs as a teaching aid!

The other demo item I want to build is a set of UHF Lecher lines. I already have a set, but need a suitable balun to feed the RF energy into them.

Thursday, 15 May 2014

Camels and needles

Im currently attempting the rather tricky task of threading a flexible, stranded wire up the inside of a 30ft fibreglass tube! But, this tube also tapers, and has a constriction in the middle!

So far ive tried various telescopic fishing rods to shove the wire up, but they are either too short, or eventually too fat to get all the way!

Next thing to try then is a thin line and a small but heavy weight!

The idea is to put an antenna element inside this tube, which happens to be Sams flagpole, which was originally put up to eventually become a vertical antenna anyway!

Part of the problem is the presence of an internal bung, which was at the top. I had thought it to be a seal of epoxy, and tried to crack it out with a chisel. Well, it just fell inside! I cant get anything past it, and without cutting more off the end of the tube I cant get it out