Thread: Balun works
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Old 12th Aug 2014, 5:06 am   #19
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: Balun works

Ideally I suppose baluns would be aerial-specific, and would provide a match between the actual aerial impedance and 75R, as well as doing the balanced-to-unbalanced transformation. In the USA it seemed to be standard practice to use a regular 300R-to-75R outdoor balun whatever the aerial. Certainly the multi-element TV and FM aerials that I used all came with such baluns. It was as if the balanced-to-unbalanced transformation was more important than impedance matching. I suspect that might have been because historically 300R twin was used for domestic aerial feeders, with 75R coax being a later arrival (1970s maybe?). So 300R twin was attached to all TV and FM aerials, including the multi-element types whose actual impedance was in some cases well below 300R. Then when 75R coax arrived, it would have seemed the natural thing to do to connect “300R” to/from it using a standard balun.

It seems to me that a balun is primarily a balanced-to/from-unbalanced transformation device, one which secondarily can also be an impedance transformation device. However, its impedance transformation properties seem to have become its dominant feature, leading to the superficial definition that it is essentially a device for transforming 300R to/from 75R. In turn this may have led to the common misperception that a balun is not needed to go from say a 75R balanced aerial to 75R coaxial cable (unbalanced) because the impedances are the same.

Cheers,
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