Jim said
The choice is not entirely arbitrary - as I recall, 50ohm coax gives the
best power handling capability for a given size of cable, while 75ohm
gives
the lowest loss. This is probably academic in amateur practice because the
cost is usually a more significant consideration than the size!
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?
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 <[email protected]>
Web <http://web.ukonline.co.uk/g3ldo>
|
