Planning a simple RF Distribution System

[ppppenguin]

Putting a bend in the inner was the bodge for when you didn't have a soldering iron to hand. The other bodge was crimping up the the inner of the connector slightly to grip the inner of the cable.

[Terry]

It has always surprised me that the only coax plugs that seem to be available are cheap rubbish. The clamp won't go over the outer of the cable so the end result is poor. The days when we were able to use genuine Belling-Lee coax plugs are long gone. They were so easy to fit and stayed fitted.

Yes, the first 'substitute' for the Belling-Lee plugs came from RS. Apart from different knurling on the screw cap, the looked identical - same aluminium construction and polythene insulator but it was let down by the cable clamp.

The B-L clamp wars hardened spring steel and the four claws were each formed into a kind of cup (best way I can think of to describe it). When fitted to the thicker 'low-loss' coax used for UHF, a bit of brute force and ignorance was usually needed to expand the jaws enough for the clamp to go over the outer jacket.

Whichever size of cable, when the cap was tightened, the claws were forced into the jacket where the cupped arrangement ensured that they bit deeply into it, making a 'guaranteed never to pull off' job of it.

The RS ones, and other copies, had terrible cable clamps made of thin, flimsy material so that when the cap was tightened, the claws turned into flat strips of metal alongside the jacket with no physical strength at all!

There was one exception (which preceded the RS one by many years) made by Egen, with a brass body, a hard plastic insulator which didn't melt when you soldered the centre pin (!) and also had a decent cable clamp.

[ppppenguin]

Those Egen ones were great for making DIY adaptors. I made quite a few BNC <> B&L and occasional PL259 <> B&L (and PL259 <> BNC) some of which worked better than others. BNC female to B&L male worked very well. The brass body of the Egen was easy to solder and the inner insulator didn't deform.

Then Egen plugs looked rather different but I've also got a very few that look like the standard B&L but with brass body. No idea who made them. May have been "borrowed" from BBC stores a few decades ago.

[Terry]

There was a poor Cinch car radio aerial plug around in the 60s but Egen also excelled there too, with both plugs and line sockets. Brass body and same clamping action as the coax plug.

We found them extremely useful.

A lot of people took to mounting car aerials at the rear and, of course, the integral cable was a standard length and too short! so, they came to us asking for extension leads.

This was not straightforward, and I'll explain why for the benefit of anybody who hasn't worked with these radios.

The capacity of the cable was part of the RF input tuning and the radios had a trimmer, often accessed via a hole behind the tuning knob or on the side, somewhere. When the radio was installed, or if the aerial was changed, it was necessary to adjust this trimmer for maximum volume on a weak station at the HF end of the Medium Wave, in exactly the same way as when aligning any radio. Even if you had the correct cable, when you added the extension, the capacity increased beyond the point at which the trimmer could be adjusted!

We found a source of the cable - it is completely unlike any other coax cable you've ever seen, and has a polythene tube inside the outer screen with an extremely thin wire inside that is bent in a zig-zag shape with the bends about a centimetre apart. When it is pulled into the tube, the bends only are in contact with the wall of the tube, ensuring that, as far as possible, as much as possible is held in the centre of the pipe, distributing the capacitance between it and the screen evenly along its length, as well as keeping it as low as possible.

Fortunately I found the spec for the cable which gave the capacity per unit length, so it was easy to work out the total for the standard length of cable. I could also work out a total figure for original plus extension for a variety of cable lengths. All simple addition, so far!

I then sat down with pen and paper - no calculators in those days! - and calculated the value of series capacitance required at the receiver end to make it look to the receiver like a standard aerial. I finally ended up with a list of preferred values against various extension lengths.

Now, this is where the Egen connectors came into their own. The Line socket was fitted normally at one end and the other end prepared in the usual way except that the polythene tube was cut much shorter than needed to fit the body of the plug and the centre conductor cut back too. A Sufflex cap of the correct value was then soldered onto the centre conductor, the other end passed through the centre pin and the plug assembled as normal. The final step was to solder the Sufflex's wire to the centre pin and cut off the surplus!

I can't find a picture of an Egen plug but if anyone is not familiar with this type of plug is trying to puzzle out eactly where the Sufflex went, this will help:

[ppppenguin]

Horrible plugs. ISTR they were also used on some Advance signal generators.

[Terry]

Horrible plugs.

The Egen ones were very nice plugs (and line sockets)!

I agree that the Cinch ones were horrible, though.