CrisPDuk

I need to move my ntl box to a place where the cable no longer reaches.

Can I just make up an extension piece from co-axial cable? & where do I get the threaded fittings from?

tonybooth

Coax extension no problem. Maplins would be a good place for the bits.

TONY

dr_ming

The connectors are 'F' type. Maplin, RS or and TV aeriel dealer will be able to sell you the bits. However, if it is a permanent cable extension, you will get lower RF losses, and a more reliable connection, by making a soldered and taped joint.

Graz

Not strictly so Dr Ming, by breaking the cable and using solder tape you spoil the dielectric properties and wave guiding effects of the cable. Using connectors ensure the dielectric and screening remains continuous.

I would get an F type coupler, a couple of F type connectors and coax for the extenison cable. You might even get a ready made up extension in Maplins. Anyone know is cable TV cable is 50 or 75 ohms?

dr_ming

75 Ohms. In theory, the connectors should be 75 Ohms too, but like the 'Belling Lee' type connectors used for your standard UHF antenna, the quality and materials are so poor, that they are unlikely to be anything close.

In addition, because 'F' type and 'Belling Lee' connectors are not intended to be soldered, the losses incurred are high, especially after the connection has had time to oxidise a little. Typical insertion loss from an unsoldered 'F' type or 'Belling Lee' connector at 500MHz is around 2dB. If you make a soldered and taped joint, and do it properly (join center conductors with as little exposed conductor as possible, tape ever joint, join braid over the top, tape over joint), the insertion loss will be around 0.5dB (agin @ 500MHz).

If you really want a plug/socket joint, ditch the 'F' types and use BNC connectors - loss will be comparable to a taped/soldered joint, and you can unplug it

Oh, and the cable isn't a wave-guide, it's a matched transmission line.

[Edited by dr_ming - 11/20/2003 5:29:02 PM]

gravelexpress

poor bloke probably wanted a reply in English not technaglese

dr_ming

LOL @ Gravelexpress - Paid you dues to Pee Cee World yet? Better watch out, the holyer than thou Leslie is on your case mate

gravelexpress

I used to know a piddly bit about RF losses in COAX from my radio op trade in the army. But we used a sheathed coax for UHF transmission we could not extend the COAX by more than 22m before losses would lead to error rates in our transmissions. probably of no use whatsover to our man!

pugoetru

i used to work for ntl

a short extension wont make any difference to the signal

dont solder use proper conectors

the coax we used is not the same as tv coax tv coax will do if it is short

philz

I just did this last night actually, cost me £5 for 10m of cable and the connectors at my local TV specialist.

CrisPDuk

Cheers chaps, can I use standard TV co-ax (as I have half a reel left overfrom putting the kids TV's in), or is it a type specific to cable applications. The extra length will be about 2m.

PS Gravelexpress, cheers for your concern, but it's only when the computer geeks start that my eyes glaze over

dr_ming

Standard 75 Ohm TV coax will do for a short run (generally poor quality compared to RG9 or RG59, but it'll do). Happy viewing

Graz

Yikes, I was just commenting, wasn't expecting a debate about insertion losses etc! We wouldn't dream of extending a coax cable for our satellite phones without using connectors, but then again we use very expensive ones made in Switzerland and it's 1.5 GHz.

Never been much of an RF engineer, mainly do digital and embedded software these days.

Anyway sounds like soldering, if done properly, is less lossy than cheapo connectors. Either should work, cable TV should have plenty of margin for signal loss.

dr_ming

Graz, Sorry mate, didn't mean to sound like I was having a rant. You are totally correct re: splicing/soldering cables at GHz frequencies and/or on anything with a transmitter in it. The main issue is that domestic antenna connectors (and cable, for that matter) are of such poor quality that they give very high insertion losses, so making soldering the better option.