Need help on balun

178

02:08:26

So here's my problem. I'm about to buy an fm transmitter. 1st what does the balun do? 2nd I have a dipole antenna from an old TV that has an in-built balun inside. Only problem is that the balun is 1:4 (75ohm-300ohm). From what i know a coaxial cable is 50 ohm and a dipole antenna is also 50ohm.a twin lead cable is what goes from transformer to antenna. I read it was 300ohm so therefore the 300ohm resistors in the circuit...... Is this how i must leave it or must i build 1:1 balun. If so, how?

published 11 years ago

faceblast

11 years ago

the balun (Balancer/Unbalancer) converts the balanced signal received from the dipole antennae into unbalanced signal for the coax. the turns ratio also converts the impedance

faceblast

11 years ago

I made my own 1:4 balun for digital TV to convert 300Ω to 75Ω. I wound 2 pairs bifilar four times around opposite sides of a toroid. connect the opposing pairs together, then the antenna connects to the longest winding. the centre of the coax connects to the first tap point and the shield connects to the half tap point

faceblast

11 years ago

http://everycircuit.com/circuit/5728998743080960

thebugger

11 years ago

But i need it for a transmitter not receiver. Do the same rules apply

thebugger

11 years ago

Don't dipole antennas have 50ohm resistance. Why do i need to convert it to 300ohm?

yra

11 years ago

Go to the ARRL website and do a little studying, this is a ham question and there are lots of answers there for you.

thebugger

11 years ago

Not so much of a ham question. Ham radios are not in the same spectrum as FM radios and i think some of the rules may be different

Sine_eyed

11 years ago

Bugger, I think @yra is on to something though- your question is about antennae, and who knows antennae better than HAM guys? I bet there's some formula our something there you could find useful..

thebugger

11 years ago

Seems plausible thanks all

bobbyhills

10 years ago

Dipole antennas do not need a balun because there impedance falls between 42.3 - 71 ohms.... just run 50 ohm coax from antenna to transmitter

bobbyhills

10 years ago

Only time you will need a balun is when the antenna design falls on a lower or much higher impedance like a halfwave antenna or a quad antenna

cdkilo

8 years ago

Basically a balun converts balanced line to unbalanced line and vice versa. Coax is an unbalanced transmission line and a dipole antenna is balanced.

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