Why Using a 75-Ohm Coaxial Cable with a 50-Ohm Antenna Matters: Understanding the Impact

When considering the performance of any electronic communication system, the importance of matching the correct cable impedance with the appropriate antenna cannot be overstated. For many systems, such as Wi-Fi, TV, and certain RF amplifiers, this matching is crucial. So, what happens if you use a 75-Ohm coaxial cable with a 50-Ohm antenna? Let's explore this issue and its implications.

Theoretical and Practical Implications

When using equipment, it's unreasonable to make significant investments and then deploy the wrong cable. It's akin to driving a car with mismatched tires, which can lead to poor performance and increased wear and tear.

Stated more technically, the impedance mismatch primarily affects wave reflections rather than direct losses. These reflections can vary with the length of the cable and can cause significant issues, especially in systems where accurate signal reception and transmission are crucial.

Implications for Communication Systems

For television and radio systems, these reflections can manifest as ghost images or erroneous collisions, significantly affecting the quality of the signal. On the other hand, in RF amplifiers, mismatches can cause energy to be fed back into the amplifier, potentially leading to overloading and severe damage to the components.

Historical Context and Antenna Standards

It's also worth noting that not all antennas are designed with specific impedances. Historically, TV antennas have been either 300-Ohm twin-lead or 75-Ohm coax. Therefore, the assumption that a 50-Ohm antenna exists may be incorrect.

Calculating Reflections and Losses

The impact of a mismatched impedance can be quantified through the analysis of reflections and standing waves. Depending on whether the signal is baseband or modulated, the concerns will differ. Baseband signals are typically more concerned with reflections, while modulated signals are more concerned with standing waves and power loss.

For a more precise calculation of the dBm loss, one would need to consult the appropriate formulas or use specific tools designed for impedance matching. However, the general conclusion is that it is not advisable to use mismatched impedances in a system.

Real-world Applications

Theoretically, the reflections at either end of the cable can cancel each other out. Yet, at certain frequencies, your section of 75-Ohm cable could act as a quarter-wave section, leading to unique and potentially problematic effects. Similarly, a half-wavelength length of 75-Ohm cable might not affect the impedance transformation, but could introduce variations in the Voltage Standing Wave Ratio (VSWR) along the cable.

In practical terms, using a large aluminum CATV 75-Ohm coax in high-power RF transmitter situations with 50-Ohm termination at both ends has been reported to work well. However, the correct connectors may be challenging to find, which could complicate the deployment.

Conclusion

In summary, while the direct loss caused by using a 75-Ohm coaxial cable with a 50-Ohm antenna may be minimal, the wave reflections can have significant negative effects on signal quality and system performance. For optimal system performance, using the correct impedance matching is critical, especially in high-stakes applications like RF amplifiers. Always ensure that your equipment and cables are well-matched to avoid potential damage to your system and related components.

Key Takeaways: Impedance matching, wave reflections, signal quality, system performance, and cable termination.