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From: "Laurence KL1X" <hellozerohellozero@hotmail.com>
To: rsgb_lf_group@blacksheep.org
Date: Wed, 12 Nov 2003 09:02:14 -0900
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Message-ID: <Law15-F21SAhlXAYPHW0008247d@hotmail.com>
Subject: Re: LF: Dummy-Load on LW
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I found a thread that says something to the effect that 50 ohm was chosen  
(in the US) during the war period because of the standard diameters 
available in copper tubing at the time (!) - that sort of makes sense D/d 
and all that....must be all hard line!

Laurence
(too young to rem WW2)


>From: "captbrian" <captbrian@ukonline.co.uk>
>Reply-To: rsgb_lf_group@blacksheep.org
>To: <rsgb_lf_group@blacksheep.org>
>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 <hajo.brandt.dj1zb@t-online.de>
>To: rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>
>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" <blanch@pncl.co.uk> 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.
> >
> >
> >
> >
> > ---
> >
> > Checked by AVG anti-virus system (http://www.grisoft.com).
> > Version: 6.0.525 / Virus Database: 322 - Release Date: 09/10/03
>
>
>
>
>
>

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