From: "captbrian" <[email protected]>
Reply-To: [email protected]
To: <[email protected]>
Subject: Re: LF: Dummy-Load on LW
Date: Wed, 12 Nov 2003 08:04:52 -0000
which raises the question I have often wondered about.
When I started in ham radio all coax was post-war surplus 75 ohm. I
assumed
in my youth it was to match a free-space dipole !.
When I came back to radio after a thirty-year absence in 1990 all was 50
ohm.
Who, when and why did someone decide to change to 50 ohm ??
Bryan G3GVB
-----Original Message-----
From: Hans-Joachim Brandt <[email protected]>
To: [email protected] <[email protected]>
Date: 12 November 2003 00:16
Subject: Re: LF: Dummy-Load on LW
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 ohms has been used and is still in use for video
equipment in TV transmitters 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
antennas 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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