07-03-2017, 05:56 PM
Many early FM radios in the UK - I'm particularly thinking of Bush - had 300 ohm inputs.
It is usually easy to convert from 300 to 75 ohm input, if required, because the balanced 300 ohm input feeds a matching transformer which has a grounded centre tap. From one side of the input to ground is a 75 ohm input.
As to why the difference, I don't know. Provided both aerial and receiver match the feeder it doesn't really matter. In this respect, you might like to note that although the centre impedance of a half wave dipole is 75 ohms, the centre impedance of a folded dipole is 300 ohms.
However, as you increase the number of parasitic elements in a Yagi array, the centre impedance drops, which is why you will find folded dipoles being used on multi-element UHF Yagis to provide a better match to 75 ohm cable.
As any good quality aerial used with unbalanced feeder, ie: coax, should incorporate a balun, this can also be used to provide an optimum match at the same time.
It is usually easy to convert from 300 to 75 ohm input, if required, because the balanced 300 ohm input feeds a matching transformer which has a grounded centre tap. From one side of the input to ground is a 75 ohm input.
As to why the difference, I don't know. Provided both aerial and receiver match the feeder it doesn't really matter. In this respect, you might like to note that although the centre impedance of a half wave dipole is 75 ohms, the centre impedance of a folded dipole is 300 ohms.
However, as you increase the number of parasitic elements in a Yagi array, the centre impedance drops, which is why you will find folded dipoles being used on multi-element UHF Yagis to provide a better match to 75 ohm cable.
As any good quality aerial used with unbalanced feeder, ie: coax, should incorporate a balun, this can also be used to provide an optimum match at the same time.
