Home - About AR - Learning Material - Exams - Clubs - Posters
This discusses several issues with receivers and transmitters.
A pre-selector is a tuneable (tunable) filter placed ahead of the receiver circuit, which only passes frequencies in the desired band, meaning it filters out strong out-of-band signals, so they do not adversely affect receiver sensitivity, etc. A front end filter might be used in a single band transceiver to help protect it from interference, especially when driving past communications sites.
These typically consist of resistors. Like a voltage divider they reduce the level of the signal going from the input to the output. In most cases they have an input and output impedance to match the system, such as 50 or 75 ohms. They can also be made for balanced systems, such as 300 ohms. It is possible to make attenuators which can match impedance between circuit sections, or between devices.
They can be used to prevent receivers from being overloaded, and may be built into radios, switched in and out via a switch, or by a relay under software control. Other uses include reducing a transmitter output before feeding a power amplifier with a sensitive input, or in the transceiver port of a transverter.
We know that the big upside of FM is that once a signal is of an adequate strength it has strong resistance to noise and interference. The downside of this is that if two hams or UHF CBers are talking without breaks between transmissions, they will not hear a weaker station trying to make a distress call, as the stronger signal has "captured" the receiver.
The practical solution is to ensure overs are not excessively long, and leave short gaps between overs. This is especially true on emergency support nets.
Suppose we have a public service station set up on 2 metres FM voice, and either a 2 metre packet station, or a marine VHF station set up on the same site, say for a canoe paddle event. Transmitting on one band will "desensitise" the receiver on the other unit. The extent to which this occurs is down to the dynamic range of the receiver(s). This can also occur with multiple HF transceivers on a field day site. The abbreviated term is "desense".
If planning an operation involving packet requiring 2 metres, the primary voice net may be best on 70 cm (avoiding harmonics), or a licensed UHF frequency. A second logistics and back-up net could be on 6 metres, or a licensed VHF mid-band channel (70 MHz), or 4 metres if you permitted it, or 1.25 metres.
This can also happen if you are trying to work a distant station on 14.226 while a bunch of lunatics are running high power on 14.222 MHz, swearing / cursing, and preaching Q-Anon and other conspiracy theories. Solutions include high quality, narrow filters, and roofing filters. I meant quality in the generic sense, but a high Q, or quality factor, is beneficial.
If you are trying to hear your own signal returning from a repeater using a second hand-held, or are testing a repeater the local transmitter may overload the receiver, or cause de-sense. It is best to get a friend to listen a few 10s of metres or more away. This person does not need to be licensed if they don't transmit back on amateur frequencies (although "supervised" third party operation does not specify a certain number of metres, so it is probably OK for them to transmit briefly, as long as they are visible, and within a distance they can hear instructions). :-)
Th exam question on this has changed from reducing bandwidth to applying an attenuator ahead of the receiver. These tend to be built into only higher-end devices.
At repeater sites the cavity filters may help to reduce desensitisation, should you say have a 2 metre repeater, and a packet digipeater or BBS on 2 metres.
A radio transmitter will generate a sequence of signals called harmonics, especially at odd multiples of their design frequency, but also on even multiples. These are supposed to be well suppressed, but 43 dB below 100 watts is 5 milliwatts, enough to be heard a fair distance.
Not mentioned on the exam, if you transmit on 146.500 kHz, and a station on site is listening on 439.500 MHz, then the 70 cm station will likely hear your harmonics. Usually annoying, this can be a quick way to test a 70 cm receiver, with a similar relationship existing to use a 70 cm HT to make a basic test of a 23 cm radio.
Given this typical factor of multiplying by 3, you could also determine the 6 metre frequency which might interfere with a Coastal Station on 156.800 MHz, Marine Ch 16, at a Lighthouse & Lightship Weekend event.
156.800 / 3 = 52.26666667 which means something like 52.255 to 52.275 MHz should be avoided at this site.
The numbering of harmonics is: The base frequency, say 1 MHz, is termed the fundamental. The second harmonic is twice the fundamental, at 2 MHz; the third is three times it, at 3 MHz; the fourth, 4 MHz; the fifth, 5 MHz; and so on. The fundamental can be termed the first harmonic, although this is not a common term.
Intermodulation, or "Intermod" is a form of interference which can be an A-grade pain in the posterior, as finding the source can be very difficult. The classic version is f = 2a-b, where a is the frequency of one transmitter, and b is that of the other, and if b is greater than a, we can end up with a signal lower in frequency than either a or b, so services above, say 2 metres can end up generating intermod on 2m.
Suppose a poor quality narrow-casting transmitter is operating on 152 MHz, and a voice communications or data link on 156.2 MHz keys up periodically. Energy from the higher frequency transmitter enters the narrow-casting transmitter, and interacts with its frequency, in the diode junctions in its transistors. This creates a third frequency, thus: f = (a x 2) - b = 152 x 2 - 156.2 = 304 - 156.2 = 147.8 MHz, a 2 metre repeater input channel.
To fix the problem the dodgy transmitter needs to be taken out of service, and either fixed, or replaced with a quality product. The exam asks about two repeaters on a site causing the same problem, with the answer being to use a circulator on the transmitter in which the mixing occurs.
A circulator is a 3 or 4 port device, in which RF or microwave energy can only travel in one direction. They typically use a strong ferrite magnet. The configuration they are discussing is a 3 port unit, so power goes from the transmitter on Port 1 to the antenna on Port 2, but energy coming into Port 2 from the other transmitters on the side (via the antenna) is sent to Port 3, as long as there is a proper dummy load (termination) on that port. Another use is to divert power reflected from a faulty antenna into a dummy load, to protect the transmitter. (I noticed a couple of them in the "flea market" at the now defunct "Mayham" hamfest in Wyong, which had only 2 external connectors, with the dummy load build into the enclosure).
The problem is that even if the transmitters are all clean, quality products, and all appropriate isolation devices are used, a naturally occurring diode in the fence, or on the tower can act as a rectifier, causing intermodulation to occur. It can even be something like an old rusty fence or telephone wire, fallen on a galvanised water trough, occurring randomly, depending on the wind. This can be termed the "rusty bolt effect". Other points can be real diodes, such as LED based tower lights or illuminations could be an issue. Even micro-diodes between braid and foil in coaxial cable are potential problems, with cable such as RG-223 (dual braid shield) or FSJ1-50 (corrugated copper shield) being ideal cables for shorter links in duplex systems. Diode junctions, intended or otherwise, are termed "non-linear devices" in the exam. Transistors junctions also appear to be diodes.
Normally the problematic transmitters are on the same site as the impacted receiver, or nearby, but in one case, intermod on a site at Pennant Hills (northern Sydney) impacted a repeater on the Central Coast, over 50 km away.
There are other combinations, some involving 3 transmitters, typically f = a + b - c.
Further combinations are listed here: Wikipedia: Intermodulation
Given many involve 3 factors, either something like twice the "a" frequency minus the "b" one; or and interactions of three frequencies, these are termed "third-order intermodulation products". These are of interest as they often land the interfering signal in the same band as the two or 3 signals which contribute to it. Taking 2 metres as an example this may include nearby parts of the "VHF-High" band, beyond 2 metres itself, although the exam uses frequencies within 2 metres. These are an example of "odd-order" products, a term introduced in the previous version of the exam.
Pro-tip for Australian hams: Narrowcasting transmitters around 152 MHz may be of questionable quality, and can be a source of interference or intermods. Searching the ACMA database, and nutting out possible combinations on the site, or on nearby sites is often part of the investigation, either by Hams, or by the ACMA. Other nearby users include government and commercial bodies (voice and data), and the VHF Marine Band. Example of data bursts may be telemetry of river heights (or the ACK / NAK reply), or telecommand of pumps in a municipal water supply system.
A large dynamic range is useful, as it allows a receiver (or the amplifiers within it) to work with signals over a wide range of levels. It can also amplify a weak signal of interest in the presence of a much stronger signal, without generating intermodulation products.
Way off the exam, valve (tube) input stages often had very good dynamic range, partly because the large supply voltage allowed large swings in output voltage without compression or distortion.
The noise floor of a receiver is expressed in dBm in one hertz bandwidth. The minimum this can be at 290 K (16.85℃), aka "room temperature" this is -174 dBm.
If a receiver has a bandwidth of 400 Hz, this is 400 times 1 Hz, expressed in dB as 26 dB. Thus the noise floor is -174 dB + 26 dB = -148 dBm.
Given 400 is 10 × 10 × 2 × 2, this translates to 10 + 10 + 3 + 3, being 26 dB. On this paper dB questions use values which can be determined using a 10 times change being 10 dB, and a doubling or halving being a 3 dB change, rather than more difficult maths.
As we know, decibels are a means to compare two signals, using a logarithmic scale. This is used for dynamic range, among other things.
If a reference level is specified, a single figure can provide an absolute value. A figure in dBm is relative to 1 milliwatt, or 1 mW. Just as freezing point of water is 0°C, and used as a reference point for a system of "degrees", the reference level of 1 mW is 0 dBm. Home audio is standardised around a signal of -10 dBm, while professional audio uses +4 dBm. In the RF world, a weak FM signal might present -128 dBm at the receiver's input socket.
Off the exam, as the US FCC specifies power limits in watts, some overseas administrations use dBW, decibels relative to 1 watt. For example, 27 dBW is 500 watts, this being 3dB less than, or half of +30 dBW or 1 kW.
Thus 1 watt is 0 dBW or 30 dBm. 10 watts is 10 dBW or 40 dBm.
A parameter by which immunity to interaction between strong nearby signals can be quantified is the Third-order Intercept Point, or IP3.
Suppose there are signals on 14.110 and 14.120 MHz. As these pass through an amplifier, harmonics, intermodulation products of the fundamentals, and intermodulation products of the harmonics, are all created in non-linear components. The two of most interest are those close-by, at 14.100 and 14.130 MHz, these being 2f₁-f₂ and 2f₂-f₁.
An increase in the signals of 1 dB results in an increase of 1 dB in their representation at the output. However, the intermodulation products increase by 3 dB in strength.
The levels of real signals and the intermodulation products can be graphed in such as way that the lines can be extrapolated to an intersection point. The intersection often occurs at 10 dB above the 1 dB gain reduction point, and often beyond the point at which the device under test would be destroyed.
A good figure for this parameter is often represented as a selling point for the radio.
It is important to emphasise that this is a theoretical figure.
This video is most helpful in understanding these processes:
You might expand it, or click on "YouTube" to open it in a new window.
See: Wikipedia: Third-order Intercept Point
While a $10,000 plus transceiver may boast a somewhat greater power output and/or other transmitter features, a high quality receiver capable of copying weak signals under arduous conditions is a significant part of the price, and of its attraction to those able to afford one. These would also be valuable if on the DX side of a dog-pile. The 40 dBm IP3 figure on the exam apparently relates to a very high-dollar Icom unit, with radios for us mortals having a 15 to 22 dBm figure.
While 47 may have 86ed much of VOA, you may have a high power SW broadcaster nearby, a few 10s of kHz from an Amateur band, so a high IP3 figure may be useful, ditto a preselector.
Software defined radios range from a low cost USB dongle designed for receiving DVB-T digital TV to a low budget "Soft Rock" device to feed a PC soundcard to a receiver of transceiver costing thousands of dollars. The latter can be a PC controlled, or have a conventional modern transceiver user interface.
A card or USB device sold to digitise home videos can apparently accept a roughly 7 MHz wide chunk of HF spectrum, or other spectrum down-converted. Amplification is needed.
While digitisation may occur at the RF frequency, it is often done at Intermediate Frequency. Whatever the system, all rely on a digital to analogue converter.
In each case an instantaneous sample of the voltage is taken, and this is converted to a numerical value.
In PCM, or pulse-coded modulation, the number of values which can be counted are 2 to the power of the number of bits, or 2≳. This may be split between positive and negative values.
An 8 bit ADC allows 256 values to be measured. 16 bits allows 65,636 to be used. 20 bits gives 1,048,576 levels, and 24 bits allows 16,777,216 values. In these audio world these correlate to telephone systems; to CD and DAT; and the last two to "high definition" or "high resolution" consumer systems such as Blu-Ray and DVD-Audio.
Off the exam, there is however a trick with 8 bit used in telephony is a logarithmic system which accurately transfers lower level speech while also being able to use coarser steps for higher level signals, which parallels human hearing. The method is to quantify at 12 bits, and convert to 8 bits using either the global A-law or US-centric μ-law algorithms in the ITU G.711 standard.
Say we want a system to quantify a maximum signal of 1 mV of RF. At 8 bits a single bit will have a value of 1/128 of a millivolt, or 7.8125 microvolts. Many real-world radio signal are well below this level. In a 16 bit system, divide 1 mV by 32,768, and 1 bit is about 0.0305 μV. 20 bit gives a resolution of 0.0019 μV. Thus, a greater number of bits increases the dynamic range of the system.
The second parameter is the sample rate, varying from 8 kHz for telephony to 48 kHz for professional audio, with 96 and 196 kHz for "HD" consumer products. Various higher rates are used for SDRs. The RTL-SDR, using DVB-T dongles, down-converts a section of spectrum, and samples it at 2.4 to 3.2 mega-samples per second. Software then extracts the desired signals, be that a voice mode, or all manner of data modes.
The sample rate must be twice the frequency of the highest frequency signal. There is often a filter placed before the A-to-D converter, to prevent false values being recorded.
Test equipment, such as digital storage oscilloscopes, samples at rates well into the Giga-samples per second range.
The various digital voice modes use compression techniques to reduce the data-rate, and thus bandwidth of the signal, such as AMBE2. CODEC2 is an open-source option.
Way off topic, until someone experiments with using it for communications, Direct-Stream-Digital is a form of pulse density modulation, operating at very high sample rates (such as 2.8224 MHz), but only 1 bit of depth. Providing storage for delta-sigma streams (without using decimation), it provides high quality audio on the Super Audio CD (SACD) format, which require specialist players. lower quality singe-bit / PDM storage is (or likely was) used in some recorded voice announcements in the telephone network. The Soviet Union's early talking clocks also used PDM marks on optical tape to have a non-contact reading method - in 1937.
Many ham repeaters consist of a couple of used ham or commercial radios, or a retired commercial repeater, or perhaps a dedicated ham device for things like Fusion or D-Star. These feed a dedicated antenna or antennas via a bunch of cavity filters or "cans".
Not on the exam, but useful: The alternative implemented by several hams who work for or own communications companies is to integrate current commercial quality repeaters tuned to 70 cm into a commercial UHF site antenna system. This consists of a receiver antenna system, typically folded dipoles or a "binary array". This is filtered, amplified and distributed to multiple receivers. Transmitter outputs are combined, often into a separate transmit antenna array. Circulators are often used, to ensure no RF power ends up where it shouldn't. This applies to modes such as FM, P25, and DMR. Sites may be linked via their corporate microwave-based IP system.
Vertical separation of antennas helps to increase isolation between transmit and receive antenna arrays.
Cross-band repeaters of various kinds can be implemented quite easily, especially during emergency situations. They can be placed in a vehicle to provide hand-held access to a repeaters, to which access would otherwise be difficult, such as due to shadowing by terrain. In the US these may be termed auxiliary stations. This can also be done using licensed frequencies belonging to emergency agencies, perhaps linking a state emergency management site with maritime rescue assets via something like a trunked UHF system to VHF marine radio link, often termed a "bridge".
You know the drill, these are the actual questions from the NCVEC's Extra licence exam pool.
E4C01
What is an effect of excessive phase noise in an SDR receiver's master clock oscillator?
A. It limits the receiver's ability to receive strong signals
B. It can affect the receiver's frequency calibration
C. It decreases receiver third-order intercept point
D. It can combine with strong signals on nearby frequencies to generate interference
This can cause strong signals on nearby frequencies to interfere with reception of weak signals, answer D.
E4C02
Which of the following receiver circuits can be effective in eliminating interference from strong out-of-band signals?
A. A front-end filter or preselector
B. A narrow IF filter
C. A notch filter
D. A properly adjusted product detector
Use a front-end filter, or a preselector, answer A.
Some HF receivers have a manually tunable filter as the first stage of the signal path. This reduces the effects of signals in other bands.
E4C03
What is the term for the suppression in an FM receiver of one signal by another stronger signal on the same frequency?
A. Desensitization
B. Cross-modulation interference
C. Capture effect
D. Frequency discrimination
This is the capture effect, answer C.
AM suffers much less from the capture effect, which is one reason that it is still used in aviation VHF. These same applies to SSB.
E4C04
What is the noise figure of a receiver?
A. The ratio of atmospheric noise to phase noise
B. The ratio of the noise bandwidth in hertz to the theoretical bandwidth of a resistive network
C. The ratio in dB of the noise generated in the receiver to atmospheric noise
D. The ratio in dB of the noise generated by the receiver to the theoretical minimum noise
This is the ratio of the noise generated by the receiver to the theoretical minimum noise, expressed in dB, answer D.
Amplifiers with a low noise factor are important when receiving weak signals. Even a 1 dB improvement is significant is systems such as EME / Moonbounce station pre-amplifiers. In radio-telescopes receivers are often cryogenically refrigerated to reduce noise production, using liquid helium.
E4C05
What does a receiver noise floor of -174 dBm represent?
A. The receiver noise is 6 dB above the theoretical minimum
B. The theoretical noise in a 1 Hz bandwidth at the input of a perfect receiver at room temperature
C. The noise figure of a 1 Hz bandwidth receiver
D. The receiver noise is 3 dB above theoretical minimum
This figure is the theoretical noise in 1 Hz of bandwidth at the input of a perfect receiver, at room temperature, answer B.
E4C06
How much does increasing a receiver's bandwidth from 50 Hz to 1,000 Hz increase the receiver's noise floor?
A. 3 dB
B. 5 dB
C. 10 dB
D. 13 dB
1000 Hz is 20 times the bandwidth of 50 Hz. 20 times expressed in dB is 13 dB. The maths is 20 = 10 × 2, which translates in dB to 10 + 3 = 13 dB, answer D.
E4C07
What does the MDS of a receiver represent?
A. The meter display sensitivity
B. The minimum discernible signal
C. The modulation distortion specification
D. The maximum detectable spectrum
This is the minimum discernible signal, answer B.
E4C08
An SDR receiver is overloaded when input signals exceed what level?
A. One-half the maximum sample rate
B. One-half the maximum sampling buffer size
C. The maximum count value of the analog-to-digital converter
D. The reference voltage of the analog-to-digital converter
The reference voltage of the A-to-D converter also represents the maximum input level and the maximum count. The current "correct" answer that it is "overloaded" when the voltage of the input exceeds the reference voltage, answer D.
Note that there is probably a margin between the maximum input that can be quantified, and that which causes damage to the converter.
E4C09
Which of the following choices is a good reason for selecting a high IF for a superheterodyne HF or VHF communications receiver?
A. Fewer components in the receiver
B. Reduced drift
C. Easier for front-end circuitry to eliminate image responses
D. Improved receiver noise figure
It is easier for the front-end to filter out the image, as it is further away in frequency, answer C.
To filter an unwanted signal under 1 MHz away from a signal of interest is harder than one tens of Megahertz away. Something like a radio-cassette player including a SW band (or SW1 & SW2) probably had a 455 kHz IF, meaning that a broadcast signal at or near 6.29 MHz or 8.11 MHz may over-power a ham one on 7.2 MHz. A 10.7 MHz IF would mean that the signal we need to reject is all the way up at 7.2 + (10.7 × 2) = 28.6 MHz, much easier to filter out.
Triple-conversion receivers may have the first IF in the 70 to 80 MHz range.
E4C10
What is an advantage of having a variety of receiver bandwidths from which to select?
A. The noise figure of the RF amplifier can be adjusted to match the modulation type, thus increasing receiver sensitivity
B. Receiver power consumption can be reduced when wider bandwidth is not required
C. Receive bandwidth can be set to match the modulation bandwidth, maximizing signal-to-noise ratio and minimizing interference
D. Multiple frequencies can be received simultaneously if desired
The correct filter can be selected for the mode being received, answer C.
CW, SSB, AM, FM, etc, each have different bandwidths. Ceramic, crystal, or ideally, Collins mechanical filters of various suitable bandwidth can be added to most HF transceivers. The last stocks of Collins filters are available from InRad, typically fitted to small "daughter boards" to plug into radios. Yaesu or its dealers may also have some left for things like the FT-857D.
E4C11
Why does input attenuation reduce receiver overload on the lower frequency HF bands with little or no impact on signal-to-noise ratio?
A. The attenuator has a low-pass filter to increase the strength of lower frequency signals
B. The attenuator has a noise filter to suppress interference
C. Signals are attenuated separately from the noise
D. Atmospheric noise is generally greater than internally generated noise even after attenuation
Distant lightning crashes, "static" and other atmospheric noise exceeds noise within the receiver, answer D.
E4C12
How does a narrow-band roofing filter affect receiver performance?
A. It improves sensitivity by reducing front-end noise
B. It improves intelligibility by using low Q circuitry to reduce ringing
C. It improves blocking dynamic range by attenuating strong signals near the receive frequency
D. All these choices are correct
A feature in more modern gear, these improve dynamic range by attenuating strong signals near the receive frequency, answer C.
E4C13
What is reciprocal mixing?
A. Two out-of-band signals mixing to generate an in-band spurious signal
B. In-phase signals cancelling in a mixer resulting in loss of receiver sensitivity
C. Two digital signals combining from alternate time slots
D. Local oscillator phase noise mixing with adjacent strong signals to create interference to desired signals
Phase noise in the local oscillator can mix with adjacent strong signals to create interference to desired signals, answer D.
E4C14
What is the purpose of the receiver IF Shift control?
A. To permit listening on a different frequency from the transmitting frequency
B. To change frequency rapidly
C. To reduce interference from stations transmitting on adjacent frequencies
D. To tune in stations slightly off frequency without changing the transmit frequency
This can help you reduce interference from stations on a nearby frequency, answer C.
This may be a specific knob on a radio with a large front panel. On a FT-857D this is the second function of the RIT (Receiver Incremental Tuner, very similar to an SSB CB's clarifier), alluded to in the last distractor.
E4D01
What is meant by the blocking dynamic range of a receiver?
A. The difference in dB between the noise floor and the level of an incoming signal which will cause 1 dB of gain compression
B. The minimum difference in dB between the levels of two FM signals which will cause one signal to block the other
C. The difference in dB between the noise floor and the third order-intercept point
D. The minimum difference in dB between two signals which produce third-order intermodulation products greater than the noise floor
Within the design frequency band an amplifier normally amplifies a signal, be it weak or strong, by a certain number of dB, determined by the nature of the active device, and things like resistors around it. However, once a certain level is reached, the amplifier "runs out of puff", and the gain is reduced. In measuring the dynamic range a common parameter must be agreed to, and this is the point where the output is 1 dB less that a straight line trend predicts, relative to the noise floor, answer A.
E4D02
Which of the following describes two problems caused by poor dynamic range in a receiver?
A. Spurious signals caused by cross-modulation and desensitization from strong adjacent signals
B. Oscillator instability requiring frequent retuning and loss of ability to recover the opposite sideband
C. Poor weak signal reception caused by insufficient local oscillator injection
D. Oscillator instability and severe audio distortion of all but the strongest received signals
Cross-modulation of the desired signal, and desensitisation from strong adjacent signals, answer A.
E4D03
What creates intermodulation interference between two repeaters in close proximity?
A. The output signals cause feedback in the final amplifier of one or both transmitters
B. The output signals mix in the final amplifier of one or both transmitters
C. The input frequencies are harmonically related
D. The output frequencies are harmonically related
This can occur when two or more repeaters (or other transmitters) are in close proximity, and the signals mix in the final amplifier of one or both transmitters, answer B.
Intermod can also occur externally, such as in rusty tower bolts, a wire resting on a feed or water trough, etc. LEDs and solar panels also contain diode junctions, so may be a point at which an intermod can occur.
E4D04
Which of the following is used to reduce or eliminate intermodulation interference in a repeater caused by a nearby transmitter?
A. A band-pass filter in the feed line between the transmitter and receiver
B. A properly terminated circulator at the output of the transmitter
C. A Class C final amplifier
D. A Class D final amplifier
A circulator ensures the transmitter's power goes to the antenna, and any signal coming in from the antenna goes to the dummy load, answer B.
A circulator can also protect the transmitter from an antenna fault by directing reflected power into the dummy load. Typically they look like an aluminium project box around 14 by 8 by 5 cm, and have three N-connectors, being for Transmitter, Antenna, and Load / Termination; or occasionally Transmitter and Antenna ports, with an internal load. They contain a magnet, and typically are marked with a circular arrow.
E4D05
What transmitter frequencies would cause an intermodulation-product signal in a receiver tuned to 146.70 MHz when a nearby station transmits on 146.52 MHz?
A. 146.34 MHz and 146.61 MHz
B. 146.88 MHz and 146.34 MHz
C. 146.10 MHz and 147.30 MHz
D. 146.30 MHz and 146.90 MHz
Assume they are using f = 2a - b, and plug 146.52 into each of a and b in turn, with 146.70 as f.
2a = f + b = 146.70 + 146.52 = 293.22; so a = 293.22 / 2 = 146.61, so answer A, but better confirm this.
-b = f - 2a = 146.70 - (146.52 x 2) = 146.70 - 293.04 = -146.34, so b = 146.34, yep, answer A.
Note that US repeaters are typically spaced by 30 kHz, rather than the 25 kHz or 12.5 kHz used elsewhere.
E4D06
What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency?
A. Reciprocal mixing
B. Quieting
C. Desensitization
D. Cross modulation interference
If you have were say running a 2 metre FM station, and a 2 metre SSB one on the same site during Field Day, they may cause each other to suffer desensitisation, answer C.
E4D07
Which of the following reduces the likelihood of receiver desensitization?
A. Insert attenuation before the first RF stage
B. Raise the receiver's IF frequency
C. Increase the receiver's front-end gain
D. Switch from fast AGC to slow AGC
If you have something like an Yaesu FT-847 it has a button with will place an attenuator ahead of the RF stages. This helps to reduce all signals, including large ones below the level at which they desense the receiver, answer A.
E4D08
What causes intermodulation in an electronic circuit?
A. Negative feedback
B. Lack of neutralization
C. Nonlinear circuits or devices
D. Positive feedback
These are non-linear devices, being an intentional or non-intentional semiconductor junction (diode), answer C.
Say you have an APRS device out in a paddock, reporting weather conditions, feeding a low cost Chinese hand-held or mobile radio radio. As there are no nearby strong signals it is unlikely to cause problems. Move that to a busy communications site, and strong signals will enter the antenna socket and reach the transmitter while it is transmitting, where this "non-linear" device cause intermod products to be produced. A professional grade transmitter, such as a Tait or Motorola should be less likely to cause this.
E4D09
What is the purpose of the preselector in a communications receiver?
A. To store frequencies that are often used
B. To provide broadband attenuation before the first RF stage to prevent intermodulation
C. To increase rejection of signals outside the desired band
D. To allow selection of the optimum RF amplifier device
A pre-selector helps the receiver to reject signals outside the desired band, answer C.
E4D10
What does a third-order intercept level of 40 dBm mean with respect to receiver performance?
A. Signals less than 40 dBm will not generate audible third-order intermodulation products
B. The receiver can tolerate signals up to 40 dB above the noise floor without producing third-order intermodulation products
C. A pair of 40 dBm input signals will theoretically generate a third-order intermodulation product that has the same output amplitude as either of the input signals
D. A pair of 1 mW input signals will produce a third-order intermodulation product which is 40 dB stronger than the input signal
Given applying two 10 watt signals (40 dBm is 10,000 mW) to a tiny FET will destroy it, the operative term is "theoretical", answer C.
This is somewhat flippant comment, but an easy way to remember the answer. An IP3 of 40 dBm means that it has a very high resistance to generation of intermodulation products within its internal amplifiers and other components. 40 dBm is the level at which graphing indicates the front-end amplifier will theoretically generate third-order intermodulation products with the same level as the input signals.
E4D11
Why are odd-order intermodulation products, created within a receiver, of particular interest compared to other products?
A. Odd-order products of two signals in the band being received are also likely to be within the band
B. Odd-order products are more likely to overload the IF filters
C. Odd-order products are an indication of poor image rejection
D. Odd-order intermodulation produces three products for every input signal within the band of interest
Odd order products, such as third-order ones, are often within the band we are operating in. As and example, if two strong signals are in or near the 2 metre band, there is a significant risk that they will produce product(s) which cause interference in that band, answer A.
In the 1970s to early 2000s in Australia high power pager transmitters operated just above 2 metres, from 148.0125 MHz and up, and these caused interference to 2 metres, through various mechanisms.
Trunked radio systems can serve multiple state / county / city government or private clients, each using a virtual channel or "mode". Each base might have four or five 400 MHz or 800 / 900 MHz voice channels, and a control channel. Thus a "rusty bolt" may cause an intermodulation product on a channel within that band.
E4D12
What is the link margin in a system with a transmit power level of 10 W (+40 dBm), a system antenna gain of 10 dBi, a cable loss of 3 dB, a path loss of 136 dB, a receiver minimum discernable signal of -103 dBm, and a required signal-to-noise ratio of 6 dB?
A. -8dB
B. -14dB
C. +8dB
D. +14dB
The received signal is 40 + 10 - 3 - 136 = -89 dBm, and the required signal is -103 + 6 = -97 dBm. The margin is thus 97 - 89 = 8 dB, answer C.
Depending on the band, this provides a reasonable margin against fading due to weather and other factors.
E4D13
What is the received signal level with a transmit power of 10 W (+40 dBm), a transmit antenna gain of 6 dBi, a receive antenna gain of 3 dBi, and a path loss of 100 dB?
A. -51 dBm
B. -54 dBm
C. -57 dBm
D. -60 dBm
40 + 6 + 3 - 100 is -51 dBm, answer A.
E4D14
What power level does a receiver minimum discernible signal of -100 dBm represent?
A. 100 microwatts
B. 0.1 microwatt
C. 0.001 microwatts
D. 0.1 picowatts
This is 10-3 × 10-10 = 10-13. Given 10-12 translates to a metric prefix of pico, this value is 0.1 picowatts, answer D.
An alternative calculation is 10-3 / 1010 = 10-13. You can also write the value in engineering notation as 100 × 10-15; or as 100 femtowatts, which is the more correct way when using metric prefixes.
A deleted question asked, "What transmit frequency might generate an image response signal in a receiver tuned to 14.300 MHz and which uses a 455 kHz IF frequency?", and gave various frequencies between 13-odd and 15-odd MHz.
The image occurs at twice the IF away from the dial frequency, and 14.300 + 0.910 = 15.210 MHz, the correct answer.
The local oscillator is 455 kHz above the desired frequency, and in this case it is set to 14.755 MHz. Adding 455 kHz (0.455 MHz) to this we get 15.210 MHz. This is a frequency in the 19 metre broadcast band. 19 metres is used in the daytime, and in the evening in summer. Power can be tens or hundreds of kilowatts.
And for completeness, subtracting gets you to 13.390, while the nearest distractor was 13.845 MHz, clearly wrong. However of the local oscillator (LO) is above the receive frequency, then the "image" or non-desired signal must be even higher, not lower. This problem was used as an opportunity to receive frequencies blocked on scanners and US market ham transceivers, still done, despite FM based AMPS services being long closed.
On to: Practices 3 - Noise suppression
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