Reg:

I am not after your scalp, trust me...

However, as I ready for bed, I was thinking--on the age of my coax...
although some may be as new as 3 years old... most is greater than 5, and I
bet the run to my 1/2 vertical is 20 years or better....

Sometime in the past, I remember reviewing data on loss in coax going up
with age.... not that it would amount to an important loss... but still, it
must be a measurable amount...

Oh, and strange how this all keeps touching on the matter I am constantly
holding at hand... but that "skin effect"... seems like copper becomes an
"impedance" at high freqs.... those little electrons in the wire just can't
keep pumping the charge fast enough... seems like that old rf there is
considering the ether itself (dielectric in coax) as a better choice of
travel than the copper atoms...

Warmest regards,
John
--
If "God"--expecting an angel... if evolution--expecting an alien... just
wondering if I will be able to tell the difference!

"Reg Edwards" wrote in message
...
| The number one reason for attenuation being higher is because the
| conductor diameter is smaller and, as a consequence, its resistance
| is
| higher.
|
| The exact simple mathematical relationship is -
|
| Line attenuation = 8.69*R/2/Ro dB.
|
| Where R is the resistance of the wire and Ro is the real component
| of
| line impedance, all in ohms.
|
| Make a note of it in your notebooks.
|
| And, hopefully, that should be the end of the matter. But, knowing
| you lot, it probably won't be. ;o)
| ----
| Reg, G4FGQ
|
| ================================
|
| To you all.
|
| As predicted, I appear to have stirred up a hornet's nest.
|
| First of all, give credit to where credit is due. The simple equation
| is not due to me but to Oliver Heaviside, 1850 - 1925. May God rest
| his soul. And mine!
|
| It applies from DC to VHF where the predominent loss is due to
| conductor resistance including skin effect. At higher frequencies, say
| above 0.5 GHz, loss in the dielectric material begins to play an
| important part.
|
| The complete equation is -
|
| Attenuation = R/2/Ro + G*Ro/2 Nepers
|
| where G is the conductance of the dielectric, which is small for
| materials such as polyethylene and Teflon. And 1 Neper = 20/Ln(10) =
| 8.686 dB.
|
| The Neper is the fundamental unit of transmission loss per unit length
| of line, familiar to transmission line engineers. It is named after
| Napier, a canny Scotsman who had something to do with the invention of
| Logarithms around the 18th Century.
|
| Attenuation is simply the basic matched loss of a particular line,
| unaffected by SWR and all the other encumbrances which amateurs such
| as W5DXP ;o) worry about. KISS.
|
| Incidentally, the additional-loss versus SWR curves, published in the
| ARRL books and copied by the RSGB, for many years, are based on an
| incorrect mathematical analysis. But they are near enough for
| practical purposes.
|
| Not that SWR matters very much. SWR meters don't measure SWR on any
| line anyway. You are all being fooled. ;o) ;o) ;o)
| ----
| Reg, G4FGQ
|
|

I have never heard of ageing effects in copper or polyethylene - or
ancient ebonite spacers even.

The very first coaxial carrier communications cable was laid in Great
Britain by the Post Office, around 1937, between the cities of
Manchester and Leeds. There were 4 coaxial tubes inside a lead
sheath. Outer conductors = 0.375". Inner conductors = 0.1", which
later became the standard. Mostly air spaced. Inner conductors were
supported by ebonite disks or similar material, spaced at about 1.5".
Polyethylene was still waiting to be invented. Working frequencies
from 60 kHz to about 2 MHz. Repeater spacing about 5 miles.

Around 1960 I had the opportunity to test sections of this cable. As
far as I could judge it was in perfect working order. Bear in mind it
is possible to detect small changes in attenuation only by looping
back on very long lengths. It cannot be done in the lab.

I imagine coax, with temperature expansion and contraction, very
slowly 'breathes' through the ends and draws in humid atmospheric
pollution. Perhaps after 50 years it may have some minute detectable
effect on attenuation and appearance. Attenuation is the last
parameter to fail. Far more serious things have to happen to a
transmission line before loss becomes noticeable.

For example, a coax line can be almost flattened with a hammer over a
length of several feet which will make a shocking mess of impedance.
Yet, provided the inner and outer conductors are not in contact with
each other, additional loss will be undetectable.

=========================

Nothing happens to metallic copper with frequency. But copper
conductors also have internal inductance in addition to conductance.
Inductive reactance increases with frequency. The increase in
inductive reactance begins at the centre of the conductor and drives
the current outwards towards the surface or perimeter. At
sufficiently high frequencies the current is forced to flow only on
the conductor's skin.

The conductance of copper remains the same. But the cross-section of
the conductor allowed to the current is very much reduced and so the
effective resistance per unit length increases together with the
inductive reactance.

It's an interesting fact that at frequencies where skin effect is
fully operative, conductor inductive reactance and resistance become
equal to each other. Measure one and you also know the other.
----
Reg, G4FGQ

I should have mentioned, the Manchester-Leeds Number 1 Coaxial Cable
had an impedance of 75 ohms. The impedance at which, for a given
price of copper, in those far-off days, had the lowest attenuation per
mile. 75 ohms has stuck as the Standard..

The distance between Manchester (then the centre of the cotton
industry) and Leeds (then the centre of the woolen industry), by road,
over the beautiful Lancashire and Yorkshire moors, is about 40 English
miles. By correct choice of impedance the conscientious engineers of
that age could have saved as much as £5,000 per mile in the price of
copper, to be formed in the manufactories into copper tapes for outer
coaxial conductors, and drawing copper wire from 3-ton billet-form
down to exact precision-size wire through water-cooled diamond dies.
It was and still is a precision manufacturing industry.

More savings occur in the distance between repeater stations. If
attenuation performance requirements can be met with one fewer
repeater station, the cost of a whole building, power supplies and
transmission equipment can be saved.

Although communications have shifted to digital, cables still matter.
But eventually optical fibers will take over the long distance
communications.

Radio Amateurs, with a little money to burn, never become involved
with such mundane matters. They are more interested in what they
imagine the SWR meter tells them. But if that keeps them happy then
so be it. I am an amateur myself. I have a call sign which sounds
very nice in morse code. Why should I disillusion them?
----
Reg, G4FGQ

Whilst on the romantic subject of coaxial attenuation -

Attenuation is the number-one characteristic of all transmission
lines. From power frequencies and upwards. Yet, quantitativly, it is
the smallest parameter per mile and the most difficult to measure
accurately. An innocent observer might think it is hardly worth
bothering about.

It is inextricably mixed up with system economics. An exact knowledge
even of the temperature coefficient of attenuation is vital to
communications system design.

Around the 1950's I was involved with measurement of attenuation (and
other characteristics) of the first oceanic submarine telephone
cables. A transatlantic coaxial cable, 2000 miles long, has an overall
attenuation at 5 MHz of around 4000 decibels. The temperature
coefficient of attenuation is half of the resistance temperature
coefficient of copper which is 0.4 percent per degree C. Which was
well known to Oliver Heaviside around 1875.

Thus, a change in temperature on the ocean bottom of 0.1 degree
results in a change of 80 dB in the signal level at the far end.
Unless corrected in the repeaters (of which there were about 100) this
is enough to shift signals between the thermal noise level of an
amplifier and its overloaded cross-modulation level.

One of the cable factories was located in Southampton Docks. As cable
came off the production machinery at about 1 mile per hour, it was
coiled in giant circular concrete tanks below ground level, the same
size as an 8000-ton cable-laying ship's hold. Attenuation and other
measurements were made in the tanks by automatic testing equipment.
The cable was then loaded onto a cable ship waiting for it in the
nearby dock.

I designed a special attenuation and phase-shift tester for research
purposes. It did not incorporate an SWR meter. It did incorporate a
phase-locked-loop but it was not until several years later that I came
by chance upon a learned paper by Gruen and discovered how a PLL
really works. The equipment was all tubes. A whole mobile rack of it!
The final output meter was a moving-coil instrument with a scale
calibrated in 0.001 decibels. There were also home-brewed 0.001 dB
stepped attenuators which I had to calibrate myself.

To determine attenuation temperature coefficients in was necessary to
bring tons of ice by lorry from Billingsgate fish market in central
London. One of the concrete tanks was flooded with sea water and the
ice dumped in. It took 24 hours for the temperature to stabilise. I
spent much of the waiting time in a pub in Southampton Town. I never
knew who organised and paid for delivery of the ice which must have
been the most intricate and illegal part of the whole operation.

The data accumulated was rushed to the boffins who immediately began
designing even higher frequency oceanic systems. I was rewarded with a
pat on the back and told to keep my mouth shut. Politics were
involved somewhere.

My tester should have eventually been installed in the Science Museum,
Kensington, London. But long after the job was finished it was stolen
by some unfeeling person and cannibalised for the spare parts. There
was a BC221, straight-line frequency variable tuning capacitor built
into it. I would have liked that for myself.

Hope you enjoyed the story. From what I remember it's mostly true.
Southampton makes a change from Manchester and Leeds. The Queen Mary
was berthed not far from the Cable Ship Monarch.
----
Reg, G4FGQ

Reg Edwards wrote:

A transatlantic coaxial cable, 2000 miles long, has an overall
attenuation at 5 MHz of around 4000 decibels. . .

Just to get a little context here. . .

Years ago when I was a little bored, I determined that the ratio of the
light output from a common two cell flashlight to the entire light
output of the Sun is a mere 280 dB (10^28). So if you attenuate the Sun
by 280 dB you get the light of a flashlight beam. Well now, if you took
that flashlight beam and attenuated it again by the same amount, then
did that again, and again, 14 times altogether, you still wouldn't quite
have totaled 4000 dB. It's a staggering number, incomprehensible except
by some pretty abstract thinking. It's real, though. I remember reading
a paper long ago about transatlantic cables, and those are the numbers
they work with.

Roy Lewallen, W7EL

Really, to all you guys:

There is sense, and there is non-sense here...

Never doubted you ALL had the the sense, just pleased you can enjoy a bit of
non-sense...

And, yes, the first time I found out I had to increase effective radiated
power by a factor of 4 to achieve a factor of 2 on someones S-Meter--I was
disapointed--not sure I have fully recoved from the meaning of that to this
very day--frankly, I expected more... I expect if I consulted a
psychiatrist on all this--he would, most likely, chalk it up to "penis
envy"... and that is why I have not... grin

Warmest regards,
John

"Roy Lewallen" wrote in message
...
Reg Edwards wrote:

A transatlantic coaxial cable, 2000 miles long, has an overall
attenuation at 5 MHz of around 4000 decibels. . .

Just to get a little context here. . .

Years ago when I was a little bored, I determined that the ratio of the
light output from a common two cell flashlight to the entire light output
of the Sun is a mere 280 dB (10^28). So if you attenuate the Sun by 280 dB
you get the light of a flashlight beam. Well now, if you took that
flashlight beam and attenuated it again by the same amount, then did that
again, and again, 14 times altogether, you still wouldn't quite have
totaled 4000 dB. It's a staggering number, incomprehensible except by some
pretty abstract thinking. It's real, though. I remember reading a paper
long ago about transatlantic cables, and those are the numbers they work
with.

Roy Lewallen, W7EL

Reg Edwards wrote:
Hope you enjoyed the story.

That's a really enjoyable story, Reg. Thanks for sharing.
During that time I was involved in smuggling operations -
smuggling girls into my Texas A&M dorm. :-) Today, there
are girls all over the Texas A&M dorms.
--
73, Cecil http://www.qsl.net/w5dxp


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http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
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Reg Edwards wrote:
Whilst on the romantic subject of coaxial attenuation -

A follow on from Reg's story, is that (in submarine the cables with
which I was familiar), there was another grade of cable also used, a
repair cable that had about two thirds the loss of the regular cable.

It was used when repair was necessary (eg repeater failure, cable damage
from anchors , earthquakes etc). It is not possible to effect a repair
without cutting the original cable in order to get the cable to the surface.

The technique used on CS Monarch and the like, was to use a special
grapnell that caught the cable (often after many days of steaming back
and forth across the suspected cable position) and hauled it up until a
pre-determine tension was reached which activated a cutter in the
grapnell, which separated each end off on a separate hauling line. One
was buoyed off, and the CS steamed toward the other cable end until
sound cable was retrieved. It was sealed and buoyed off. They then
steamed back to the other buoy and retreived the other end, again
steaming until sound cable was found. Calculations were then done of
depth, position, losses to find how much more cable was to be removed so
that when the repair cable was inserted, the S/N into each of the
affected repeaters was sufficient to allow normal operation (these were
linear FDM or carrier telephone cables). (In some cases, so much cable
was affected that a mix of original cable and repair cable was used.)

This operation could take several days in good weather, worse in bad seas.

Owen

PS: 4000 dB sounds a lot, but when it is stated as 40dB between
repeaters it sounds more manageable.

Reg, G4FGQ wrote:
"A transatlantic coaxial cable, 2000 miles long, has an overall
attenuation at 5 MHz of around 4000 decibels."

That sounds reasonable as it is only 2 dB per mile. A mile is 52.8
increments of 100 feet, so that would produce about 0.038 dB/100 feet.

The lowest loss 75-ohm cable I found listed in the ARRL Antenna Book
table is 7/8-inch Hard-line at about 0.1 dB/100 feet at 5 MHz. For an
intercontinental link, you would strive for better as Reg indicated.

One problem of cable is that it has constant loss. Every cebtimeter of
length takes the same percentage loss of the remaining energy. Hence,
dB/ 100 feet.

Not so with radio in free-space. Getting rid of the wires ends their
attenuation. Loss is then due to decreased signal in a square unit of
the wavefront caused by expansion or thinning of the signal. The
"unattenuated" signal decline is 6 dB every time distance from the
source doubles, be it one mile or 1000 miles. The signal power level at
a point is 1/4 the power it had for the same area at 1/2 the distance
from the source.

We could`not communicate by wire with our space probes due to too much
loss even were the wires a practical alternative.

To cross an ocean, cable solves the problem of repeater placement. The
signal must be regenerated before it falls into noise. The repeaters are
"simply" integrated into the cable at proper intervals. The first
transatlantic cable message was sent by Queen Victoria to the American
President.

Ashore and on distant offshore platforms, I`ve puzzled why microwave was
not used instead of cable. Privacy may be one reason, but encryption,
route switching and other techniques could make theft of information
from thin air more difficult than other theft. There are always
beneficiaries of the status quo who make change difficult to impose.

In the early 1950`s, Houston`s Transcontinental Gas Pipeline Company
(Ken Lay was an officer of "Transco" before moving to Enron) built a
private microwave system from its heasdquarters to New Jersey along its
pipeline. I recall looking the new system over. It was supplied by
Philco Corporation and used Pulse Code Modulation, if I recall. The
microwave system was sold to a communications common carrier (now
Sprint) after a few years but it is still in service, I believe. Transco
(now Williams Pipeline Company) is one of many subscribers to the
service I believe.

Microwave repeaters located at about 20-mile intervals can provide
low-noise and high-reliability communications when properly designed.

In the 1950`s, I marveled as I commuted to work on a stretch of road
which ran between Lisbon and O`Porto, watching the Portuguese Post,
Telephone, and Telegraph Company laying coaxial cable alongside.

Cable is more vulnerable to damage, harder to repair, and surely costs
more than microwave. It was none of my business. I was a foreigner in
their country.

Best regards, Richard Harrison, KB5WZI

Cable doesn't fade from atmospherics.

--

73
Hank WD5JFR
"Richard Harrison" wrote in message
...
Reg, G4FGQ wrote:
"A transatlantic coaxial cable, 2000 miles long, has an overall
attenuation at 5 MHz of around 4000 decibels."

That sounds reasonable as it is only 2 dB per mile. A mile is 52.8
increments of 100 feet, so that would produce about 0.038 dB/100 feet.

The lowest loss 75-ohm cable I found listed in the ARRL Antenna Book
table is 7/8-inch Hard-line at about 0.1 dB/100 feet at 5 MHz. For an
intercontinental link, you would strive for better as Reg indicated.

One problem of cable is that it has constant loss. Every cebtimeter of
length takes the same percentage loss of the remaining energy. Hence,
dB/ 100 feet.

Not so with radio in free-space. Getting rid of the wires ends their
attenuation. Loss is then due to decreased signal in a square unit of
the wavefront caused by expansion or thinning of the signal. The
"unattenuated" signal decline is 6 dB every time distance from the
source doubles, be it one mile or 1000 miles. The signal power level at
a point is 1/4 the power it had for the same area at 1/2 the distance
from the source.

We could`not communicate by wire with our space probes due to too much
loss even were the wires a practical alternative.

To cross an ocean, cable solves the problem of repeater placement. The
signal must be regenerated before it falls into noise. The repeaters are
"simply" integrated into the cable at proper intervals. The first
transatlantic cable message was sent by Queen Victoria to the American
President.

Ashore and on distant offshore platforms, I`ve puzzled why microwave was
not used instead of cable. Privacy may be one reason, but encryption,
route switching and other techniques could make theft of information
from thin air more difficult than other theft. There are always
beneficiaries of the status quo who make change difficult to impose.

In the early 1950`s, Houston`s Transcontinental Gas Pipeline Company
(Ken Lay was an officer of "Transco" before moving to Enron) built a
private microwave system from its heasdquarters to New Jersey along its
pipeline. I recall looking the new system over. It was supplied by
Philco Corporation and used Pulse Code Modulation, if I recall. The
microwave system was sold to a communications common carrier (now
Sprint) after a few years but it is still in service, I believe. Transco
(now Williams Pipeline Company) is one of many subscribers to the
service I believe.

Microwave repeaters located at about 20-mile intervals can provide
low-noise and high-reliability communications when properly designed.

In the 1950`s, I marveled as I commuted to work on a stretch of road
which ran between Lisbon and O`Porto, watching the Portuguese Post,
Telephone, and Telegraph Company laying coaxial cable alongside.

Cable is more vulnerable to damage, harder to repair, and surely costs
more than microwave. It was none of my business. I was a foreigner in
their country.

Best regards, Richard Harrison, KB5WZI