Understanding and Fixing Interference

The Two Sides of EMI

As an Advanced licensee running up to 400W, you're more likely to encounter interference issues — both ways:

Your signal interfering with others: TV interference (TVI), affecting neighbour's electronics
Other things interfering with you: Plasma TVs, LED lights, switch-mode power supplies, solar inverters

How Interference Gets In

There are two main paths:

Conducted: Interference travels along cables — power leads, audio cables, Ethernet cables, antenna feedlines. The cables act as unintentional antennas.
Radiated: Interference travels through the air as electromagnetic waves directly into the affected device.

In amateur radio, conducted interference via common-mode currents is the most frequent culprit. RF gets onto the outside of cables and enters equipment where it's not wanted.

Your EMC Toolkit

1. Ferrite Chokes (The Universal Fix)

Snap-on or toroidal ferrite cores around cables are the single most useful EMC tool. They block common-mode currents while having minimal effect on the wanted signal.

Use them on:

DC power leads to your radio
Coax feedline at the shack entry
Cables on affected equipment (TV, computer, etc.)

2. Low-Pass Filter (After Your Transmitter)

Suppresses harmonics of your transmitted signal. A 7 MHz signal has harmonics at 14, 21, 28 MHz etc. These must be suppressed to legal limits. A good LPF gives 40-60 dB suppression.

3. High-Pass Filter (On the Affected Device)

If your HF signal is getting into a neighbour's TV, a high-pass filter at their TV antenna input blocks your HF signal while passing VHF/UHF TV signals.

4. Band-Pass Filter (For Specific Problems)

Passes only your operating frequency and rejects everything else. Maximum filtering but only works on one band.

Troubleshooting order:

First, make sure YOUR station is clean — check harmonics and spurious emissions
Add ferrite chokes to cables in your shack
If the problem is at the neighbour's end, suggest ferrite chokes on their equipment cables first
If needed, offer to install a high-pass filter at their TV

Intermodulation at Your End

If you overdrive your amplifier, it creates intermodulation products — spurious signals that interfere with other users. Keep your drive levels within the linear range of your PA. If someone reports "splatter" from your signal, turn down the drive!