Chapter 28: Electromagnetic Compatibility

The man-made electromagnetic noise around it

EMC is about devices sharing an environment full of man-made electromagnetic noise: keeping what each radiates down and its immunity up. It is not limited to natural static such as lightning.

To cut the electrical noise polluting the radio spectrum

EMC works on both sides: less noise generated by devices and more immunity to it, easing the electromagnetic smog of cities. It sets limits rather than demanding zero radiation, and sharing out bands is frequency allocation.

Deciding who is responsible for solving it

The technical side, the causes and the cures, is the easier one; the legal and social question of who must fix it is harder, because a legal approach often brings unwanted social results.

Whether the source must radiate, and whether the device is a receiver

Sources are intentional radiators (transmitters) or unintentional ones (fences, ignition), and affected devices are receivers or not, like a hi-fi playing CDs; these, not power or distance, decide who must act.

An unintentional radiator

The ignition system radiates although it does not need to in order to work, so it is an unintentional radiator, held to strict radiation limits. Intentional radiators, such as transmitters and garage remotes, must radiate.

Its owner must fix it and keep it off until it complies

A fence need not radiate, so strict limits apply; if it interferes it is probably over them, and its owner must have it repaired and keep it off until it complies. Filters at your end are not your duty.

Switch contacts that spark as they break heavy currents

A switch that makes or breaks a heavy current may spark, sending out a burst of noise that can travel a long way. Fibre carries light, not RF currents, so it radiates nothing.

They slowly raise the noise level, hampering radio across the city

Each switch adds a little noise, and millions of them raise the urban noise level until radio communication across the city is hampered. Random bursts of noise add up; they do not cancel.

With the hi-fi, which should not respond to radio signals

A hi-fi playing CDs is not meant to receive radio, so if it responds to RF the fault is in its design, not in your licensed transmitter: the hi-fi’s makers did not design it for strong fields.

It is legal, and works where only weak signals reach it

Such equipment may legally be sold, and it works in about 99% of locations, which see only weak, distant signals. The trouble starts when a licensed amateur sets up next door.

Radiation

Though technically correct, the word radiation calls up ionising radiation and nuclear warning signs, and alarms people. Interference is a neutral word that describes the problem without frightening anyone.

Goodwill: the neighbour may have a say in your tower plans

Curing it is the neighbour’s job, but helping is diplomatic: it keeps good relations, and the same neighbour may later be asked for comment when you apply to put up a tower. No licence condition requires it.

The TV is faulty: it responds to a signal outside its band

When the transmission is legal in every respect (in band, within power, clean), the receiving equipment is the defective part: it reacts to a signal outside its band, and its owner must have it repaired.

Inspect your transmitting equipment

ICASA will probably check the transmitter first: if it is faulty or does not comply with your licence, you may be held responsible and it can be confiscated. So check your own signal before blaming the TV.

You are entitled to carry on operating

Offering help is good diplomacy and good for amateur radio’s name, but if it is refused and you are sure your equipment is legal, you may keep operating despite the interference.

Later faults in the system may be blamed on you

Any change you make to a neighbour’s installation may be blamed for later trouble: if the sound system fails months afterwards, fingers will point at you. A filter at their speakers does nothing to your signal.

Secondary; industrial, scientific and medical users are primary

Amateurs hold 13 cm as secondary users, and industrial, scientific and medical (ISM) use is primary. So amateurs must not interfere with ISM users and must accept interference from them.

Nothing: you must put up with it

A secondary user has to put up with interference from the primary user, and must not cause it any, stopping if that is the only way. A complaint against a primary user working as intended would fail.

2,3 to 2,45 GHz

λ = 300 ÷ F gives 300 ÷ 2 300 = 0,13 m, hence the name 13 cm; the band runs from 2,3 to 2,45 GHz. 1,24 to 1,3 GHz is the 23 cm band.

Operating courtesy and a clean transmitted signal

Every band is shared with fellow amateurs, so avoid interfering with them: operate with courtesy and keep your signal clean. No licence class has priority, and there is no rota or booking.

A transmitter radiating where it should not

The transmitter is putting out energy where it should not, the manual’s first cause, so the cure is at the transmitter. The other service’s receiver is working on its own frequency, as it should.

A corroded gutter rectifying it and re-radiating harmonics

Corrosion can make a metal joint act like a rectifier, a non-linear device, turning a strong nearby signal into harmonics that it re-radiates; this cause is rare and hard to trace. Absorption creates no new frequencies.

Mixing products: sums, differences, harmonics

A non-linear device distorts the waveform, creating harmonics and sum and difference frequencies, as in a mixer (chapter 20); corroded joints do the same. A linear amplifier would only make the signals bigger.

It is uncommon and needs special equipment and expertise

In the third cause something else, such as a corroded joint, converts the signal; it is quite uncommon, and tracing it usually takes special equipment and expertise. It is not the electricity supplier’s concern.

Temperature and knocks affecting an LC oscillator

LC oscillators drift as the temperature changes and shift when knocked, so they need temperature compensation and solid construction. Crystal oscillators are very stable, which is why crystal-controlled synthesisers seldom drift.

It is no longer held steady and may wander

A synthesiser is only as stable as the reference it is locked to; once unlocked, its VCO runs free and is unstable. The crystal steadies the output only through the locked loop.

A high-impedance buffer stage following the oscillator

Chirp comes from the keyed stages loading the oscillator, or its supply sagging, at each key-down; a buffer amplifier isolates the oscillator, and a regulated supply helps too. No filter can remove a frequency shift.

The fourth

582,4 ÷ 145,6 = 4, so it is the fourth harmonic, found as in the manual’s 144 × 5 = 720 MHz example. The fifth would fall on 5 × 145,6 = 728 MHz.

14,2 MHz, in the 20 m band

The second harmonic is twice the fundamental: 2 × 7,1 = 14,2 MHz, inside the 20 m band (14,000 to 14,350 MHz). 21,3 MHz is the third harmonic, 3 × 7,1.

145,2 MHz

A harmonic is a whole multiple of the fundamental, so the station is on 435,6 ÷ 3 = 145,2 MHz, in the 2 m band. Multiplying by 3 instead (1 306,8 MHz) mixes up a harmonic and a sub-harmonic.

The second harmonic of your signal

100,8 ÷ 50,4 = 2: your second harmonic lands on that FM channel, a transmitter fault to cure at your end. Overload would spoil reception across the band, not at one spot.

When harmonics remain with the drive set correctly

Harmonics that come and go with the mic gain or CW drive mean the amplifier is overdriven, and the cure is less drive. Only if they persist when it is driven correctly is an extra lowpass filter the best cure.

A lowpass filter

Harmonics lie above the wanted frequency, so a lowpass filter after the final amplifier passes the signal and weakens them, typically by 50 dB or more at 50 MHz for HF filters. A highpass filter would pass the harmonics.

A Pi network with one inductor

The ATU must work as a lowpass filter: a Pi network with one series coil, or a T with two series coils. The common T with two series capacitors is a highpass circuit and lets harmonics through.

Some of its output finding its way back to its input

Oscillation needs a loop: unintended feedback from the output back to the input of an amplifier with tuned circuits makes it oscillate at their resonance (self-oscillation). Input leaking forward to the output forms no loop.

Negative feedback, to bring the loop gain down

Negative feedback lowers the loop gain below what oscillation needs; cutting the coupling, say with shorter component leads, also works. Positive feedback is what makes an amplifier oscillate.

Self-oscillation

Oscillation on or close to the working frequency, caused by feedback through the tuned circuits, is self-oscillation. Parasitic oscillations usually occur at VHF or UHF, far from the working frequency.

A parasitic oscillation

152,3 ÷ 7,05 = 21,6, not a whole number, so it is no harmonic; an unrelated VHF signal is typical of a parasitic oscillation from a hidden resonance. Self-oscillation would be near 7,05 MHz.

A low-Q, lossy choke

A lossy, low-Q choke damps any hidden resonance so that it cannot sustain a parasitic oscillation; ferrite beads do a similar job. A high-Q choke makes a sharper resonance and oscillation more likely.

A ferrite bead on a lead of the transistor

A bead on the transistor’s emitter or base lead adds inductance and loss at VHF, right where the parasitic runs, damping it without affecting the working frequency. A bead on the microphone lead is nowhere near it.

It adds RF impedance, blocking RF but not DC

The ferrite adds inductance and loss at RF, so RF on the wire is blocked or damped while DC passes freely; that is how beads stop VHF parasitics. A bead is not a screen like a coax braid.

They keep RF out of the DC wiring, while passing the DC

Each bead adds series inductance at RF, and with its decoupling capacitor makes a little lowpass filter: DC reaches the stage, but RF is kept out of the supply wiring, where it could couple elsewhere.

Splatter caused by intermodulation products

Driven too hard, a linear amplifier flattens the peaks, and the speech frequencies mix into intermodulation products close to the signal, widening it into splatter. Harmonics appear too, but far away, where the output filter deals with them.

Its signal is far wider than it needs to be

Splatter is excessive bandwidth: the signal spreads onto neighbouring frequencies, usually through overdriving or overmodulation. Harmonics fall far away, on whole multiples of the frequency, and are a separate problem.

100%

100% is the limit: the carrier just reaches zero at the troughs; beyond it the carrier cuts off and the distortion splatters onto adjacent channels. The manual’s 80% is only an example.

3 kHz

Communications-quality speech needs only about 300 Hz to 3 kHz; audio above that just widens the signal and interferes with others, the price of chasing fidelity. 6 kHz is an AM signal’s whole bandwidth, both sidebands.

So that the ALC reading stays within its marked range

The ALC voltage shows how hard the transmitter is being driven; keeping it inside the range marked on the meter means no stage is overdriven. Chasing the highest power reading overdrives it and widens the signal.

Smoothly, over about 5 ms each way

Keying the carrier abruptly spreads it into clicks either side of the signal; shaping each edge over about 5 ms keeps it narrow. Half a second would smear the Morse beyond reading.

Key a low-power stage, or use a transistor or relay

Keying a lower-power stage, or letting a transistor or relay do the switching, keeps the heavy current and voltage off the key. Bigger contacts would still spark at the same voltage and current.

More smoothing capacitance, or a voltage regulator

Hum is 100 Hz ripple left on the DC; a large smoothing capacitor (thousands of µF) or a regulator removes it. Small RF bypass capacitors have far too little capacitance to matter at 100 Hz.

To keep RF voltages off the equipment and feedline

With the earth as half of the antenna system, a good, separate RF earth carries the RF return, so that the equipment and feedline do not sit at RF voltages. Protection from mains faults is the mains earth’s job.

As short and as direct as possible

An earth lead has inductance, so every extra centimetre and every turn raises its impedance at RF: keep it short and straight. Coiling the spare length makes an inductor, the opposite of what is wanted.

The mains earth has become part of a poor RF earth path

Mains-earthed equipment lets station RF flow on the mains earth, which runs beside live and neutral, couples into them and carries RF to neighbouring buildings. The cure is a good separate RF earth; harmonics are not the issue.

So that a fault does not leave a lethal voltage on its case

The mains earth is a safety earth: it carries fault current so the case cannot become live. It often has a high impedance at RF and runs beside live and neutral, so it is a poor and troublesome RF path.

A steady heterodyne whistle along with the speech

An SSB signal should carry no carrier; a leftover carrier beats with the receiver’s BFO or with other signals, giving a steady tone (a heterodyne) on the audio. Wrong pitch comes from mistuning the receiver.

Intermodulation in the first mixer creates spurious signals

A signal stronger than the receiver was designed for drives the first mixer into non-linearity, and intermodulation creates spurious products that land among the wanted signals. An image is a separate defect, set by the tuning and the IF.

They overload easily on strong signals

Inexpensive masthead preamplifiers, used to improve TV reception in marginal areas, are very prone to overloading; models with good signal handling exist but cost more. Low gain is not their problem.

On the mast, ahead of the preamplifier

The filter must stop the strong signal before it reaches the preamplifier, which would otherwise overload and make spurious products that no later filter can remove. A filter behind the set comes too late.

The products are already made in the preamplifier

Once the preamplifier overloads, its spurious products fall inside the TV channels, so no filter after it can separate them, though a suitable filter would pass the TV signals. Replace or remove the preamplifier.

A highpass filter in the TV’s antenna lead

TV signals lie at VHF and UHF, well above HF, so a highpass filter passes them and blocks the HF signal. A lowpass filter would do the opposite, keeping the HF signal and blocking the TV channels.

A highpass filter cutting off at 470 MHz

UHF television starts at 470 MHz, above the 2 m band, so a highpass filter with that cutoff passes the TV channels and blocks the 144 MHz signal. A 100 MHz highpass would let 2 m straight through.

A trap: a bandstop filter for that band

With both signals at VHF, a highpass filter is unlikely to separate them, so a bandstop filter (a trap) for the interfering amateur band is needed. A choke stops currents on the braid, not signals arriving through the antenna.

It blocks one band of frequencies and passes those either side

A trap is a bandstop or notch filter: it removes a band of frequencies, such as one interfering amateur band, while passing those above and below. Passing only one band is what a bandpass filter does.

A stub

A quarter-wave length of line connected across a feeder is a stub; open at the far end, it shorts the feeder at the frequency where it is exactly a quarter wave. A phasing line joins antenna elements.

A quarter wavelength of that signal, measured in the cable

The stub must be a quarter wavelength at the interfering frequency, and as waves travel more slowly in coax, that is the free-space length times the velocity factor. Cut to the free-space length it would resonate too low.

35 cm

λ = 300 ÷ 145 = 2,07 m; a quarter is 0,517 m, and in coax 0,517 × 0,66 = 0,341 m, nearest 35 cm. Forgetting the velocity factor gives the free-space 52 cm.

1 950 kHz

The LO is at 1 040 + 455 = 1 495 kHz, and the image one IF above it, at 1 950 kHz, in the 160 m band. 1 495 kHz, f + IF, is the LO itself.

123,6 MHz

With the LO above the signal, the image lies two IFs above the tuned frequency, so the scanner is on 145,0 − 21,4 = 123,6 MHz. Subtracting one IF (134,3 MHz) gives the LO, not the tuned frequency.

In your own receiver, which was overloaded

Interference made in the transmitter would fall by only the attenuator’s 20 dB, like the signal; a drop of 40 dB or more means your receiver was making it through overload. Then the attenuator is a cure.

Sensitivity is lost, but the signal-to-noise ratio improves

Any cut in signal level costs sensitivity, but with the receiver no longer overloaded the interference falls further than the wanted signal, so the signal-to-noise ratio improves. Selectivity is set by the filters.

In the same direction, in phase, on both conductors

Common-mode signals flow the same way, in phase, on both conductors: live and neutral, both speaker wires, or a coax inner and its braid. Opposite currents are differential, like the wanted signal in a TV downlead.

Currents flowing opposite ways in the two conductors

The wanted signal’s currents run opposite ways in the two conductors, so their magnetic fields cancel and it passes unaffected; currents flowing the same way in both are common-mode and meet the choke’s inductance.

Its opposite currents’ fields cancel in the core

In the wanted, differential signal the two currents flow in opposite directions, so their magnetic fields cancel and the core sees no net field; the choke is then no inductor to it. A weak signal would still meet the inductance.

Turns of the cable itself on a ferrite ring

Turns of the mains, speaker or coax cable on a ferrite toroid form an inductor for common-mode current; despite the name braid breaker, nothing is cut. A capacitor across the conductors would act on the wanted signal.

A braid breaker fitted to the downlead

Current on the outside of the screen is common-mode, flowing the same way on inner and braid, so a common-mode choke on the downlead (a braid breaker) stops it before it reaches the set. A bead elsewhere misses its path.

A braid breaker fitted near the radio

Signals induced on the braid travel as common-mode current, which a toroidal choke (braid breaker) on the downlead blocks; a filter at the socket acts only on the differential signal inside. More power makes it worse.

More turns on its core, to raise its inductance

A choke rejects a signal only if its impedance at that frequency is high enough, and XL = 2πfL falls with frequency; more turns, or a better core, raise L. Fewer turns would make it worse.

Receivers such as TV sets, and also audio amplifiers

A common-mode choke works on any lead that carries interference in, so it suits TV sets and radios and also equipment that is not a receiver at all, like an audio amplifier troubled by strong signals.

1,3 kΩ

XL = 2πfL = 6,283 × 7,05 × 30 = 1 329 Ω (megahertz times microhenry gives ohms), nearest 1,3 kΩ. Leaving out the 2π gives 7,05 × 30 = 212 Ω.

The interference shows on every channel the set receives

Every channel passes through the same IF stages, so a signal coupled straight into them is present whatever channel is chosen. Interference on one channel points instead to a harmonic or an image landing there.

E in V/m, and H in A/m

The electric field E is given in V/m and the magnetic field H in A/m, and a field strength meter reads them. Swapping them confuses the electric field (voltage) with the magnetic field (current).

Good design, using decoupling capacitors

Direct pickup bypasses the leads, so the cure lies in the circuit itself: good design with decoupling components, as well as a shield around it. Beads on the leads stop only what travels along the wires.

A Faraday cage: a conductive box that encloses it

At HF and above the coupling is mainly by the electric field, which any conductive enclosure (a Faraday cage) keeps out. Thick Mu Metal also works but is far harder to use; it is needed only for mainly magnetic coupling.

About 1,3 GHz

Holes pass signals whose wavelength is not much longer than their circumference: at 1,3 GHz, λ = 300 ÷ 1 300 = 0,23 m, about twice 10 cm; at 145 MHz and below it is over 20 times as long.

Thick shields of special metals, such as Mu Metal

Electric fields are easily stopped by any conductive box, but magnetic coupling needs relatively thick shields of special materials such as Mu Metal or Permalloy. Luckily, magnetic fields are seldom the main problem at HF and above.

Cabinet radiation, cured by better screening

Stray currents inside a badly designed cabinet radiate directly, bypassing the antenna socket and any filter there; the cure is better screening, a Faraday cage round the transmitter. Matching the antenna does nothing for it.

Screened cable with its screen properly earthed

The earthed screen keeps the RF inside the cable, so the feeder itself neither radiates nor picks up much: shielding works both ways. An unscreened feeder radiates, whatever its length.

It cuts the noise the equipment itself radiates

Shielding works in both directions: a conductive enclosure that keeps strong fields out also keeps the device’s own noise in, reducing objectionable radiation. It does nothing for wanted reception through an antenna.

Where it gives the weakest field in nearby homes

Interference falls off with distance, so the antenna should go where it gives the lowest field strength in homes, its own and the neighbours’: high, and as far from them as possible. Hiding it low often does the opposite.

The 15 m mast at the far end of the garden

Field strength falls with distance, so the far end helps, and height lifts the strongest field over nearby houses instead of through them: the 15 m mast there does both. A low mast puts the beam nearer everyone’s wiring.

On its own route, well clear of other cables

Running the feeder clear of mains, telephone and TV cables keeps it from coupling RF into them, where it would travel to other equipment. Bundling it with the TV downlead invites exactly that.

Using the least power the contact needs

The manual’s simple rules are: antennas high and far from others, the minimum power the contact needs, and listening before transmitting. Full power raises the field strength in every nearby home.