Diagnosis by ear
Interference has a sound, and the sound identifies the source.
| Sound | Cause |
|---|---|
| Distorted speech | RF from a nearby SSB transmitter |
| On-and-off humming or clicking | RF from a nearby CW transmitter |
| Hum on your transmitted signal | a ground loop in your audio connections |
| Wide-frequency crackling | arcing at a poor electrical connection |
What sound is heard from an audio device experiencing RF interference from a single sideband phone transmitter?
Distorted speech
What sound is heard from an audio device experiencing RF interference from a CW transmitter?
On-and-off humming or clicking
Both are the same mechanism — RF rectified somewhere in the audio device — and the difference is only what the RF was doing. SSB carries speech, so you hear distorted speech. CW switches on and off, so you hear clicks in the rhythm of the sending.
Which of the following could be a cause of interference covering a wide range of frequencies?
Arcing at a poor electrical connection.
A spark is a broadband impulse generator. A failing power-line insulator, a loose neutral, or a corroded connection produces hash across the entire HF spectrum, and it is the single most common source of unexplained noise in a modern station.
Fixing RF where it should not be
Which of the following might be useful in reducing RF interference to audio frequency circuits?
Bypass capacitor
A small capacitor across the offending input shorts RF to ground while leaving audio untouched — the same principle as the bypass capacitor in the reactance lesson, applied as a cure.
Which of the following would reduce RF interference caused by common-mode current on an audio cable?
Place a ferrite choke on the cable.
Common-mode current flows the same direction on both conductors, using the cable as an antenna. A differential filter cannot see it, because there is no difference between the conductors to filter. A ferrite choke raises the impedance of the cable as a whole, which is exactly what common-mode current has to flow through.
Knowing the mode tells you the cure. Differential noise: filter. Common-mode noise: ferrite.
Why a ground wire can be the problem
This is the counterintuitive material in the group, and it is worth understanding rather than memorising.
What is a possible cause of high voltages that produce RF burns?
The ground wire has high impedance on that frequency.
What is a possible effect of a resonant ground connection?
High RF voltages on the enclosures of station equipment.
A “ground” wire is a conductor with length, and at RF a conductor with length has inductance and can be resonant. A ground lead near a quarter wavelength long presents a high impedance rather than a low one — so instead of holding the chassis at ground potential, it lets a substantial RF voltage appear there.
The result is RF burns from touching a microphone, RF in the audio, and equipment that behaves differently on one band than another. The station is “grounded” and the chassis is at RF potential.
The fixes are all about length and bonding:
What technique helps to minimize RF “hot spots” in an amateur station?
Bonding all equipment enclosures together.
How can the effects of ground loops be minimized?
Bond equipment enclosures together.
Bond everything to a common point with short, wide straps — flat copper, as the Technician station-setup lesson noted, because perimeter beats cross-section at RF. The goal is not a connection to earth so much as every enclosure at the same potential as every other.
Ground loops
What could be a symptom caused by a ground loop in your station’s audio connections?
You receive reports of “hum” on your station’s transmitted signal.
Two pieces of equipment grounded at different points, with an audio cable between them, form a loop. Mains-frequency current circulates in that loop and appears in the audio as hum — which you cannot hear, because it is on your transmitted signal rather than your receiver.
Reports of hum from other stations, with nothing audible locally, is the signature.
Safety grounding is separate — and mandatory
Why must all metal enclosures of station equipment be grounded?
It ensures that hazardous voltages cannot appear on the chassis.
That is safety grounding, at mains frequency, and it is non-negotiable. It is a different job from RF bonding, done with a different conductor for a different reason, and neither substitutes for the other.
Why should soldered joints not be used in lightning protection ground connections?
A soldered joint will likely be destroyed by the heat of a lightning strike.
Solder melts at a few hundred degrees. A lightning surge delivers tens of thousands of amperes, and the joint vaporises — leaving the ground disconnected at exactly the moment it was needed. Lightning grounds use mechanical connections: clamps, bolts, exothermic welds.
The three-grounds summary
| Purpose | Frequency | Method |
|---|---|---|
| Safety | mains | third-wire safety ground, mandatory |
| Lightning | broadband impulse | short direct runs, mechanical joints, all rods bonded |
| RF | your operating bands | wide short straps bonding every enclosure to a common point |
They are three different jobs. Doing one does not accomplish the others, and confusing them is where most station grounding problems originate.
Check yourself
- Other stations report hum on your signal but your receiver sounds fine. What do you suspect?
- You get a small burn touching the microphone while transmitting on 15 metres only. What is happening?
- Why must a lightning ground connection not be soldered?
Answers
- A ground loop in your audio connections. Bond the equipment enclosures together.
- Your ground lead is near resonant on that band, presenting a high impedance, so RF voltage appears on the chassis. Shorten it and bond with wide strap.
- Lightning’s heat destroys solder, disconnecting the ground exactly when it is carrying the surge. Use mechanical connections.