Impedance/Resistance dipole qustion

[Dave84]

Hi,

I've decided to make a dipole antenna and I've seen conflicting information over what coax to use. Should it be 50ohm or 75ohm? Also, should the coax be the same for the centre wire from antenna to radio?

Many thanks

[thedeerhunter270]

Use 50 ohm coax. The will be a slight mismatch at the dipole feedpoint of 72 ohms, but nothing to worry about.

[Dave84]

Thanks

[Dragonfly]

I dont no if its needed but i hand wound a little ballon on my wire dipole to stop any common mode affecting the tuning . Use it for sidebands , SWR is 1.2 , resonant at 18 high band .



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[bigpimp347]

A Balloon ??

you mean Balun ??

[Dave84]

Yeah I've seen them, I'm making it out of 50ohm per this link hxxp://www.radiobrandy.com/dipole1.html as we speak....

[Anthony123]

You can use a Balun or choke as it 'Can' help to stop 'Unwanted' RF currents from upsetting the radiation pattern and also help stop the unwanted currents from running back down the coax .. making your coax act as a antenna, Something you dont really want...

As for the Impedance, A perfect dipole in free space has a Impedance of around 72 ohms or so... This is in a perfect world, In reality height above ground will effect the impedance so will structures near the antenna especially anything metalic... I would feed it with normal 50 ohms coax as this is the normal impedance of cb coax and your transciever, Use the balun if you want, It wont make things worse but can add extra weight if its outside and the dipole is held at either end with the balun in the middle, The balun can make the dipole sag a bit... There will be a mismatch between the coax and the dipole but it will be small ... you should be able to get a low vswr though... and the dipole does work well for such a simple antenna.... I use a resonant one for 40 mtrs... fed with Mini 8 and a choke hung from a tree in the garden... It works well for what it is... There cheap simple and hassle free...

Depending on how your dipole centre is.....Make sure you weatherproof the joint between the coax and the dipole wires... or rain will quickly ruin your coax.

[Dave84]

Thanks anthony, I haven't needed to weatherproof as she's in a converted loft. All up and running, just a few adjustments to be made