Understanding ARRL’s Clean Signal Initiative: What It Means for Every HF Operator

If you have spent any time on the HF bands, you have probably noticed the problem. There is that sudden click from a nearby CW station, the SSB signal that seems to splatter 5 kHz wide, or the background hiss that rises when a strong station transmits. For years, receiver technology has made huge leaps with better dynamic range, sharper filters, and amazing DSP. But transmitters have not kept up.

ARRL is tackling that problem head on with the Clean Signal Initiative (CSI), officially launched in the April 2026 issue of QST. Here is a comprehensive breakdown of what CSI is, how it works, and why it matters to you whether you are contesting, DXing, ragchewing, or just listening.

1. Why CSI Exists: The “Dirty Transmitter” Problem

For decades, HF receiver performance has steadily improved. Modern radios can pull weak signals out of noise that would have been impossible 20 years ago.

Transmitters did not keep up. The result is a band environment where many signals are wider and noisier than they need to be. Three main culprits cause this:

CSI was conceived by Mike Ritz, W7VO, and Ward Silver, N0AX, then developed by ARRL Lab staff with RF experts and radio manufacturers. The goal is to create agreed benchmarks so manufacturers design cleaner transmitters, and operators understand what clean actually means.

2. How CSI Certification Works

The ARRL Lab is the only entity that can certify a radio as CSI compliant. Starting with the April 2026 QST, every HF transceiver reviewed will be tested against CSI.

Two prerequisites before a radio is even considered:

1. It must comply with FCC rules for spurious emissions on every band it transmits.
2. Amplifier key line timing must prevent hot switching. That is when an amp changes T/R state while RF is still present, causing wideband interference.

The Three CSI Benchmarks: A radio must pass tests on every band it can transmit to earn compliance in that category.

CSI Badge System: You will see these in QST Product Review Table 1 and near the review title.

• Gold: Meets all 3 benchmarks
• Silver: Meets 2 of 3
• Bronze: Meets 1 of 3
• No badge: Fails all 3, but the graphs and masks are still shown. If a radio misses by 1 dB or less on one band, Lab Notes will call it out as close and still a good performer.

Manufacturers can use CSI graphics on packaging and marketing, so you will start seeing it in catalogs and at hamfests.

3. What Is Changing in QST Product Reviews

ARRL Lab is updating how it reports transmitter data so you can compare cleanliness at a glance.

New and Changed Items:

• Figure B: Now shows CW bandwidth for the worst performing band, not the same reference band every month.
• New Figure D: Two tone transmit IMD graph of worst band, referenced to dBc.
• Key Measurements Summary: IMD now reported in dBc instead of relative to PEP.
• Transmit Keying Sidebands: Shows max and min power in each CSI mask bandwidth for worst band.
• Transmit Composite Noise: Replaces Transmit Phase Noise. Reports max and min in the two CSI mask bands at 20m full power.
• Figure C: Now displays composite noise. Separate phase and amplitude noise graphs moved to www.arrl.org/qst-in-depth.
• Removed: Audio Output section. It was deemed too subjective.

Data for all bands stays available in Table 1 and online at QST In Depth. The graphs just highlight worst case to keep manufacturers accountable.

4. What Clean Actually Looks Like on Graphs

CW Bandwidth example: A CSI compliant CW signal has the blue trace completely under the red mask at all offsets. This ensures key clicks are down 60 dB by 675 Hz and inaudible to adjacent stations.

Two Tone IMD example: Non compliant signals have 3rd through 11th order products that poke above the red mask. That is what causes SSB splatter. Predistortion technology in newer radios is one way manufacturers are fixing this.

Composite Noise example: A non compliant result at 10 kHz offset might be -120 dBc/Hz, but CSI requires -136 dBc/Hz or better. That 16 dB difference means the radio’s noise would mask weak signals 10 kHz away.

5. What About Radios Already on the Market

If you own a radio reviewed before April 2026, it was not tested to CSI. Manufacturers can send previously reviewed models back to ARRL Lab, at their expense, for CSI testing. The Lab will only re test the three CSI benchmarks and add badges to the original review online.

New transceivers get priority, so re tests are first come, first served. Do not expect every old model to get a badge overnight.

6. Why This Matters to You, the Operator

1. More QSOs in Less Spectrum: If everyone’s signal is 3 kHz wide instead of 6 kHz, the 20m phone band doubles its capacity. Contests get less chaotic.
2. Better Weak Signal DX: Lower composite noise means that rare DXpedition is not buried when a big gun 20 kHz away transmits.
3. Informed Buying: Bronze, Silver, and Gold give you a quick way to compare on air friendliness beyond receiver specs. A Gold badge radio is less likely to cause complaints.
4. Operator Education: CSI is not just for manufacturers. Poor ALC settings, overdriving, and dirty amps still cause splatter even with a Gold radio. ARRL plans more tech articles and videos to help operators run clean.

7. The Bottom Line

CSI does not ban older radios or force you to upgrade. It sets a clear, measurable bar for good neighbor performance and gives manufacturers incentive to hit it. As George Spatta, W1GKS, ARRL Lab Manager, put it, the goal is ensuring modern amateur transmissions occupy no more bandwidth than necessary.

If you are shopping for a new HF rig in 2026 or later, look for the CSI badge. If you are keeping your current radio, use CSI graphs to understand its limits and make sure you are not the one splattering.

Clean signals mean more contacts for everyone. And that starts now.

Want to dig deeper? Check https://www.arrl.org/files/file/QST%20Binaries/April%202026/Spatta%20Clean%20Signal%20Initiative.pdf