Author Topic: 75 Ohm why?  (Read 2953 times)

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Offline tchicago

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75 Ohm why?
« on: May 13, 2022, 04:45:20 pm »
Why does the TV antenna and cable TV uses the 75 Ohm standard for impedance versus the rest of RF industry that uses the 50 Ohm? Is there a specific characteristic that makes it more advantageous for TV?


Funny story. I have a background from a former Soviet union, and the explanation I've had there is that it was intentionally made incompatible so the people would not steal the 50 Ohm military cable for TV antenna cabling. That explanation no longer computes, as after I moved to US found out that the TV cabling here is also 75 Ohm :)
 

Offline TimFox

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Re: 75 Ohm why?
« Reply #1 on: May 13, 2022, 07:04:59 pm »
I was taught that with traditional solid-polyethylene dielectric, 50 \$\Omega\$ was optimal for power loss at high frequencies (e.g., RG-58/U and RG-8/U).
I assumed that 75 \$\Omega\$ (e.g., RG-59/U and RG-11/U) became popular because it is the theoretical impedance for a 1/2-wave center-fed antenna (73 \$\Omega\$).
With solid polyethylene, it is difficult to obtain a higher characteristic impedance due to the logarithmic dependence on diameters.
Higher impedances usually have either partial-polyethylene (e.g., RG-62/U at 93 \$\Omega\$) or PTFE or foamed plastic dielectrics.
(After posting, I see that the Wikipedia article cited above has the same information.)
 

Offline A.Z.

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Re: 75 Ohm why?
« Reply #2 on: May 13, 2022, 07:14:17 pm »
if I recall it correctly. after the cited study, it was decided to use 75 Ohms for RX sytems, since it offered lower losses and 50 Ohms for TX since it had some advantage... my old brain isn't working right now; at any rate, the good old ladder line works much better, but then mils had to find a line which could be used w/o the limitations of ladder line
 

Offline TimFox

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Re: 75 Ohm why?
« Reply #3 on: May 13, 2022, 07:16:20 pm »
Also, a common simple VHF antenna is the horizontal folded dipole, which matches 300 \$\Omega\$.  It is easy to make a broadband 4:1 impedance ratio balun to match 75 \$\Omega\$.
 

Offline BigBoss

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Re: 75 Ohm why?
« Reply #4 on: May 14, 2022, 02:22:07 am »
Because 75 Ohm Characteristic Impedance has Lowest Attenuation for Coaxial Cables
33 Ohm has best Power Handling Capability for Coax.
50 Ohm is the Geometrical Mean between These two
 
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Offline David Hess

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Re: 75 Ohm why?
« Reply #5 on: May 14, 2022, 03:31:33 am »
Because 75 Ohm Characteristic Impedance has Lowest Attenuation for Coaxial Cables
33 Ohm has best Power Handling Capability for Coax.
50 Ohm is the Geometrical Mean between These two

That is what I remember.  And then 92 ohm cable was used in digital systems because it is easier to drive and lower power.
 

Offline E Kafeman

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Re: 75 Ohm why?
« Reply #6 on: May 15, 2022, 07:23:11 am »
because it is the theoretical impedance for a 1/2-wave center-fed antenna (73 \$\Omega\$ ).

 An ideal a 1/2 wave center-feed dipole antenna in free space have an impedance around 83 Ohm.
 Antenna impedance is a complex value consisting of antenna resistance and reactance.
 By making a dipole antenna shorter then 0.5 lambda can reactance part be compensated for. Typical 0.47 lambda and reactance is very low and antenna resistance have dropped to around 65 Ohm. A possible problem is that matching coaxial cables at 65 Ohm are rare.
 For TV-reception will that mismatch make minimal difference but if transmitting several kW and a such mismatch can start a fire.
It can in any case be seen that a dipole antenna impedance is not something that did decided coaxial cable impedance for some kind of natural impedance matching.

 Cable characteristic impedance (resistance when reactance i zero) is selected depending on what cable properties are preferred for each situation, however are 33 and 75 Ohm two optimal values using air-filled coaxial cable  (max voltage and min loss).
 It is mostly when low losses are of high value that air-filled coaxial cable is used today due to cost and these cables are designed solid or semi solid.
 If replacing air with something else must its dielectric properties be taken in account which affects cable impedance and  velocity factor.

 There are many stories why 50 Ohm and not something else. Probably are many of these stories correct and part of the history.
 17, 33, 51.5, 60, 75 and 93 Ohm have all been used or is in use as coaxial cable impedance because specific advantages. A bit fun equipment designed for 51.5 Ohm coaxial cable is this BIRD restive load.

 In Europe was 60 Ohm a common impedance in European military radios during WWII and US military selected 50 Ohm. Quality transmission lines was a new need due to relative powerful transmitters was developed in a never before seen numbers why rigid 50 Ohm coaxial cables (steel tubes) was developed.
 It was not practical to use ladder line inside airplanes, tanks, ships and submarines (was more common during WWI).
Steel tubes was also very reliable kind of cables, survives rough handling, even fireproof and can be bolted directly at other steel structures so it fitted military needs very well.

 Hewlett Packard did became a mayor player for measurement of radios a few years after WWII and they offered only instrument with 50 impedance as US military was its greatest customer.
European military had no choice, they switched from 60 to 50 Ohm due to practical measurement reasons.
 Same numbers as 50/60 Hz.
 55 Hz is maybe a diplomatic halfway switch from both sides  ::) .
When TV at higher frequencies did became popular did HP add 75 Ohm alternative for its measurement instruments as low loss impedances was more important for TV reception then transmitting a lot of power and TV and broadcast FM radio seemed to be a big thing.
For that was a flexible coaxial cable needed between radio and antenna as well for cable TV. Fixed steel tubing is way to expensive and it would become unpractical to move around a TV if connected with steel tubes. Have to call a plumber to move around the coaxial cable.
Selected cable was instead a more flexible type, a foam filled cable with properties very similar to air-filled cable as the foam mostly contained air.This cable did replace older antenna cables for TV-set, typical a 300 Ohm twin lead.
Table FM radio was born during same period but there did twin lead cable survive for very long time as at 100 MHz are twin lead cable losses not as high as for UHF (>300 MHZ).
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Offline MikeK

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Re: 75 Ohm why?
« Reply #7 on: May 15, 2022, 09:41:43 pm »
As mentioned, 75 ohm has the least attenuation.

Researchers at Bells Labs in 1929 produced this graph:


 

Offline jjoonathan

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Re: 75 Ohm why?
« Reply #8 on: May 16, 2022, 01:53:39 pm »
That graph gets copied everywhere, but for some reason the assumptions behind its derivation never come along for the ride. Guess: teflon dielectric, copper conductor, 100MHz.
 

Offline TimFox

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Re: 75 Ohm why?
« Reply #9 on: May 16, 2022, 03:29:32 pm »
The graph purports to be from 1929.  DuPont scientists invented Teflon (PTFE) in 1938.
 
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Offline bsdphk

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Re: 75 Ohm why?
« Reply #10 on: May 16, 2022, 03:37:24 pm »
Not Teflon: Air.

Telco coax used air as dielectric, with insulating discs spaced as far apart as practical holding the center conductor in place.

There are many reasons for for that design, but the main two are that A) It allowed them to pressurize the coax to hold moisture out, the same way the did with twisted-pair cables and B) air is a pretty damn good dielectric material to begin with.

The discs were made from "the best material at the time", and among other materials they used porcelain, pertinax, natural rubber and various plastics, until they settled on styrofoam, which is as close to air as one can get for a solid.

For AT&T choosing the physical proportions with the lowest attenuation was a total no-brainer:  The difference between a repeater every 10 miles or every 8 miles makes a huge difference in cost when you build a cable from Chicago to San Francisco.

The periodicity of the discs did cause them some grief, even a 0.0001dB unevenness adds up, when you have a million of them one after each other, so there were significant work on randomizing the spacing of the insulating disks during manufacture.

As the top frequency increased, the need for more and more detailed equalization of other variable effects overshadowed the disc-periodicity.

Lots of good and interesting articles in BSTJ about it.
 
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Offline TimFox

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Re: 75 Ohm why?
« Reply #11 on: May 16, 2022, 03:47:08 pm »
With long coaxial cables, a periodic defect (either in extruding a solid dielectric or a periodic spacing of insulators) can cause lots of grief with constructive interference from the small reflections at the change in Zo.
 

Offline MikeK

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Re: 75 Ohm why?
« Reply #12 on: May 16, 2022, 04:43:32 pm »
The graph purports to be from 1929.  DuPont scientists invented Teflon (PTFE) in 1938.

What does that have to do with anything?  Modern coax was invented in 1929 at Bell Labs.
 

Offline jjoonathan

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Re: 75 Ohm why?
« Reply #13 on: May 16, 2022, 06:37:28 pm »
Hmm, teflon will have a higher dielectric constant and loss tangent than air. I wonder how the graph looks under conditions more typical of modern cables.
 

Offline TimFox

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Re: 75 Ohm why?
« Reply #14 on: May 16, 2022, 06:39:43 pm »
The graph purports to be from 1929.  DuPont scientists invented Teflon (PTFE) in 1938.

What does that have to do with anything?  Modern coax was invented in 1929 at Bell Labs.

I was replying to the post immediately above it, so I didn't see the need to specifically cite jjoonathan
 

Offline jjoonathan

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Re: 75 Ohm why?
« Reply #15 on: May 16, 2022, 06:48:00 pm »
Haha yeah I knew it was old but I didn't know it was old enough to predate teflon!
 

Offline gbynum

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Re: 75 Ohm why?
« Reply #16 on: May 16, 2022, 06:53:03 pm »
Guess: teflon dielectric, copper conductor, 100MHz.
I doubt Teflon was around in 1929 :)
 

Offline jjoonathan

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Re: 75 Ohm why?
« Reply #17 on: May 16, 2022, 07:06:38 pm »
I wonder how the graph looks under conditions more typical of modern cables.
 

Offline A.Z.

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Re: 75 Ohm why?
« Reply #18 on: May 16, 2022, 07:16:19 pm »
Guess: teflon dielectric, copper conductor, 100MHz.
I doubt Teflon was around in 1929 :)

but nylon was...
 

Offline HighVoltage

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Re: 75 Ohm why?
« Reply #19 on: May 16, 2022, 07:32:47 pm »
There are 3 kinds of people in this world, those who can count and those who can not.
 
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Offline jjoonathan

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Re: 75 Ohm why?
« Reply #20 on: May 16, 2022, 07:48:28 pm »
Nice! It looks like there have been a few additions since I last visited. The additions, credited to "Per," make the figures relevant to 21st century test equipment  :-+

Quote
for solid PTFE (ER=2.2, yellow line) the minimum loss occurs near 52 ohms
 

Offline TimFox

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Re: 75 Ohm why?
« Reply #21 on: May 16, 2022, 08:23:06 pm »
Guess: teflon dielectric, copper conductor, 100MHz.
I doubt Teflon was around in 1929 :)

but nylon was...

Nylon (1935 to 1938) is a crummy dielectric, and absorbs way too much water for good insulating properties.
Polyethylene (from 1936) and PTFE (1938) are much better.
 

Offline A.Z.

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Re: 75 Ohm why?
« Reply #22 on: May 16, 2022, 08:38:10 pm »
Guess: teflon dielectric, copper conductor, 100MHz.
I doubt Teflon was around in 1929 :)

but nylon was...

Nylon (1935 to 1938) is a crummy dielectric, and absorbs way too much water for good insulating properties.
Polyethylene (from 1936) and PTFE (1938) are much better.

sure, but when they started they tried several materials for the dielectric, and you cited other two possible ones
 

Offline TimFox

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Re: 75 Ohm why?
« Reply #23 on: May 16, 2022, 08:53:06 pm »
Polystyrene was discovered way back in 1839, but commercialized much later, in 1930.  StyrofoamTM was invented post-war, in 1954.
 

Offline radiolistener

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Re: 75 Ohm why?
« Reply #24 on: May 16, 2022, 10:18:55 pm »
An ideal a 1/2 wave center-feed dipole antenna in free space have an impedance around 83 Ohm.

No, ideal half-wave dipole with center feed and wire thickness 0.001 wavelength placed in a free space has Z = 73.1 + j*42.5 Ω.

If you're use dipole length 0.4775 wavelength (for wire thickness 0.001 wavelength) it will be Z = 63.7 + j*0 Ω (resonant frequency). So you're needs to use a little shortened half wavelength dipole to catch the best resonance.

Also it is interesting that at resonant frequency you can feed half wavelength dipole with 75 or 50 Ω cable, both will be matched with SWR about 1.2 :)

Note that impedance of dipole also has dependency on a wire thickness.

Regarding to the question about 75 vs 50 Ω, I think that the good explain is a mentioned reason that 75 Ω has a better attenuation for receiver and 50 Ω has a better power handling for transmitter. Both 75 Ω and 50 Ω can be good matched with half wavelength dipole. And probably this is why these coax impedance are most popular.
« Last Edit: May 16, 2022, 10:41:23 pm by radiolistener »
 


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