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(Read about Balanced / Unbalanced first)
BALUN= Balanced to unbalanced ( vise versa )!!
Coax cable etc are unbalanced Transmission line whereas twin lead 300 ohm etc are Balanced one... Antenna however is also balanced, and receiver's terminal is unbalanced too. So in order to connect any balanced and unbalanced together we need a Balun in between.
when you say you need Balun for antenna coupling this means that you are converting balanced antenna to unbalanced wire but with Balun in between.
for this purpose a 1:1 Balun will be required (one that doesn't transform impedance but keep as it is that is why 1:1... one that transform let say 300 ohm to 75 ohm will be 4:1 ) ...!!!
search on Google on making a trifilar 1:1 Balun .. you will get links to make one... there will be alot if information which can't be written here!!