Latin America. If you work with coaxial cable, you probably know that it is available in a number of different impedances. The most common is 75 ohms, as a video cable, but in fact our products range from 32 ohms to 124 ohms. Why so much difference?
It's not an accident, of course, and there's a reason for each. Let's take a quick look at the 50 ohm coaxial. Belden manufactures hundreds of 50-ohm cables, including an entire line with ultra-low losses (Belden 7805 to Belden 7977). The two larger versions (Belden 7976 and 7977) are huge. The 7977 has a diameter of 15mm. It's the biggest coaxial cable we make.
But first of all, why 50, or any other number? It was demonstrated by two researchers, Lloyd Espenscheid and Herman Affel, working for Bell Labs in 1929. They had to send RF signals (4 MHz) to hundreds of kilometers with thousands of phone calls. They needed a cable that carried high voltage and high power.
For high voltage, the perfect impedance is 60 ohms. For high power, the perfect impedance is 30 ohms. This means, clearly, that there is no perfect impedance to do both.
Which ended with a commitment number, and that number was 50 ohms. 50 ohms is closer to 60 than 30, and that's because voltage is the factor that will kill your wire. Ask any transmitting engineer. They talk about VSWR, standing wave ratio, all the time. If your coax drops, voltage is to blame.
Why not 60 ohms? Just look at the feed management at 60 ohms - below 50%. It's horrible! With a commitment value of 50 ohms, the power improves a bit. So 50 ohm cables are meant to be used to carry energy and voltage, like the output of a transmitter.
If you have a small signal like video, the graph shows that the lowest loss or attenuation is 75 ohms. However, I get a lot of feedback from people who use 50 ohms for small signals; Although they are suffering an attenuation of 2-3 dB. The excuses I hear are "It's too late to change now!" Or "That's the impedance of the box itself." This is only true in most test equipment, which is universally 50 ohms.
Belden 7977 can carry more than 5 kW at 30 MHz and more than 600 watts at 6 GHz. So even a cable of this caliber can be used for low-power TV or FM, boosters, two-way radios, security alarms, "ham" frequencies, microwaves up to 6 GHz and many other applications where the signal is being delivered with high voltage and high power.
Most of the time, these signals end up in antennas. For example, sections in transmitters where exciter power is available also require 50 ohm cable. It is surely where the smallest 50 ohm cable can be used.
Many of these cables come in three versions: for outdoor applications, for indoor applications (riser) and for applications with "water blocking" (underground or water). Some are certified even for boats. These onboard versions are also LSZH (low-smoke zero-halogen), which is often a legal requirement in some countries.
Text written by Steve Lampen, Belden product manager.
