A couple of weeks ago, a neighbor asked me a weird question. It came right out of the blue. They know that I test a lot of antennas, and that I’m sort of a local expert on this stuff. So they asked me, “Do I need a 75 ohm antenna or a 300 ohm antenna?”
It took me a little while to suss out what they really meant. They meant, would the antenna on their roof, which still used that flat wire, be enough for them. Otherwise they would buy a new one. At some point they had picked up the idea that the flat wire has an impedance of 300 ohms and that coax cable has an impedance of 75 ohms. Points for that, definitely. I was able to help them out and it turned out that old antenna on the roof wasn’t worth saving. But it wasn’t because of the wire it used.
There are no stupid questions
I don’t blame people for asking questions. I don’t even blame them for asking dumb questions. After all, that’s how we learn. No one, myself included, was born knowing everything there is to know about antennas or anything else. I had the help of some very patient people to get me to the point I’m at now.
Still, when someone says, “do you sell 75-ohm antennas,” it does make me shake my head a little bit. And then I listen and try to figure out what they’re actually asking.
What’s the difference between 75 ohm and 300 ohm anyway?
First of all a word about impedance. Impedance is a measurement of how effective a material (like a cable or antenna) is at sending power through it. More precisely, it’s a measure of the total ability that a material is at stopping power from being sent through it. That’s a very simple explanation but it will suit for now. Really understanding impedance in a real-world way is a lot harder than you think. It’s only important to understand that it’s something you have to measure, and everything in your wiring has to work together or it gets really messed up.
The native impedance of most antennas is about 300 ohms. This is a factor of the way transmissions happen and the material used to receive them. There isn’t really such a thing as a “75 ohm antenna.” What happens is actually a bit sneakier.
Enter the balun
Modern antennas use a device called a balun that serves two purposes. First and foremost, it converts a BALanced line to an UNbalanced line, in other words, it lets you use a coaxial cable instead of those flat wires everyone used to use when your dad put up your TV antenna in the 1970s (or before.) It also contains a small transformer that decreases the impedance by a factor of 4 (for most antennas) or by a factor of 6 (for some commercial antennas.) The result is that the 300 ohm impedance is reduced to 75 or 50 ohms which is a much more compatible result for copper coaxial cable.
Almost every antenna made today has this balun built into it. It’s been pointed out to me numerous times that in many cases that flat wire does a better job of getting signals from point A to point B. However, coaxial cable is better at shielding signals from interference. In the real world I wouldn’t say it makes much difference, except to say that pretty much everyone uses coaxial cable today.
So, I guess it does sort of seem like today’s antennas are 75-ohm antennas, because that’s what they output. They’re not, but I guess that isn’t the point. What is the point? The point is that if you’re looking for the right antenna for your needs, the perfect place to look is SolidSignal.com, where you’ll find an antenna for virtually every need.
