Co-ax V 300 ohm flat twin.

[colly0410]

I understand that North America used 300 ohm flat twin aerial downlead till the 80's before they drifted over to co-ax, however the UK used 75 ohm co-ax, anyone know why this was? Also what did Australia & New Zealand use? I have a 1980's Sony music centre with both 300 & 75 ohm inputs for FM & it still works OK..

[Terry]

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.

[Synchrodyne]

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

[Terry]

Mention of 75 ohm twin feeder reminds me of the quick'n'dirty method of making an indoor FM aerial:

- Choose a wall that is as close to broadside onto the transmitter as possible;

- Take a suitable length of flat two-core mains lead and strip the outer sheath back 2' 7";

- Separate the two conductors and stretch them out along the picture rail, securing them with drawing pins at intervals as required;

- Connect the other end of the flex to a plug to suit the input of the FM radio.

I've no idea how close twin flex is to 75 ohm feeder but it worked well, anyway!

Although the 2' 7" measurement has been fixed in my mind for over 50 years, I'd never bothered about checking it but, allowing for the elements being only 95% of the wavelength in free air, I calculate it to be 91MHz which would have been the centre of the small portion of Band II that HMG grudgingly allowed the BBC to use in those days!

Two other figures rattling around at the back of my mind are 10' 10" for the length of a half-wave dipole for channel B1 and 2' 3½" for channel B9.

Funny thing, the mind. It can pluck figures like that out of the dim, distant past yet if Helen asks me "How much did you have to drink last night?" I can never remember the answer ...

[peter scott]

I've no idea how close twin flex is to 75 ohm feeder but it worked well, anyway!

We used to use twisted pairs at work and that generally worked out at about 120 ohms.
Peter

[Terry]

We used to use twisted pairs at work and that generally worked out at about 120 ohms.

As the impedance is related to the log of the ratio of conductor diameter and spacing multiplied by a figure derived from the relative permittivity of the insulating material, varying any one of the three parameters can have quite an effect on the result.

Assuming you are referring to telephone type twisted pairs, the conductor size and spacing will be considerably smaller than for twin flex, although the ratio between the two might be very similar - who knows? Even then, variations in the relative permittivity of different insulating materials can affect the result.

[Hartley118]

Did we adopt coax in the UK because we were preoccupied with ignition interference problems? Presumably the coax shield helps to reduce feeder-induced intereference'

So many UK houses are pretty close to roads. Maybe the situation was different in the US.

Martin

[ppppenguin]

As the impedance is related to the log of the ratio of conductor diameter and spacing multiplied by a figure derived from the relative permittivity of the insulating material, varying any one of the three parameters can have quite an effect on the result.

The log function reduces the effect of size and separation. That's why co-ax is only available over a fairly narrow range of impedances. Much below 50R and the inner gets very close to the outer. Much above 90R and the inner gets very thin.

Twisted pair seems to end up in the 100R to 120R range unless you explicitly do something very different. I think Cat5 etc is 110R, which is also the impedance specified for AES digital audio. I wonder why

[Terry]

Did we adopt coax in the UK because we were preoccupied with ignition interference problems?  Presumably the coax shield helps to reduce feeder-induced intereference

I also wondered about that, Martin, but dismissed it because of the poor screening provided by the coax used - more like leaky feeder than screened - and wondered if the inherent interference cancelling ability of a balanced feeder would not have performed better under such circumstances?

Obviously, the feeder would have to have been balanced at both ends but the cost savings from using twin feeder as against coax would surely have cancelled out the cost of the extra balun at the aerial end?

[Synchrodyne]

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