Identifying a source by its signature
Noise has a fingerprint, and the fingerprint names the culprit.
| What you hear or see | Source |
|---|---|
| Carriers at regular intervals across a band | switch-mode power supplies |
| Unstable modulated or unmodulated signals at specific frequencies | computer networking equipment |
| AM broadcast signals combining on unrelated frequencies | corroded metal connections mixing and re-radiating |
| Intermittent loud roaring or buzzing | arcing — power lines, appliances, or a defective doorbell transformer |
| Sharp regular pulses | ignition, electric fences, some chargers |
What causes interference received as a series of carriers at regular intervals across a wide frequency range?
Switch-mode power supplies
A switching supply runs at a fixed frequency and produces a comb of harmonics — evenly spaced carriers marching up the band. Once you have seen the pattern on a waterfall it is unmistakable, and counting the spacing tells you the switching frequency, which sometimes identifies the device.
What could be the cause of local AM broadcast band signals combining to generate spurious signals on the MF or HF bands?
Nearby corroded metal connections are mixing and reradiating the broadcast signals
The rusty bolt effect. A corroded joint is a crude diode, and a crude diode is a mixer — so two strong broadcast stations produce sums and differences that land in the amateur bands. Gutters, fences, and tower joints are the usual offenders, and the fix is mechanical: clean and bond the joint.
Suppression, matched to the mechanism
What current flows equally on all conductors of an unshielded multiconductor cable?
Common-mode current.
Which of the following can cause shielded cables to radiate or receive interference?
Common-mode currents on the shield and conductors.
Common mode is the recurring villain. A shield stops differential pickup and does nothing for common mode, because common-mode current uses the shield itself as an antenna.
How can conducted noise from an automobile battery charging system be suppressed?
By installing ferrite chokes on the charging system leads.
What is used to suppress radio frequency interference from a line-driven AC motor?
A brute-force AC-line filter in series with the motor’s power leads
Two different cures for two different mechanisms:
- Common-mode current on a cable → ferrite choke, raising the impedance of the cable as a whole.
- Conducted noise on power leads → line filter, a differential low-pass in series with the supply.
Fitting the wrong one is the usual reason an interference fix “does not work.”
Noise reduction, and its costs
Which of the following types of noise are removed by a noise blanker?
Impulse noise.
What undesirable effect can occur when using a noise blanker?
Strong signals may be distorted and appear to cause spurious emissions
A blanker gates the receiver off during a pulse. A very strong signal can look like a pulse to the blanker’s detector, so it triggers on the signal itself and chops it — producing distortion that appears as splatter from a station transmitting perfectly cleanly. If a strong station “splatters” only when your blanker is on, it is your blanker.
Which of the following types of noise can often be reduced by a digital noise reduction?
All these choices are correct — broadband white noise, ignition noise, and power line noise.
What problem can occur when using an automatic notch filter (ANF) to remove interfering carriers while receiving CW signals?
Removal of the CW signal as well as the interfering carrier
An automatic notch filter hunts for steady carriers and removes them. A CW signal is a steady carrier. So an ANF that helps enormously on SSB will delete the very signal you are working on CW. Switch it off for CW; it is not a subtle effect.
Single-point grounding
What is the purpose of a single point ground panel?
Ensure all lightning protectors activate at the same time.
The reason behind the “one bonded ground, one entry point” rule from the foundations safety module. If feed lines enter at different points with separate grounds, their arresters see different potentials and fire at different moments — so during that interval the difference appears across your equipment.
A single panel where every line enters, with every arrester bonded to it, means they all rise together and nothing sees a difference.
Where should a station AC surge protector be installed?
On the single point ground panel.
The AC line is a feed line too. Protecting the coax while leaving the mains unprotected leaves the surge a path straight through your equipment to the coax ground.
Check yourself
- Evenly spaced carriers every 40 kHz across 20 metres. Source?
- A neighbour’s electric gate motor generates hash on your power line. Ferrite or line filter?
- Your noise blanker is on and a strong local station sounds like they are splattering. Whose problem?
Answers
- A switch-mode power supply — the comb spacing is its switching frequency.
- A brute-force AC-line filter in series with the motor’s power leads. Ferrites address common-mode current on cables; this is conducted noise on the supply.
- Yours. The blanker is triggering on the strong signal and chopping it. Switch it off and listen again.