Coax Cable Loss and the Impact of Bad SWR

The coaxial cable is critical when setting up a radio communication system. This cable transmits RF signals from the radio to the antenna. We use coax cables because they have a conductive shield surrounding the inner conductor. This shielding minimizes electromagnetic interference (EMI) and radio frequency interference (RFI) when correctly installed. Coax cables are relatively easy to install. They can be run through walls, ceilings, and other spaces without significant signal degradation. While coaxial cables are essential for efficient signal transmission, they are not without their limitations, particularly when it comes to signal loss. In this article, I will explore the basics of coaxial cable loss and how a poor SWR¹ can worsen these losses.

What is Coaxial Cable Loss?

Coaxial cable loss refers to the reduction in signal strength as it travels through the cable. The total loss is usually measured in decibels (dB) and specified per unit length (e.g., dB per 100 meters or feet). The loss increases with frequency, meaning higher-frequency signals will experience greater attenuation.

What is SWR?

SWR (Standing Wave Ratio) or VSWR (Voltage Standing Wave Ratio) occurs when there is a mismatch between the impedance of a transmission line and the load (such as an antenna). This mismatch causes a portion of the transmitted signal to be reflected back toward the source, creating standing waves along the transmission line. The amplitude of the standing wave corresponds to the sum of the forward and return waves.

The degree of mismatch and the resulting SWR value indicate how efficiently RF power is transmitted from the source through the transmission line and into the antenna. A perfect match between the transmission line and the load would have an SWR of 1:1, meaning all the power is transmitted with no reflections.

However, a perfect match is rare in the real world, and some of the power is reflected towards the source. This reflection creates standing waves along the transmission line, leading to an increase in SWR. The greater the mismatch, the higher the SWR.

Interpreting VSWR values

• VSWR = 1:1: Perfect match, no reflected power. All power is transmitted to the load. A perfect SWR is hard to reach, especially on multi-band/multi-frequency systems used by ham radio operators.
• VSWR < 2:1: Generally considered acceptable for most applications.
• VSWR > 3:1: Indicates a significant mismatch and the potential for high reflected power, which can lead to inefficiency and possible damage to equipment.

Ultimately, the increase in losses due to poor SWR reduces signal strength at the antenna, leading to poorer communication system performance.

Transmission line losses

Even though the energy lost in the transmission line is not uniform², we can calculate the total energy lost using the following formula^{3 4}:

Where \(\alpha\) (Alpha) is the matched-line loss ratio:

And the reflection coefficient \(\Gamma\) (Gamma) is related to the SWR by:

Example

Suppose you have a system with an RG-58 coaxial cable with a loss of 1.95 dB per 100 feet at 21 MHz, and the SWR at the antenna⁵ is 3:1.

1. Calculate the feed line loss for our 75 feet of feed line. The total cable loss will be 1.462 dB:
   $$ \text{Line Loss} = \left( \frac{1.95_{db}}{100_{feet}} \right) \times 75_{ft} = 1.462 \text{ dB} $$
2. Then calculate the matched-line loss ratio:
   $$ \alpha = 10^{\frac{-1.46}{10}} \approx 0.714 $$
3. Then, calculate Gamma, the reflection coefficient:
   $$ \left| \Gamma \right| = \frac{3 - 1}{3 + 1} = \frac{1}{2} = 0.5 $$
4. Finally, the total loss, including matched-line and the additional loss due to standing waves, is:
   $$ \begin{aligned} \text{Total Loss}_{dB} &= -10 \log_{10} \left( 0.714 \cdot \frac{1 - 0.5^2}{1 - 0.714^2 \cdot 0.5^2} \right) \\[4pt] &= -10 \log_{10} \left( 0.714 \cdot \frac{0.752}{0.872} \right) \\[4pt] &= -10 \log_{10} \left( 0.615 \right) \\[4pt] &\approx 2.1 \text{ dB} \end{aligned} $$
5. The total loss (cable + SWR) is 2.1 dB. Therefore, we can use the following formula to calculate the losses due to a SWR of 3:1:
   $$ \begin{aligned} \text{SWR Loss}_{dB} &= \text{Total Loss}_{dB} - \text{Cable Loss}_{db} \\ &= 2.1 - 1.46 \\ &= 0.64 \text{ dB} \end{aligned} $$

Cable Loss Calculator

Instead of running all these fastidious calculations by hand, you can use the following calculator.

Mitigating Coax Cable Loss and SWR Issues

Although a minor increase in loss is noted with SWRs up to 2:1, this is generally negligible, especially in HF bands where the difference is less than 0.5 dB. Higher SWRs in non-resonant antennas can cause significant feed line losses. Even minor mismatches can lead to substantial transmission line losses in VHF and UHF ranges, making an additional 0.5 dB loss unacceptable.

1. Use Low-Loss Coaxial Cables: Opt for high-quality cables with low attenuation, especially for high-frequency applications or long coax runs.
2. Ensure Proper Connectors: Use high-quality connectors and ensure they are correctly installed to minimize connector losses.
3. Match Impedances: Carefully match the transmission line's impedance with the load (antenna) to achieve a low SWR.

You can use the following tool to compare coaxial cables.

Conclusion

Understanding and reducing coaxial cable loss and SWR is essential. Choosing low-loss cables, using high-quality connectors, and matching impedances are crucial for minimizing signal loss and ensuring optimal system performance. By focusing on these factors, you can improve the reliability and efficiency of your radio communication setup.