Foundation Course Feeders & Antennas Flashcards
Feeders
Two basic deefer types Coax and Twin Wire
Inner conductor is shrouded by dieletric with outer (braided) screen
For Radio 50 ohm coax is used and for TV 75 Ohm is used
Twin Feeder
Two conductors kept at constant seperation by insultation -no screen [Balanced Feeder]
Balanced/Unbalanced
Coax is unbalanced - Inner has voltage, Outer is earthed
Coax is widely used as its outer acts as a screen
Twin is balanced - conductors have equal and opposite voltages/current fields
In order to connect an unbalanced feeder to a balanced antenna (coax feeding a dipole) a transformer known as a balun is needed
Without a balun RF currents flow on the outside brain and the screening properties of Coax are lost
Coax Connectors
A wide variety exists
Common RF connectors include BNC, PL259, N-type, SMA etc
Ensure inner conductor and outer brain are assembled correctlty
Poor condition connectors are a major cause of bad SWRs
Screening must be continous through plugs and sockets
Foundation Licence requires good understanding of two connectors - BNC, PL259
BNC connectors
BNC connectors have a Bayonet locking action and are commonly used for lower power inteconnections.
Take care not to mix incompatinlee 50 and 75 ohm versions
which have different inner pin sizes
PL259 Connectors
Common HF/VHF connector with reasonable power handling
Antennas transform AC signals into propogating radio waves
Gain is the directing of power in the wanting direction.
Need to know the following types
1) Dipole
2) Quarterwave ground plane
3) Five-Eights ground plane
4) Yago
5) End-Fed wite
Simple - but requires a balanced feed via a balun
Each leg l/4 long - l/2 across in total.
Quarter Wave lamba/4
Radials simulate a groundplane are also lamba/4 long
Sometimes called a groundplane antenna
•5l/8 - Common antenna for mobile use
Better impedance match and gain that basic quarterwave
Radials emulate groundplane like the quarterwve
Dipole acts as a pickup
Front directors focus to give gain
Rear Reflector gives back/front isolation
Rear reflector gives back/front isolation
