Why Some Equipment Causes Interference to Shortwave and Ham Radio Receivers

Modern computers, monitors, and accessories can cause interference to shortwave and ham radio receivers due to electromagnetic noise they emit. This issue is compounded by the excellent receive sensitivity of today's radios, making even low levels of noise problematic. The key factor is whether the equipment meets Part 15 Class A or Class B regulations, with Class B devices designed for residential environments and being less likely to interfere with radio reception. Understanding the difference between these classifications can help mitigate interference in your radio setup.

Electromagnetic Noise from Electronic Devices

Electronic devices, such as computers and their peripherals, generate electromagnetic interference (EMI) during their operation. Switching power supplies, LED lighting, and high-speed digital processing create high-frequency emissions that can affect sensitive radio receivers operating in the shortwave and ham bands. These emissions often manifest as buzzing, humming, or static across a range of frequencies, disrupting clear radio reception.

Part 15: Class A vs. Class B

Under FCC regulations, devices that emit RF energy must comply with Part 15, which outlines acceptable emission limits. Devices are categorized as Class A or Class B:

• Class A devices are meant for industrial or commercial use and have higher allowed emission levels. These can interfere with nearby radio receivers, especially in non-industrial environments.
• Class B devices are intended for residential use and are held to stricter emission standards, making them less likely to interfere with consumer electronics, including radio equipment.

Impact on Sensitive Radios

The sensitivity of modern shortwave and ham radios has improved significantly, allowing them to pick up weak signals from distant stations. However, this sensitivity also makes them more susceptible to interference from nearby electronic devices. Even small amounts of noise from Class A devices, or poorly shielded Class B equipment, can drown out weak signals or add background noise.

Mitigating Interference

To reduce interference, consider using Class B-rated equipment in the vicinity of your radio gear, and place noise-generating devices as far from your antenna and receiver as possible. Additionally, the following techniques can help minimize unwanted noise:

• Use ferrite chokes on cables to suppress high-frequency interference.
• Properly ground your radio and associated equipment.
• Utilize shielded cables where possible to reduce RF noise pickup.
• Avoid placing electronic devices close to your radio antenna.

It’s also important to note that end-fed antennas and random long-wire antennas are particularly prone to interference. Unlike dipole antennas, these setups often use the coaxial cable as a counterpoise, meaning the coax becomes part of the receiving antenna. Because the coax is not purely acting as a shielded transmission line, it can pick up noise from nearby equipment, exacerbating the interference problem.

Typical Example

Refer to this illustration, from an actual station that is reporting computer interference.

There are five (5) flat panel displays visible in this picture, not counting the one embedded in the transceiver. The laptop is in a plastic case, which is invisible to RF. The four large monitors are all Class A and have connecting cables to the controlling computer. The system at far left consists of a Raspberry Pi computer driving that world time display, also a Class A device, with zero shielding.

The person with this station who is reporting the interference did not say what type of antenna and feedline he is using. But for certain, the amount and effctiveness of shielding with coax cable, as well as power supply filtering, will make a big difference in troubleshooting and permanently fixing the interference problem.

Interference is measured in something like "microvolts per meter". This means, among other things, that you want your receiving antenna as far as possible from the interference source. This is why we put our antennas outside, since the interference is usually inside. But if your antenna is not matched, or resonant, this makes the coax cable to stop acting like a transmission line, and it becomes just a part of the receiving antenna, from the place it attaches to the antenna, all the way into your house. Any interference along the way is no longer shielded from your receiver and is simply picked up and added to your interference problem.