What Is the Biggest Effect of a Line Isolator?

The Primary Purpose: Eliminate Common Mode Currents

For most users, the biggest immediately noticeable effect of using a line isolator is a drastic reduction in noise levelson receive — often by several S-units, especially on the lower HF bands such as 160m, 80m, and 40m. These bands are more prone to common mode noise from household electronics, power lines, and other local sources. On VLF/LF (below 500 kHz), a line isolator can significantly reduce hum and low-frequency noise in active antenna systems. On the higher HF bands (20m–10m), the benefits are typically less dramatic but still useful — especially in dense urban or industrial environments where local RF noise and RFI are present. This leads to clearer audio, better weak-signal reception, and in many cases, a lower noise floor that makes your entire station feel more "quiet" and responsive. On transmit, users frequently report elimination of RF feedback issues, more stable SWR readings, and cleaner signals, especially on vertical or end-fed antennas.

The most significant benefit of a line isolator is the elimination of common mode currents flowing on the outside of your coaxial cable. These unwanted currents can cause a wide range of issues:

  • Unstable or unexpectedly high SWR
  • RF feedback into the shack (microphone issues, hot chassis, etc.)
  • Increased noise pickup, especially on receive
  • Radiation pattern distortion (particularly on vertical antennas)
  • RFI into other electronics or equipment in the house

By inserting a line isolator into your feedline, you break the path of these currents, preventing your coaxial cable from acting as an unwanted radiator or noise collector.

Line Isolators for Transmit (TX)

We offer a powerful range of TX line isolators, all engineered to handle high power levels and provide superb common mode rejection across HF:

  • TX 6kW ICAS Band-Specific Line Isolator – Available in Low, Mid, or High band versions. Offers +30 dBcommon mode rejection, optimized per frequency range. Compact and efficient for specialized use.
  • TX 9kW Wide Band Line Isolator (30 dB) – Handles up to 9kW ICAS from 1.5 to 30 MHz, with +30 dB CMR. Perfect for legal-limit stations looking for full-band coverage with robust construction.
  • TX 9kW Wide Band Line Isolator (45 dB) – Same power handling and bandwidth as the 30 dB model but with enhanced +45 dB common mode rejection. Ideal for demanding contest or multi-transmitter environments.

All our TX isolators use high-grade ferrites, heavy-duty enclosures, and premium components, built for both performance and durability.

Line Isolators for Receive (RX)

Even small amounts of common mode current can degrade receive performance — especially with high-gain active antennas or long coax runs. We offer two models tailored for receive-only lines:

Both are compact, purpose-built, and perfect for reducing unwanted noise pickup on receive lines.

Where to Place Line Isolators?

Placement is key to maximizing the benefit:

  • At the feedpoint of your antenna to prevent coax radiation
  • At the shack entry point to stop unwanted RF from entering the operating position
  • On receive antenna lines to block noise ingress via coax shield

A very common and highly effective use is placing a line isolator directly behind a UNUN or transformer. This forces common mode currents to remain confined within the antenna system rather than traveling down the coax shield. Depending on the antenna design:

  • If the coax braid is not part of the counterpoise, the isolator is placed immediately after the transformer using a short jumper.
  • If the braid is used as part of the counterpoise, the isolator is placed a fraction of a wavelength away, typically between 0.05λ and 0.15λ, to allow the intended current distribution and then choke off unwanted RF beyond that point.

This technique is especially effective for end-fed half-wave, long wire, and vertical antennas with transformer feeds.

Placement is key to maximizing the benefit:

  • At the feedpoint of your antenna to prevent coax radiation
  • At the shack entry point to stop unwanted RF from entering the operating position
  • On receive antenna lines to block noise ingress via coax shield

For some installations, using more than one isolator is the most effective approach.

Summary

Biggest effect? 👉 Eliminates common mode currents on coaxial cable

Why it matters?

  • Lower noise floor on receive
  • Cleaner transmit signals
  • Reduced RFI and shack feedback
  • More predictable antenna performance

Explore the full range of RF.Guru isolators here: 🔗 https://shop.rf.guru/collections/line-isolators

Need help choosing the right one? We're happy to assist!

Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.