50, or 75 Ohms?

[G6Tanuki]

Looking through various UK ham-radio magazines/books from the 60s/early-70s [for example the RSGB "Radio Communication Handbook"] I'm intrigued that many of their transmitter-designs are specified to work into a 75-Ohm load.

Whereas all the HF/VHF 'professional' gear I worked with [from Pye Cambridges/Vanguards/Westminsters, through to the more-exciting Kilowatt-rated HF gear from RACAL, Raytheon, Rockwell, Atlas, Hallicrafters etc] was always specified to feed a 50-Ohm load.

So, why was 75-Ohms the general design-load on UK amateur gear? Was it perhaps because of the ubiquity of 75-Ohm TV coax?

[Jon_G4MDC]

Perhaps balanced line, baluns and 300/4 might be a good start?

I think 50 Ohms in Co-ax came from across the pond but you are right, Pye did adopt it quite early on.

Not that it matters much in Class C AM modulated TX PA stages.

[Cruisin Marine]

I guess it was to match best in to a dipole, which from memory should be about 72 ohms?

[G3PIJpeter]

Originally Posted by Cruisin Marine

I guess it was to match best in to a dipole, which from memory should be about 72 ohms?

Yes - The website at https://antenna-theory.com/antennas/halfwave.php states: "The input impedance of the half-wavelength dipole antenna is given by Zin = 73 + j42.5 Ohms. … by reducing the length slightly the antenna can become resonant. If the dipole's length is reduced to 0.48 , the input impedance of the antenna becomes Zin = 70 Ohms, with no reactive component. This is a desirable property since the antenna will be better matched to the radio (transmitter or receiver), and hence is often done in practice ..."

To my memory, 1950s KW, Labgear, Panda, Minimitter etc. equipment all assumed 70 ohms output impedance and 70 ohm coax. The US and Japanese stuff that swept the board in the 1960s was all for 50 ohms (why?). So we threw away the brown semi-airspaced coax with Belling-Lee plugs and bought ourselves RG-xxx cable and fitted PL529 connectors to it. The rest, as they say, is history.

[ex seismic]

The post war Larkspur range of military sets were all 75 ohms as was the late war American BC1000 which became our WS31. The subsequent Clansman range was 50 ohms.

[Radio Wrangler]

75 Ohms:

There is a mathematical analysis showing that for copper conductor and polythene dielectric 75 Ohms gave an advantage in loss per mile for a given cost (or vice versa!) and this was enough to sway telephone system designs for FDM etc.

Close to dipole centre impedance

Cheap TV coax for 50s amateur setups.

50 Ohms:

Someone picked a round number and it's stuck ever since.

David

[Alan_G3XAQ]

Around 75 ohms has lowest loss for air dielectric. Around 30 ohms has highest peak power handling (voltage breakdown), again for air.

Polythene dielectric has lowest loss and highest peak power around 50 ohms.

https://www.microwaves101.com/encycl...why-fifty-ohms

But the chronology doesn't support a logical choice. Probably just manufacturing convenience. I've seen talk of 2" copper pipe for example. Far too much myth and urban legend to be dogmatic about the standard.

[Craig Sawyers]

And then there is 93 ohms https://www.belcom.co.uk/cable-range/coaxial/93-ohm-rg

Craig

[Keith956]

Originally Posted by Craig Sawyers

And then there is 93 ohms https://www.belcom.co.uk/cable-range/coaxial/93-ohm-rg

Craig

A highly technical explanation, must have been written by a salesman! 'Capacitance tends to “store” the peaks of the square waves'

[Craig Sawyers]

Only reason I knew about 93 ohm coax was that Tektronix specified it for some applications, like the Type 130 LC meter. Perhaps because of its low capacitance of 47pf/metre

But yes - the nonsense at the start of the link was clearly something a marketer made up based on what someone who understood the technology told them. Been there, done that, and insisted that they showed me the copy for correction before it went live.

Craig

[Craig Sawyers]

Ha ha! Same company, who don't check their site - 95nF/m!
https://www.belcom.co.uk/cables/rg14...-coaxial-cable

Craig

[G6Tanuki]

93-Ohm RG62A/U was the coax used for the IBM "3270" display-terminals in the 70s/80s/90s; it was intriguing in that it used a copper-plated-steel centre core, which was supported inside a polythene tube by a helically-wound polythene thread - so it counts as a 'semi-airspaced' coax.

I remember quite a few office-blocks in the 90s which were essentially kept upright by virtue of bunches of _thousands_ of such coaxes in the risers. The coming of the IBM 3299 8-into-1 coaxial multiplexer was a big relief for some of us...

I used short lengths of RG62A/U coax in 'impedance transformers' for RF applications: it was 'close-enough' to 100-Ohms that we could fudge it.

Did anyone ever come across 50- or 75-Ohm twin-lead? It was supposedly rather like the old bell-wire or figure-of-8 single-insulated flex, or 'Zip-wire' as our US colleagues called it. Sometimes mentioned in elderly antenna-designs [where it was occasionally also shown as a twin-twisted construction] - the only place I _ever_ came across it was in the 'field dipole' antenna kit issued with the Australian A510 HF transceiver.

[G6Tanuki]

Thinking of this, did any of you get to play with the US 75-Ohm CATV[1] Hard-line coax; I remember one ham station just ouiside Fort Worth who had something like 200 yards of this running up the hill outside his shack to a 5-ele 14/21/28MHz beam.

"Hard-line" was about 3/4 inch diameter, with the outer part being extruded-aluminum tube. There was an ARRL-Handbook-designed toroidal transformer which everyone used when stuffing a Kilowatt at 50-Ohms into a length of cheap hardline.

Search "Methods-foe-using-cable-tv-hardline-for-amateur-radio-Version3.pdf"

[1] note that "CATV" historically means "Commumity Antenna TV" - where signals were gained from a well-positioned antenna on a hill, amplified, and fed via coax and amplifiers to those in the project whose geography meant they couldn't get a good signal.

[Graham G3ZVT]

I've still got a reel of 50 Ohm Thinnet cable left over from when 10Base2 was a thing.

Currently I have about 20m of the stuff feeding a vintage 405 line TV on the sideboard in the hall, from the modulator in the conservatory at 51.75Mhz vision carrier frequency.

[m0cemdave]

Originally Posted by G6Tanuki;

Did anyone ever come across 50- or 75-Ohm twin-lead? It was supposedly rather like the old bell-wire or figure-of-8 single-insulated flex, or 'Zip-wire' as our US colleagues called it.

I use the 75 ohm stuff to feed a fan dipole in my loft, and I think it's still available. Mine came from Westlakes when they used to have a stall at the major rallies, around 20 years ago.

[Synchrodyne]

Regarding 75 ohm (or thereabouts) twin cable, in the early post-WWII period, Belling & Lee promoted its use, instead of 75 ohm coaxial, as a TV aerial feeder. Lower cost was one reason in its favour.





It was also offered for amateur HF aerial installations:




One or two of the UK setmakers, including Murphy, fitted their early FM receivers with 75 R balanced inputs, and accordingly specified the use of 75 R twin aerial feeder.

Screened twin of about 75 R was also the customary feeder for antistatic AM aerial systems, as noted in the concurrent thread: https://www.vintage-radio.net/forum/...d.php?t=191780.


Cheers,

[Steve_Withnell]

77 Ohms is around optimum for receive systems, which gets overlayed with the standard dipole feedpoint impedance. On transmit, the optimum surge impedance for the feeder is around 35ohms. The geometric mean is 52 ohms. 93 Ohm coax tends to be used for phasing/matching (as does 75 ohm).

As a BTW, HF dipoles in amateur use are very rarely 72/3 ohms anyway and typically vary from 35 to 100 ohm due to installation conditions (Soil types, height above ground, miscellaneous 'furniture' in proximity.

I designed a 27MHz dipole for a guy with a 50 ohm transceiver. The design demanded 75 ohm coax as a feeder to properly match the dipole...

[SiriusHardware]

50 Ohms: Someone picked a round number and it's stuck ever since.

50 ohms (or thereabouts) is the 'natural' impedance of a simple quarter wave vertical mounted on a downward sloping metal ground plane - a vehicle body, in other words. I have always assumed this is why 50 ohms became a kind of standard, to keep mobile antennae cheap and simple.

[Hans PE1KWH]

The output impedance of a tube transmitter is not that critical and they can be well tuned for 50 and 75 ohms.

Transistor outputs are much more bound to 50 ohms because of internal impedances of power transistors ( at least the impedance is always lower than that of a tube as it is a current output ).

Early post WW2 German measuring equipment has 60 ohm output and inputs.
The Nordmende UWM-346 ( a wobble generator with indicator tube build in is such an example ).