12-03-2017, 12:28 AM
Regarding the US use of 300 R flat twin in the early days, Fink (in “Television Engineering”, 2nd, 1952; see pp.382,383) stated that most TV receiver makers adopted 300 R balanced inputs, for use with 300 R flat twin, because it was cheaper and of lower loss, particularly at higher frequencies, than the available coaxial feeders. Those of the latter might be used in domestic installations were more costly and had higher losses than was the case for flat twin, the more so at UHF, and larger diameter, lower loss coaxial cables were prohibitively costly.
In the European case, Kerkhof & Werner (in “Television”, 1952; see pp.348-350) did not comment upon receiver maker choices, but the detailed discussion was about balanced feeders and also the advantages of using balanced RF and mixer/oscillator stages, with conversion to unbalanced thereafter, rather than at the input. The closing comment in that section was: “An unbalanced (coaxial) aerial feeder may also be used, but in that case the aerial must also be unbalanced or a balancing transformer must be incorporated between the dipole and the coaxial cable”.
In respect of the US change to 75 R coaxial, these pages from a 1969 Heathkit catalogue give some sense of its timing. Evidently it was a relatively new idea (for the US market) in 1968-69.
I imagine that the rapid growth of cable TV after about 1970 also helped with the change, given that the cable systems mostly use coax as far as I know. When I was living in the USA, RG6 seemed to be the norm for cable installations and also for domestic aerial installations. The thinner and lossier RG59 seemed to be used more for short interconnects.
And one might also observe, in respect of the “righteousness” of using baluns to connect balanced aerials to coaxial cables, given that most aerials had a nominal impedance of 300 R, some kind of impedance matching device was required anyway to get to 75 R, and that being the case, it was no more difficult to make it a balun than say a “bal-bal”.
Cheers,
Steve
In the European case, Kerkhof & Werner (in “Television”, 1952; see pp.348-350) did not comment upon receiver maker choices, but the detailed discussion was about balanced feeders and also the advantages of using balanced RF and mixer/oscillator stages, with conversion to unbalanced thereafter, rather than at the input. The closing comment in that section was: “An unbalanced (coaxial) aerial feeder may also be used, but in that case the aerial must also be unbalanced or a balancing transformer must be incorporated between the dipole and the coaxial cable”.
In respect of the US change to 75 R coaxial, these pages from a 1969 Heathkit catalogue give some sense of its timing. Evidently it was a relatively new idea (for the US market) in 1968-69.
I imagine that the rapid growth of cable TV after about 1970 also helped with the change, given that the cable systems mostly use coax as far as I know. When I was living in the USA, RG6 seemed to be the norm for cable installations and also for domestic aerial installations. The thinner and lossier RG59 seemed to be used more for short interconnects.
And one might also observe, in respect of the “righteousness” of using baluns to connect balanced aerials to coaxial cables, given that most aerials had a nominal impedance of 300 R, some kind of impedance matching device was required anyway to get to 75 R, and that being the case, it was no more difficult to make it a balun than say a “bal-bal”.
Cheers,
Steve
