Why is 50 ohms the industry standard for all modern two way radios?

Prior to Word War Two there was no standard output impedance for two way radios.

Early radios used tubes which had a high input and output impedance. The output tube or Final was adjusted each time the operator changed frequency. By "Loading" the plate and the grid using variable coils and capacitors, the output tube could be matched to the antenna, insuring a maximum transfer of energy. When the transmitter output impedance is the same as the antenna input impedance you have a 1 to 1 SWR or "Standing Wave Ratio"

Due to war time rationing it was determined that an impedance of 52 ohms used the minimum amount of materials to make coaxial cable. Copper and other metals where used for the war effort. During that time no cars were made and everything was recycled, even pennies were made of steel in 1943.

Nickels made between 1942 and 1945 use 35% silver instead of nickel. After the war the standard was kept at 52 ohms and eventually was rounded down to 50 ohms. This is still the standard used today.

With the development of transistors in 1948 the output impedance was fixed to 50 ohms, which worked out well because the output impedance is low unlike the vacuum tube.

Author Rick Lewis - The CB Doctor
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An expert at Radio Repair, Rick of the CB Doctor can repair or tune your CB Radio. A license Radio Technician, Rick has more than two (2) Decades of Experience and many satisfied customers.

As you may know many of the older radios are capable of going above and below the normal CB band; the most popular ones being the MB8719 chassis. This includes the Cobra 2000 GTL, Cobra 142 GTL, and the Uniden or President Washington.

The older Washington used the 858 chassis and can be identified by the use of a 4 pin standard microphone jack instead of the newer 5 pin jacks. Regardless of the radio used the most important part of the station setup is the antenna. The trick to getting out is an antenna system with the minimum amount of reflection. As you go above or below the CB band the SWR’s will increase; reducing the radio’s efficiency.

The idea is to get most if not all of the radio energy to radiate from the antenna without any energy being reflected back. This is known as a 1 to 1 SWR. In this condition your antenna acts the same as a tuning fork and resonates maximizing the radio’s performance; as the reflection or “SWR” increases the performance of your station decreases. The following is a list of the ratio of an SWR to its lose. (more...)