Section 9.2: Secondary Allocations and Interference #

Picture this: You’re enjoying a quiet QSO on 30 meters when suddenly a powerful digital signal obliterates your conversation. You’re about to complain when you remember—wait, we’re secondary users here. Time to spin the dial and find a new frequency.

Welcome to the reality of shared spectrum.

Primary vs. Secondary: Understanding the Pecking Order #

Every slice of radio spectrum has a hierarchy, like a carefully choreographed dance where some partners always lead:

• Primary Users: The VIPs with first dibs on frequencies
• Secondary Users: Must work around the primary users

Think of it as renting a room in someone else’s house. You can use the space, but when the homeowner needs it, you step aside—no questions asked.

Key Information: When the FCC rules designate the amateur service as a secondary user on a band, amateur stations must not cause harmful interference to primary users and must accept interference from primary users.

This isn’t just bureaucratic red tape—it’s the price we pay for access to prime spectrum real estate. As secondary users, we operate under a simple two-part contract:

1. Never interfere with the primary tenant
2. Accept whatever interference they cause us

If your signal disrupts a primary service, you must immediately fix the problem—which usually means changing frequency or going QRT. And if they disrupt you? Smile and spin the dial. It’s their house; we’re just visiting.

Where Amateur Radio Takes Second Place #

Several popular bands treat amateur radio as the junior partner:

• 60 meters (5 MHz): We share with government operations who definitely outrank us
• 30 meters (10.1-10.15 MHz): Fixed services get priority
• 70 centimeters (420-450 MHz): In some areas, government radiolocation rules the roost

Each secondary allocation comes with its own personality. On 60 meters, you’re limited to specific channels and power levels. On 30 meters, no phone allowed—data and CW only. It’s like each band has house rules posted on the door.

The Interference Avoidance Dance #

Operating as a secondary user is just one scenario where you need to tread carefully:

Key Information: When operating within one mile of an FCC Monitoring Station, when using a band where the amateur service is secondary, or when a station is transmitting spread spectrum emissions, a licensed amateur radio operator must take specific steps to avoid harmful interference to other users or facilities.

Let’s break down these special situations:

Near FCC Monitoring Stations: These facilities are like the FCC’s ears, constantly listening for rule violations and interference. Operating nearby requires extra caution—their receivers are incredibly sensitive, and your “perfectly clean” signal might still cause them problems. It’s like trying to have a conversation next to someone with superhuman hearing.

Secondary Band Operations: As we’ve discussed, when you’re the secondary user, interference avoidance isn’t just polite—it’s mandatory.

Spread Spectrum Emissions: These signals spread across a wide swath of spectrum like butter on toast. While they offer advantages like interference resistance, they can also interfere with multiple services at once if not carefully controlled.

Your interference-prevention toolkit includes:

• Running minimum necessary power (QRP when possible)
• Choosing operating times when primary users are less active
• Being ready to QRT at a moment’s notice
• Using directional antennas to keep your signal focused
• Monitoring before transmitting—always

The Beacon Network: Amateur Radio’s “Do Not Disturb” Frequencies #

Some frequencies are so important to the amateur community that we’ve all agreed to leave them alone:

Key Information: Amateur operators should normally avoid transmitting on 14.100, 18.110, 21.150, 24.930, and 28.200 MHz because a system of propagation beacon stations operates on those frequencies.

These beacons are like lighthouses for radio waves. The International Beacon Project runs a synchronized network where beacons transmit in sequence, allowing you to instantly assess propagation to different parts of the world. Imagine trying to navigate by lighthouse, only to have someone shine a spotlight in your face—that’s what happens when you transmit on beacon frequencies.

The beacon cycle is elegant: Each beacon transmits for 10 seconds, identifying itself and its location. By listening for just a minute, you can hear beacons from six continents and know instantly which paths are open. Mess with this system, and you’re not just causing interference—you’re dismantling a global propagation tool.

Robots on the Air: Automatically Controlled Stations #

When computers take the controls, special rules apply to prevent runaway interference:

Key Information: Automatically controlled stations transmitting RTTY or data emissions may communicate with other automatically controlled digital stations anywhere in the 6-meter or shorter wavelength bands, and in limited segments of some of the HF bands.

Think about it: An unattended station could theoretically transmit forever if something goes wrong. That’s why automatically controlled digital stations face restrictions:

VHF/UHF Freedom: On 6 meters and up, automated stations can operate anywhere digital modes are allowed. The shorter range of these bands limits potential interference.

HF Restrictions: On HF, where signals can circle the globe, automated stations are confined to specific sub-bands. It’s like designating certain highways for self-driving cars—keeps them from causing problems across the entire road system.

These rules enable amazing capabilities like:

• Winlink email gateways that forward messages automatically
• APRS digipeaters that relay position reports without human intervention
• Weak signal propagation reporters that log band openings 24/7

When Interference Happens: A Practical Guide #

Despite everyone’s best efforts, interference happens. Here’s your response playbook:

Step 1: Check Yourself Before pointing fingers, ensure your station is clean. Overmodulation, spurious emissions, or a mistuned antenna can make you the interference source.

Step 2: Document Everything Note the time, frequency, type of interference, and any identifying characteristics. This information becomes crucial if you need to track down the source.

Step 3: Try Technical Solutions First

• Change frequency (easiest solution)
• Adjust operating times
• Try different modes
• Reduce power
• Use directional antennas

Step 4: Communicate If another ham is involved, a polite conversation often resolves the issue. Most operators want to be good neighbors.

Step 5: Know When to Fold If you’re secondary and causing interference to a primary user, there’s no debate—you must cease operation on that frequency.

Beyond Your Shack: Neighborhood Interference #

Your General privileges come with more power, and with more power comes… more potential to disrupt your neighbor’s Netflix binge. While consumer electronics interference isn’t “harmful interference” in the legal sense, maintaining neighborhood harmony keeps amateur radio welcome in your community.

Common solutions:

• Install proper station grounding (solves many RFI issues)
• Add ferrite cores to problematic cables
• Help neighbors install filters
• Operate during less sensitive hours
• Keep good relations—they’re more patient with friends

Remember: Today’s interference complaint could be tomorrow’s antenna restriction. Handle RFI issues promptly and professionally.

International Spectrum Tetris #

Cross a border, and the spectrum map changes. What’s primary in the US might be secondary in Canada, or completely off-limits in Mexico.

When operating internationally:

• Research the host country’s allocations
• Respect their band plans, not yours
• Remember you’re a guest in their spectrum
• Some countries have regional variations—check local rules

Band Plans: The Gentleman’s Agreement #

Beyond FCC regulations lie the voluntary band plans—amateur radio’s social contract. The IARU (International Amateur Radio Union) coordinates these plans globally, creating order where regulations leave flexibility.

Following band plans isn’t legally required, but it’s like following trail etiquette while hiking. Sure, you could blast music on the peaceful trail, but you’d quickly become that person everyone avoids.

Band plans typically specify:

• Where to find CW, digital modes, and phone
• Calling frequencies for each mode
• DX windows for international contacts
• Special event and emergency frequencies

The Bigger Picture #

Every time you yield to a primary service, every time you QSY to avoid interference, every time you check the beacon frequency before transmitting—you’re participating in a massive, global spectrum-sharing experiment that actually works.

This cooperation isn’t just polite; it’s strategic. Spectrum battles happen regularly at World Radiocommunication Conferences. When delegates see amateur radio operators successfully sharing spectrum without constant enforcement actions, it strengthens our position. We demonstrate that self-regulation works.

Your actions on secondary allocations today influence whether amateur radio keeps those allocations tomorrow. No pressure, but the future of our hobby partially depends on how well we play with others.

Ready to explore what happens when amateur radio goes digital? In the next section, we’ll decode the rules that keep our digital modes from becoming digital chaos.