Natural Radio listening would be easy if Natural Radio signals were the only signals in the band. Unfortunately, the dominant signals that you will hear are the sounds of a technological civilization.
Powerline Hum & Buzz
If we would have stuck with Edison’s idea for transmitting DC power, Natural Radio listening might have been a bit easier, but AC won out. Long wires carrying lots of amps of AC current tend to radiate a bit despite the best attempts of the electric companies to keep all the current in the wires. Now, if the problem were only the fundamental frequency of 60 Hz. in North America and 50Hz. in most other places, we could filter that out with out too much trouble.
But sadly, people like to connect lots of non-linear devices to the power grid, insulators get dirty and start to arc, and loads constantly shift. The result being that not only do we hear the fundamental frequency of 50 or 60Hz., but we hear all the harmonics that get generated all the way up beyond the range of our hearing.
Hum generally refers to the fundmental frequency and maybe the first couple harmonics. It sounds like a pure tone of fairly low frequency. Buzz is rich in harmonics and is very raspy sounding. While we can effectively filter out hum because it is below the range of signals that we are normally interested in listening to, buzz extends well into the range of signals we want to hear and is difficult to remove without degrading the wanted signals.
This is why it is important to be well away from power lines for the best listening conditions.
Miscellaneous Electronic Interference
Power line noise isn’t the only problem we contend with. There are lots of other signals that don’t arise directly from the power line frequency but because they are generated by devices connected to AC power, they may propagate down and be radiated by power lines. Transformerless fluorescent ballasts are notorious for their noise, and many of the cheaper new LED bulbs are noisy as well. Computers and monitors generate a variety of noise, as does any device with a switching power supply.
A less troublesome issue is that power companies may send different tones down the power line for telemetry or control of certain equipment. You may occasionally pick these up and wonder what they are.
Again, here’s another reason to head for the country to do your listening. But be aware that passing cars and trucks can generate noise with their ignition systems as well as their alternators.
The static electricity built up by certain tires on the road can be heard in a Natural Radio receiver. This usually isn’t an issue and can only be heard when you are within tens of feet from the source. It fades away quickly as the distance increases beyond about 100 feet. I’ve heard this effect from both automobiles and bicycles. It sounds like a faint swish or increase in hiss as the vehicle passes.
The beating of insect wings can disrupt the charge around your E-field antenna so that you actually hear the buzzing of the insect. Again, this is not usually a problem, unless a bumblebee gets fixated on your antenna, and then it can be very irritating because it can get quite loud. You’ll probably throw off your earphones the first few times it happens, because the illusion it creates is that a large insect just flew into your ear. The bug needs to be within inches of your antenna for this to occur.