Horizontal Antenna Setup

Figure 1 illustrates an optimized horizontal antenna setup, including the recommended cabling path between the antenna (left) and transceiver (right). The various components depicted in the figure play a crucial role in mitigating unwanted common-mode currents.

Components and Their Functions

BAL (Green Block) – Balun

The balun ensures a proper transition between the balanced antenna and the unbalanced coaxial cable. It prevents unwanted RF currents from flowing along the outer surface of the coaxial cable during transmission and suppresses common-mode noise from external sources during reception. A high-quality current balun (common-mode choke type) is recommended for this purpose.

LI1, LI2, LI3 (Yellow Blocks) – Line Isolators

These devices suppress residual common-mode currents on the coaxial transmission line.

LI1 is typically placed at the tower base or directly beneath a wire antenna, near the ground. It should be connected using the shortest and thickest possible conductor to a good RF ground. This isolator primarily eliminates common-mode currents induced in the coaxial cable in the antenna’s near field.

LI2 is used along long coaxial cable runs and functions similarly to LI1 and LI3. Its placement is optional and depends on the specific installation.

LI3 is installed where the coaxial cable enters the house or shack. This is the most critical location for a line isolator, as it separates the noisy safety ground (connected to the transceiver) from the “clean” RF ground on the antenna side. It is important to orient the isolator correctly, with the clean RF ground side toward the antenna.

Note: Although LI1, LI2, and LI3 may have different names based on their placement, they are functionally identical coaxial common-mode chokes designed to provide high impedance at the frequencies of interest.

TRX (Blue Block) – Transceiver

The transceiver is typically grounded via its power supply to the house’s safety ground, which protects against electrical hazards. However, this ground has a high RF impedance and is ineffective at draining RF common-mode currents or noise. Improved RF grounding can be achieved by:

• Installing a local, low-impedance RF ground near the radio equipment.
• Relying on the RF grounding provided by the line isolators further down the transmission line.

Vertical Antenna Setup

Figure 2 illustrates an optimized vertical antenna setup, including the recommended cabling path between the antenna (left) and transceiver (right). As with the horizontal setup, the blocks depicted play a vital role in minimizing unwanted common-mode currents.

Components and Their Functions

ZM (Green Block) – Impedance Matching Circuit

Vertical antennas installed above a good ground plane offer excellent low-angle radiation, making them highly effective for DX operation. This is particularly beneficial on lower HF bands, where horizontal antennas become less practical. However, loaded vertical antennas often have low feedpoint impedances (sometimes below 25Ω), requiring impedance matching for acceptable VSWR. Matching can be achieved using:

• A narrowband L/C network
• A wideband UNUN impedance matching transformer

LI1, LI2, LI3 (Yellow Blocks) – Line Isolators

LI1 is placed immediately after the impedance matching circuit, close to the vertical antenna and ground. It should be connected using the shortest and thickest possible conductor to a good RF ground. This isolator ensures that the coaxial outer shield does not act as a radial, thereby preventing significant common-mode current from flowing back to the transceiver and causing interference. It also blocks common-mode noise from coupling into the received signal.

LI2 is used along long coaxial cable runs and functions similarly to LI1 and LI3. Its placement is optional, depending on the specific installation.

LI3 is installed at the house or shack entry point. As with the horizontal setup, this is the most critical placement for a line isolator, as it isolates the noisy safety ground (connected to the transceiver) from the “clean” RF ground on the antenna side. Proper orientation is crucial, with the clean RF ground side toward the antenna.

Note: While LI1, LI2, and LI3 are deployed at different locations, they are technically identical coaxial common-mode chokes, providing high impedance at the operating frequency.

TRX (Blue Block) – Transceiver

As in the horizontal setup, the transceiver is connected to the house safety ground, which is ineffective for RF grounding due to its high impedance at radio frequencies. Proper RF grounding can be achieved by:

• Installing a local, low-impedance RF ground near the radio.
• Using the RF grounding provided by the line isolators along the coaxial transmission line.

General Considerations

RF Grounding vs. Safety Grounding

An RF ground (RF GND) is distinct from a safety ground:

• •Safety ground is designed for low resistance at AC mains frequency (50Hz or 60Hz) to protect against electrical hazards.
• RF ground must have a low impedance at radio frequencies, which requires interconnecting multiple ground rods spaced apart and connected with a short, thick conductor to the antenna or transmission line.

Additional Common-Mode Current Paths

Common-mode RF feedback and noise can also be introduced via:

• Antenna rotor cables
• Control cables

Since these cables are positioned in both the antenna near-field and radio room, they should also be equipped with wideband common-mode chokes for optimal performance.

Summary

By strategically placing baluns, line isolators, and impedance matching components, unwanted common-mode RF currents and noise can be effectively suppressed, leading to:

• Improved signal clarity
• Reduced interference
• Enhanced overall system performance

Article written by Joeri Van Dooren, ON6URE – RF engineer, antenna designer, and founder of RF.Guru. With extensive experience in active and passive antenna systems, high-power RF transformers, and custom RF solutions, Joeri shares insights into cutting-edge radio communication technologies.