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may 2010
Making co-ax connections
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Making co-ax connections Follow some simple rules for fitting coaxial cables and you’ll save yourself a lot of bother
Initial installation and almost any alteration to a digital TV setup will mean making new connections with co-ax cable. Whether it’s the LNB cable or the aerial downlead, a multiswitch system or a UHF distribution system, plugs need to be fitted to the cables and if this is not done correctly it can seriously damage your signal health.
Fitting an F-connector is an easy skill to pick up
Co-ax connection basics 01 Co-axial cable carries the signal in a central conductor surrounded with a metal (foil or braid) shield that prevents interference from or to the signal and provides the return connection. A co-ax connector should maintain this shield all the way through a cable joint or connection to equipment. The two sorts of connectors that digital TV viewers commonly meet are
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the aerial plug (above left, properly called a Belling-Lee or IEC 169-2 connector) used for terrestrial UHF connections and the F-connector used for satellite IF connections, such as the downlead from the LNB (above centre). These are similar in size, design, and effect but the aerial plug has a centre pin constructed as part of the plug and is a slip-on fit to the socket, whereas the F-connector uses the central conductor from the co-ax cable as its centre pin and has a screw-on collar to hold it in place on the socket. With both sorts of plug it is essential that a good connection is made between the conducting parts of the plug and the cable, and that there is no connection, or risk of connection, between the two conductors in the plug – the centre pin and the shielding body of the connector.
When joining one cable to another (if it’s not possible to install a new, single run of cable) you should always use a double-ended female-female barrel connector to join the male plugs on the end of the two cables (above right). How to fit an F-plug
02 F-connectors either screw onto the cable or are ‘crimped’ on. Crimped connectors are inherently more reliable but require a special tool to fix them and so screw-on F-connectors are more commonly used by DIYers. You can also buy ‘push-fit’ connectors, which are easy to fit and remove with a simple tool. Make sure that you have the right screw-on plug for the cable you are using. Small differences in the cable diameter can make a large difference to the reliability of the connector.
a) Cut the cable to length. Allow sufficient for a drip loop for external cables at the LNB. b) Strip off about 20mm of the cable’s outer plastic sleeve, being careful to not cut through the braid beneath. c) Peel back the exposed braid and remove the foil underneath. d) Remove the inner insulator (the dielectric) to leave about 5mm remaining. e) Screw on the F-plug, clamping the loose braid between the sleeve and the plug body. f) The plug body should screw all the way onto the cable until the end of the inner insulator contacts or just passes through the baffle inside the
abc guide to... damage as it is unplugged and plugged in, and equipment is moved around. Much better is to fit a faceplate on the inside of the wall where the cable enters the room. This provides a socket where a replaceable cable of the correct length can be connected. A faceplate is particularly useful if you have two or more co-ax cables entering the room in the same area. Several sockets can be combined on a single faceplate in a neat, reliable arrangement.
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How to choose a faceplate
05 When buying a faceplate for
plug body. g) Trim off any exposed braid and cut the inner conductor to just protrude from the end of the plug. Finally, if possible, check the connection along the cable and the isolation between the conductors. How to fit an Aerial plug
03 Aerial plugs come in many different guises but most follow roughly the same assembly procedure. Don’t use plasticbodied plugs; they provide poorer shielding and are generally weaker.
a) Cut the cable to length. b) Slide the plug cap onto the cable. Strip off about 25mm of the cable’s outer plastic sleeve, being careful to not damage the braid beneath. c) Slide the collet over the braid to the end of the sleeve. Strip off the braid to leave about 10mm protruding from the sleeve d) Fold the braid back over the collet. Remove the exposed foil and strip back the inner insulator to leave about 5mm exposed.
e) Push on the pin holder so the cable’s central conductor goes through the pin. If the pin holder has a grub screw to bite into the conductor, tighten this. Otherwise, carefully solder the conductor in the end of the pin. f) Remove any conductor and solder protruding from the end of the pin. Slide the plug body onto the pin holder. g) Screw the plug cap onto the plug body, catching the braid and tightening the collet on the cable. Finally, if possible, check the connection along the cable and the isolation between the conductors.
Screw-on Belling-Lee UHF plugs are a simple fit, but soldering will give you a better connection
How to make a faceplate
06 You can simply construct your own
Why use a faceplate?
04 The traditional method of cabling satellite dishes and aerials leaves the downlead extending from a hole in an outside wall with enough free to reach the equipment in the room; sometimes with far more than enough coiled up behind the TV. Although this works it is far from elegant and leaves the cable prone to
F-connector or aerial connections, shielding is the key consideration. Poor commercial faceplates use separate screw terminals to connect the two conductors in a co-ax cable. If the terminal for the central conductor is exposed then the shielding will not be continuous across the connection. Select a faceplate in which the shield surrounds the centre wire’s terminal so the interference is kept out.
Commercial co-ax faceplates are available, but it’s not difficult to make your own
faceplate with a mains socket/switch blanking plate. This should be metal and used with a deep metal wall box/pattress. The blanking plate is drilled to accept a double-ended female barrel connector, which is held in place with a nut, and the cable from the wall connected to the inside socket. This provides a neat finish and a good continuous shield through the wall plate connections. Barrel connectors for F-plugs can be bought at most installers but the equivalent aerial connectors are less widely available. Instead, you can use an aerial-to-F-type barrel converter with the F-connection used inside n Geoff Bain
Glossary Co-ax cable
Co-axial cable contains a central conductor to carry the signal and a surrounding shield, insulated from the central core, to provide the ground connection and prevent radio interference picked up by or radiated from the central conductor.
Collet
Crown-shaped part of a plug that grips the cable when the plug is assembled and tightened.
IF (Intermediate Frequency) 06
Signal in the LNB cable generated by the down-conversion of the broadcast signal by the LNB. Usually 9502150MHz.
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