I am enjoying this but was there no consideration of the likely load
impedances needing to be fed? Surely all the finer points of line loss are
wiped out if a mismatch or lossy matching network becomes necessary??
bryan g3gvb ?
-----Original Message-----
From: Dave Brown <[email protected]>
To: [email protected] <[email protected]>
Date: 12 November 2003 20:52
Subject: Re: LF: Dummy-Load on LW
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" <[email protected]>
To: <[email protected]>
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" <[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.
> >
> >
> >
> >
> > ---
> >
> > 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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