Presentation on theme: "Feeders and Antennas. A. Outer plastic sheath B. Woven copper shield C. Inner dielectric insulator D. Copper core CoaxCable Coaxial cable, or coax is."— Presentation transcript:
A. Outer plastic sheath B. Woven copper shield C. Inner dielectric insulator D. Copper core CoaxCable Coaxial cable, or coax is a type of cable that has an inner conductor surrounded by a tubular insulating layer, surrounded by a tubular conducting shield. Coaxial cable differs from other shielded cable used for carrying lower- frequency signals, such as audio signals in that the dimensions of the cable are controlled to give a precise, constant conductor spacing, which is needed for it to function efficiently as a radio frequency transmission line. From Wikipedia
Twin Feed Twin-lead cable is a two conductor used as a transmission line to carry radio frequency (RF) signals. It is constructed of two multi stranded copper or copper clad steel wires, held a precise distance apart by a plastic ribbon. The uniform spacing of the wires is the key to the cable's function as a parallel transmission line; any abrupt changes in spacing would reflect radio frequency power back toward the source.
Coax is an unbalanced feeder - the inner carries the voltage and the outer is the earth. Coax is used by many stations as the outer acts as a screen Twinn feeder is considered to be a balanced feeder. It has equal and opposite voltages/currents/fields. If you wish to connect an unbalanced feed to a balanced antenna (coax feed a dipole) you need an interface. This interface is known as a BALUN (BALanced – UNbalance. The BALUN prevents RF currents flowing on the braid. If this occurred the screening properties our lost.
When using a PL259, BNC plugs and sockets it is essential that the inner conductor and outer braid are connected correctly. Poor connections can lead to equipment mal functioning and can also lead to unwanted expense - having to get equipment repaired. It is essential that the screening is continuous from start to finish.
PL259,s can be used when developing a HF or VHF station. Most amateur stations will use these plugs somewhere. They have a screw thread to secure the connection.
BNC connectors have a bayonet connection and mainly used in circuits that are running low power. There are two types a 50 and 75 Ohm. The difference – different inner pin sizes
Some ant are designed to be directional and therefore give maximum gain in a set direction – beams. Many wire ant will be fixed and it will soon be established which geographical areas the ant favours. Recall that these are polarised according to the orientation of the antenna, e.g. a horizontally oriented antenna will radiate horizontally polarised waves Review following antenna Dipole Quarter wave ground plane Five-eights ground plane Yagi End Fed wire
Dipole: The most common form of dipole is two straight rods or wires oriented end to end on the same axis, with the feed line connected to the two adjacent ends HF - Would need to be fed via a BALUM
Quarter wave ground plane Radials represent ground plane and are a quarter wave long
Polarisation of Antennas Polarisation of antennas are normally Horizontal or vertical. For the receiving station to get maximum signal strength it needs to have the same polarisation as the transmitting station. Vertical antenna give vertical polarisation – mobiles with mobile 5/8 mag mounts Yagis and dipoles may be set up to be either polarisation – depends on how they are erected. In some complex situations the above rules may not apply.
Antenna Matching and SWR Antennas preform at their best when they are designed for a particular frequency and used on that frequency. However the challenge come when the antenna is to be used on more than one band. It is essential that the SWR (Standing Wave ratio) is kept as low as possible. This measurement of the antenna system to the normal impedance of the radio. If the SWR is high the output power will be reflected back and could damage the transceiver Some HF antennas will cover two bands but normal practice is to use ATU (Aerial Tuning Unit). Many ATU’s also have a SWR meter which enable you to monitor any transmission and take appropriate action should the SWR change. Dummy Loads permit radio tests without radiating a signal