Why 50 ohms?
 

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?

Why 50 ohms?
 

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

Why 50 ohms?
 

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

73

Tony I0JX

Why 50 ohms?
 

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.

Why 50 ohms?
 

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

Why 50 ohms?
 

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

Why 50 ohms?
 

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

Why 50 ohms?
 

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

Why 50 ohms?
 

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

Why 50 ohms?
 

Fred McKenzie wrote:
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.

loss, at constant frequency, is proportional to
1/Z * (1/b + 1/a)
b= diameter of outer , a = diam of inner

This reflects the fact that at lowish frequencies (HF), the loss is
ohmic, so it's the series combination of the resistivity of the center
and outer conductors (the 1/b+1/a term), and the current flowing (the
1/Z term.. higher Z means lower current, so less IR loss)



Z is proportional to log(b/a)
so
loss = k/log(b/a)*(1/b+1/a)

Run the numbers and you see that a ratio of 1:3.6 gets you 50 ohms with
epsilon=2.3 and a ratio of 1:6.7 gets you about 75 ohms.

At 10 MHz loss is about 0.028 dB/meter for the 50 ohm, and 0.017
dB/meter for the 75 ohm.

but you can go higher in Z...and the loss keeps going down, but even at
1:20 diameter ratio, the impedance is 118 ohms, and the loss is 0.010
dB/meter. So 75 isn't a "lowest loss" frequency. More likely, 75 ohms
happens to be close to 72 ohms, which is the characteristic impedance of
a dipole in free space, or, more usefully, 1/4 of the 300 ohm impedance
of a folded dipole.

In air, the dimensions and loss a
50 ohm 1:2.3 0.033 dB/meter
75 ohm 1:3.5 0.019 dB/meter

If you use the same mechanical
dimensions but with a polyethylene dielectric, the impedance becomes 52
Ohms.


I think that is just coincidence:

Z = 138/sqrt(epsilon)* log (b/a)

air has epsilon=1 (or very close)
polyethylene has 1/sqrt(epsilon) about 0.66, so, in fact, it happens to
work out (e.g. 50/75 = 0.66)