Dear all, Here's a useful tip -

The loss along any sort of HF transmission line, SWR or not, increases with
line temperature according to just ONE HALF of the resistance-temperature
coefficient of the conductors. Why ? It's something to do with skin
effect.

RTC of copper is near enough 0.4 percent per degree C.

So loss along any line, in dB or nepers, increases by 0.2 percent per degree
C.

Now you may not think this matters very much. But if you consider a seasonal
change in temperature on the ocean bottom of only 2 degree C along a
2000-mile transatlantic cable which has an attenuation at 5 MHz of 1.6 dB
per mile, total attenuation = 3,200 dB, then you will appreciate the
responsibilty laid on the shoulders of the design engineers of the first
oceanic telephone cable systems.

An uncertainty of 0.4 percent of 3,200 dB = 13 dB which is enough to wreck
system operation. Omission of a submerged repeater allows signals to fall
below noise level at the last repeater. Addition of one more repeater
overloads the last repeater causing cross-modulation, cross-talk and noise.
Bear in mind repeaters are both-direction amplifiers and the lowest speech
channels are at 60 KHz where overall attenuation over the same distance is
only about 350 dB.

Repeater power is fedover the the inner cable conductor from constant
current sources at both ends, maximum voltage = 10 KV. +ve from one end, -ve
from the other. During magnetic storms and other disturbances the potential
difference between ground electrodes in N.America and W.Europe can rise to
several thousand volts. Although the last time I measured it on an AVO-8 it
was only 1.3 volts. I did, of course, make use of the safety grounding
stick before using the crocodile clips on the ends of the meter leads.
Depended on the tides and the flow of the Gulf Stream across the Earth's
magnetic line of force.

It always struck me as being highly incongruous, even absurd, that in normal
operation, cables of the highest possible quality materials, manufactured by
automatically controlled, specially-designed precision machinery, laid at
great expense by an 8000-ton, specially-design ship over thousands of miles,
should end up by being terminated with a foot or so of soldered, screwed-up,
cotton-covered 22-gauge wire rescued from the terminal-station scrap bin.
This is true. I have seen both ends with my own eyes. On one occasion I even
did the soldering after completing overall tests! But I was careful to use
a fairly straight length of wire with sufficient sag to eliminate any
possible tension beyond that due to its own weight.

Dear readers, believe me, there's no time to worry about SWR when loss in
revenue amounts to $100,000 per hour + repair-ship expenses every time a
flatfish trawler scoops up a cable in its net, cuts it free with an axe, and
the skipper sneaks away at top speed without telling anybody in case he has
to pay for the damage.

Coax cable Zo = 43 ohms. Diameter over polyethylene = 1 inch. Inner
conductor = longitudinal overlapping crimped copper tape, laid over the
cable's principal strength-member of a number of high-tensile steel wires,
overall inner diameter about 1/3 inch. Outer conductor = 6, touching,
longitudinal aluminium tapes with a small spiral lay. Sheath = 0.1-inch
thick extrusion of black polyethylene if I remember correctly. For shallow
water and continental shelves there was a number of protective heavy iron
wires laid on a bed of tarred hessian as had been used for 100 years on the
first of the Atlantic telegraph cables.

I sometimes think of (the later) Lord Kelvin who followed his calculations
with the recommendation to investors "Go ahead, make and lay the bloody
stuff". But it was Heaviside, a generation later, a genius who died of
neglect, who eventually described how the "bloody stuff" and radio
propagation really worked.

Folks, just a little light-hearted digression, a respite from so-called SWR
meters. Please continue with your discussion. ;o)
----
Reg, G4FGQ