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?

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?

Twinlead transmission lines have to be kept away from other conductors,
can't be taped to a tower leg, run through a metallic trough, or be
buried. If there's a substantial amount of solid insulation between the
conductors, it can get very lossy when wet. Even when properly balanced
(often not easy to do), some field escapes so it can be subject to
undesired radiation and signal pickup.

Coaxial cable has none of these limitations. It's a good trade for most
people.

Roy Lewallen, W7EL

Additional information can be found at
http://www.microwaves101.com/encyclopedia/why50ohms.cfm

73

Tony I0JX

phaedrus wrote:

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?


One story is that when radar was first developed coaxial feed lines were
made by buying copper pipe at the local plumbing supply and the
impedance resulting from using two standard sizes (one as the center
conductor and one as the shield was 50 ohms.

Geoff.

--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or
understanding, as in he has a sub-wikipedia understanding of the situation.
i.e possessing less facts or information than can be found in the Wikipedia.

In message , Geoffrey S.
Mendelson writes
phaedrus wrote:

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?


One story is that when radar was first developed coaxial feed lines were
made by buying copper pipe at the local plumbing supply and the
impedance resulting from using two standard sizes (one as the center
conductor and one as the shield was 50 ohms.

Well I've made 50 ohm line samplers ( Bird 43 lookalikes) with standard
UK copper tee pipe fittings and pipe.

Brian GM4DIJ
--
Brian Howie

On Sun, 14 Feb 2010 01:33:58 -0800 (PST), phaedrus
wrote:

why are transmitters normalised to 50 ohms
when clearly 450 ohms would enable us to enjoy cheaper, do-it-
yourself, lower loss feeders?

A 450 ohm coaxial cable would be rather large.
Zo = 450 = 138 log(b/a) (where b=OD and a=ID)
For an inner conductor diameter of 1 mm, the outer shield diameter
would need to be 1800 mm or about 71 inches.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Sun, 14 Feb 2010 11:14:29 -0800, Jeff Liebermann
wrote:

A 450 ohm coaxial cable would be rather large.
Zo = 450 = 138 log(b/a) (where b=OD and a=ID)
For an inner conductor diameter of 1 mm, the outer shield diameter
would need to be 1800 mm or about 71 inches.

This solution for the formula also reveals the diminishing increase in
Zo for two-line or one line against earth as the separation grows
astronomically.

Let's put a 1mm wire 1000m in the air,
Zo = (138/e^.5) · log(4h/d)
(138/1.667) · log(4000/0.001)
82.8 · log(4000000)
82.8 · 6.6
546 Ohms

73's
Richard Clark, KB7QHC

On Feb 14, 2:14*pm, Jeff Liebermann wrote:
On Sun, 14 Feb 2010 01:33:58 -0800 (PST), phaedrus

wrote:
why are transmitters normalised to 50 ohms
when clearly 450 ohms would enable us to enjoy cheaper, do-it-
yourself, lower loss feeders?

A 450 ohm coaxial cable would be rather large.
* Zo = 450 = 138 log(b/a) * (where b=OD and a=ID)
For an inner conductor diameter of 1 mm, the outer shield diameter
would need to be 1800 mm or about 71 inches.

--
Jeff Liebermann * *
150 Felker St #D * *http://www.LearnByDestroying.com
Santa Cruz CA 95060http://802.11junk.com
Skype: JeffLiebermann * * AE6KS * *831-336-2558

I understand the Navy used to have some coaxial line you could drive
through. The center conducter was hung from huge ceramic insulators
similar to those used by the power company.

Jimmie

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

In article
,
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?

As others mentioned, the origin is probably based on military standards.

It is my understanding that the lowest loss air dielectric Co-Ax would
have an impedance of around 75 Ohms. If you use the same mechanical
dimensions but with a polyethylene dielectric, the impedance becomes 52
Ohms.

Fred
K4DII