Here's a problem that crops up from time to time when you are listening to broadcast AM stations on short wave. (Medium frequencies can be affected as well.) Perhaps you made some rearrangement to your receiving equipment, you tune to your favorite AM station, the signal strength is good and strong -- but there is a terrible hum on the carrier.
Now the BBC, CBC and VOA are not in the habit of broadcasting hum on their shortwave transmissions so something must be wrong with the receiving equipment. Your first reaction might be to blame the radios power supply -- perhaps the smoothing capacitor is failing or the supply is overloaded? The chances are this is not the cause. Instead you should be looking for a nearby source of hum modulation.
Here's the explanation high frequency radio waves from your favorite shortwave broadcaster are propagating hundreds or thousands of miles through the air until they reach your location with just the usual mixture of fading, phase distortion and noise from their passage through the ionosphere. Unfortunately something close to your radio's antenna is picking up the strong short wave signal and rebroadcasting it with power-line frequency hum superimposed.
If you have a portable receiver with a built-in antenna, the most likely source of hum modulation is the receiver's own power supply. A good quality shortwave radio such as a Sony -- with the recommended Sony power supply -- probably won't be affected. But a cheap power supply not intended for shortwave reception can be the source of plenty of hum modulation.
The offending devices are the silicon diodes inside the power supply. They are being switched on and off at twice the power line frequency (2 x 60 Hz in the USA, 2 x 50 Hz in Europe). However, if the diodes also have a high frequency shortwave signal passing through their circuitry, the shortwave signal will be distorted as the diodes change from conducting to non-conducting. That distorted signal is then coupled into your receiver or re-radiated by the power supply's cables.
If your receiver is a shortwave portable set, you can test this theory very easily. Tune in a signal with hum modulation, then switch off the AC power supply and listen to the station using the set's built-in batteries. If the hum disappears then the power supply is involved. Be aware that unplugging the power supply may reduce the strength of the short wave signal -- the power supply cabling is often part of the antenna! If the hum does not disappear, you will have to search wider for your source of hum modulation -- see later.
Power supply fixes
If the receiver power supply is the source of your problem, shortwave signals are entering via the power supply cables, being hum modulated by the rectifier diodes then exiting on the same cable. There are two approaches filter the cables or stop the hum modulation at the diodes.
First, try to prevent shortwave signals entering and exiting on the power supply cables. You can wind the cable around a ferrite rod, thread the cable through a ferrite toroid, or use one of the "snap-on" ferrite choke cores available from Radio Shack (273-104, 273-105), MFJ (MFJ-701) and some computer fairs. The more turns of wire through the core, the higher the attenuation of HF signals.
External filtering with ferrite chokes is safe and convenient, but may not cure the problem completely. The second, more drastic approach is to bypass the power supply diodes to high frequency signals. This will require disconnecting and opening the power supply, identifying the rectifier diodes and soldering suitable disc ceramic capacitors across each diode. If you are not confident of your abilities to solder and work near line voltage components, then leave well alone! If the power supply is a molded and sealed unit, you may want to leave that alone as well. I wont be responsible for damage you cause to your power supply, to other equipment or yourself!
If you decide to bypass the diodes, here is a typical DC power supply with the four diodes of the bridge rectifier identified, then bypassed.
Each of the rectifier diodes should be bypassed with a disc ceramic (= ceramic disc) capacitor -- typical values would be 0.01-0.05 microfarad (mF). Make sure the working voltage of the capacitor is substantially higher than the reverse voltage across each diode.
Switch off and unplug the power supply before doing any work on it. Your supply might have a four-terminal bridge rectifier unit rather than four separate diodes. Another possibility is a two-diode full wave rectifier with a center-tapped transformer. Just make sure that each power diode is bypassed to radio frequencies with a disc ceramic capacitor.
Incidentally, my first encounter with modulation hum was back in the '70s, trying to service a car radio while running from a 12 volt ex-computer power supply. The hum on MF broadcast stations had to be heard to be believed -- but it disappeared as soon as I bypassed the rectifier diodes in the power supply.
A more up-to-date example concerns a Sony ICF-2010 receiver, which worked fine on its own with its Sony power supply, but suffered terribly when connected to amplified loudspeakers -- in this case a Cambridge SoundWorks "PCWorks" system. The culprit was the 12 volt power supply for the amplified speakers. I fixed the problem by substituting a different power supply with bypassed diodes.
Other sources of modulation hum
Lets suppose you have filtered and bypassed your power supply -- but you are still experiencing modulation hum on strong shortwave stations. Its likely that some other item of line-powered equipment is picking up the short wave signal, hum modulating the carrier, then re-radiating the distorted result.
The difficult part is determining exactly which item of equipment is at fault. A modern home might have ten or more items of electronic equipment plugged in and running continuously. If you have a portable receiver, you can try searching for the source -- perhaps the hum level will grow louder in certain rooms. You can also try turning off each electrical circuit at the circuit breaker while monitoring the shortwave signal -- just remember to reset all the clocks in those appliances that hate to lose power.
Some of my own experiences with tracing external hum modulation include a Honeywell electronic timer, and a house air conditioner. The Honeywell timer was a sealed unit, so the fix was to plug it into a power strip with "EMI/RFI protection". The mylar capacitor in the power strip was sufficient to prevent shortwave signals entering and leaving the timer.
The air conditioner problem was tracked down to the attic, where the control unit had a long cable running to a relay in the external compressor. It was easy enough to bypass each wire to ground with disc ceramic capacitors.
- 73 de Malcolm, NM9J
Radio Designers Handbook, F. Langford Smith, Iliffe & Sons, 4th impression (1957) p. 1239-1240.
An HF Hum Interference Mystery Solved!, by KC5KBG, QST (official Journal of the ARRL), April 1995 p. 35-37.
G3VNQ-NM9J amateur radio site, 05-Jun-2007