aerial baluns, please explain

jonnygee

On any old aerials (contract type) that i have used then then the inner conductor of the coax simply connected to one side of dipole and the outer braid connected to the other side.
Now i see with these balun types that the outer braid connects to nothing but the pcb while the conductor is connected to what seems to be the centre of a dipole split in two. (if you know what i mean).
I,M confused.

The Cush

http://www.aerialsandtv.com/aerials.html#Balun

jonnygee

Yeah, i,ve read all that site, but i still cant understand why the braid connects to nothing, it,s sort of like that the clamp over the braid is irrelevant and the only important connection is the centre conductor. i thought that the conductor and braid should form a loop.

watty

The braid/ screen / outer isn't connected to nothing. There maybe be a PCB track transformer underneath if not on top.

jonnygee

On the aerial that i have with a balun then the centre conductor connects with the dipole only, that is in the centre of the left and right sides of the dipole on the aerial, the braid only connects with the pcb but has no connection with the dipole whatsoever.
If i could explain it further(maybe i will post a pic.) then the right and left sides of the dipole are joined by the squiggly bit of the balun and the centre connector connects to the middle of this but the braid just screws to the pcb which has no electrical connection to the dipole whatsoever.

watty

You'd need to post pictures of both sides of PCB. Of course it could be a badly made counterfeit aerial

jonnygee

www.aerialsandtv.com/_wp_generated/wp2f1c9759.jpg
It,s similar to these, what i am curious about is why the old type on the left has conductor and braid connected to the dipole wheras the balun types seem to have only the centre conductor connected to a split dipole with the braid terminated at the clamp.

Sam Russell

jonnygee wrote: »

www.aerialsandtv.com/_wp_generated/wp2f1c9759.jpg
It,s similar to these, what i am curious about is why the old type on the left has conductor and braid connected to the dipole wheras the balun types seem to have only the centre conductor connected to a split dipole with the braid terminated at the clamp.

These baluns work at high frequency, not DC. They are designed to act as a transformer from a balanced load at 300 ohms (that is, it has no ground or earth) to a grounded unbalanced load at 75 ohms. (The impedance is the ratio of voltage to current at the working frequency). A folded dipole has a balanced impedance of 300 ohms, and if it is not matched using a balun, much of the signal is lost. Where you connect the cable (ubnbalanced, 75 ohms) to the dipole (balanced, 300 ohms) depends on the design. Remember we are dealing with frequencies above 400 MHz.