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G4C

AMATEUR RADIO PRACTICES

- Interference to consumer electronics; grounding and bonding

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G4C011 of 12

Which of the following might be useful in reducing RF interference to audio frequency circuits?

Why A capacitor's reactance falls as frequency rises (Xc = 1/(2 pi f C)), so a small capacitor connected from a signal line to ground looks like a near short to RF while remaining a high impedance at audio frequencies. That shunts the unwanted RF energy to ground before it can be detected and rectified in the audio stage, and the wanted audio passes through untouched. Bypass capacitors at amplifier inputs, speaker leads, and power pins are a standard cure for RF getting into stereos, telephones, and intercoms.
Watch out An inductor used as a shunt would do the opposite, presenting low impedance at audio and high impedance at RF, which kills the wanted signal and leaves the interference; inductors help when placed in series with a lead, as in a ferrite choke.
Caps shunt highs to ground: bypass capacitor = RF drain, audio stays.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C022 of 12

Which of the following could be a cause of interference covering a wide range of frequencies?

Why An arc is a rapid, repetitive breakdown of air that switches current on and off in microseconds. Those sharp, fast pulses contain energy spread across a huge span of the spectrum, so arcing at a loose power line connection, a corroded ground clamp, or a bad tower joint shows up as buzzy noise from the AM broadcast band up through VHF. Narrowband problems, by contrast, come from oscillators or harmonics that land on specific frequencies.
Watch out Skipping a balun or line isolator does cause RF on the feedline shield and can create interference, but that is your own transmitted signal appearing where it should not, not broadband hash across the spectrum. Rectification at a corroded joint can also mix signals, but note the choice about rectification says 'lack of' it, which would be a good thing.
Fast sparks make wide noise: an arc is a broadband transmitter you never licensed.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C033 of 12

What sound is heard from an audio device experiencing RF interference from a single sideband phone transmitter?

Why RF interference gets into an audio device when a nonlinear junction (a transistor input, a poorly bonded joint) rectifies the RF and recovers its envelope. An SSB signal has no carrier, so the recovered envelope is not a faithful copy of the modulation, and what comes out of the speaker is garbled, unintelligible speech that follows the rhythm of your voice.
Watch out Clearly audible speech is what you get from an AM signal, because the carrier travels with the sidebands and lets a simple rectifying junction demodulate it properly. On-and-off humming or clicking points to a keyed CW or pulsed signal, and a steady hum suggests a constant carrier such as FM.
SSB has no carrier to rectify against, so the audio comes out mangled: SSB in, scrambled speech out.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C044 of 12

What sound is heard from an audio device experiencing RF interference from a CW transmitter?

Why Audio gear doesn't have a receiver's BFO, so it can't turn a CW carrier into a musical tone. What happens is that a nonlinear junction in the audio circuit rectifies the strong RF, and the recovered DC follows the keying envelope of the transmission. The result is a buzz, hum or click that switches on and off in step with the dits and dahs.
Watch out The choice describing a pure audio tone assumes the audio device beats the signal against a local oscillator the way a ham receiver does, which it cannot. Severely distorted or garbled audio is the classic symptom of SSB phone interference, not CW.
No BFO in a stereo: CW comes through as clicks and hum that follow the key, SSB comes through as garbled speech.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C055 of 12

What is a possible cause of high voltages that produce RF burns?

Why A wire is only a good ground at DC and low frequencies. At RF, a ground conductor has inductance and can be an appreciable fraction of a wavelength long, so it presents a high impedance. Current flowing through that impedance develops significant RF voltage on the chassis or 'ground' connection, and touching it gives an RF burn. The cure is short, wide, low-inductance bonding straps, or common-mode chokes and counterpoise to keep RF off the equipment.
Watch out The resonant ground rod choice sounds plausible but it is the wire between the equipment and the rod, not the buried rod, whose length and inductance matter; flat braid is actually better than round wire because it has lower RF inductance.
Ground wires can be hot at RF: short and wide equals low impedance, long and skinny equals RF burns.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C066 of 12

What is a possible effect of a resonant ground connection?

Why A ground conductor is not automatically at zero RF potential. If the wire or strap running to the ground rod happens to be near a quarter wavelength (or odd multiple) at the operating frequency, it acts like a resonant stub: the far end is a low impedance, but the equipment end presents a high impedance, so RF voltage appears on chassis and enclosures. That is why you can get RF burns or erratic equipment behavior even with a 'good' ground, and why ground leads should be kept short or bonded in a way that avoids resonance.
Watch out A ground loop is a different problem, caused by multiple ground paths of differing potential creating circulating currents (often hum), not by resonance in a single conductor.
Quarter-wave ground lead = hot chassis. Low impedance at the rod means high RF voltage back at the rig.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C077 of 12

Why should soldered joints not be used in lightning protection ground connections?

Why A lightning strike dumps tens of thousands of amperes into the ground conductor, and the resulting I-squared-R heating in the joint far exceeds the melting point of solder (around 180-190 C for common tin-lead alloys). The joint would melt and open the path just when it is needed most, letting the surge find another route through the station. Lightning ground connections must be made with mechanical means, brazing or exothermic welds, and the bonding conductors should be heavy and as straight as practical.
Watch out The flux and dielectric constant answers are nonsense distractors: flux aids a good connection rather than preventing one, and solder is a metal, so a dielectric constant does not apply to it.
Lightning melts solder. Bolt it, clamp it or braze it, never solder it.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C088 of 12

Which of the following would reduce RF interference caused by common-mode current on an audio cable?

Why Common-mode current is RF flowing in the same direction on every conductor of the cable, including the shield, so the cable acts like an antenna feeding RF into the audio gear. A ferrite core clamped around the whole cable adds series impedance only to that common-mode current, because the desired audio signal travels as equal and opposite currents whose fields cancel inside the core. Several turns of the cable through the ferrite increase the choking impedance further.
Watch out Shorting center conductor to shield would simply kill the audio signal, and grounding the center conductor does the same; neither addresses the RF riding on the outside of the cable. Extra insulation does nothing, since common-mode current is conducted on the wire, not leaking through the jacket.
Ferrite chokes the common mode, passes the differential signal. Think of it as a one-way speed bump for RF on the outside.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C099 of 12

How can the effects of ground loops be minimized?

Why A ground loop happens when two pieces of equipment reach ground by different paths, so RF or AC currents circulate between them and appear as hum or RF in the signal path. The fix is to tie all the equipment enclosures together with short, heavy bonding straps so everything in the station sits at the same potential and there is no voltage difference to drive current through the interconnecting cables. Good bonding practice is a single-point station ground with everything bonded, not a collection of separate ground paths.
Watch out Connecting grounds in series (daisy-chaining to one rod) is exactly what creates the loops and differing potentials; and tying the AC neutral to the ground wire anywhere but the service entrance is a serious code violation that can put current on your ground conductors.
Bond everything together, short and heavy. Equal potential means no loop current.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C1010 of 12

What could be a symptom caused by a ground loop in your station's audio connections?

Why A ground loop happens when equipment is grounded at more than one point, so the shields and ground leads form a closed loop that picks up stray AC power-line currents. Those currents appear as a small 60 Hz (and harmonics) voltage in series with your audio, which gets amplified and modulated onto the signal. The classic symptom is other stations telling you there is hum on your transmitted audio. The cure is bonding all equipment to a single common point so there is only one ground path.
Watch out High SWR points to an antenna or feedline fault, and harmonic interference comes from overdriving or inadequate transmitter filtering; neither is caused by audio wiring ground paths.
Ground loop = loop of wire near AC power = 60 Hz hum in the audio. Hum on audio, think loop.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C1111 of 12

What technique helps to minimize RF "hot spots" in an amateur station?

Why RF "hot spots" are points where you can get an RF burn or stray RF current because two pieces of gear sit at different RF potentials. Short, wide bonding straps tying all chassis and enclosures together put everything at the same RF potential, so there is no voltage difference to burn you or to couple RF into audio and control lines. This bonding is separate from safety and lightning grounding, though the bonded bus should also connect to the station ground panel with a short conductor.
Watch out Metal enclosures alone do not help if the cabinets float at different RF potentials from each other; the shielding only works once the boxes are bonded. Low-pass filters address harmonic output, and surge suppressors handle AC line transients, neither of which is an RF potential problem.
Hot spots come from voltage differences. Bond everything together and there is no difference left.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G4C1212 of 12

Why must all metal enclosures of station equipment be grounded?

Why Bonding every metal chassis to the station's safety ground means that if a hot conductor inside the equipment contacts the case, the fault current has a low-impedance path back to the source, tripping the breaker or blowing the fuse instead of leaving the case energized. Without that bond, a chassis can float at line potential and shock anyone touching it, especially when they simultaneously touch another grounded item like a mic or a rack. The purpose is personal safety: the metal surfaces you touch stay at or near ground potential.
Watch out The choice about preventing a blown fuse is backwards, since a proper safety ground is exactly what lets the fuse or breaker open on an internal short. Grounding the neutral is the job of the service entrance panel, not of your radio's chassis, and RF overload is handled by filtering and attenuation, not by the green wire.
Green wire = people protection. Chassis ground keeps the metal you touch at zero volts.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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