Message-ID: <004201c3a969$d7dab240$72c428c3@bryan2>
From: "captbrian" <captbrian@ukonline.co.uk>
To: rsgb_lf_group@blacksheep.org
Date: Wed, 12 Nov 2003 22:10:53 -0000
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Subject: Re: LF: Dummy-Load on LW
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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 <tractorb@ihug.co.nz>
To: rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>
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" <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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>
>
>