Higher antenna vs. transmission-line loss: an RF (and engineering) tradeoff example

11/21/2009 12:00 PM EST

Engineering is about tradeoffs, some obvious, some vague, some quantifiable, some qualitative, some based on judgment and sense of the situation. Here's one I just did some thinking about, though I haven't decided what to do–since the problem is not at all critical.

The over-the-air reception for my digital converter box ahead of my analog TV is decent, not great. (Yes, some of us don't have cable or dish or even a digital TV.) Although I am nearly to line-of-sight to various TV towers about 20 miles away (about 30 km) with just some trees in the way, I get image breakup and freeze on some of the lesser channels, especially when there is rain or snow.

Since my antenna is indoors in the attic (much easier to deal with than outside, for sure) I have been thinking about moving it to another part of the attic, or maybe moving it outside and up, to see if there is improvement. Right now, I have a run of about 100 feet (30 meters) of coax from the antenna to the converter connector, and I'd have to add another 50 feet (15 meters) if I moved it around inside, or perhaps another 100 feet if put it outside.

So the question becomes: is the improved signal strength I may get worth the additional loss in the transmission line? This is a simple, yet realistic, example of the kinds of tradeoffs engineers must make in their system and circuit design over and over.

I did some quick research and found that the typical loss for various coax cable types is roughly between 0.1 and 1 dB per 100 feet (about 0.15 and 1.5 dB per 50 meters) at 1 MHz, and goes up to between 5 and 15 dB at 1 GHz (about 7.5 to 22 dB per 50 meters); it varies depending on the type and quality of cable. I'll assume I am at the higher-loss side, it's actually hard to find better-quality cable locally (I have looked for it) and I'm not ready to buy it online, yet. The TV signal is in the 1-GHz range, of course, so we are at the higher end of the losses.

I haven't decided what to do, and it is very likely I will do nothing, since the solution may be more of a nuisance and cost in money and headache than the problem is worth to solve. And no, I won't looking to add a preamplifier at the antenna, since that involves more cost, plus a power supply, and assumes the problem is signal strength itself rather than SNR (a likely but not assured assumption).

My TV reception is a very modest problem, of course. But it reminds us that in engineering, as in life, it's often about tradeoffs in performance, effort, and cost. That's message engineers should both keep in mind, and make clear to others. ♦