Message-ID: <0ea301c3a95a$5d8abcb0$6a00a8c0@athlon>
From: "Dave Brown" <tractorb@ihug.co.nz>
To: rsgb_lf_group@blacksheep.org
References: <Law15-F21SAhlXAYPHW0008247d@hotmail.com>
Date: Thu, 13 Nov 2003 09:20:07 +1300
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Subject: Re: LF: Dummy-Load on LW
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 Laurence and others-
While it's quite likely that the standard copper pipe story is true, the
basic physics of the issue are fairly simple- for a given coax outer
diameter it can be shown there is a broad minimum in loss for an outer to
inner diameter ratio of  approx. 3.6.  With polyethelene dielectric this
gives a characteristic impedance of approx. 50 ohms.(actually nearer
52-sound familiar to you old timers?) Take out the dielectric and the
resulting air line has a characteristic impedance of roughly 75 ohms.(close
to 77)
The loss minimum is very broad-such that in an airline, there is only about
a 10-12% loss increase going from about 50 ohms up to 110 ohms as the
outer/inner diameter ratio changes. So if using standard pipe sizes gave a
50 ohm line, they were only compromising  by 10% or so wrpt use of  the 77
ohms ideal figure.
73
Dave, ZL3FJ


----- Original Message ----- 
From: "Laurence KL1X" <hellozerohellozero@hotmail.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Thursday, November 13, 2003 7:02 AM
Subject: Re: LF: Dummy-Load on LW


> 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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