Radio frequency interference was a problem fifty years ago. It is a problem today. We can actually learn a lot from this very old presentation. Enjoy!
I recently came across this amazing video on YouTube. It appears to be a U.S. Army training film from 1961, somehow recovered and posted by the folks at quickfound.net. “Warring forces created by ourselves, and not the enemy, threaten catastrophe.”
The battleground today is your house and neighborhood. Tactically, the problem is not usually other intentional transmitters that need to be coordinated. Rather, it is unintentional transmitters. The main enemy is the modern power supply for consumer and household equipment. In other words, it is the combination of modern power supplies and the systems in which they operate.
Modern power supplies, and the equipment they power, are increasingly manufactured to save energy. Saving energy is a good thing to do. Saving energy is frequently mandated by government regulations. This happens often by setting minimum standards that manufacturers must meet in order to sell their products. It also happens by government-sponsored incentives for consumers to buy equipment that exceeds these standards.
Modern power supplies are generally known as switched mode power supplies. SMPS are typically those little cubes you use to power or charge your consumer electronics. They work by manipulating electrical power at radio frequencies. By their very design, they cause radio frequency interference as an unintended side effect. There are standards which manufacturers are supposed to meet. These standards govern both the conducted and radiated emissions from power supplies and associated consumer equipment. The standards are higher for devices used in the home, than for devices used in industry. Many devices meet these standards, but many don’t.
What typically happens is that SMPS give off radio frequency interference that enters household wiring. Household wiring acts as a great antenna. It amplifies and broadcasts the radio frequency interference around your home and around your neighborhood.
Listen to the radio frequency interference in your home
Check this out for yourself. Take your portable radio and tune it to a clear channel the AM broadcast band. Hold your radio next to a power cube. If you don’t hear a strong buzzing sound right away, adjust the frequency a bit. You will hear it. Then, if you put your radio back against the wall next to the power cube, you will be able to trace your household wiring through the wall. Follow the noise. When you get to the power panel in the furnace room, you will hear a large collection of radio frequency interference from all of the power supplies in your house.
Normally, when consumer devices meet emissions standards, the level of the radio frequency interference will be quite low. Radio signals decrease quickly by distance – the inverse square law. But sometimes, when the power supply is not meeting standards, or when it is being forced to work too hard, radio frequency interference power increases exponentially. Some consumer devices that can give off exceptionally strong radio frequency interference are:
- Washing machines and treadmills. These systems work hard to get the most physical power from electrical power efficiently. Their power supplies manipulate household current in ways that create square waves with fast rise and fall times. Without really good filtering, the low frequency square waves emit a spectrum of higher frequency harmonics. These harmonics enter your household wiring and travel a great distance.
- Low voltage lights. Since many if not most electricians do not understand much about radio frequency interference, they install long lengths of wiring between the halogen pot lights and the power supply. Normally, to meet standards, wiring harnesses for these lights should not exceed 8-10 feet. However, you need 25-50 feet to wire halogen pot lights in most kitchens. The voltage drop across all of this wiring stresses the power supply, causing increased radio frequency interference. This RFI is then broadcast the by the wiring. Incidentally, an easy way to find out if your neighbor is growing pot is to just turn on your radio. Grow lights are amazing emitters of radio frequency interference.
Why does all of this matter to ham radio operators and shortwave listeners? Simple. The level of the radio frequency interference is typically stronger than the level of the radio signals travelling around the world. It can be like trying to hold a conversation while standing next to a construction site.
Why should all of this matter to ordinary people? Simple. It’s the invisible battleground shown in the 1961 film. We are moving into the Internet of Things. Potentially, all of our devices are going to increasingly interfere with each other. One recent example concerns arc fault circuit interrupters (AFCI) which are safety devices now being mandated by some electrical codes. They can often be “tripped” by radio frequency interference from plasma televisions, low voltage lights and switched mode power supplies. Manufacturers refer to these as problems “nuisance trips”. The invisible battleground is radio frequency interference. Your local ham or shortwave listener is just the early warning device.