[time-nuts] 50 vs 75 ohm cables
WB6BNQ
wb6bnq at cox.net
Thu May 10 13:50:59 EDT 2007
John Day wrote:
At 11:00 AM 5/10/2007, WB6BNQ wrote:
>Hi Peter,
>
><snipped>
>
>In an effort to standardize, the industry selected the mid point
between
>the 35 Ohms and the 72 Ohms, that being 50 Ohms. This forced the
antenna
>manufacturers to design their antennas for 50 Ohms or provide a
matching
>network.
Nice thought, but in fact the comments made earlier are more
correct.
Early co-axial connectors go back to Belling-Lee in the UK in the
early 20's with what has now become the IEC-692 or 'PAL' connector.
Although not a controlled impedance connector it made the use of
coaxial cable convenient. But like most coaxial components it need
to
wait for radar to become really useful.
During the war it was used for video and IF connections in radar
equipment.
The reason that coax was not widely used was due to cost. It was
readily available in large quantities, so the only people able to get
and use it were the Government and government funded research.
52 ohms was in fact the compromise. In 1929 experimental work at
Bell
Laboratories found that the ideal impedances for coaxial cable
were 30 ohms for high power, 60 ohms for high voltage and 77 ohms
for low attenuation. Thirty ohm cable is very difficult to make,
not
very flexible and is expensive. So the Bell folk decided that 52
ohms
was the best compromise between 30 and 60 ohms. This has become the
50 ohm cable we know today.
I do not directly know of the referenced Ma Bell experiments.
However, if such claims were made, it was certainly taken out of
context ! Without the context or application that was being
researched these numbers have little meaning.
Why make coax for 30 Ohms when no systems were in existence that
utilized such an impedance ?
How does 60 Ohms differ from 77 Ohms when discussing high voltage ?
How much high voltage are we talking about ?
How does 77 Ohms provide lower attenuation then 60 Ohms ?
The fact of the matter is that some concept was being studied by MA
BELL utilizing what was currently available at the time of that
study. Those values could have been determined based upon the quality
of available materials of that time frame.
If the high voltage was "DC" then the cable impedance has little
importance, except during the rise time from off to on. Not
withstanding dielectric leakage. If it was "AC" then :
What was the applied frequency ?
With no facts as to the targeted application or the design criteria,
the reasons for the above quoted values have no merit. What does make
sense is you provide a medium that satisfies the maximum transfer of
power between two points no matter what the matching impedance's are.
This would mean the cable impedance would match whatever the system
(source and terminating) impedance is.
Seventy-five ohms is also a compromise, in fact a folded dipole
has,
if I recall correctly because I cant check any books as I am away
from the office, a feed point impedance of 73 ohms.
SORRY, a folded dipole is referred to as a 300 Ohm feed point.
So 75 ohms as we
know it now is a compromise between the low attenuation 77 ohms and
the 73 ohm dipole feed-point.
We also tend to think in terms of ONLY 50 and 75 ohm. But in fact
(again IIRC) RG-8 cable is actually 52 ohms, RG-59A is 73 ohms
(wonder why!) but RG-58B is 75 ohms. RG-11, which found use for
video
cable during the war is 75 ohms. But amongst the older cables their
are cables at 52.5, 51, 76 as well as 50, 52 and 75 ohms. Newer
types
tend to be either 50 or 75 ohm with 93, 120, 125 and even 950 ohm
available for special uses.
>The television world originally used 300 Ohms at the antenna,
along with
>300 Ohm "Twin Lead" and the 300 Ohm input to the television
itself. The
>reason for the 300 Ohms was due to the use of a folded dipole
which has a
>characteristic impedance of 300 Ohms.
Well, again IIRC, it is not actually 300 ohms, if memory serves it
is
in fact more like 273 ohms - plus or minus the effect of conductor
sizes.
OK, if we are going to split hairs then the folded dipole is a 4:1
transformation and the nominal free space impedance is 72 Ohms. So a
4:1 ratio would make the folded dipole, in actuality, 288 Ohms. 288
is an awkward number to roll off your tongue, that is why it is called
300 Ohms.
> The folded dipole design could be
>tweaked to provide a wide frequency range needed to cover all the
TV
>frequencies, especially with the UHF channels.
>
>The switch to coax for TV use came about in an effort to prevent
or
>greatly reduce ghosting problems and for cable systems as a
reliable
>means of transporting the signals to many locations. "Twin Lead"
cannot
>tolerate being near metal objects and is unable to be buried.
"Coax"
>contains the signal entirely within its own shielded structure and
>therefore can be buried and laid next to other metal objects
without
>degrading the signal quality.
>
>The reason 75 Ohms was selected for the TV world was because a
simple,
>easily constructed, 4:1 balun (transformer) would transform 300
Ohms to
>75 Ohms.
That factor accelerated its acceptance. But in fact 75 ohms was
well
established in the TV industry before coax found its way to the
receiving antenna - as the cable for carrying video signals due to
the lower losses it exhibited when coax ran all the way around
buildings carrying video.
SORRY, again this is simply not so !
Base band video is not "RF" at 60 Mhz and up. Base band video barely
made it to 5 MHz in the Black & White days.
Again, "Twin Lead" was dirt cheap compared to the manufacturering cost
of coax back in the 1950's and 1960's. "Twin Lead," if made with
quality material, has much lower loss then coax, especially at very
high frequencies. The TV transmitters were of lower power and their
antennas were not all that high in gain in those early years. So,
with relatively insensitive TV's of the early years and the poor gain
of the receiving TV antennas, you really needed all the help you could
get.
Although other cable types (especially a
shielded twisted pair amongst them) were tried, manufacturers were
already making lots of 75 ohm coax so it was cheap and readily
available. The fact that blind freddy could wind baluns cheaply
just
made it more economical. It is said by some historians of
television
that 75 ohms was adopted early on because of low losses and the
fact
that 75ohms could easily be made in resistors (for terminations and
feed resistors) by paralleling two 150 ohm units before the E-24
values became common.
> Trying to go from 300 Ohms to 50 Ohms would require a 6:1 ratio
>with increased I/R losses and greater difficulty in obtaining wide
band
>operation in the early days of ferrite mixes.
Remember also that early television antenna baluns didn't use
ferrite
at all! Many of them were reasonably large and used 'air' cores.
Baluns with ferrite cores only became common much later because
they
could be wound by automatic machines and were better on UHF,
although
many viewers would never have noticed the difference.
A few points of reference: Coaxial cable was invented in 1884 and
patented in Germany by Ernst von Siemens, the founder of Siemens.
But
nothing really happened as they couldn't figure out a use for it.
About a decade later Tesla took out a US patent , which you should
be
able to find at the USPTO website. Crudely made coaxial cable, or
perhaps more correctly, shielded cable of coaxial construction is
found in the UK in the early 20's. In the US it was not until the
work at Bell Labs in 1929 that coax became widely known there.
Connectors were an issue and whilst the Brits had Belling-Lee from
1922, the Americans didn't have a standard'ish style of connector
until the advent of the 'UHF' (PL-259, SO-239) connector during the
war - as a result of needing a connector for carrying radar video
signals.
John
OK, I am ready for round two.
Bill....WB6BNQ
More information about the time-nuts
mailing list