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