09-03-2017, 11:27 PM
Screened 75R twin may have been the best when it came to interference reduction. It would have required a balun at the receiver end, assuming that receivers had unbalanced inputs, but not at the aerial end. On the other hand, coaxial cable required a balun at the aerial end, without which it would be very prone to picking up interference on the outer.
From the above-mentioned Cocking article, referring to aerial baluns (matching transformers):
“This form of transformer is very effective in reducing the pick-up of interference by a coaxial cable and in cases of bad interference can improve the signal/interference ratio by 20 dB or more. It is not a method commonly adopted, however, largely because of the practical difficulty of weather-proofing the transformer. More often an equivalent result is achieved with resonant sleeves.”
So if the objective of using coax was to reduce interference pickup, the omission of the balun, as was normal UK practice, resulted in a major fail. I wonder how many apparently intractable ignition interference problems could have been solved quite simply with an aerial balun, but which were addressed by using more complex aerial arrays, relocating the aerial and so on.
The waterproofing difficulty argument looks to be a bit lame though. Since the 1930s the UK industry had been supplying antistatic AM aerial systems that included a suitably waterproofed aerial-end transformers.
I wonder if the use of 300R twin in the USA and elsewhere had something to do with the popularity of folded dipoles, of nominally 300R impedance, in those countries. As an aside, here in NZ it would have been a very rare event to see a dipole that was not folded. On the other hand one did see some three-bar folded dipoles.
Against that, the use of simple dipoles in the UK indicated that a 75R feeder be used, although that does not address the question as to why coaxial was chosen rather than twin.
A corollary question is why the apparent UK aversion to folded dipoles at VHF? They have a mechanical advantage, in that one bar of the folded dipole may be firmly attached to the boom without the need for any insulation.
Maybe there was not a deep technical reason for the choice of 75R coax. Perhaps someone simply thought that 75R matched a simple dipole in impedance terms, and that with a vertical dipole it was logical to attach the inner to the upper half and the outer to the lower half, as that was closer to “earth”.
Cheers,
Steve
From the above-mentioned Cocking article, referring to aerial baluns (matching transformers):
“This form of transformer is very effective in reducing the pick-up of interference by a coaxial cable and in cases of bad interference can improve the signal/interference ratio by 20 dB or more. It is not a method commonly adopted, however, largely because of the practical difficulty of weather-proofing the transformer. More often an equivalent result is achieved with resonant sleeves.”
So if the objective of using coax was to reduce interference pickup, the omission of the balun, as was normal UK practice, resulted in a major fail. I wonder how many apparently intractable ignition interference problems could have been solved quite simply with an aerial balun, but which were addressed by using more complex aerial arrays, relocating the aerial and so on.
The waterproofing difficulty argument looks to be a bit lame though. Since the 1930s the UK industry had been supplying antistatic AM aerial systems that included a suitably waterproofed aerial-end transformers.
I wonder if the use of 300R twin in the USA and elsewhere had something to do with the popularity of folded dipoles, of nominally 300R impedance, in those countries. As an aside, here in NZ it would have been a very rare event to see a dipole that was not folded. On the other hand one did see some three-bar folded dipoles.
Against that, the use of simple dipoles in the UK indicated that a 75R feeder be used, although that does not address the question as to why coaxial was chosen rather than twin.
A corollary question is why the apparent UK aversion to folded dipoles at VHF? They have a mechanical advantage, in that one bar of the folded dipole may be firmly attached to the boom without the need for any insulation.
Maybe there was not a deep technical reason for the choice of 75R coax. Perhaps someone simply thought that 75R matched a simple dipole in impedance terms, and that with a vertical dipole it was logical to attach the inner to the upper half and the outer to the lower half, as that was closer to “earth”.
Cheers,
Steve
