On Feb 14, 4:33*am, phaedrus wrote:
Hi again,

Well I have read your informative replies on the problems with making
a low Zo twin feed with much interest. Clearly it's not really a
practical proposition. Shame.

So the obvious question is: why are transmitters normalised to 50 ohms
when clearly 450 ohms would enable us to enjoy cheaper, do-it-
yourself, lower loss feeders? Was this some oversight at the time, or
good practice for some obscure reason that I simply cannot think of?


The RG8 (and RG58 RG6 RG 56) standards were developed during WW2 as US
military standards.
The RG8 52 OHM feedline standard goes back to the original RG8 MIL
SPEC, which has long since been abandoned.

The demand for COAX cable during the early days of WW2 FAR outstripped
the existing manufacturing capacity.

RG8, given the existing constraints on the amount and type of
available matierials available and the volume needed; was deemed the
most economical and easiest to produce.

HENCE RG8's impedance (52 ohms) became the standard.

Many RF devices outputs and and RF connectors were harmonised to work
easily with RG8 cables.

Several factors have led to 52 ohms CONTINUING IN USE FOR ALL THESE
YEARS:

1)Because of the large installed base of RF devices, after WW2 52
ohms became the de facto standard.
2) large numbers of RG8 compatible connectors and accessories are
available.
3) RG8 cable does not "leak" RF like 450 open ladder feedlines,
consequently,several cables of the RG8 family can be run physically
close to reach other without encountering cross talk problems.
4) RG8 cable is easier the thread through contricted spaces like
bulkheads and walls.
5) RG8 is physically more rubust than 450 ohm ladder line