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HomeTips & GuidesCoaxial Cable Sizes: How Size Matters in the Performance

Coaxial Cable Sizes: How Size Matters in the Performance

Coaxial cables are part of everyday use. Their unique design makes it possible to use them to transmit high-frequency radio signals with minimal loss. When you use big coax cables, the signal loss is lesser along a given cable length than a small coax cable, despite the former being convenient.

So, how do coaxial cable sizes affect signal quality? Let us know the details.

Do coaxial cable sizes matter?

The size of the coaxial cable matters significantly alters the performance of the cable. The size refers to the inner and outer conductors and the relative size of the two conductors.

Comparison of the size of two coaxial cables

Comparison of the size of two coaxial cables

How do coaxial cable sizes affect the performance?

An electrical circuit consists of two wires (go and return) connecting the power source and the electronic device. The “GO” wire carries the power from the power source to the device. On the contrary, the “Return” wire grounds at the junction box and thus does not have any volts in it.

Skin effect

The coaxial cable also works on the same principle. The cable has two conductors (outer and inner). The inner wire is live while the outer wire is earthed. Further, the outer wire has a plastic protective covering to make the cable waterproof.

Now, the current flows in the inner conductor in one direction while in the opposite direction in the other. Thus, at any point and time along the coax cable length, there are opposite electric currents in the two conductors. These two currents attract each other. As a result,  the electric current is present on the outer surface of the inner conductor and the inner side of the outer conductor. This uneven current distribution along the conductor’s surface is known as the skin effect.

Transverse Electromagnetic Waves

Now, due to the presence of opposite currents, there is some potential difference between the two conductors. This leads to the generation of an electric field. Now, when the inner conductor is positive, the electric field points radially outwards. On the other hand, in the latter half of the cycle, the electric field points radially inwards. In addition, there is a magnetic field along the circumference of the conductor. This magnetic field is perpendicular to the radial electric field. The interaction of electric and magnetic fields results in the formation of electromagnetic waves between inner and outer conductors, known as Transverse Electromagnetic Waves.

Role of dielectric in coax cable

Generally, the transmission line has air filled between the outer and the inner conductor. However, to reduce dielectric loss, there is a plastic material (foamy) or intermittent spacers in the space between the conductors. 

A dielectric insulating material that ensures a consistent distance between these two conductors. This low-loss dielectric also serves three other purposes:

  • The refractive index of the material slows down the wave
  • It reduces the characteristics impedance of the coax cable
  • It increases the breakdown voltage, thus increasing the maximum power handling capacity of the cable.

N (refractive index)=  √k (dielectric constant of the material used).

Signal Loss:

You can use coaxial cables as feeder lines to transmit radio frequency signals between a radio frequency generator and its load.  For instance, as a feeder line between a transceiver and the aerial. Here, the aerial or the load has some feed point impedance, approx 72 ohms for a dipole.

Now, if you want to radiate maximum RF power towards the load with minimal reflection towards the transceiver,  a similar characteristic impedance in the coaxial feeder is essential. In this case, the value is approximately 75 ohms. 

Radio frequencies are lower than microwave frequencies. Hence,  the major loss in this case is because of the conductor loss in both the conductors.

As the skin depth and the current are the same, the signal loss is more toward the inner conductor. Also,  the radio frequency current mainly flows through it due to its smaller circumference.

The loss depends on the skin depth, dielectric loss, velocity factors and the material’s resistivity.

Characteristics impedance:

Fundamentally, the characteristic impedance is the ratio of inductance/length to the capacitance of the inner conductor to the outer conductor per unit length. However, you can determine it with the ratio of the diameters of the inner and outer conductors and the dielectric constant. 

The characteristic impedance Z is represented as

Z = (138.059/√K) * Log 10(Do/Di).

Here, 

K= dielectric constant of the material,

Di= diameter of the inner conductor

Do= diameter of the outer conductor.

Breakdown Voltage:

Assuming that you ground the outer conductor, the voltage at the inner conductor becomes high.

Now, when the strength of the electric becomes more than the breakdown strength of dielectric material, breakdown will start. (if the dielectric is air, the breakdown strength is 3 megavolts per meter).

The strength of the electric field near a conductor is inversely proportional to its radius. Because of its lesser radius, the electric field is stronger towards the inner conductor.

Thus, the breakdown starts at the inner conductor’s surface. So, the larger the coax cable, the higher its capacity of handle voltage for a given ratio of inner to outer diameter.

However, in practice, this breakdown may occur before the theoretical value. This is because of the air gaps in the junction between cables and the space between cables and other components.

Optimum designs:

The optimum design of a coax cable is one with the highest power and lowest loss. Now, when you fix the outer diameter, the maximum size of the cable is also fixed.

On increasing the characteristic impedance of the cable transmitting RF signals, the loss will be lower. This is because the resistances of the conductors would be lower than the characteristic impedance.

However, raising the impedance means reducing the diameter of the inner conductor, which ultimately increases the loss. Thus, the signal loss per unit length of the cable depends on the characteristic impedance and the resistances of both conductors.

Considering these two conflicting relations, the minimum loss occurs at a particular ratio, which is approximately 3.591. At this ratio, the characteristic impedance of the cable is 76.653 ohms for an air-filled cable.

This ratio is independent of the resistivity of the material used and the size of the cable.

Suppose we assume the size of the outer conductor is not changeable.  Here, increasing the diameter of the inner conductor can lead to two contrasting effects on the cable’s power handling capacity. These are:

  • If you increase the inner conductor diameter in a fixed outer conductor size, the space between the two conductors decreases. As a result, the electric field rises, which increases the probability of breakdown between the two conductors.
  • Also, on raising the inner conductor diameter, characteristic impedance reduces, lowering the voltage at the inner conductor. As a result, the probability of breakdown decreases.

To balance these conflicting factors, again, there is an optimum ratio of diameters of inner and outer conductors. This value is approximately 1.6474, corresponding to a characteristic impedance value of 29.918 ohms.

(Note: These figures also do not depend on the resistivity of the material and the size of the cable.)

Why 50 ohms?

It is still unclear why the standard of 50 ohms impedance is prevalent. However, during the 1940s, researchers at the Massachusetts Institute of Technology studied coaxial systems with air as a dielectric.

At that time, low-loss plastic was not available. Based on the optimum impedance of the highest possible power and lowest possible loss, they calculated geometric and arithmetic means of 53.29 ohms and 47.87 ohms, respectively.

And the mean average of the above two values is 50 ohms, and that’s why it is the accepted norm.

Coaxial cables with connectors

Coaxial cables with connectors

The most coaxial cable sizes:

Mainly, coaxial cables fall into two categories: RG and LMR. Their use depends on the application type.

RG coax cables:

Here, RG stands for Radio guide, which indicates the military specification for these cables used originally. The RG number of the coax cable mainly signifies its diameter. The cable with a higher RG means it has a thinner central core. On the contrary, a lower RG number indicates a thicker central conductor.

RG coaxial cable typeimpedanceCore sizeNominal attenuationPer Mhz dB/100 ftDielectric typeFunction
RG-6/U75 ohms1.024 mm850/8.5dB;1900/13,6dBPFCommon coaxial cables used in different commercial and residential applications like the internet, cable TV and more
RG-850 ohms2.17mm850/6.4dB;1900/10.4dBPFCannot carry pure video signals Useful in audio control rooms, extra radio antennas, and radio stations
RG-1175 ohms1.67mm850/6.25dBPFHigher gauge cable Used in TV antennas, video distribution, HDTV and CATV
RG-5975 ohms0.64mmNot applicablePFFlexible and easy installation Not suitable for long runs Best for low-frequency audio, video signals and CCTV

LMR coax cables:

These are updated versions of RF coaxial cables with higher flexibility, lower cost, and greater installation cost. LMR cables are mostly used for antennas on satellites, missiles, airplanes, and other communication systems. The LMR cables are indicated as LMR (number); here, the number indicates a rough idea of their thickness.

LMR coaxial cableImpedanceCore sizeDielectric typeNominal attenuationFunction
LMR 20050 ohms1.12mmPF850/9.6dB;1900/14.6dBOutdoor-rated flexible cableKnown for low-loss communication Good for short antenna feedlineLower passive intermodulation
LMR 24050 ohms1.42mmPF850/7.2dB;1900/11.2dBSuitable for outdoor applications Lesser signal lossSuitable for short feedlines like WLAN, GPS and mobile antennas
LMR 40050 ohms2.74mmPF850/3.8dB;1900/5.8dBFlexible cables used in applications with repeated bending and flexingUse in jumper assemblies and antenna feeders
LMR 60050 ohms4.47mmPF850/2.4dB;1900/3.8dBDesigned for outdoor applicationsFlexible than hardline cables
LMR 90050 ohms6.65mmPF850/1.6dB;1900/2,6dBLarger cables suitable for medium antenna feeder lines where flexible and easily routed cables are required
LMR 120050 ohms8.66mmPF850/1.2dB;1900/1.9dB
LMR 170050 ohms13.39 mm`PF850/0.9dB;1900/1.5dB

Conclusion:

Coaxial cable sizes have standard properties and if you follow application guidelines from the manufacturer, you may not face any issues with these cables.

If you need any information regarding coax cables or want customized options, Cloom is there for you. 

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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