VHF Coax Cables: Essential Guide for Optimal Signal Transmission

A VHF coax cable, or coaxial cable for VHF radio, is a crucial component in connecting radio transmitters, receivers, antenna tuners, or SWR meters in the marine frequency band.

Since its inception in the 1880s, coax cables have been a reliable solution for delivering high-frequency signals over long distances with minimal loss.

A coax consists of a center conductor enclosed in a shield and protected by an outer plastic jacket.

The conductor and shield are separated by a thick insulation layer, sharing a single common axis, hence the term “coaxial.”

In VHF radio applications, the importance of impedance matching and signal loss minimization cannot be overstated, as it directly affects the quality of communication.

Additionally, knowing how to connect and decode PL-259 connectors and preventing damage to your coax cables will ensure the longevity and performance of your system.

Key Takeaways

• VHF coax cables are essential in connecting radio communication components with minimal signal loss.
• Understanding the types and specifications of coax cables contributes to an optimized communication system.
• Proper handling and connection techniques help prevent damage and prolong cable life.

Essential Knowledge of VHF Coax Cables

When dealing with VHF radios, it’s crucial to choose suitable coax cabling to ensure reliable communication and performance. Those designed for marine VHF applications, usually with a 50-ohm impedance and operating in the frequency range of 156 MHz to 164 MHz.

Selecting the right VHF coaxial cable depends on the distance between your radio and antenna, signal strength requirements, and cable attenuation. Commonly used cables for VHF radio are RG-58 for shorter runs and for longer RG-213 or LMR-400. 

These cables have different characteristics that affect signal transmission, and you should base your selection on a balance between price and performance.

Apart from choosing cables, proper connections are important too. Common connectors used with VHF antennas include PL259 plugs and SO239 sockets.

Ensure the connectors are compatible with your antenna and radio equipment and make secure, weather-resistant connections. You should follow my how-to wire vhf coax cables guide to ensure the best practice when installing the cables.

Remember, a properly installed VHF coaxial cable not only provides clear communication and stable radio frequency but also lowers the chances of interference and signal loss. 

Coax Cable Types and Their Specifications

In this section, I’ll introduce you to the various types of VHF coax cable and their specifications, focusing on RG-58, RG-58U, RG-8U, RG-8X, RG-213, and LMR-400.

RG-58 and RG-58U

RG-58 and RG-58U are commonly used for VHF applications. They are lightweight and flexible, making them ideal for temporary installations or portable setups. While both cables are pretty similar, there are some slight differences to consider:

• RG-58: This cable features a solid center conductor and a plain polyethylene dielectric.
• RG-58U: The “U” stands for “universal,” and this cable features a stranded center conductor and a foam polyethylene dielectric, providing slightly better signal quality and flexibility.

However, RG-58 and RG-58U have higher signal loss than the other cable types discussed below. They are best suited for short cable runs, typically under 50 feet.

RG-8U and RG-8X

RG-8U and RG-8X are also popular choices for VHF coaxial cables, offering better performance and lower signal loss than RG-58 and RG-58U. These cables are suitable for more extended cable runs and have some key differences:

• RG-8U: This cable has a solid center conductor, a foam polyethylene dielectric, and typically a double shield, improving overall performance and durability.
• RG-8X: Often considered a lighter, more flexible alternative to RG-8U, RG-8X features a stranded center conductor, making it easier to work with for portable or mobile installations.

Due to their improved performance, RG-8U and RG-8X are suitable for cable runs up to 100 feet in VHF applications.

RG-213

RG-213 is a high-quality coaxial cable option with low signal loss characteristics, making it an ideal choice for VHF and HF applications. 

This cable features a solid center conductor, a solid polyethylene dielectric, and double shielding, making it a durable and reliable choice for long cable runs and permanent installations. 

RG-213 is typically suitable for cable runs up to 200 feet in VHF applications with minimum signal loss.

LMR-400

LMR-400 is a top-of-the-line coaxial cable option with a 50-ohm impedance, ideal for VHF and UHF applications. 

It boasts very low signal loss, high power handling capabilities, and excellent shielding. With a solid copper center conductor, a foam polyethylene dielectric, and an aluminum foil outer conductor, LMR-400 is designed for high performance and versatility in various applications.

Given its superior performance characteristics, LMR-400 is suitable for cable runs of over 200 feet in VHF applications and can handle power levels up to 5.66 kW.

Impedance and Signal Loss in Coax Cables

Exploring the 50 Ohm Impedance

The impedance of a coaxial cable is an important parameter that affects the transmission of radiofrequency waves, and in the case of VHF coax cables, the standard impedance is 50 ohms. 

This value has been chosen because it balances the requirements for low signal loss (attenuation) and manageable voltage standing wave ratio (VSWR). 

A 50-ohm impedance in VHF coax cables is commonly used for antenna systems, allowing for more efficient transmission between the transmitter and receiver.

When using a VHF coax cable for your marine radio, it’s important to ensure that your entire system matches the 50-ohm impedance, including your type of marine VHF antenna and connectors. 

Mismatched impedance can lead to signal reflections and increased loss, negatively impacting your communication range and performance.

Mitigating Signal Loss in Coax Cables

There are several steps you can take to reduce signal loss. Ensuring your cabling has a proper 50-ohm impedance is just the beginning. Here’s a list of possible actions to help you maintain the best possible signal strength:

✅ Choose a low-loss cable type: Some coax cables, such as LMR-400 or 9913, have lower signal loss than standard RG-8, especially at VHF frequencies. Opt for these cables for longer cable runs or when performance is critical.

✅ Keep cable runs as short as possible: Signal loss increases with cable length, so minimize this where possible. Also, use the most direct path between your antenna and transceiver to avoid unnecessary bends or cable extensions.

✅ Use quality connectors: Cheap or poorly made connectors can increase signal loss, so invest in good quality connectors designed for your specific cable type.

✅ Check for damage: Periodically inspect your coax cables for wear, corrosion, or damage that might compromise their performance. If you find any issues, replace the cable immediately. A good antenna testing procedure will cover how to perform the checks.

✅ Proper cable termination: Ensure your coax cables are correctly terminated with connectors matched to the cable’s impedance. Improper termination can lead to increased signal loss.

Decoding PL-259 Connectors

PL-259 are popular connectors typically used in marine communication systems, and they have been around for many years, making them a preferred choice among mariners. 

In this section, you’ll learn about PL-259 connectors and how to install them using crimp and solder methods.

A PL-259 connector, also known as a UHF (Ultra High Frequency) connector, consists of a central pin, a threaded body, and a hollow outer shell. 

This design ensures a secure connection to your coaxial cable, enhancing signal transmission. When correctly installed, PL-259 connectors provide a reliable, low-loss connection that optimizes your VHF antenna’s performance.

To install a PL-259 connector using the crimp method, you’ll need a crimp tool. 

1. First, strip back the outer insulation and mesh shielding on the coaxial cable. Then, insert the cable through the connector, ensuring the inner conductor is aligned with the center pin. 
2. Push the connector onto the cable until the braid shielding contacts the connector’s body. Finally, use your crimp tool to secure the connection by applying pressure to the outer shell.

Alternatively, you can install a PL-259 connector using the soldering method. For this, you’ll need a soldering iron and some solder. 

1. Start by preparing your cable just as you did for the crimp method. Position the connector onto the cable, with the inner conductor inserted into the center pin. 
2. This time, you must apply solder to the center pin and the connector base to create a robust connection. Be careful not to overheat the connector, as excessive heat can damage the cable or connector’s insulation.

Both crimping and soldering are effective techniques for installing PL-259 connectors on your VHF coaxial cables. 

Preventing Coax Cable Damage

To ensure the longevity and proper functioning of your VHF coax cable, it is crucial to protect it from damage. 

One of the primary sources of destruction for cables is moisture. Any moisture penetration into the cable or within the connectors can cause RF cable deterioration. 

To safeguard against moisture build-up, apply silicone grease to the connectors, which will help maintain the intended level of performance.

Another vital aspect is shielding. Both the outer shield and inner conductor of the coaxial cable serve to decrease signal interference. Proper shielding ensures minimal loss and distortion of the received signals. Make sure the shield and conductor are intact and free from corrosion.

The dielectric material surrounding the inner conductor plays a vital role in carrying signals effectively. Always examine the dielectric for any wear or damage. Any irregularity in the dielectric can compromise the performance of the VHF coax cable.

UV-proof materials help protect your coax cables from damage caused by long-term exposure to sunlight. Some coaxial jackets feature UV-resistant materials, increasing durability when used outdoors. Be sure to choose cables with UV-proof jackets to prevent degradation from sunlight, especially when installing on a boat or other exposed locations.

Finally, consider where to mount your VHF antenna on the boat, as the ideal location will ensure safe and efficient cable management. Avoid unnecessary bending or coiling of the cable, as this may lead to performance issues or a weakened signal.