Why 50 ohm is used eveywhere? How to measure other values?

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randy_sl

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Hi,

I've got two questions.

1. Why almost all RF & microwave systems use 50 ohm as the standard impedance value?

2. Is it possible to measure a device deigned to 75 ohm using a standard network analyzer which is having 50 ohm connectors?
(eg. how to measure a 75 ohm coaxial power divider with a standard network analyzer)

thanks,
randy
 

we use a 50 ohm transmission line to connect a 50 ohm impedance antenna to a transmitter rated at 50 ohms output impedance. In that case everything is matched and as long as we make sure there are no currents flowing on the coax shield, everything should work great. Since all parts of the system are matched, transmission line losses are minimized, the transmitter can operate at its designed efficiency and almost all of the power output by the transmitter will get to the antenna and be radiated.

But what happens when we connect a 50 ohm transmission line to an antenna with a feed point impedance that is not 50 ohms? Let's say, for example, that the antenna has an input impedance of 10 ohms resistive, which is not too uncommon in short antennas. Notice that the transmission line is 50 ohms and is matched to the 50 ohm transmitter output. However, the impedance mismatch between the 50 ohm transmission line and the 10 ohm antenna causes an SWR of 50/10 = 5:1 and a substantial amount of power is reflected from the antenna back down the transmission line. More than likely the protective circuits in the transmitter will cause it to reduce power.

i hope it is helpful for you.
 
vishal_sonam said:
we use a 50 ohm transmission line to connect a 50 ohm impedance antenna to a transmitter rated at 50 ohms output impedance. ................. snip...........i hope it is helpful for you.
Click to expand...

I dont think thats what the OP was asking
he's asking WHY 50 Ohms impedance is used ?

Randy_sl
this standard has its origins in coax cable construction
50 Ohms is a compromise between 30 ohms for best power handling and 77 ohms for lowest loss. 53.5 Ohms is the mean value between these 2 values ( 30 and 77). Thus the choice of 50 ohms is a compromise between power handling capability and signal loss per unit length, for air dielectric.

Of coure 50 Ohm coax isnt the only impedance value out there, the other common one is 75 Ohms.
Low cos commercial cables such as CATV, and other TV and video applications, 75 ohms is the standard. These cables don't have to carry high power, so the main characteristic that should be considered is a cable with low loss. The answer to the "why 75 Ohms?" Knowing that 77 Ohms gives the lowest loss for air dielectric coax, it is probably safe to assume that 75 ohms might be a rounding off of the value.

cheers
Dave
 
Thanks Dave for your great explanation... This question was in my mind for a quite some time... only today I got a good explanation...

I'm still wondering is there a way to measure 75 ohm devices (I've seen power dividers designed for 75 ohm coax cables used in TVs in electronics shops) with a standard device such as a network analyzer?

I've seen all of the microwave signal generators and spectrum analyzers also have 50 ohm output and input ports.
What I've understood is these devices have 50 ohm connectors for I/O ports.

So, to measure 75 ohm devices do I need to use an external impedance transformer?
or is there any simple way such as an option provided in the measurement device to choose the correct impedance?


cheers,
randy
 
Last edited:

A 75 ohm device measured with 50 ohm equipment will have a nominal VSWR of 1.5:1 (14 dB return loss), which is not a terrible VSWR. Many "50 ohm" devices might have worse VSWR over at least part of their range.

There will be some inaccuracy in some measurements because of the mismatch though. What parameters do you want to measure, and what accuracy do you need?
 
Rhodes & Schwartz, a German manufacturer of high grade test and transmission equipment used 60 ohms.
Frank

- - - Updated - - -

Rhodes & Schwartz, a German manufacturer of high grade test and transmission equipment used 60 ohms.
Frank

- - - Updated - - -

Rhodes & Schwartz, a German manufacturer of high grade test and transmission equipment used 60 ohms.
Frank

- - - Updated - - -

Rhodes & Schwartz, a German manufacturer of high grade test and transmission equipment used 60 ohms.
Frank
 

Yes, there are network analyzers that are designed to be 75 ohm systems. You can find them on ebay at a great discount because....nobody else wants them!

But any calibrateable network analyzer can be used to make non 50 ohm system measurements. The more modern ones have software inside of them to change the effective port impedance. The older ones---you need to carefully calibrate the analyzer to a connector interface plane, make the 50 ohm S parameter measurements, and then export those S parameters. An external program can modify the S parameters to make is appear like the measurement system was 75 ohm.

One such program for post processing is SPViewII:
https://www.arkrfsystems.com/SPview.htm


Of course, if you have a standard linear analysis program, like ads, genesys, etc, you can modify the 50 ohm S parameters inside of it too. For Genesys, you just click on the PORTI, etc, and change the default port impedance from 50 to whatever.
 
LOL Chuckey

did your auto repeat key get stuck ? ;-)

Dave


 

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