How long can a TV extension cable be?

[The Natural Philosopher]

How odd. I bought several from the shop that supplied my dis amp.

[Terry Casey]

That is hardly a terminator - and, unless they do the uncommon male-male coupler, it won't even fit!

What do you suggest he puts on the other end of the coupler, anyway ...?

[Terry Casey]

Teleste used IEC (belling) connectors on their CATV kit and I'm sure they also provided a 75 ohm termination ...

A resistor and Belling plug would be much cheaper though!

[Ian Jackson]

In message , Terry Casey writes

I did have a bit 'contact' with the Teleste headend equipment. I remember the short purpose-made jumpers, but I don't recall finding any IEC/B&L terminations being used with it. While I'd be surprised if Teleste didn't supply them (for use where required), I've never seen them offered in any vendors' catalogues etc (although 'The Natural Philosopher' has).

[Ian Jackson]

Terry Street says that Teleste did them (from what he suggests, at 'professional' prices!), but I'm sure I have never personally seen any - anywhere.

[MM]

That's handy, because I was about to ask how I might split the cable and renew part of it, cheers.

MM

[Terry Casey]

Well, the reactance of a 100pF capacitor at Ch 21 is 3.4 ohms, dropping to 1.8 ohms at Ch 68, so it wouldn't have a major effect on matching.

But the tiny capacitor inside the plug has a much smaller capacity! One picofarad, perhaps? Or less?

Which means multiplying the figures above by 100 for 1pF or even 10,000 for

0.1pF. so, 300 ohms to about 3k at Ch. 21. I don't think it is going to match to 75 ohms any more - do you?

[The Natural Philosopher]

Yep. I just checked, and I've got one on a spare port on my dis amp. Its just a knurled block with a belling lee plug integrated into it - no makers markings.

this place seems to have em

"IEC169-2 CoaxPlug 75 ohm Terminator each" @ 60p a hit.

Which if you are not happy to source resistors and solder, is a good price, I reckon.

I have to say that that is the ONLY source I found on the web tho, which supports the proposition that these are rare birds indeed.

[Ian Jackson]

And, at 60p a go, a bargain!

BTW, I wonder how many people terminate unconnected outputs from Freeview set-top boxes? With the impending demise of all analogue transmissions, and a dearth of STBs with RF modulators, there is no longer any reason to connect the output of an STB RF bypass to the TV set aerial input. I feel it would be 'nice' to terminate it (using the 'IEC169-2 Coax Plug to Plug Coupler' in the row above the 'IEC169-2 CoaxPlug 75 ohm Terminator'.

[Terry Casey]

I don't know what the postage would be for a small order but, at that price for one-off, especially considering the rarity, that has got to be a bargain!

I can't be bothered to check but I bet Maplin would want more for the plug and resistor for the DIY approach!

[Alan]

In message , Ian Jackson wrote

I found that I had to terminate the unused outputs from a wall plate where Satellite, UHF and FM were triplexed into one down cable.

Without the 75 ohm terminations on the unused satellite outputs the frequency response on the UHF was very "peaky" and with the terminations the response was virtually flat (response taken from the Freeview box signal strength figures channel 24 to 59).

[John Rumm]

Also you can (with most sockets) leave a little inner poking out the end of the B&L plug F connector style. So the inner itself will make contact with the socket.

[Ian Jackson]

Indeed. There's more one way of (incorrectly) skinning a cat. But while 'crimping' the pin with a pair of side-cutters or 'kinking' the centre conductor before insertion are pretty sound methods of achieving a reliable contact, I really can't approve of this one.

Anyway, now we have three ways of bodging/botching the centre conductor, perhaps it's time to turn our attention to question of how not to deal with the braid?

[Terry Casey]

That isn't at all surprising. The triplexer - same thing applies to diplexers too - consists of tuned circuits. Both frequency and impedance are important factors in the design of these tuned circuits, so they are designed with both input and output impedances of 75 ohms. Leaving the output impedance of one of them at infinity, i.e.: unterminated, will make it perform in an entirely unpredictable way which can also upset the response of the section which is actualy in use - as you have both found and proved!

[Terry Casey]

Ooh! Nasty!

Seconded!

The responses to that could be VERY interesting ...!

I will sit back and wait with interest ...

[tony sayer]

In article , Terry Casey scribeth thus

Loosen up Terry it was only intended as a joke;)

Don't take it all too seriously... Chill out bro;!....

[tony sayer]

In article , BartC scribeth thus

Makes no odds it should be connected properly not half done or bodged.,.

You won't find the likes of Bill Wright doing that..

[Nick Leverton]

I now use a brand (can't remember which offhand but available from a B&Q near you) which has a grub screw for the inner conductor. Not cheap at shed prices, but then I only need them once or twice per year and they do seem neatly made - all metal except for the insulating bush.

Nick

[Dave Liquorice]

But not just any 75 ohm resistor. Many use a spiral track on an insulating former, nice little inductor... Heaven knows what the actual impedance will be at UHF frequencies.

[Ian Jackson]

In message , Nick Leverton writes

In nearly types all I've seen where there is a screw, the tiny screw (or its hole) is easily stripped, or is already stripped before you touch it. You (well, I) usually end up by having to soldering the inner!