Tuesday, September 20, 2005

Why 50 ohm? - Part 1 of 2

Back in the old days where people had just started to have Local Area Network in the office, the Thin Ethernet was the popular choice. It was widely used becuase it was one of the simplest and cheapest implementations. It is also known as the 10 Baseband 2 or the IEEE 802.3a. All computer are connected to each other on RG-58 coaxial cable using the BNC T-connector. One end of the bus is terminated with a grounded 50-ohm terminator, while the other end has an ungrounded 50-ohm terminator. On Saturdays, I followed some friends, who owned a small "IT solution" company, to perform LAN installation at those small/medium industry (SMI) companies. That was back in the year 1993 ~ 1995.

RG-58 coaxial cable has been long enough before the invention of Thin Ethernet. As you and I know, it is the transmission cable used by many hams. It is the life line between the rig and the antenna. Besides RG-58, there are RG-174, RG-223, RG-8, Belden 9913, etc. Other than they are all coaxial cables, they share one thing in common, i.e. they are all having the characteristic impedance of 50 ohms.

When I started to involve in this ham radio hobby, the "why 50-ohm" question is revisited. In the quest for an explanation, I never stop asking people on this topic. Many tried their best to share, thank you very much. Some, on the other hand, tried to bull shit or tried to beat around the bush.

seh-jai-ming: Why do we use 50-ohm coaxial cable?

dai-mah-seng: It is for matching purpose. If you use coaxial cable with other value, your SWR will be high.

seh-jai-ming: Why they choose the value of 50-ohm ?

dai-mah-seng: Because it is the standard.

seh-jai-ming: How do they come out with such a standard?

dai-mah-seng: All the cable manufacturer decided to standardize it.

seh-jai-ming: Why don't they choose other value?

dai-mah-seng: It is because 50-ohm is the output impedance of the transceiver.

seh-jai-ming: Why the output impedance of the tranceiver must be 50-ohm?

dai-mah-seng: So that, there will be no reflection of the transmitted power.

seh-jai-ming: Why not make everything to be, let's say, 40-ohm?

dai-mah-seng: No, because 50-ohm gives the lowest SWR.

seh-jai-ming: If everything is 40-ohm, then you will also get the lowest SWR too, won't you?

dai-mah-seng: Where can you find 40-ohm coaxial cable?

seh-jai-ming: If you make 40-ohm as the standard, I am sure you will be able to get 40-ohm coax.

dai-mah-seng: Lima Lima coming in, talk to you later.

seh-jai-ming: 73.


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