I got a friend in Montana who wants to lay a video cable across his Ponderosa. I think I heard somewhere that video signal degrades over distance, but if this is true, how does the cable company <spit on ground> lay cable across the city? Are there boosters or something?
And what about cable weight? I have seen spy cameras with skinny cable, how come I gotta use the fat coaxial line commonly found in hardware stores?
Any cable jockeys out there?
Myself
All electronic signals lose strength over distance. The cable company’s main trunk lines transmit at a much higher voltage to compensate. They then go through transformers to regular lines, which have repeaters along the way to boost the signal up.
The cable company may well be using optical fibre anyway, which suffers much less from signal loss & carries more information too. These too will have signal regeneration units and amplifiers at suitable distances anyway, just further apart than for electrical signal cables.
Fibre optics are also immune to interference from surrounding electrical equipment, unlike electrical signal cable. So that is another point to bear in mind - whether there is any other machinery/cables around where your friend wants to lay the cable.
More conducting metal = lower internal resistance = less signal degradation per cable length. Skinny cable is OK for a short distance, but not for crossing the Ponderosa.
Coaxial cables are almost completely immune to electrical interference.* The signal is carried between the inner and outer conductors, and the outer conductor shields the signal from outside interference.
(*) If it’s installed correctly, anyway. Also, if it get’s hit by lightning, all bets are off.
friedo wrote:
The cable company doesn’t use transformers like in electric power transmission. A transformer converts high-voltage, low-current to low-voltage, high-current, which means that the impedance is converted by the square of the transformer ratio.
The cable company uses 75-ohm coaxial transmission lines with repeating amplifiers. Coaxial because it’s nearly immune to interference (either causing it or being susceptible to it), and 75-ohm because that makes for the least power loss in a coax cable. 75-ohm is not the best, however, for handling large amounts of power, because the skinny center conductor concentrates the electric field too much around it - something like 30 ohms would be best for power handling, that’s why general-purpose coaxial is often 50 ohms, which is a compromise between the two.
Also, the cable company uses some pretty hefty cable out there on the pole (not on the drop, that’s standard RG-6). I remember when they were installing it in my area, probably about 25 years ago - heavy, solid aluminum shield (essentially an aluminum pipe), with a thick (12 gauge?) copper center conductor. I believe the diameter of the cable was more than 1/2".
Additionally, in a spy satellite electrical components will use higher grade materials. A gold wire is superior to a copper/aluminum one, but is not practical for cable television.
make that spy cameras, rather than spy satellites
He’ll need signal amplifiers but will have to accept some signal degradation if he sends it too far, anyway, because amplifiers, obviously, amplify whatever analog signal they receive and that can include introduced noise. Good amps will minimize that, but…
As usual, I will need weeks of reading followed by days of thought…you guys are the best.
Thanks,
Myself
I have a great quote on this desk somewhere, give me a minute.
Are you sure that spy cameras don’t use some sort of fiber optic line from the lens to the actual camera, instead of having the camera on the end of the line?
I know they can make cameras pretty small, but thatsmall?