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Old 8th Mar 2006, 4:36 pm   #3
YC-156
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Join Date: May 2005
Location: Aarhus, Denmark
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Default Re: Fighting VHF interference with a DIY balun.

Quote:
Originally Posted by GMB
But can you explain exactly how the aerial relates to the coax so that things are unbalanced at that end too? Do you need another balun at the dipole?
Yes, indeed you do, a pair in total. If you didn't have a balun at the antenna, the signal difference between the cable screen and the antenna could travel down the inside of the cable, still causing interference.

Quote:
Originally Posted by GMB
Also, at these frequencies, why do you need a core at all?
You are trying to make a wideband transformer. Here the core helps ensuring a very strong, magnetic coupling between the different windings (the trifilar winding helps too). Also you need less copper wire for a given amount of inductance, around 50% less in this case, reducing the risk of unwanted, parasitic resonances. Finally a coil wound on a magnetic toroid core is pretty much self shielding. An airwound inductor/balun would need to be externally shielded if you didn't want to run the risk of the balun itself being a possible source for picking up interference.

Hope this helps a bit.

Frank N.
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