Dear Lowfers, the reason for a characteristic resistance of coaxial cables is based on material constants and by finding a minimum of the maximum field strenth in the cable. This is true as well for high frequency cables and high voltage cables for energy transport. The maximum field strength is just on the surface of the inner conductor. Material constants are the conductivity of the copper or silver for the inner conductor and the outer shield. As well as the dielectic losses of the insulation (leakage and polarisation losses). In United States an optimum of about 50 Ohm was calculated in the forties, and kept on in transmitting equipment. In the sixties 60 Ohm was the resistance for a minimum of losses, in cable broadcast nets in the eigties 75 Ohm is a!
n optimum. 55, Hans-Albrecht DK 8 ND may [email protected] schrieb am 12.11.03 01:18:51:
>
> Dear all,
>
> around 1959, when I joined Rohde&Schwarz in Munich, 60 ohms has been the standard cable impedance in Germany for all broadcast and television transmitters in the fifties and for other commercial RF equipment. Cage and other transmitting antennas were also designed for 60 ohms. I have also built my first amateur radio dummy load for 60 ohms.
>
> I do not know exactly the reason for 60 ohms, somebody has told me that this impedance has been a compromise between 50 ohms and 75 ohms, (perhaps following the idea that 75 ohms allows the production of cables with minimum losses whilst 50 ohms would allow to send higher power through a coaxial cable).
>
> On the other hand 75 ohm
!
s has been used and is still in use for video equipment in TV transmit
ters and studio equipment; higher video voltages needed for TV modulator stages have often been terminated with 150 ohms to save power, generating double the voltage with the same current.
>
> But in the sixties a change to 50 ohms impedance for RF equipment became standard also in Germany, there was a need to match to the international market standards.
>
> Concerning receiving antennas, the first VHF FM and TV antennas in the early fifties in Germany were folded dipoles with a 300 ohms symmetrical cable feed. But soon the folded dipole with reflector (and some directors) became popular, using 240 ohms symmetrical cable for about two decades (thus confirming Wolf's [DL4YHF] contribution), TV distribution systems in houses used 60 ohms with a 4:1 balun to connect the antennas (for the TV receivers with 240 ohms symmetrical input another 1:4 balun was needed to connect them to the 60 ohms house system). Later on TV receivers and TV receiving antenna
!
s were changed to 75 Ohms coaxial, CATV systems used 75 ohms from the beginning, the CATV start in Germany has been rather late, about 1984.
>
> By the way, a lot of russian Surplus HF equipment which became available in Germany after the breakdown of the iron curtain, even automatic antenna tuners, were designed for 75 ohms impedance.
>
> 73 Ha-Jo, DJ1ZB
>
>
> "Walter Blanchard" <[email protected]> schrieb:
> > DK8ND's email raises an interesting point. Can any of our German friends tell me why an impedance of 60 ohms was popular in Germany for some time but not anywhere else?
> >
> > Walter G3JKV.
> >
> >
> >
> >
> > ---
> >
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>
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