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Old 9th Dec 2007, 8:40 pm   #1
Al (astral highway)
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Default Aerial impedance in vintage TVs

Please can anyone tell me if TV aerial input impedances have always been 50 ohms, or if they were ever something else.

(Actually, are they 50 ohms now, btw?)


Many thanks
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Old 9th Dec 2007, 9:08 pm   #2
Ed_Dinning
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Default Re: Aerial impedance in vintage TVs

HI AL , we had a Baird set in the 60's that used twin feeder from an "X" aerial. This was 300R I think, later converted to ITV and then on std coax .

Ed
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Old 9th Dec 2007, 9:13 pm   #3
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Default Re: Aerial impedance in vintage TVs

I thought they were all 75 ohms.
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Old 9th Dec 2007, 9:39 pm   #4
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Default Re: Aerial impedance in vintage TVs

AFAIK always 75R except for a few which were 300R balanced.
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Old 9th Dec 2007, 9:52 pm   #5
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Default Re: Aerial impedance in vintage TVs

Most NZ sets had 300R balanced inputs until the mid 70's and the introduction of colour, since then its been 75R on PAL connectors.
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Old 10th Dec 2007, 10:21 pm   #6
Ed_Dinning
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Default Re: Aerial impedance in vintage TVs

No this Baird had balanced twin feeder into the tuner chassis with two screw treminals to attach it to.
When the tuner chassis was swopped to get ITV the new tuner chassis was fitted with a coax socket.

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Old 11th Dec 2007, 12:05 am   #7
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Default Re: Aerial impedance in vintage TVs

A few musings on this 75 ohm question, which may serve to cloud the issue a little:-

* receive aerials designed to use 50 ohm unbalanced co-ax are by no means unknown, but it's unlikely that you'd ever see one in a domestic environment. They're widely used by the broadcasters: the famous (some would say notorious) log-periodic aerials are 50-ohm devices, widely used for both RX and low-power TX arrays. It represents a simplification in stock-keeping to only have to stock one type of spare...

* I seem to remember that some 300-ohm balanced-feeder receivers used to sport a centre-tap on the input winding - this meant that you could also use them on 75-ohm unbalanced, by using one input connection and the tap (half the turns ratio = one quarter the impedance).

* In any case, 75-ohms is very much a notional value. If you were unwise enough to actually check the input match of a TV tuned to a particular channel, you might see a VSWR ratio of anything up to 2:1 across a given channel. By any standards, this doesn't represent a particularly marvellous match...

Philosophically, this last point leads to some interesting surmises. What is the match going to look like, at any channel to which that TV is not tuned? Pretty poor, I suspect - and this means that most of the energy on channels to which the set is not tuned, will be reflected back up the feeder, and in theory at least will be re-radiated... At a very low power level, to be sure. But if there are other aerials mounted very close-to (how many times have you seen a chimney bristling with multiple arrays?) it might be possible for that signal to be picked up by those other aerials, and produce apparent reflections on their RXs when tuned to different channels. Does anybody have any evidence of this actually occurring?
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Old 15th Dec 2007, 5:02 pm   #8
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Arrow Re: Aerial impedance in vintage TVs

Hi Al,

I thought you'd come back and say something like that!

Time for a bit of theory on transmission lines.

If we have a source of impedance Zs and a load of impedance Zr, we can match these impedances by connecting a quarter-wave length of transmission line between Zs and Zr whose characteristic impedance is given by the sq. root of (Zr*Zs). Note that the length of this section of co-ax must be a quarter-wave long - or any number of odd quarter-wavelengths long. Therefore, the impedance match is frequency-sensitive. What we have here then is an impedance-matching transformer, which will be quite freq. sensitive - i.e. high Q

In actual point of fact, at the freqs. we are talking about here, the bandwidth over which this match will be 'reasonable' will be relatively wide.
8 metres of co-ax at these freqs. will accomodate several quarter-waves. It sounds like this length has a number of odd quarter-waves on it - hence, the above bit of theory. From this theory then, a quick calculation indicates that the input impedance of your receiving device is in the region of 33 ohms.

Al / Skywave
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Old 15th Dec 2007, 6:27 pm   #9
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Default Re: Aerial impedance in vintage TVs

Hi Al,

Not being a radio amateur I may be on dangerous ground but I seem to remember that a simple half wave dipole has a natural impedance of something like 73Ω and if you make it as a folded dipole its impedance rises by a factor
of about 4 times i.e. 300Ω. Hence the popularity of these values, leastwise in the Band 1 only era.

Then again when you start to bring reflectors and directors close to the folded dipole its impedance is significantly reduced and is normally arranged to match 75Ω cable, which is what you'll see for Bands 3, 4 & 5.

Peter

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Old 15th Dec 2007, 6:35 pm   #10
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Default Re: Aerial impedance in vintage TVs

Quote:
Originally Posted by peter_scott View Post
Then again when you start to bring reflectors and directors close to the folded dipole its impedance is significantly reduced and is normally arranged to match 75Ω cable, which is what you'll see for Bands 3, 4 & 5.
Correct. Which is why decent aerials have a matching element of some kind. Also a balun, to convert the balanced signal from a dipole to unbalanced co-ax.

Coming back to the original question. Ever considered that your original length of 75R co-ax was duff?
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Old 16th Dec 2007, 12:15 pm   #11
Skywave
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Arrow Re: Aerial impedance in vintage TVs

Quote:
Originally Posted by astral highway View Post
Why do we assume a 75 Ohm input impedance for these devices . . .
The short answer is that 75-ohms is the standard impedance for domestic receiving equipment that uses a co-axial cable input.

My understanding as to why 75-ohms has become the de facto standard for domestic receiving equipment is simply because a simple half-wave dipole (if located at least one wavelength above ground) presents a source impedance of 75-ohms (strictly speaking, it is 73.2-ohms) to its load. Hence, 75-ohm cable and a 75-ohm load: all impedances are equal = maximum power transfer & zero reflections = 1:1 VSWR and an infinite return loss. The only snag here is that a half-wave dipole is a balanced source: co-ax cable isn't. (75-ohm balanced cable is quite common in the USA). Ideally, what is required is a balun at the aerial to change this. However, in most applications, the effect of the length of the downlead co-ax and its attenuation is such that this downlead acts as a crude balun - and in the domestic broadcast reception scenario, it is assumed that field strengths are high enough to make these consequent losses neglectable.

As to why 50-ohm co-ax cable came into existence is a Q. that I have asked on this Forum (and elsewhere). There are a number of theories - some sound plausible, some don't - but that's another issue.

Al / Skywave.
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Old 16th Dec 2007, 12:41 pm   #12
Skywave
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Arrow Re: Aerial impedance in vintage TVs

Quote:
Originally Posted by astral highway View Post
And is one way of looking at the improvement I noted simply to say that we don't quite have a 1:1 impedance match now (33:50R), but it's a lot better than it was before (75:33 > 2:1)? and so we don't have standing waves? Or not
Transmission line theory - especially when mis-matched loads are considered - can get quite complex (pun not intended!) - as a quick look around the 'Net (or reading any standard text book on Radio Communications Theory will show) - so I'm a little bit reluctant here on this Forum to delve into all that and provide a technically-competant answer that your Q. really requires.

However, I'll attempt an answer as simply as I can.
With the quarter-wave transformer in situ, there are reflections taking place - it's just that these reflections become self-cancelling on account of the phase relationships which then exist on the line. The overall effect is to produce a maximum power transfer from the source to the load - albeit at a reduced bandwidth compared to the 'everything is matched' scenario.

And yes - the thought that you might have had a duff piece of co-ax did cross my mind - but I thought I'd leave others to pick up on that! . Mind you, that doesn't explain why you get a significant improvement when you then used 50-ohm cable (which should, per se have made matter worse on account of the mis-match) - which brings me back to where I came in

Al / Skywave
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Old 16th Dec 2007, 1:11 pm   #13
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Default Re: Aerial impedance in vintage TVs

Many TV aerials incorporate a balun in the terminating box. A bit confusing if you try to measure the resistance between the inner and outer conductors of the coax cable.
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Old 16th Dec 2007, 3:51 pm   #14
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Default Re: Aerial impedance in vintage TVs

As a matter of interest, the coaxial cable impedance used in the original EMI AP installation was 100 ohms and not 50. I suppose that the higher Z was better for matching to valve drivers. The transmission level was 10V p-p which meant 1 watt of video!
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Old 16th Dec 2007, 4:27 pm   #15
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Default Re: Aerial impedance in vintage TVs

Quote:
...the coaxial cable impedance used in the original EMI AP installation was 100 ohms and not 50...
Take care. When people say '50 ohms', this immediately conjures up an image of RF distribution to mind. The 100-ohm stuff you're talking about here was used for the video baseband signal, as distributed about AP.

For the record, the twin-balanced cable used for the OB installation was quoted as 186 ohms characteristic impedance. Moving on to the 'real' world of RF, the main RF distribution feeders from the TXs to the transmitting aerials was 78.5 ohm char Z, and yet the vision TX itself was designed to run into a 50 ohm load, and so had to have a 50/78.5 ohm matching system connected to its output...

...and to complete the mix, the co-ax feeder from the OB receiver aerials on the mast-top was 80 ohms char Z. So many different values - no wonder they later tried to standardize.

(Now this is what I call real vintage TV...)
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