4 Factors Affecting Signal Loss When RF Coaxial Cable Transmits Signals

In the process of transmitting the signal of the RF coaxial cable, the signal will be continuously lost, which will cause the signal to be greatly reduced after reaching the end, and sometimes it may not meet the normal working requirements.

The main factors affecting signal loss are the resistance loss, dielectric loss, and mismatch loss of the coaxial cable. At the same time, leakage loss is also a problem that cannot be ignored when low-quality cables work at high frequencies. We will analyze these losses separately below.

1. Resistance loss of RF coaxial cable

Resistance loss is the consumption of signal energy by the DC resistance of the radio frequency cable and the eddy current generated by the high-frequency induction of the conductor. The resistance value is related to the material and production process of the cable. To solve this problem, you can find a professional RF coaxial cable manufacturer. It will change with the change of the transmission frequency because the conductor has a skin effect in the transmission of AC signals. As the frequency increases, the effective resistance will continue to increase.

When an alternating current passes through the conductor of the RF coax cable, an alternating magnetic field is generated around the conductor. This magnetic field will generate a new induced current (eddy current) inside the conductor, which is opposite to the signal current in the center of the conductor and the same direction as the signal current on the surface of the conductor. In this way, the signal current inside the conductor is offset by the reverse eddy current, and the current is reduced; the signal current on the surface of the conductor is the same as the same direction eddy current, and the current increases. This is the skin phenomenon of AC passing through the conductor.

As the frequency of the RF cable signal increases and the induced current increases, this phenomenon becomes more obvious. It causes the current to flow only in a small section of the surface, causing the effective resistance of the conductor to increase significantly.

The skin depth of a signal is related to frequency and material. The lower the frequency, the deeper the skin depth; the higher the frequency, the shallower the skin depth. Iron has a much smaller skin depth than copper. Hongsen is a professional RF coaxial cable supplier, you can contact us to solve this problem.

2. Dielectric loss of RF coaxial cable

Dielectric loss is the signal loss of the dielectric (insulator) between the center conductor and the outer conductor of the radio frequency cable. An important parameter for measuring dielectrics is the dielectric constant. It refers to the ratio of the capacitance when a substance is used as the dielectric in the same capacitor to the capacitance when it is a vacuum, which is called the "dielectric constant" of the substance. The dielectric constant generally varies with temperature and the frequency of electromagnetic waves propagating in the medium.

The inner and outer conductors of the coaxial cable are equivalent to the two poles of the capacitor. Since the cable dielectric in practical use has resistance, the dielectric constant is usually greater than 1. Therefore, the loss of signal during transmission is inevitable. The size of the dielectric constant is related to the material and processing technology (such as coaxial cable foaming). The greater the dielectric constant, the greater the loss to the signal. The higher the temperature and the higher the frequency, the greater the dielectric loss of the RF coaxial cable.

3. Mismatch loss of RF coaxial cable

The mismatch loss is mainly closely related to the physical structure of the radio frequency coaxial cable. If the coaxial cable causes the cable to deviate from the nominal impedance or the cable impedance is uneven during the design and production, it will cause signal mismatch loss. During construction, excessive bending, deformation, damage, and water ingress of the cable will also cause mismatch loss.

The characteristic impedance (not DC resistance) of a coaxial cable has nothing to do with the length of the cable, it is determined by the equivalent capacitance and inductance in the coaxial cable. The equivalent capacitance and inductance are determined by the diameter of the inner and outer conductors and the dielectric constant of the medium.

Uneven cable impedance or mismatch with the signal source and load will cause part of the signal energy to return to the opposite direction of the transmission direction when the RF coaxial cable transmits signals, that is, reflection. It will affect the original signal and cause the transmission efficiency to drop. In severe cases, it will directly affect the normal operation of the system.

The degree of signal reflection in the transmission is usually expressed by standing wave ratio or reflection loss (return loss).

4. Leakage loss of RF coaxial cable

Leakage loss is the signal radiated through the braided gap of the cable shield. It also causes energy loss during signal transmission. This is a problem that cannot be ignored in high-frequency transmission. For this reason, the braided coverage of the radio frequency coaxial cable cannot be too low.

To sum up, there are many factors for the signal transmission loss of the RF cable. Its final loss is the sum of the above-mentioned various losses. This comprehensive loss can be tested with a network analyzer. The DC resistance of the cable plays a major role in signal attenuation only at low frequencies; at high frequencies, the signal attenuation is mainly determined by the skin effect and dielectric loss. With the increase of the transmission signal frequency, the signal attenuation of the coaxial cable increases exponentially. Therefore, the transmission loss of coaxial cable is important to consider high-frequency loss. In addition to the design, production, and processing of coaxial cables, improper construction during use will also have a significant impact on the normal use of the cable.

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