I've been reading a lot about early trans-Atlantic telegraph cables, circa 1858. These cables only transmitted a few words per minute, and because they had no in-line amplifiers, *all* the energy for the signal had to be supplied from the transmitting end. Because the cables were so long and small, there was tremendous attenuation - so that a sensitive mirror galvanometer had to be used to detect the signal. Looking at the specs of the first trans-Atlantic cable, it would be the a little smaller than a No.12 AWG wire (only 80 lbs of copper per statute mile!). WIth today's copper standard this would be about 20,000 ohms of resistance one-way - and copper wire of that era was made for mechanical, not electrical, purposes and could vary in resistance by a factor of 2 in any given run. I haven't seen or worked out what the total capacitance was, but assuming it was comparable to Belden 9913 what *is* the dielectric constant of gutta-percha, anyway?), I get over
200 microfarads of total capacitance. This goes a long way to explaining the slow keying rates.I then got to thinking how well radio waves run through a cable. Supposing you bought a couple of miles of Belden 9913, a low-loss, premium grade coaxial cable. Suppose you hooked up a pair of those little FRS radios to the cable. They put out about 1/2 watt - call this -3 dB(W). I don't know what the sensitivity of these little guys is, but it can't be much less than 1 uV in 50 ohms - which works out to -136 dBW. So, an FRS radio has a power budget of 133 dB to burn.
Here's the part that surprised me...if you hooked up a pair of "2-mile" radios to this very good grade co-ax, you wouldn't even get 1 mile range! The cable has an attenuation at 400 MHZ of 2.7 dB in 100 feet - so you would use up the whole 133 dB of power margin in only 4900 feet. (And in free space the "2-mile" FRS radios are good for a lot more than two miles.)
Well, I thought it was interesting...until doing this calculation I think I would have guessed that the range in a cable would be better than free space.
Bill
( Check out John Griesemer's "Signal & Noise: A Novel" for the epic brawling saga of the men who laid the trans-Atlantic cable and the women who loved them...cliched, but accurate description. Would make a great movie, though there may not be enough telegraphers and electrical engineers out there to make it a profitable movie)