Thursday, July 1, 2010

Can Cable TV coax be used for Amateur Radio applications? ...

Q Ancel, W4KF, asks, “Can Cable TV coax be used for Amateur Radio applications?”

A Cable TV coax is, for the most part, RG-6, RG-59 and a few varieties of hardline. All of these cables have characteristic impedances of 75 Ω as opposed to the 50 Ω feed lines that hams use.

Does this mean that you cannot use Cable TV coax? Not at all. Assuming you have a modern-day rig, its output network is designed to accommodate a 50-Ω load, but the network can usually handle a 75 Ω impedance with little difficulty (you may see an SWR of approximately 1.5:1). Most dipole antennas, for example, have feed point impedances of 50 to 100 Ω, depending on a number of factors such as height above ground. So, a 75 Ω feed line should present a reasonably good match.

So why don’t hams use Cable TV coax? The answer is that some do, but there are several factors that make the traditional 50 Ω cables more attractive. Cable TV coax uses an aluminum shield and it is often difficult to solder. And as I’ve already mentioned, the 75 Ω impedance isn’t suitable for all amateur antennas. Having said all that, the price of Cable TV coax is often too low to refuse—as in free! Your local cable company often has “remnant” spools containing 100 feet or more of coax that they may be willing to sell at a nominal fee, or simply give away.

From QST August 1999