Home - About AR - Learning Material - Exams - Clubs - Posters
A noise-blanker, often marked "NB", is a function in many radios, even 27 MHz CBs, used to reduce impulse noise, most notably, vehicle ignition noise. Such noise is wide-band, and I have certainly heard it from passing vehicles on 2 metres SSB while pulled over beside the road on a ridge before heading down the hill to get home. These have traditionally been analogue circuits, but DSP can perform this task.
The cousin of ignition noise is the noise from the strong electrical pulses required to drive high pressure injectors in modern diesel engines.
Power lines, especially of higher voltages, often generate 50 or 60 Hz modulated RF interference, causing a loud buzz.
Digital Signal Processing can do a range of tasks, most notably reducing interference on received radio signals, but can also be used to process outdoing signals.
The FT-857D, the D indicating included DSP, for example, includes wideband noise reduction (with adjustable aggressiveness), a bandpass filter with adjustable lower and upper frequencies, and automatic notch filters to remove unwanted carriers, "heterodyne whistles", and other tones.
While better radios have DSP at IF, some do so at audio frequency only. External DSPs are also available, either as modules or built into speakers.
They do however need to be set up correctly, as they can also reduce desired signals, especially CW. They may also do this to sustained notes in music in broadcast signals. At the highest setting they make voices sound robotic.
The examiners have decided to add a couple of questions which belong in Safety, regarding the idea of a "single point ground panel". Whether forming the antenna entry method, or nearby to it, many stations employ something like a fairly heavy aluminium (or maybe copper) panel which is well grounded via the station ground system. To these are fitted with lightning arrestors, typically with N connectors on each side, although other options are available. There are units designed to mount so the signal passes through the panel, or to have both cables on the same side of the mounting point.
Oddly, the examiner states the reason to have everything mounted on one plate is that the MOVs or gas discharge tubes will all fire at the same time.
For coaxial cable protection specifications include the power they pass for various bands. If you have a combiner for say 70 cm FM / P25, DMR, Packet, and a licensed UHF frequency for your volunteer agency, plus one for a client organisation, the voltage peaks which occur if all were to key up at once must be taken into account. Whether they pass or block DC is another factor, important if you have a masthead amplifier, or use DC to power remote relays. This would include 50 or 60 Hz at 24 volts or so, which is used in TV receiver masthead amplifiers, such as
Potentially useful protection devices sold by DX-Engineering include: PolyPhaser Coaxial Lightning Protectors IS-50NX-C0 with N for lower powers, PolyPhaser Coaxial Lightning Protectors IS-B50HU-C0 with "UHF" connectors for higher power, DX Engineering Balanced Feedline Protectors. The also sell replacement Coaxial Lightning Protector Components. They also have Terminated Braids. Don't forget your rotator control lines: DX Engineering Rotator Control Line Protectors DXE-IS-RCT, which is probably not too hard to duplicate at home.
List of products direct from Polyphaser: RF Surge Protectors, and Data Surge Protectors including for phone lines. That also have units for mitigating energy from high altitude nuclear explosions: High-Altitude Electromagnetic Pulse (HEMP) Tested Protection RF Coax Series.
An example sold by RFI in Australia: PolyPhaser Type N F/F Bulkhead Coaxial RF Surge Protector, 1.5MHz - 700MHz, DC Block, 3kW, 20uJ, 50kA, Blocking Cap, Hole Mount
While Polyphaser are doing what they can to minimise the impact of tariffs, which are a huge tax on consumers, including export customers, by making what they can in the US, and changing sources for parts; for people outside the US avoiding anything which has passed through the US may be the best option.
This is a listing of surge protectors on Diamond Antenna's Japanese website, in English: Surge Protectors. Diamond antenna dealers should be able to source these.
As an example, Phoenix Contact is a German company with offices around the world, including Sydney, Australia. An example products is the CN-UB-280DC-BB - Surge protection device.HUBER+SUHNER are a Swiss company. Their products are available from RS: Huber+Suhner, SSC-N230 Coaxial Surge Protection. If you buy direct you must buy 10, rather then one at RS.
Opek is a Taiwanese company. They sell antennas and Lightning Surge Protectors. Higher power units have a black case. The LP-230A / LP-230B with N connectors are rated for 100 watts; LP-350A / LP-350B with "UHF" connectors are rated at 300 watts. The metal barrel ones only 15 watts. A US seller marked eth N versions as not being suitable for CB, I suppose because people have mangled the Ns by trying to insert a PL-259. With the right cables they would work well at both 27 and 477 MHz.
ARRL has a list of articles on Lighting Protection.
For mains over-voltage devices, be aware of the voltage rating of the device. The Metal Oxide Varistor is a device which looks a lot like ceramic capacitor, typically with a glossy blue coating, but brands other than Siemens & Halske AG or Bourns are often other colours. Others are in a rectangular block. They do have a capacitance not inconsistent with a higher voltage ceramic capacitor of a similar size, so are for DC and power frequency AC only. They start at a few volts up to over 1000 volts. Some are suitable for use across the US 120 volt supply, or for 208 volts, 240 volts, 277 volts, 415 volts, 480 volts, or 690 volts, as examples.
Voltages used on rotators and relays are also available, maybe 18 or 20 volts for 13.8 volt systems, and 30-odd volts for 24 or 28 volt systems.
For those in the North America Morgan systems have a good range of protection products. These include Morgan Systems M-474 AC line filter with 3 or 6 NEMA 5-15R outlets (and a 15 amp pug), a 20 amp plug version, single point grounding plates called Morgan RF Systems M-350 mounting fixtures in various sizes. For rotators and other control lines they have several 8 and 12 line arrestors for up to 20, 50, or 120 volt lines, an example being: Morgan Systems M-348 rotor line/control lines (20 V), They have a range of Coaxial, balanced line, and single wire arrestors.
It appears that these are constructed in the US, with much of the value local, so hopefully tariffs will have limited impact.
The mains protection products from Morgan include an RF filter, which as much of the energy in lightning is at around 1 MHz probably helps reduce the level of energy reaching connected devices, compared to those with just MOVs.
Electrician supply shops have DIN rail mount overvoltage protection / surge protection devices to be placed in switchboards. These could perhaps be placed in a surface mount housing on the shack wall, with mains power passing through it. With 2 and 3 module capacity, Clipsal 4CC2-WE and 4CC3-WE are examples which can also hold one or more combined breaker and earth leakage protection device(s). Thus you can have one circuit for radios, the other for loads such as a heater, vacuum, etc. Tell your family that this is how to isolate power to the room.
I remember an "interesting" story: Some engineers in 240 volt Australia had a large and fancy computer in a multi-storey building, and decided that they should add some MOVs across the supply. Said devices were proudly produced and installed. Only the humble technician noticed a problem with them, but was shushed and ignored. Yes, they were rated as suitable for a 110 volt supply. At power-up things indeed became VERY exciting. While the Active / hot being pulled toward neutral and/or ground would be inconvenient, the neutral and/or electrical service earth being pulled toward the active, and thus above things such as the telecommunications earth used for the PABX and other 'phone and telecommunication systems, and above the building frame resulted in much equipment in the building being damaged. I am unsure if there were bangs and smoke as well.
Not mentioned are gas discharge tubs (GDTs) or that spark gaps in air can be used in some cases. It is possible to build your own protection devices, typically in a die-cast aluminium case. As well as using GDTs you can provide a DC break by including a capacitor consisting of metal strips soldered to the centre pins of a the input and output connectors, and bent to face each other. You need to work out if the capacitance provides a suitably low reactance at the operating frequency.
Morgan Systems also have a filter for noisy electrical products, or ones which are sensitive to noise, without surge protection. I suppose it is what the examiner wants as a solution to a noisy motor, and calls a "brute force" filter: Morgan Systems M-471 AC line /appliance filter. As their page points out, these only provide benefit where the RF noise is conducted along the mains lead.
These work by rectifying and filtering the mains input to produce filtered DC at around 170 volts (in the US), or around 340 volts in most other places. 277 volts produces nearly 400 volts DC. An oscillator and power FETs generate a chopped signal at around 25 kHz. These feeds a transformer, much smaller than of the equivalent mains frequency one, then high speed rectifiers and small filtering capacitors. An optical coupler is used to feed information back to the control IC on the high voltage side, which varies the drive to the transformer, in order to regulate the output voltage.
While Yaesu made well filtered, metal cased units as far back as the 1980s, many generate a LOT of RF interference. This includes harmonics around every 25 kHz across the HF bands, perhaps to the FM broadcast band or beyond.
Off the exam, cigarette lighter socket to USB adaptors sold into the automotive market generate hash and garbage in the AM band, HF, and potentially the FM band, although a strong FM signal may have some immunity. Ditto things like DAB+. What does NOT have immunity to this interference are VHF AM communications radios on light aircraft. The use of iPad and similar devices in place of conventional charts, etc, means that pilots may use dodgy plug-in converters, until an expensive service call identifies them as the reason their VHF-Comm is no longer working. Proper aviation plug-in converters are available, with the Pilot CO Charger including a carbon monoxide detector for just A$90.00. Installed ones are in the high $100s, plus engineering and installation for certified aircraft.
I bought one designed to be fitted under the seats on a coach, and despite CE marking, it does generate noise into the VHF band. For 12 volt vehicles building a devices containing something like a 7809 feeding a 7805 will provide slow charging without hash, with the cost of heat to be dissipated.
An unbalanced cable consists of a single wire with a shield, and is very common in domestic audio, even if duplicated in a stereo or multi-channel audio system, (5.1 surround, etc). Composite, RGB (including VGA, SCART, etc), and YPbPr "component" video. Ditto S-Video aka Y/C. RS-232 is an example of a a data transmission system which is unbalanced. Having multiple unbalanced signals in one shielded this is also an unbalanced system, and can suffer crosstalk.
Balanced signals are also termed differential signals. This is used in professional audio, such as signals from microphones, and DI boxes, and signals from a mixing desk to power amplifiers, where each pair of wires is shielded. USB and RS-422 use differential signalling. Not all balanced systems are shielded.
When noise, mains hum / buzz, or other interference is radiated near a conductor it typically affects all conductors equally, and even penetrates shielded cables to some extent. Where an unbalanced cable is impacted this often appears as noise or crosstalk on the input of the device the cable is connected to.
Where RF gets into control lines on a radio with a remote head cable consisting of flat telephone-style cable it can cause erratic behaviour, such as the radio power-cycling, or locking up in transmit mode.
Where a signal is balanced, as used in professional audio the difference between the wires is taken, so common mode noise is much less likely to cause crosstalk or interference.
These are the actual questions from the Extra licence exam pool, as published by the NCVEC, for use up to and including June 30, 2028
E4E01
Which of the following types of noise can often be reduced by a digital noise reduction?
A. Removal of the CW signal as well as the interfering carrier
B. Any nearby signal passing through the DSP system will overwhelm the desired signal
C. Excessive ringing
D. All these choices are correct
This function, often part of a DSP system, can filter out the desired CW signal, answer A.
E4E02
Which of the following types of receiver noise can often be reduced with a digital signal processing noise filter?
A. Broadband white noise
B. Ignition noise
C. Power line noise
D. All these choices are correct
DSP reduces white noise, ignition noise, and garbage from power lines, answer D.
E4E03
Which of the following types of noise are removed by a noise blanker?
A. Broadband white noise
B. Impulse noise
C. Hum and buzz
D. All these choices are correct
These are effective against impulse noise, answer B.
These interfering signals typically occur across a wide bandwidth. Ignition noise is a prime example, although injectors in newer diesels (or DIEsels, given the course particulates they generate, along with the ultrafine ones from petrol / gasoline) also generate strong electrical impulses. Time to go electric!
E4E04
How can conducted noise from an automobile battery charging system be suppressed?
A. By installing filter capacitors in series with the alternator leads
B. By installing a noise suppression resistor and a blocking capacitor at the battery
C. By installing a high-pass filter in series with the radio's power lead and a low-pass filter in parallel with the antenna feed line
D. By installing ferrite chokes on the charging system leads
Install ferrite chokes in the alternator leads, answer D.
E4E05
What is used to suppress radio frequency interference from a line-driven AC motor?
A. A high-pass filter in series with the motor's power leads
B. A brute-force AC-line filter in series with the motor's power leads
C. A bypass capacitor in series with the motor's field winding
D. A bypass choke in parallel with the motor's field winding
The solution is to install a "brute-force" AC-line filter in series with the motor, answer B.
These typically consist of a parallel capacitor, inductors in series with the current carrying lines, and another parallel capacitor. It is possible to add further stages, and perhaps MOVs. The components in the distractors are placed opposite to the required connection to form a filter which blocks RF, while passing low frequencies. Off the exam, a grounded metal enclosure may also help. As things like pool filter pumps use a lot of power third parties now sell variable frequency drive boxes as "energy savers" or some such thing, which can generate RF hash. If our neighbour has one causing problems you can report the problem to the FCC or ACMA, etc, pointing out interference with the AM broadcast band.
E4E06
What type of electrical interference can be caused by computer network equipment?
A. A loud AC hum in the audio output of your station's receiver
B. A clicking noise at intervals of a few seconds
C. The appearance of unstable modulated or unmodulated signals at specific frequencies
D. A whining-type noise that continually pulses off and on
Such equipment can cause unstable modulated or unmodulated signals at various frequencies, answer C.
PCs also cause such interference, which may vary with keyboard input, scrolling, disk access, etc.
E4E07
Which of the following can cause shielded cables to radiate or receive interference?
A. Low inductance ground connections at both ends of the shield
B. Common mode currents on the shield and conductors
C. Use of braided shielding material
D. Tying all ground connections to a common point resulting in differential-mode currents in the shield
Most consumer electronics is interconnected using shielded, unbalanced cables, and these can pick up or radiate common-mode interference, answer B.
Interconnects between radios and soundcards or modems are also typically unbalanced cables, and these occasionally pick up RF. Professional audio equipment typically uses balanced cables, which tend to reject common-mode currents.
E4E08
What current flows equally on all conductors of an unshielded multiconductor cable?
A. Differential-mode current
B. Common-mode current
C. Reactive current only
D. Magnetically-coupled current only
These are common-mode currents, answer B.
E4E09
What undesirable effect can occur when using a noise blanker?
A. Received audio in the speech range might have an echo effect
B. The audio frequency bandwidth of the received signal might be compressed
C. Strong signals may be distorted and appear to cause spurious emissions
D. FM signals can no longer be demodulated
High level, often nearby, signals may be distorted, answer C.
E4E10
Which of the following can create intermittent loud roaring or buzzing AC line interference?
A. Arcing contacts in a thermostatically controlled device
B. A defective doorbell or doorbell transformer inside a nearby residence
C. A malfunctioning illuminated advertising display
D. All these choices are correct
All these arcy-sparky dodgy devices will cause this problem, answer D.
Neon signs operate at around 15 kV, so arcing could occur if wires are damaged, etc.
E4E11
What could be the cause of local AM broadcast band signals combining to generate spurious signals on the MF or HF bands?
A. One or more of the broadcast stations is transmitting an over-modulated signal
B. Nearby corroded metal joints are mixing and reradiating the broadcast signals
C. You are receiving skywave signals from a distant station
D. Your station receiver IF amplifier stage is overloaded
This is intermodulation, caused by an unintentional diode formed in a corroded joint mixing and re-radiating the signals, answer B.
This is termed the "rusty bolt" effect.
E4E12
What causes interference received as a series of carriers at regular intervals across a wide frequency range?
A. Switch-mode power supplies
B. Radar transmitters
C. Wireless security camera transmitters
D. Electric fences
If you buy a lower cost Jaycar switchmode power supply, or one from AliExpress / Temu, it will generate a carrier at around 25 kHz, then harmonics at 25 kHz across all of the HF bands, answer A.
The interference tends to wander a little, as the supply's oscillator frequency varies with temperature, etc.
E4E13
Where should a station AC surge protector be installed?
A. At the AC service panel
B. At an AC outlet
C. On the single point ground panel
D. On a ground rod outside the station
The examiner states that the AC power surge protector should be placed on the common earth panel, answer C.
Bonding it via a reasonably heavy strap or insulated wire may be more convenient. Anything with MOVs inside should be enclosed within metal, as they can explode if they attempt to dissipate too much energy.
E4E14
What is the purpose of a single point ground panel?
A. Remove AC power in case of a short-circuit
B. Prevent common-mode transients in multi-wire systems
C. Eliminate air gaps between protected and non-protected circuits
D. Ensure all lightning protectors activate at the same time
A properly engineered home station or repeater site often includes a well grounded metal panel with lightning arrestors for each coax or other cable coming into the station. Despite being a bit J.D., oops, I mean weird, answer D gets the mark.
Normally tariffs and duties are low, and protect specific industries which a country has, say building low precision lathes, but not high precision ones (as was the case in Australia), so high rate nation based ones are highly irregular. For political reasons normally sanctions in the form of outright bans are used, such as against South Africa in the apartheid era. With the current system importers may have hundreds of thousands tied up in tariffs on stock held on shelves. Either interest must be paid on this, or interest is lost. Thus they form a part of the landed price and must be passed on, with mark-up, to consumers, retailers, or wholesalers.
The other weird thing is that a trade imbalance is NOT a country taking advantage of the US; US buyers pay money and get diamonds from Lesotho in a mutually beneficial transaction, but they have little need for oversized, overpriced pick-up trucks, hence the imbalance.
The final fallacy being relied on, (or a factor which is misunderstood by a certain person) is that a national VAT or GST is a form of tariff. When these are imposed on all products of a certain type, be they local, Chinese, or US made, they are clearly NOT a tariff. It is perhaps a fact that raw and basic foods like bananas or meatless baked beans are not subject to such a tax, while more highly processed foods (say meat and beans in a can, or confectionery), or manufactured goods are taxed, which adds to the confusion.
On to: Electrical Principles 1 - Resonance, Impedance, and Phase Angles
You can find links to lots more on the Learning Material page.
Written by Julian Sortland, VK2YJS & AG6LE, August 2025.
Tip Jar: a Jefferson (US$2), A$3 or other amount / currency. Thanks!
You can also buy me a non-coffee beverage: ko-fi.com/ag6le