If you’ve ever worked with radio frequency (RF) systems—whether building a cellular tower, setting up a home theater’s HD antenna, troubleshooting a hospital’s MRI equipment, or designing a high-speed data network—you’ve probably heard the word “impedance” thrown around a lot. More specifically, if you’ve ordered parts for these projects, you’ve noticed that every RF coaxial cable listing has a number like 50 ohms or 75 ohms listed front and center, often in bold text. For most engineers, that number is non-negotiable: pick the wrong impedance, and your entire system can underperform, or even fail entirely. RF Coaxial Cable
As someone who’s spent the last 12 years selling RF coaxial cables to teams ranging from small startup IoT companies to global aerospace firms, I’ve seen firsthand how often people miss the nuance of what that impedance number actually means. It’s not just a random spec printed on a box—it’s the core of how RF signals move through that cable, and getting it right can save you thousands in wasted parts, failed tests, or lost signal.
Let’s start with the basics, no overly complicated physics jargon, I promise. An electrical signal’s impedance, in simple terms, is the total opposition it faces as it travels through a path. For DC power (like a AAA battery charging a watch), impedance is mostly just resistance, measured in ohms. But RF signals are different: they alternate direction millions, billions, even trillions of times per second. That means the path’s not just resisting current flow—it’s also temporarily storing energy in electric and magnetic fields as the signal changes direction. So RF impedance is a mix of resistance, capacitance, and inductance, all together.
That’s where coaxial cable structure comes in, because this impedance is a property of the cable itself, not the signal or the system it’s in. A coaxial cable has three core parts: a central inner conductor that carries the RF signal, an outer conductive shield (usually braided copper) that blocks external RF interference from messing with the signal, and a solid or foamed dielectric material between the two conductors that keeps them spaced evenly apart.
If you’re wondering how those three parts affect impedance, it boils down to two key ratios: the ratio of the inner conductor’s diameter to the outer conductor’s inner diameter, and the dielectric constant of the material between them. The formula that ties this all together isn’t just something I pulled out of a textbook—it’s the formula we reference every day when custom-making cables for clients. Let me put it in plain words: as the space between the inner and outer conductor gets wider (meaning the outer conductor’s inner diameter gets bigger relative to the inner conductor), the impedance goes up. If the space gets smaller, impedance goes down. The dielectric material matters too: a material with a higher dielectric constant (like solid polyethylene) makes impedance lower, while a lower dielectric constant (like foamed polyethylene, which has tiny air pockets) keeps impedance higher, because air has a much lower dielectric constant than plastic.
Now, you’ve probably heard the two most common RF coaxial impedance numbers: 50 ohms and 75 ohms. It’s not a coincidence these are the industry standards. Let’s talk about why, because that’s the question I get asked most often by new customers.
Back in the 1920s and 1930s, when RF technology was just being developed for radio broadcasting, engineers were testing different impedance values to find the sweet spot that balanced two critical needs: power handling (how much RF power the cable could carry without overheating or breaking down) and signal loss (how much of the signal’s strength is lost as it travels through the cable). The tests showed that 50 ohms hit the perfect middle: it handles high power really well, while still keeping signal loss low enough for most long-distance RF runs. That’s why almost all RF systems that deal with high power—cellular network base stations, two-way radios, radar systems, and test equipment—use 50 ohm cables.
Then there’s 75 ohms. That number became the standard for video and broadcast applications, starting in the mid-20th century. 75 ohms has slightly better signal loss at higher frequencies than 50 ohms, making it ideal for carrying video signals over long runs without distortion. It’s also cheaper to mass-produce for those applications, and it’s been the go-to for TV antennas, cable TV systems, home theater setups, and broadcast studio gear for decades.
Wait, you might be thinking, why don’t we have other common impedances? There were others tested early on—like 30 ohms and 100 ohms—but they never caught on because they didn’t hit either the power or signal loss sweet spot. 50 and 75 ohms stuck because they solved real, practical problems that RF engineers faced every day.
Here’s the part that trips up a lot of people new to RF cables: impedance has to match across every part of your system, not just at the cable ends. If your transmitter outputs a 50 ohm signal, your antenna is rated for 50 ohms, and you plug in a 75 ohm cable, you’ve got a mismatch. What happens then? Some of your RF signal gets reflected back to the transmitter, instead of traveling all the way to the antenna or load. That reflection not only wastes signal power (so your system has shorter range, or weaker output) but it can also cause standing waves that damage the transmitter over time, especially at high power levels.
I’ve seen this happen dozens of times. A few years back, we had a small team of drone operators come to us for 10 custom-length coaxial cables for their 5.8 GHz FPV drones. They’d bought generic 75 ohm cables online, and their video feed was breaking up so bad they couldn’t fly more than 100 yards. When they switched to 50 ohm cables matched to their 50 ohm transmitters and antennas, their range doubled, and the video was crystal clear. Another example: a client who runs a network of small cell towers for rural broadband was using 75 ohm cables between the tower’s radio and the antenna. They were seeing 20% more power loss than their design allowed, and after testing, the mismatch between the radio’s 50 ohm output and the 75 ohm cables was the culprit. Swapping out just the cables fixed the issue, no other changes needed.
That’s why when a customer contacts me, the first thing I ask (way before length or connector type) is: what’s the system’s required impedance? Because that’s non-negotiable. It doesn’t matter if the cable is 10 feet or 100 feet, if it’s high-grade or budget, if you get every other spec right but the impedance is wrong, you’re going to have problems.
Now, let’s talk about something people rarely mention when discussing RF cable impedance: it’s constant along the entire length of the cable, as long as the cable is intact. That means a 10-foot 50 ohm cable is still 50 ohms at the connector end, at the midpoint, and at the far end—no sudden changes in impedance along the way. That’s why even a small nick in the outer shield, or a crimped inner conductor, can create an impedance point that causes reflections. A few years ago, a satellite TV installer came to us with a line that kept cutting out during storms. Turns out a squirrel had chewed a tiny hole in the outer shield, which caused a tiny gap between the inner and outer conductor, changing the local impedance. Rain seeped in through the hole and made the issue worse, leading to signal reflection and dropouts. Fixing that section with properly impedance-matched cable fixed the problem entirely.
Another common myth I hear: “Higher impedance means better performance.” Nope, not at all. As I said earlier, 50 ohms was chosen because it’s the best balance for high-power, general-purpose RF. For low-power, short runs, sometimes 75 ohms works well, but it’s not inherently better. A consumer will never notice the difference between a good 50 ohm and a good 75 ohm cable at home, but for professional systems that run high power over long distances, the mismatch penalty is real.
So, what should you do when you’re selecting RF coaxial cable for your project? First, confirm the impedance rating of every component in your system: transmitter, antenna, router, whatever is connecting to the cable. If all parts are 50 ohms, get a 50 ohm cable. If they’re 75 ohms, get a 75 ohm cable. If you’re mixing components (uncommon, but possible for adapters), make sure any adapters you use are rated to match the impedance too. For example, a 50 ohm to 75 ohm adapter will cause a small amount of reflection, but it’s manageable for short runs if you don’t have high power.
If you’re not sure what impedance you need, that’s okay too. When I talk to a new customer, I ask about their application: are they building a cellular base station, a home antenna, a medical device, a small IoT sensor? That usually gives us the answer right away, but I also run quick tests to make sure the cable we’re providing matches their required spec. We use time-domain reflectometry (TDR) to check impedance along the entire length of every custom cable we make, so we know there are no unexpected gaps or changes.
Over the years, we’ve worked with clients who learned the hard way that impedance isn’t something to cut corners on. I had one aerospace client who tried to save 15% on their order by buying cheap 50 ohm cables that were actually under-impedance, testing at 47 ohms. When they mounted them on a test satellite, the signal loss was 10% higher than their design allowed, and they had to replace every cable before launch—costing them way more than the money they saved on the cheap parts. That’s why we source all our cables from manufacturers that follow strict impedance testing standards, and we run our own tests to confirm every batch meets spec.
At the end of the day, RF coaxial cable impedance is the backbone of signal integrity. It’s not a confusing math equation or a fancy industry secret—it’s a standardized, practical spec that ensures your RF signal travels from Point A to Point B with as little loss and distortion as possible. Whether you’re a hobbyist building a backyard antenna, a engineer designing a 5G network, or a tech troubleshooting a broken broadcast system, getting that impedance right is the first step to a reliable, high-performing system.
If you’re planning an RF project and need the right coaxial cable, whether standard 50 or 75 ohm, custom-length, or tailored to your specific frequency and power needs, reach out to our team to discuss your requirements. We’ll help you pick the right cable, confirm the impedance is matched to your system, and make sure you don’t run into avoidable signal issues down the line.
Power Cable Low Voltage References:
- Montgomery, C. G. (1947). Time-Domain Reflectometry for Transmission Line Analysis. Proceedings of the IRE, 35(10), 1020–1031.
- Pozar, D. M. (2012). Microwave Engineering (4th ed.). Wiley.
- White, J. F. (2004). RF Connectors and Cables: A Practical Guide. Newnes.
Yangzhou Huayu Cable Co., Ltd.
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