Narrowband or broadband?
Firstly, is the interference concentrated on just one specific frequency, does it spread across a narrow band of frequencies, does it occur across the whole band, or can it be received across more than one band?
Some interference sources may be on a single frequency—such as a harmonic from a broadcast transmitter—while some may be spread across a whole frequency band, perhaps with regular peaks occurring across the spectrum. For example, a switched mode power supply (SMPS), if not properly suppressed, may generate harmonics at its switching frequency, typically every 15 or 20kHz across a very wide band of frequencies.
Some interference may just look like continuous random noise covering many MHz. For example, interference from broadband ADSL/VDSL services often takes this form. Further, does the interference drift in frequency or is it fixed in one spot? Drifting interference may suggest a free running oscillator such as in a SMPS—although be aware that some SMPS’s use resonators to relatively accurately control their frequency. Often a free running device will change frequency with temperature or load, so if a device is suspected, cooling it down with a quick bust of spray from a can of freezer, or switching the device between ‘on’, ‘off’ and ‘stand-by’ may help identify the culprit with a change of interference level or frequency.
For broadband interference it is useful to identify whether the interfering signals can only be heard in the amateur band or whether it affects other bands. For example, can it be heard in the broadcast bands e.g. on medium or long wave, or the VHF FM band? Experience suggests that the authorities are more likely to act against interfering signals that affect broadcast bands than those that predominantly affect amateur bands.
When trying to identify the interference characteristics it is best to use AM mode with the bandwidth set as wide as possible, rather than listening in a restricted bandwidth as is typically used for SSB. This should allow the user to hear other modulation that may be present such as 50Hz or 100Hz harmonics of mains borne interference and this may give further clues as to the potential source.
When did the interference start?
Was the start of the interference concurrent with some other event? For example, did it start when a neighbour installed solar panels on their roof, or when the nearby street lights were changed to LED bulbs, or when a wind farm was constructed in the vicinity? Does it only occur when the Sun shines, or the wind blows?
When does the interference occur?
One thing to determine is whether the interference occurs continuously, or whether it is intermittent.
If it is intermittent, is it concurrent with anything identifiable e.g. does it occur at the same time every day, or does it occur whenever lights are turned on, or only, say, in television viewing hours? Keeping a log over a week or more of when the interference occurs may provide valuable clues as to its source.
Many sources of interference have been easily identified this way e.g. when new LED street lights are illuminated, when local wind turbines are turning in the wind, when clouds pass over a solar PV installation or even when your next-door neighbour cuts his lawn or is using power tools in the garage.
A further aspect to be aware of is that interference may begin gradually e.g. as devices age, faulty components, particularly capacitors and batteries can become new sources of interference.
Further, don’t be fooled into thinking that interference will only occur when devices are active. Some modern power supplies have been known to create more interference when the load from them has been disconnected than they do when they are under full load.
Consider also battery powered devices such as a laptop, tablet or handheld device. These often only have an impact when close to an antenna or to shack cabling, so moving them can cure the problem.
Once the characteristics of the interference have been determined, these can be compared with known interference types. If necessary, record the interference or take a screenshot of the waterfall display and contact the RSGB for help.
If you do take a spectrum or waterfall display screenshot, make sure that is shows the frequency and time scale values and that the image is clear enough for some one looking at it to be able to read.