Coax Transmission Lines – Calumet Radio

Main Coaxial Line

For the long run from the repeater shack up to the antenna on the tower, you need the lowest loss cable you can afford, with a focus on PIM performance.

Heliax Hardline: This is the gold standard for repeater installations. Heliax (and similar cables from CommScope, Andrew, or other brands) has a solid outer conductor (not braided) and a foamed dielectric, which provides extremely low loss at UHF frequencies. It’s also specifically designed to have excellent PIM performance, as its construction avoids the rubbing of dissimilar metals.
Alternatives: For shorter runs (under 50-75 ft), some hams use high-quality, solid-dielectric cables like RG-214. While it has more loss than Heliax, its double-braid, all-copper/silver-plated construction avoids the PIM issues of other cable types.

Jumper Cables

The short jumpers used to connect the Heliax line to the duplexer and antenna require flexibility while maintaining low loss and PIM resistance.

Superflex Heliax: This is a flexible version of the Heliax hardline. It’s the top choice for jumpers as it provides the same low-loss and PIM-free characteristics as the main line.
RG-400 or RG-142: These cables are excellent choices for jumpers. They are a smaller diameter, very flexible, and use a double-braid, silver-plated shield. This construction prevents PIM and makes them ideal for short runs in a repeater cabinet.
N-Type Connectors: For both the main line and the jumpers, N-type connectors are the professional standard for UHF applications. They provide a more consistent 50-ohm impedance and superior weather sealing compared to the common PL-259/SO-239 “UHF” connectors, which are not true 50-ohm connectors at UHF frequencies.

For connections between a repeater and a duplexer, RG-400 and RG-142 are generally the best choices. RG-213 is not recommended for this application. The primary reason for this is related to passive intermodulation (PIM), which is a key concern in duplex systems.

Why RG-400 and RG-142 are Superior

Repeaters transmit and receive simultaneously. This means a powerful transmit signal is being generated very close to a sensitive receiver. The strong transmit signal can create “intermodulation” products (unwanted new signals) in the receive frequency band, which can significantly degrade the repeater’s performance.

RG-400 and RG-142 are excellent for this task due to their construction:

Double Shielding: Both cables have a double-braid, silver-plated copper shield. This superior shielding is critical for preventing the strong transmit signal from leaking out of the cable and interfering with the receive path.
High-Quality Materials: The use of a PTFE dielectric and silver-plated conductors in both cables minimizes the potential for PIM. PIM is often caused by dissimilar metals touching each other, which can happen with lower-quality cables and connectors. The consistent, high-quality materials in RG-400 and RG-142 are designed to prevent this.

The main difference between the two is their center conductor. RG-400 has a stranded center conductor, making it very flexible and ideal for short jumper cables that might need to be bent or moved often. RG-142 has a solid center conductor, making it stiffer and more prone to damage from repeated flexing, so it’s best for permanent, non-moving connections.

Why RG-213 is Not Recommended

While RG-213 is a very good low-loss cable for long antenna feedlines, it is not suitable for jumper cables connecting a repeater to a duplexer.

Single Shielding: RG-213 has only a single copper braid shield. This is not sufficient to prevent signal leakage in a duplex environment, which can lead to PIM issues and a desensitized receiver.
Lower-Quality Materials: The use of a polyethylene dielectric and a bare copper conductor makes RG-213 more susceptible to PIM compared to the silver-plated and PTFE construction of RG-142 and RG-400.

Ultimately, for repeater-to-duplexer connections, the superior shielding and PIM-resistant construction of RG-400 make it the most popular and reliable choice, especially for short, flexible jumpers within an equipment cabinet.

For connections between a repeater and a duplexer, RG-400 is the better choice over RG-142, primarily due to its greater flexibility and resistance to noise generation from repeated bending. While both are high-quality, double-shielded, 50-ohm coaxial cables with similar electrical performance, their core conductor material is the key difference.

Key Differences and Why They Matter

Center Conductor: This is the most critical distinction. RG-400 has a stranded, silver-plated copper center conductor, which makes it very flexible and durable. In contrast, RG-142 has a solid, silver-plated copper-clad steel center conductor. While the solid core of RG-142 is rigid and provides good performance, it is susceptible to fracturing if it is repeatedly flexed. A fractured conductor can create a “crackling” noise and degrade signal quality.
Flexibility and Durability: The connection between a repeater and a duplexer, often called a “jumper,” is located inside a cabinet where it may be handled and bent during maintenance or adjustments. The stranded core of RG-400 is specifically designed for such applications, as it can withstand repeated flexing without the risk of breaking or becoming a noise source.
Electrical Performance: Both cables are built with a PTFE (Teflon) dielectric and dual silver-plated copper braided shields, which provide excellent RF characteristics. They have similar impedance (50Ω), attenuation (loss), and power handling capabilities. Therefore, from a purely electrical standpoint, they perform similarly, and neither offers a significant advantage in signal transmission for the short jumper lengths used in this application.

In summary, while both cables are high-quality options, the superior flexibility and resistance to mechanical failure of RG-400 make it the most reliable and recommended choice for the dynamic environment inside a repeater cabinet.

For repeater-to-duplexer connections, RG-400 is the better choice over RG-142. Both cables offer excellent electrical performance, but their physical differences make RG-400 more suitable for this application.

Key Differences

The main difference between the two cables is the center conductor.

RG-400 has a stranded, silver-plated copper center conductor. This makes it highly flexible and resistant to repeated bending without fracturing.
RG-142 has a solid, silver-plated copper-clad steel center conductor. While it’s great for rigid applications, it’s more prone to breaking and becoming a noise source if it’s flexed repeatedly.

Repeater and duplexer “jumpers” are often located in tight spaces and may be handled during maintenance. The stranded core of RG-400 can withstand this without degrading, making it the more reliable choice for the long term.

Electrical Performance

Both RG-400 and RG-142 have the same electrical characteristics, including:

Impedance: 50 Ohms.
Shielding: Double silver-plated copper braids.
Dielectric: PTFE (Teflon).

Because the cable runs between a repeater and duplexer are short, the minimal differences in attenuation (signal loss) are not significant. The greater flexibility and durability of RG-400 are the deciding factors.

Why to Avoid LMR-Series Cables

While LMR-series cables (like LMR-400) are very popular for their low loss, they are not recommended for duplex repeater systems. Their construction uses a copper-braid outer shield over an aluminum foil shield. The rubbing of these two dissimilar metals can create intermodulation noise when the system is transmitting, which is then picked up by the receiver on a different frequency. This noise can desensitize the receiver, severely impacting the repeater’s performance.

Short to Medium Runs (Up to 150 ft)

For runs of around 150 feet or less, 1/2-inch Heliax (LDF4-50A) is the most common and practical choice. It offers a great balance of low loss, flexibility, and cost.

Low Loss: At 450 MHz (a common UHF frequency), the loss is approximately 1.5 dB per 100 feet. This means for a 100-foot run, you would lose less than 1.5 dB of signal, which is excellent. For a 150-foot run, the loss is still very manageable.
Manageability: 1/2-inch Heliax is relatively easy to work with on a tower compared to larger sizes, making it a popular choice for most amateur radio and commercial applications.

Medium to Long Runs (150-300+ ft)

For longer runs, especially those on tall towers or for very high-frequency applications, the signal loss of 1/2-inch cable becomes more significant. In these cases, 7/8-inch Heliax (AVA5-50) is the ideal choice.

Ultra-Low Loss: At 450 MHz, the loss for 7/8-inch Heliax is approximately 0.8 dB per 100 feet. For a 200-foot run, this results in only 1.6 dB of loss, which is half that of a 1/2-inch line for the same distance.
Professional Standard: This size is often considered the professional standard for high-performance repeater and broadcast applications where every fraction of a decibel of signal is critical.

Very Long Runs or High-Power Applications

For runs exceeding 300 feet or for very high-power broadcasting, even larger sizes like 1-1/4 inch or 1-5/8 inch Heliax are used. These cables have even lower loss but are much heavier, more expensive, and extremely difficult to handle and bend.

Cable Type	Diameter	Loss @ 450 MHz (dB/100 ft)	Typical Use Case
1/2″ Heliax	0.5″	1.5 dB	Most ham radio and light-duty commercial runs up to 150 ft.
7/8″ Heliax	0.875″	0.8 dB	Longer runs (150-300+ ft), high-performance repeater, and commercial sites.
1-5/8″ Heliax	1.625″	0.5 dB	Very long runs, high-power broadcast, and microwave links.

Choke Balun

A choke balun is a device used in radio frequency (RF) systems to block unwanted current from flowing on the outside of a coaxial cable’s shield.

How it Works

The term “balun” is a portmanteau for “balanced to unbalanced.” Its primary purpose is to convert a signal from an unbalanced transmission line (like coaxial cable) to a balanced antenna element.³ A choke balun achieves this by creating a very high impedance (resistance to RF) for current that flows in the same direction on both the center conductor and the shield, known as common mode current.

The desired RF signal, called differential mode current, flows in opposite directions on the two conductors and is unaffected. By “choking” the common mode current, the balun ensures the antenna operates as designed.

Why is a Choke Balun Necessary?

Without a choke balun, common mode current can cause several problems:

Distorted Antenna Performance: The coaxial cable itself can become part of the radiating antenna, which changes its intended radiation pattern and can reduce efficiency.
RF Interference (RFI): The radiating coax can cause static, noise, and interference in nearby electronic equipment, such as TVs, computers, and telephones. This is often referred to as “RF in the shack.”
Inaccurate SWR Readings: The presence of common mode current can lead to false readings on an SWR meter, making it difficult to properly tune the antenna.

Ferrite Core

A ferrite core is a type of magnetic core used to form the windings of inductors, transformers, and other electronic components. It is a ceramic compound made of iron oxide mixed with other metals, such as manganese and zinc (MnZn) or nickel and zinc (NiZn).

Key Properties of Ferrite Cores

High Magnetic Permeability: This property allows a ferrite core to concentrate magnetic flux lines effectively, which significantly increases the inductance of a coil wound around it. A higher inductance in a smaller space is a key advantage over air-core inductors.
Low Electrical Conductivity: Unlike metallic magnetic materials (like iron), ferrites have very high electrical resistance. This prevents the flow of eddy currents, which are a major source of energy loss and heat generation in high-frequency applications. This makes ferrite cores ideal for high-frequency circuits.

Common Applications

Ferrite cores are essential in modern electronics due to their ability to manage and suppress unwanted high-frequency currents.

EMI/RFI Suppression: Ferrite beads or rings are commonly seen on power and data cables (like USB, HDMI, and laptop power cords). They act as a common-mode choke to suppress high-frequency noise and prevent the cable from acting as an antenna that radiates or receives radio frequency interference.
Transformers: Ferrite cores are used in the transformers of switched-mode power supplies (SMPS) for devices like laptops, phones, and TVs. Their low losses at high frequencies allow for smaller, lighter, and more efficient power supplies.
Inductors and Antennas: They are used in radio receivers to create compact, high-performance inductors and antennas, such as the ferrite rod antenna (or loopstick antenna) found in AM radios.

Ferrite Core vs. Choke Balun

A ferrite core is designed to be a lossy material, meaning it dissipates unwanted high-frequency energy as a small amount of heat, rather than storing it. This is how it effectively “chokes” out noise.

A ferrite core is a type of magnetic core made of ferrite, a ceramic compound of iron and metal oxides. Ferrite cores are used to create the windings of inductors and transformers due to their high magnetic permeability and low electrical conductivity. This low conductivity prevents eddy currents, a source of energy loss. Ferrite cores come in various shapes and sizes, such as toroids (rings), rods, and beads, and are essential components for many electronic circuits, including power supplies and radio frequency (RF) equipment.

A choke balun is a specific application of a ferrite core. A balun is a device that converts a balanced electrical signal to an unbalanced one, and vice versa. An “ugly balun,” or air choke balun, is a type of balun made from coiled coaxial cable that provides a high impedance to unwanted common-mode currents. When a ferrite core is added to this design, it significantly increases the inductance, making the balun more effective at choking off unwanted currents. The ferrite core acts as a high-impedance “choke” to common-mode currents, preventing the outer shield of the coaxial cable from radiating and affecting the antenna’s performance.

In summary, a ferrite core is a material, while a choke balun is a device that utilizes a ferrite core (or an air core) to perform a specific function. The ferrite core is the component that provides the inductive properties needed for the balun to work effectively.

Ferrite Core: A material with high magnetic permeability used in various electronic components.
Choke Balun: A type of balun that uses a high impedance to suppress unwanted currents. Often, but not always, it uses a ferrite core.

The primary purpose of a choke balun is to ensure that the feedline doesn’t become part of the antenna system, which would distort the antenna’s radiation pattern and can cause radio frequency interference (RFI) in your shack.

In fact, using ferrite cores is often more effective and broadband than an air-wound loop (ugly balun), especially with thick cables.

Why Use Ferrite Cores Over Air-Wound Loops?

Higher Impedance: Ferrite cores significantly increase the common-mode impedance of the choke. This means they are much better at blocking unwanted currents from flowing on the outside of the coaxial cable shield.
Wider Bandwidth: A well-designed ferrite choke balun can be effective across a very wide range of frequencies, from the 80-meter band to the 10-meter band and beyond, depending on the ferrite mix and the number of turns. An air-wound choke balun, in contrast, is typically only effective on a single band or a very narrow range of frequencies.
Compact Size: Ferrite cores allow you to achieve a high choking impedance with a much shorter length of cable, making the balun more compact and easier to install. This is particularly beneficial with stiff, large-diameter cables like Heliax, which are difficult to coil into a tight loop.

How to Do It

Instead of winding the Heliax cable into a large coil, you can slide ferrite beads or toroid cores directly onto the cable. You may need to use multiple cores or make a few turns through a large toroid to achieve sufficient choking impedance.

When selecting a ferrite core, the key is to choose the correct material (mix) for the frequency range you intend to operate on. For general HF use, mixes like #31 or #43 are common and effective.

A single ferrite core on its own may not be enough. The effectiveness of the choke depends on the number of turns and the properties of the ferrite. For best results, you’ll need to use a sufficient number of cores to create a high impedance choke that prevents common-mode currents.

It is important to understand that a choke balun is a type of common-mode choke. Its purpose is to present a high impedance to common-mode currents, which are unwanted currents flowing on the outside of the coaxial cable shield. This prevents the feedline from radiating RF and interfering with your radio shack.

Grounding Kit vs Ferrite Core

a grounding kit and a ferrite core do not accomplish the same task. They serve two completely different purposes in a radio or antenna system. A grounding kit is for safety and protection from lightning and static buildup, while a ferrite core is for suppressing radio frequency interference (RFI) and improving antenna performance.

Grounding Kit

A grounding kit is a device used to provide a direct electrical path for static electricity and lightning strikes to a building’s or tower’s main ground system. Its purpose is to protect equipment and people by safely diverting a high-voltage surge away from your radio and into the earth.

Function: To provide a low-resistance path to ground for high-current, high-voltage events.
Primary Use: Lightning and static discharge protection.
Mechanism: It creates a direct metallic connection from the coaxial cable’s outer shield to a grounding rod or a building’s ground bus bar. This connection should be made at the base of the tower and at the point where the cable enters the building.

Ferrite Core (Choke Balun)

A ferrite core, when used as a choke balun, is a type of common-mode choke. It’s designed to increase the impedance to unwanted currents that flow on the outside surface of the coaxial cable. These are not lightning currents; they are radio frequency (RF) currents that can cause RFI and distort an antenna’s radiation pattern.

Function: To suppress common-mode RF currents and prevent the feedline from acting as part of the antenna.
Primary Use: RFI reduction and antenna system optimization.
Mechanism: The ferrite core absorbs the unwanted RF energy and dissipates it as a small amount of heat, effectively choking off the current’s path. It does not provide a path to ground.

Grounding Kit

The purpose of a grounding kit on repeater coaxial transmission lines is to provide a low-resistance path for stray electrical energy to be safely discharged to the earth. This is a critical safety and equipment protection measure.

Primary Functions of Grounding Kits

Lightning Protection ⚡: This is the most important function. Repeater towers are often located on high ground, making them susceptible to lightning strikes. A grounding kit provides a direct path for the immense current from a lightning strike to travel down the tower and into the ground, bypassing the repeater equipment. Without proper grounding, a lightning strike could travel down the coax shield, enter the equipment, and cause catastrophic damage.
Static Discharge: Static electricity can build up on the coaxial cable shield from wind, rain, and snow. A grounding kit continuously drains this charge to the earth, preventing it from building up to a level that could damage sensitive electronic components or cause a static shock to personnel.
Safety: In the event of a fault in a power system near the repeater, a grounding kit ensures that the coaxial cable shield remains at a safe electrical potential, protecting maintenance personnel from dangerous “touch” voltages.

Where and How They Are Installed

Grounding kits are typically installed at specific points along the coaxial cable run to be effective:

At the Top of the Tower: A grounding kit is installed near the antenna to provide a path for lightning and static buildup that happens at the highest point of the installation.
At the Base of the Tower: Another grounding kit is installed where the coaxial cable exits the tower and begins its run toward the shelter or building. This ensures that any energy collected on the tower is shunted to ground before entering the structure.
At the Building Entry: A final ground kit is often installed at the building’s entry point to provide a final opportunity to divert any remaining surge energy to the building’s ground system.

A grounding kit consists of a copper strap that clamps around the coax shield, a heavy-gauge insulated ground wire, and a lug for connecting to the tower or ground bus bar. The connection to the ground system must have a very low impedance to be effective against the high-frequency energy of a lightning strike.

per Google Gemini