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Date: Sun, 04 May 2003 13:05:11 +0100
To: rsgb_lf_group@blacksheep.org
From: "Fabian Kurz" <fk@diolog.de>
Cc: dj1yfk@telegraphy.de
In-reply-to: <000601c3123c$8e7b7860$68c428c3@erica>
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Subject: Re: LF: Mains Cable and other transmission lines
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>Over the years I had come to think of 50ohms for coax cable and RF equipment
>as some sort of
>Fundamental Constant and never questioned it. When I described test
>equipment in 'The Antenna Experimenter's Guide' it was always 50ohms where
>applicable. After reading Jim's e-mail I searched through all my radio books
>(Terman, Kraus et al) for enlightenment and the reason for 50ohms as a
>standard is not discussed at all.
>Any references Jim?

This might help:

http://www.tele.ntnu.no/radio/fag/SIE2080/Why_50_ohm_coax.pdf

73, Fabian DJ1YFK

PS: google -> +Why 50 ohms standard


> >
> > Standardisation is not an insignificant advantage however - designing
> > everything to match to 50ohms means any transmitter, antenna tuner, power
> > meter, tuning aid, low-pass filter, dummy load etc., etc., can be
> > interconnected with predictable results, rather than having to devise
> > suitable matching for each piece of equipment, or conversion factors for
> > measurements.
> >I take the view that you have to do some sort of impedance
> > matching whether you want to or not, so designing the system to be 50 ohm
> > throughout is rather like designing everything to run from 230V, 50Hz - it
> > saves a lot of headaches!
>
>I guess what is used depends on how one's station evolved. All my
>experimental work, starting from generating RF at 73kHz in 1996 (using a
>signal generator and car stereo amplifier into a car headlight bulb load)
>took place in the shed at the bottom of the garden where the XYL doesn't
>care how messy it gets!
>The next stage was a wire antenna and a loading coil, all in close proximity
>to one another.
>Most existing RF test was not much use. Resonance was originally done by
>tuning for maximum noise in the receiver. RF power was measured using a
>current meter with a dummy load. The amplifier FET current indicated load on
>the amplifier. Power to the antenna was measured by RF current, which was
>also a final check of antenna resonance. An interim 50ohm impedance section
>seemed inappropriate.
>Now if the transmitter in some distance from the loading coil then that's a
>different matter. A 50 or 75ohm system then  makes sense.
>
>So, using the analogy of designing everything to run from 230V, 50Hz - fine;
>unless of course you want to operate mobile or portable - in which case
>12Vdc might be more appropriate.  Test equipment for outside antenna
>measurements is often more convenient if battery powered.
>
>Regards,
>Peter, G3LDO
>
>e-mail <g3ldo@ukonline.co.uk>
>
>Web <http://web.ukonline.co.uk/g3ldo>