Coaxial cables do a pretty good job carrying signals, but let’s be real—no cable is perfect. As your signal travels down the line, some of its energy just slips away as heat or leaks out.
Cable length and signal frequency both play big roles in how much energy you lose. Higher frequencies usually get hit harder by attenuation, which can chip away at signal strength and mess with your system’s performance.
You get loss in coaxial cables from a few places. Resistance in the conductors, energy sucked up by the insulating dielectric, and a bit of radiation sneaking out through the shield all contribute.
These effects don’t all act the same way at every frequency. At lower frequencies, resistive losses take center stage. When you get up to higher frequencies, dielectric losses start to matter more.
Knowing how frequency affects attenuation makes it easier to pick the right cable for the job. It also helps you decide on cable length, shielding, and installation so you can keep signal integrity in check.
When you look at the physical and electrical properties of coaxial cables, you can actually predict and cut down on these losses for more reliable performance.
Understanding Coaxial Cable Losses
Coaxial cables carry signals pretty efficiently, but every cable introduces some energy loss. Cable construction, length, and signal frequency all factor in.
If you use higher frequencies or longer cables, you’ll see more loss, which weakens the signal at the receiving end.
What Is Attenuation?
Attenuation means your signal gets weaker as it travels through the cable. People usually measure it in decibels (dB) per unit length, like dB per meter or dB per 100 feet.
Signals lose energy in coaxial cables because some of it turns into heat or just disappears. As frequency goes up, so does attenuation. High-frequency signals fade faster than low-frequency ones across the same distance.
Manufacturers publish attenuation charts that show loss at different frequencies. For example, a cable might lose 1 dB at 10 MHz, but lose 5 dB at 500 MHz for the same run.
If you know these numbers, you can pick a cable that actually fits your application.
Types of Losses in Coaxial Cables
Coaxial cable losses mainly come from two places:
- Resistive (Conductor) Loss – The resistance in the inner and outer conductors causes this. When current flows, some energy just turns into heat. At higher frequencies, the skin effect pushes current to the surface, which bumps up resistance.
- Dielectric Loss – The insulation between conductors soaks up some energy as heat. This loss climbs steadily with frequency and doesn’t really care about conductor size.
You might get some minor losses from bad shielding or lousy connectors, but resistive and dielectric losses usually dominate. Cable diameter, conductor metal, and dielectric type all play a part in how much you lose.
Impact of Signal Loss on Performance
When you lose signal, less power gets to the load, which can shrink your transmission range and clarity. In RF systems, too much attenuation can also mess with the signal-to-noise ratio (SNR), making weak signals tough to pick up.
At really high frequencies, even short cables can sap a lot of signal. Satellite TV and microwave links often need low-loss coax or even waveguide to keep attenuation reasonable.
To keep losses down, you might go for a thicker cable, a better dielectric, or just keep your runs short. Matching the cable to your frequency and use case is key for solid performance.
Frequency Dependence of Coaxial Cable Losses
As signal frequency climbs, coaxial cable losses go up. Both resistive effects in the conductors and dielectric effects in the insulation drive this change.
You can predict and measure the relationship between frequency and attenuation, so you can pick a cable that fits your needs.
How Frequency Affects Attenuation
At low frequencies, resistive loss in the conductors rules. The cable’s DC resistance and the skin effect (which pushes current to the surface as frequency rises) cause this.
When frequency goes higher, dielectric loss in the insulation between the center conductor and shield starts to matter more. This loss is tied to frequency and comes from the electric field working the dielectric material.
High frequencies also make imperfect shielding and little impedance mismatches more of a problem, which can chip away at your signal. For most cables, conductor loss grows with the square root of frequency, while dielectric loss rises linearly.
Typical Attenuation vs Frequency Values
Manufacturers usually give attenuation values in decibels per unit length for certain frequencies. You can use these to estimate total loss for a cable run.
For instance, a 100‑foot length of RG‑58 coax might show:
| Frequency (MHz) | Attenuation (dB) |
|---|---|
| 10 | ~0.7 |
| 50 | ~1.5 |
| 100 | ~2.3 |
| 400 | ~4.8 |
| 1000 | ~7.9 |
You can see that doubling the frequency really bumps up the loss. The rate depends on cable size, conductor quality, and dielectric. Bigger cables with low-loss dielectrics usually do better at high frequencies.
Frequency Ranges and Cable Performance
Different coaxial cables work best in different frequency ranges.
Low-frequency applications below 30 MHz can get by with smaller cables and higher resistive loss, since attenuation stays low over short runs.
In the VHF and UHF ranges, loss becomes more noticeable, so people often choose low-loss cables like LMR‑400 or RG‑213.
At microwave frequencies above 1 GHz, dielectric loss takes over. You’ll need specialized cables with precise builds and low-loss materials like PTFE dielectrics. Cable length really matters here—even a few meters can chew up a lot of signal.
Physical and Electrical Properties Influencing Loss
Both physical and electrical factors cause signal attenuation in coaxial cables. Resistance in the conductors, how current flows at high frequencies, and the properties of the insulating dielectric all add up.
Cable size and geometry also have a direct impact on how much energy you lose as the signal moves down the line.
Role of Resistance and Skin Depth
The resistance of both conductors rises with frequency because of the skin effect. At higher frequencies, current sticks to the surface instead of flowing through the whole conductor.
Skin depth (δ) is how deep the current goes before it drops to about 37% of its surface value. As frequency rises, skin depth shrinks, so less conductor area gets used and resistance goes up.
For copper, skin depth at 1 GHz is just a few micrometers. That’s tiny. Surface smoothness and plating matter a lot here. Any extra resistance per unit length means more attenuation, especially if your cable run is long.
Dielectric Material and Its Effects
The dielectric between the conductors stores and releases electric energy as the signal passes by. Its properties, like relative permittivity (εr) and loss tangent (tan δ), set how much energy turns into heat.
Materials like polyethylene have low loss tangents, so they work well for high-frequency stuff. Some foamed plastics lower the dielectric constant and cut capacitance per unit length, which can also reduce loss.
Dielectric loss ramps up at higher frequencies, since the alternating electric field causes more molecular motion and heat. This adds to conductor losses and increases total attenuation.
Conductor Size and Geometry
The diameter of the inner conductor affects resistance and capacitance. Bigger diameters lower resistance by giving current more surface area under the skin effect.
Spacing between the conductors and the dielectric type set the cable’s characteristic impedance. You’ll usually see 50 Ω or 75 Ω, which balance power handling, loss, and signal quality.
Geometry impacts shielding, too. A solid or tight braided shield can cut leakage and interference, though it might bump up capacitance a bit. Getting conductor size and geometry right is crucial for minimizing loss without making the cable too stiff or fragile.
Characteristic Impedance and Its Frequency Response
Characteristic impedance in a coaxial cable affects how signals move and how efficiently energy passes between devices. Cable dimensions, materials, and signal frequency all set its value.
If impedance shifts with frequency, you can see changes in signal quality and system performance.
Definition and Importance of Zo
Characteristic impedance (Zo) is just the ratio of voltage to current for a wave traveling down a transmission line with no reflections.
It’s measured in ohms (Ω) and doesn’t depend on how long the cable is.
For coaxial cables, Zo comes from:
- Inner conductor radius
- Outer conductor inner radius
- Dielectric constant of the insulation
Most RF systems use standard cables designed for 50 Ω (common for radio and data) or 75 Ω (common for video and broadcast).
Keeping Zo right is essential to cut down on reflections, avoid standing waves, and make sure power moves efficiently from source to load. Even small mismatches can cause signal loss or distortion.
Impedance Variation with Frequency
Zo isn’t always perfectly constant. It can shift with frequency because of conductor resistance, inductance, capacitance, and dielectric loss.
At high frequencies, skin effect keeps current at the surface, inductive reactance takes over, and Zo settles near its nominal value.
At low frequencies, current goes deeper, and DC resistance matters more. Zo can get more complicated, with a bigger reactive (capacitive) part. For example, a 50 Ω coax might measure over 200 Ω at very low frequencies.
Where this shift happens depends on cable geometry and materials. For RG58U, it’s somewhere between tens of kilohertz and a few megahertz.
Impedance Matching and Signal Integrity
Impedance matching is about making sure the source, cable, and load all have the same Zo. That stops reflections, standing waves, and signal degradation.
If Zo and the load impedance (ZL) don’t match, some of the signal bounces back toward the source. The reflection coefficient shows how bad the mismatch is.
In digital and RF systems, mismatched impedance can cause:
- Reduced power transfer
- More insertion loss
- Distorted waveforms
Using cables with the right Zo and proper termination resistors keeps your signal clean, especially at high data rates or RF frequencies.
Reflection Coefficient and Return Loss
Signal reflections in coaxial cables pop up when the cable impedance doesn’t match the gear it’s hooked to. You can measure these reflections using the reflection coefficient and return loss. Both tell you how much of your transmitted signal just bounces back toward the source.
Reflection Coefficient Explained
The reflection coefficient (Γ) is the ratio of the reflected signal voltage to the incident signal voltage at a point on the line.
Here’s the formula:
[
\Gamma = \frac{V_{reflected}}{V_{incident}}
]
Γ ranges from 0 (perfect match, no reflection) to 1 (total reflection). So if Γ is 0.1, that means 10% of the voltage gets reflected.
Reflection coefficient links directly to impedance mismatch between the cable’s characteristic impedance (Z₀) and the load impedance (ZL). The formula goes:
[
\Gamma = \frac{Z_L – Z_0}{Z_L + Z_0}
]
Smaller Γ means better matching and less signal loss from reflections.
Return Loss in Coaxial Cables
Return loss (RL) means pretty much the same thing as the reflection coefficient, but you’ll see it in decibels (dB). It basically tells you how much power bounces back compared to what you send down the cable.
The formula looks like this:
[
RL(dB) = -20 \log_{10}|\Gamma|
]
If you see higher return loss values, you’re looking at better performance. For example:
| Return Loss (dB) | Reflection Coefficient | Reflected Power (%) |
|---|---|---|
| 20 | 0.1 | 1 |
| 26 | 0.05 | 0.25 |
| 14 | 0.2 | 4 |
In a well-matched coaxial setup, return loss usually goes above 20 dB for the frequency range you care about. That means you don’t have to worry much about reflections.
Minimizing Reflections for Optimal Performance
If you want fewer reflections, keep the impedance steady from start to finish. That means the cable, connectors, and terminations all need to match.
Some good habits:
- Pick cables with the right characteristic impedance (like 50 Ω or 75 Ω)
- Put proper termination resistors at the load end
- Don’t use damaged or badly fitted connectors
Sometimes, adding attenuators can help with matching by knocking down mismatch effects a bit.
When you cut down the reflection coefficient, you boost return loss. That helps keep your signal clean and reduces distortion, especially at high frequencies.
Measurement and Mitigation of Coaxial Cable Losses
You need accurate measurements to really know how your coaxial cables perform. Picking the right cable type matters, but so does careful installation and using connectors that fit. Match the cable to your frequency range for best results.
Methods for Measuring Cable Loss
Technicians usually measure attenuation in decibels (dB) with a signal generator and either a spectrum analyzer or a vector network analyzer (VNA).
One popular method is the insertion loss test. You send a known signal through the cable, then compare the output to the input. The difference shows the loss.
Another way is to check return loss. This catches reflections from impedance mismatches. If return loss is high, that’s a good sign.
Out in the field, portable cable and antenna analyzers get the job done quickly. These can even measure distance-to-fault (DTF), so you can track down damage or moisture that’s causing extra loss.
It’s important to measure at the frequency you’ll actually use, since attenuation climbs as frequency goes up. Testing at a few different frequencies gives a better picture of what’s happening.
Selecting Low-Loss Coaxial Cables
Cable loss comes down to conductor resistance, dielectric quality, and shielding design. If you go with a larger diameter cable, you’ll usually see less attenuation because it cuts down on resistance and skin effect.
For instance, at 1 GHz over 100 feet:
| Cable Type | Approx. Loss (dB) |
|---|---|
| RG-58 | ~6.7 |
| RG-213 | ~3.0 |
| LMR-400 | ~2.0 |
Low-loss cables use high-quality copper and low-loss dielectrics like foamed polyethylene. Solid center conductors have lower resistance than stranded ones, but you lose some flexibility.
If you pick a cable that’s rated for the highest frequency you’ll use, you’ll keep signal loss down over time. And don’t forget about environmental factors—UV protection and moisture sealing make a real difference for outdoor runs.
Best Practices for Reducing Loss
Try to keep cable runs short, since longer cables just add more attenuation. Don’t let cables get all twisted up or bent too tightly—those kinks can mess with the dielectric and bump up your signal loss.
Pick high-quality connectors and make sure you install them right. If you rush the terminations or use cheap parts, you’ll probably end up with reflections and more loss than you bargained for.
If you can’t avoid long runs, you might want to look into signal amplification at the source or somewhere along the way. Sometimes, it’s easier to just use a thicker cable, or maybe even a waveguide, especially when you’re dealing with really high frequencies.
Check your cables regularly. If you spot damage, corrosion, or water sneaking in, swap out the bad sections before things get worse.