08-03-2017, 04:27 AM
New Zealand initially followed North American practice and used 300 ohm twin aerial feeder for TV. Approximately with the arrival of colour in the early 1970s there was a swing to 75 ohm coaxial feeders, typically using a balun at the aerial end. Thus 75 ohm coaxial was well established when FM arrived and was used for it.
I can’t say that I have seen a definitive statement as to why 300 ohms was chosen in North America, but three reasons seem likely.
Firstly is cost, in that 300 ohm twin was typically noticeably cheaper per unit length than 75 ohm coaxial.
Secondly is loss, in that, at least when properly mounted (e.g. with standoffs) and in dry conditions, 300 ohm has noticeably lower loss per unit length than 75 ohm coaxial, particularly at higher frequencies. Back in the late 1940s, this difference may have been more marked, as low-loss coaxial cables were either unavailable or prohibitively costly. When there were fewer TV stations, and so proportionately more viewers in fringe areas, the need for low loss feeders would have been paramount.
Thirdly is that aerial and feeder (and receiver) should be matched in balance state as well as impedance terms. When the USA swing to 75 ohm coaxial feeders, it was normal to use baluns at the aerial end to ensure that there was no balance mismatching. (The aerials that I bought when I was living in the USA all came with baluns as part of the kit. That included a vertically polarized VHF communications aerial as well as TV and FM aerials.)
Why the UK chose 75 ohm coaxial I don’t know. Perhaps that it used only Band I initially, and was a relative latecomer to Band III had something to do with it, in that feeder losses at Band I were lower and so of less concern. But for whatever reason, it landed upon what was the durable choice.
Also, why the UK chose generally to ignore the balance mismatch by direct connection of balanced TV aerials to unbalanced feeders I don’t know. Wireless World for 1950 December ran an excellent article by W.T. Cocking, namely “Aerial Feeder Connections – Balance-to-Unbalance Transformer”, which advocated the use of balance matching as well as impedance matching. Its opening paragraph read: “The connection between the aerial and the receiver does not always receive the attention it deserves, even in these days when so much is known about cables and their terminations. The need for matching is generally recognized but the desirability of including balance-to-unbalance transformers is all too often unrecognized.” Lack of knowledge in the aerial community of corner-cutting, perhaps induced by post WWII austerity?
Nevertheless, there was apparently some use of 75 ohm twin, as evidenced by this Belling & Lee advertisement from 1952:
And when FM arrived in the UK, some setmakers, including Murphy, designed their receivers to have 75 ohm balanced FM aerial inputs, with aerials connected by 75 ohm twin cable or 75 ohm screened twin cable. I suspect that to some extent this may have been to facilitate the use of simple indoor dipole aerials and/or internal compressed dipoles. 75 ohm (or thereabouts) twin and screened twin feeders were evidently standard inventory items of the period, as shown in this BICC advertisement. Belling & Lee offered the same screened twin as was used for its antistatic radio aerial systems.
From first-hand experience I’d say that 300 ohm twin is a real pain to mount properly, with standoffs and twisting. I can recall changing over my parents’ TV aerial in Auckland from 300 ohm twin to 75 ohm coaxial circa 1970, before its use in NZ became general, in order to obtain an easier and neater installation. There was no problem with obtaining a head-end balun, but this was one that did not attach to the aerial, but was mounted on a standoff on the pole. So a short 300 ohm line was needed to connect the channel NZ2 aerial folded dipole to the balun. When TV2 in Band III came along shortly thereafter, the only Band I/Band III diplexer that I could buy was for 300 ohm twin. So it was mounted between the existing Band I and (new) Band III arrays on a standoff, with a 300 ohm connection to the balun. What a palaver! I had assembled the whole thing on the fairly long pole down below, then somehow managed to get it on to the edge of roof without damage, wherefrom the idea was to lift and swing it over to the chimney lashing. I all but swung myself off the roof in the process. Still, it provided a strong and clean signal, free of ignition interference, which was a bane in those days. Then came the Philips K9 colour TV receiver with its awful bipolar VHF tuner that needed something like 18 dB of signal attenuation to avoid cross-modulation, even though the previous valved monochrome receiver (with a standard 6Ln, 6Cm turret tuner) had swallowed the strong signal whole and with ease.).
Cheers,
Steve
I can’t say that I have seen a definitive statement as to why 300 ohms was chosen in North America, but three reasons seem likely.
Firstly is cost, in that 300 ohm twin was typically noticeably cheaper per unit length than 75 ohm coaxial.
Secondly is loss, in that, at least when properly mounted (e.g. with standoffs) and in dry conditions, 300 ohm has noticeably lower loss per unit length than 75 ohm coaxial, particularly at higher frequencies. Back in the late 1940s, this difference may have been more marked, as low-loss coaxial cables were either unavailable or prohibitively costly. When there were fewer TV stations, and so proportionately more viewers in fringe areas, the need for low loss feeders would have been paramount.
Thirdly is that aerial and feeder (and receiver) should be matched in balance state as well as impedance terms. When the USA swing to 75 ohm coaxial feeders, it was normal to use baluns at the aerial end to ensure that there was no balance mismatching. (The aerials that I bought when I was living in the USA all came with baluns as part of the kit. That included a vertically polarized VHF communications aerial as well as TV and FM aerials.)
Why the UK chose 75 ohm coaxial I don’t know. Perhaps that it used only Band I initially, and was a relative latecomer to Band III had something to do with it, in that feeder losses at Band I were lower and so of less concern. But for whatever reason, it landed upon what was the durable choice.
Also, why the UK chose generally to ignore the balance mismatch by direct connection of balanced TV aerials to unbalanced feeders I don’t know. Wireless World for 1950 December ran an excellent article by W.T. Cocking, namely “Aerial Feeder Connections – Balance-to-Unbalance Transformer”, which advocated the use of balance matching as well as impedance matching. Its opening paragraph read: “The connection between the aerial and the receiver does not always receive the attention it deserves, even in these days when so much is known about cables and their terminations. The need for matching is generally recognized but the desirability of including balance-to-unbalance transformers is all too often unrecognized.” Lack of knowledge in the aerial community of corner-cutting, perhaps induced by post WWII austerity?
Nevertheless, there was apparently some use of 75 ohm twin, as evidenced by this Belling & Lee advertisement from 1952:
And when FM arrived in the UK, some setmakers, including Murphy, designed their receivers to have 75 ohm balanced FM aerial inputs, with aerials connected by 75 ohm twin cable or 75 ohm screened twin cable. I suspect that to some extent this may have been to facilitate the use of simple indoor dipole aerials and/or internal compressed dipoles. 75 ohm (or thereabouts) twin and screened twin feeders were evidently standard inventory items of the period, as shown in this BICC advertisement. Belling & Lee offered the same screened twin as was used for its antistatic radio aerial systems.
From first-hand experience I’d say that 300 ohm twin is a real pain to mount properly, with standoffs and twisting. I can recall changing over my parents’ TV aerial in Auckland from 300 ohm twin to 75 ohm coaxial circa 1970, before its use in NZ became general, in order to obtain an easier and neater installation. There was no problem with obtaining a head-end balun, but this was one that did not attach to the aerial, but was mounted on a standoff on the pole. So a short 300 ohm line was needed to connect the channel NZ2 aerial folded dipole to the balun. When TV2 in Band III came along shortly thereafter, the only Band I/Band III diplexer that I could buy was for 300 ohm twin. So it was mounted between the existing Band I and (new) Band III arrays on a standoff, with a 300 ohm connection to the balun. What a palaver! I had assembled the whole thing on the fairly long pole down below, then somehow managed to get it on to the edge of roof without damage, wherefrom the idea was to lift and swing it over to the chimney lashing. I all but swung myself off the roof in the process. Still, it provided a strong and clean signal, free of ignition interference, which was a bane in those days. Then came the Philips K9 colour TV receiver with its awful bipolar VHF tuner that needed something like 18 dB of signal attenuation to avoid cross-modulation, even though the previous valved monochrome receiver (with a standard 6Ln, 6Cm turret tuner) had swallowed the strong signal whole and with ease.).
Cheers,
Steve
