antenna-theory.com

[laurent92]

Hi to everyone

I would like to build a current "balun" for a receiving antenna but am a bit confused about how to do it . My first idea was merely to take the same one as the transmitting antenna and connect it the same way !
I think this should work since current transformers work both ways balanced-unbalanced / unbalanced-balanced .

Am I right ?

[admin]

Yes, there's no difference in balun design for a transmit or receive antenna.

But....
* how do you know you need a balun?
* if you need a balun, how will you know it is effective (i.e. doing anthing useful)?

[laurent92]

Thanks very much for your response

I need a wideband balun to feed a bowtie antenna the current on both arms must be equal for the antenna to radiate properly .

For the receive one I assume it is needed too for the same reason : the positive and negative current must be equalized on both arms before entering the receiver .

For the later I am not sure though . I 'll have to take a test and analyse the signal with an oscilloscope with and without the balun ...

[helix]

You are of course both correct. It makes little difference for the design if a balun is used for transmitting and receiving (power handling is usually a consideration for the transmit case but not for the receive case). And for a dipole you want properly balanced currents on the two arms when feeding with an unbalanced t-line (such as coax).

You will find that the dipole may "work OK" without the balun. In fact, because the (unbalanced) feedline may radiate, it's possible that the antenna will "work better" without it. But in this case you may also find that how the feedline is run, how long it is, etc., may ALSO impact the antenna performance (e.g. VSWR, pattern, polarization, etc). If you're OK with that, then you may be able to skip the balun.

I have heard stories where unbalanced transmit currents on antenna feed lines can cause RF to show up other places you don't expect, e.g., on a microphone. You may be able to feel that on your lips (ouch).