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Satellite Communications/Characteristic Impedence 50 Ohm vs 75 Ohm

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Question
Dear Sir,

My questions are more related to impedance matching in the ground segment.

1. Is there any signal degradation if there is impedance mismatch in the L-band or IF-signals?

2. Do we really need to install impedance matcher device?

3. How does this impedance mismatch effect the quality of signal?

4. Why 75 ohm impedance is widely used in the end-user applications and 50 ohm is used for the high frequency radio signals?

Thanks

Rgds-John

Answer
John:

QUESTION 1: Is there any signal degradation if there is an impedance mismatch in the L-band or IF-signals?

Please refer to following for more detailed explanation.
http://en.wikipedia.org/wiki/Impedance_matching
http://en.wikipedia.org/wiki/Signal_integrity

Yes. There is signal degradation if there is an impedance mismatch. The mismatch causes reflections that create standing waves that, in turn, cause power loss and frequency dependent attenuation.

At high bit rates and over longer distances (compared to the wavelength) or through various mediums, various effects can degrade the electrical signal to the point where errors occur and the system or device fails.

QUESTION 2: Do we really need to install an impedance matcher device?

Is it required? No. Is it recommended? It depends on various factors…

In general, impedance Z is a complex value with a resistive component R (the real part) and a reactance component X (the complex part). In simple cases such as low-frequency transmission the reactance may be negligible or zero; the impedance can be considered a pure resistance.

If you are concerned about the ground segment transmitter, you could use extra power to overcome losses due to impedance mismatch between high-power amplifier and antenna.

If you are concerned about the ground segment receiver, losses due to impedance mismatch would be a serious concern, especially at the front end between antenna output and downconverter or between antenna output and LNA. In either case, the mismatch would degrade the SNR at the receiver.

QUESTION 3: How does this impedance mismatch affect the quality of the signal?

In the case of the ground segment receiver, the loss of some SNR would lead to a higher bit error rate.

QUESTION 4: Why 75 ohm is widely used in the end-user applications and 50 ohm is used for high frequency radio signals?

Please refer to following for more detailed explanation.
http://www.microwaves101.com/encyclopedias/why-fifty-ohms

WHY 50 OHMS?

50 ohm impedance is the result of a compromise between a coaxial cable’s power handling capability and loss. Peak power handling occurs at Z0 = 30 ohms. Lowest loss occurs at Z0 = 77 ohms.
The arithmetic mean is 53.5 ohm, and the geometric mean is 48 ohms. Thus the choice of 50 ohms is a compromise between power handling capability and signal loss per unit length.

WHY 75 OHMS?

For low-cost commercial cables such as those that bring CATV to your home, 75 ohms is the standard. Because these cables do not have to carry high power signals, the key characteristics are low loss and low cost.

The center conductor of low-cost cables is made of steel with some copper plating whereas the center conductor of a 50 ohm cable is made from copper. The lower the impedance, the bigger the diameter of the center core. An impedance 75 ohms probably was a compromise between low loss and cable flexibility.

I hope that I have answered your questions to your satisfaction. If not, please advise. If so, I would appreciate it if you could complete the satisfaction survery.

Regards,

Thomas E. Burke, Ph.D., PMP

Satellite Communications

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Thomas E. Burke

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Experience

Forty years of experience with satellite communications. Have held roles as system engineer on JPL Mariner 9 program and a program manager for a number of classified communication satellite programs. Served as head of TRW / Defense Communications Division / Engineering Development Operation, a 1,400-person organization responsible for all aspects of classified communication satellite design and development.

Education/Credentials
Ph.D. Chemistry, California Institute of Technology (1969) B.A. magna cum laude Chemistry, University of Minnesota (1962)

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