Ever wondered where those annoying persistent signals come from when you are trying to listen to your shortwave receiver?. They always seem much stronger than everything else on the bands and never seem to have the fading that normal transmissions at HF frequencies produce.
It would be nice to put the blame on some harmonic ridden poorly designed transmitter somewhere but the reality of the situation is that ITS ALL OUR OWN FAULT.
Unfortunately our extensive use of electronics in all aspects of our daily lives has also generated a massive amount of low power transmissions that go unnoticed by almost everybody else but us long suffering radio enthusiasts. Take a look around your home and count the devices you use everyday and consider that some of these will still produce these pesky signals even when they are not being actively used.
Things are getting better all the time as we now recognize these signals as problems and have standardized ratings in place to try and curb them but sadly even within these stray signal restrictions there is still more than enough scope to cause problems to our hobby.
What Can You Do?
If one of these signals is spoiling the enjoyment of your hobby there are a couple of things you can do to try and minimize any disruption each with its on difficultly level and required skills.
Taking the most obvious first start powering down all the electrical and electronic items that are closest to your shortwave receiver and checking after each turn off to see if the offending signal has disappeared. Give each one a few minutes to turn off completely as some low power devices may take a while to eat through the current stored in the eternal power supply.
Rearranging Your Shortwave Station
Once the cause (or causes) of the interference has been identified it may sometimes be easier to move the shortwave receiver away from the problem than the other way around, even more so if the problem piece of electronics is a more permanent fixture or very large.
Even though most quality coax cable has good shielding putting it close enough to a rouge signal source will play havoc with your shortwave receiver. Take a look at where the transmission line between radio and antenna runs through the house taking note of anything that could be a problem and if possible re-route the coax to minimize interference.
Modifying the Offender
Many of the problems we experience with modern electronics are from local oscillators and if the evil signal is in a part of the radio spectrum you desperately need you may have the option to modify any equipment by shifting the frequency of the oscillator slightly with a few exceptions.
Timing or clock oscillators are very crucial for the operation of electronic devices especially when there is a CPU involved but many other signals can be adjusted without any performance loss. Obliviously this will depend completely on the entire circuit and is not something to be attempted if you don’t know the difference between diodes and resistors.
Making a choice between flawless use of your shortwave receiver or losing some piece of electronic equipment may not be that easy if you share the house with other people. Some of the worst sources of stray signals are televisions and computers so best of luck explaining to your family that these have to be switched off while you go DX hunting.
A good example of a compromise is we used to decode HF data signals on a big honking beast of a PC that caused more than a few stray signal problems. By downgrading it to a old smaller 500 MHz model the interference was cut right back and its become a stand alone system devoted entirely to our shortwave hobby.
Signal photo courtesy Wikipedia