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In days of old, a station consisted of several pieces of equipments, often having their own cabinet, perhaps put into a rack.
A receiver, suitable for the desired band was perhaps the first component bought, or more often build. Now we can hear operation in the Amateur bands. This would have consisted of thermionic valves (electron tubes), rather than being a simple crystal set, and would usually be tune-able across one or more bands. Next was a transmitter, again consisting of valves, and likely crystal locked. This would get us on air with a few watts of CW, but we can add in an audio amplifier, driven by a microphone, and we have AM voice transmission. A power amplifier may also be added. Originally a couple of switches might be needed to change between receive and transmit, these replaced with relays controlled by a single "Push-to-talk" switch. Initially, for a particular band each person in the area had a particular crystal-locked frequency, and operation was by Fred calling, on say 7.105 MHz, then tuning up and down the band, looking for replying, so he might find John on 7.120 MHz, and knowing John was replying, call him personally on 7.105, while listening on 7.120 MHz.
|A modern Crystal which I bogeyed into some sort of connector, which happens to fit an older crystal holder. The silver coloured case is 10mm wide, 13mm high. Normally these are soldered to a PCB. These are also socketed into older (1970s) radios for channel selection, and Tandy / Radio Shack sold standard crystals for CB channels. Inside is a small disc of quartz, looking like cloudy glass, with metal glaze on parts, to allow connection to the wires. The crystal sliver physically vibrates at the marked frequency (fundamental), or at a fraction, such as the CB ones which vibrate at, say 9.01833 MHz, and also at the harmonic, 27.055 MHz (Ch 8) - these are "overtone" crystals. Even older radios used crystals approaching the size of a matchbox, or a octal valve / tube.|
But NO, blue-tacking a quartz crystal to your computer monitor will NOT protect you from whatever "dangerous radiation" it supposedly generates...
Note that the term "crystal" in crystal set, or crystal receiver, a usually simple radio, where reception refers to the crude diode made from a lump of galena crystal (Lead(II) sulfide, PbS) and a metal contact, which due to the fine wire used, is called a "cat's whisker". In modern versions this uses a point-contact germanium diode. This is a radio frequency rectifier, not a frequency control device.
A VFO, meaning a variable frequency oscillator could be added, so our transmit becomes frequency agile. Changing bands may have required changing plug-in coils, to interact with the large tuning capacitor connected to its knob, generally via a reduction or "vernier" drive, a simple gearbox allowing finer tuning, as it now take several turns to fully mesh or un-mesh the variable capacitor. The low powered transmitter stage driving the amplifier is also called an "exciter", as it excites, or drives, the amplifier. Thus now, if John heard Fred on 7.106 kHz, he would speak to him there, a practice which continues on most Amateur operation, including regular contests. The exception is DX pile-ups, or "dog-piles", where a station many amateur want to contact, like VK0DX (in Antarctic) might transmit on 7.115, and listen for replies over the range of 7.105 to 7.125, so the stations calling can hear the "DX" station. A station thus might repeat there call, among others on 7.120 MHz. Suddenly I might hear "go the VK2 Yankee", or "station ending in Juliet Sierra only", and others are supposed to stop, so I can give my call, and a report, like 57. He or she would then reply with my call, and report, and I would reply that this was the correct call, ending with 73 (best wishes); and thus I have just added Antarctica to my list of DXCC "countries" contacted. Other stations must not transmit lower than 7.119, or above 7.111, or they will interfere with the DX stations signals.
As an aside, should you obtain a valve based transceiver, or "hybrid" one, meaning transistor circuits feeding a valve driver and final, these need to be "tuned up", for each frequency, meaning the output stage is adjusted by means of knobs on the panel, to adjust the plate current, load, and drive from the previous stage. The manual, another Ham, or You-Tube can show you how.
Now all these parts, including a solid state amplifier of around 100 watts PEP is included in a box the size of a 1970's CB, handing multiple bands. This transmitter - receiver unit is called a "transceiver". Some transceivers, with advanced signal processing, a large screen for displaying various parameters, and multiple dials, knobs, buttons, and a keypad for direct entry of frequencies can be the size of a family box of cereal.
Back to out old-school station, and we hear that this new VHF thing is quite the lark, but unless we can find some ex-military gear to adapt, we have to once again build our own. However, we really don't want to build all the gear over again. What if we could convert on-air 50 MHz, or 144 MHz, or whatever, signal down to an HF band, and use the receiver we already have? And we could sample the output of our fancy AM transmitter, step it up to these bands too? YES! This device is called a "Transverter", a transmitter-converter, I suppose.
These devices remain in use, as, as far as I am aware, not being a band used in Japan, there are no commercially made single-sideband transceivers for 222 MHz (1.25m). Thus we need a transverter. These are also used from 144 MHz up to the various microwave bands. Some still claim that a transverter from, say 28 MHz to 144 MHz or 432 MHz are better than using a radio designed for these bands, as the receiver component of the HF transceiver is superior to that in VHF radios. For Europeans, the same applies to 70 MHz (4 metres) in the UK, although some newer radios now include this band (noting ex-PMR or Chinese radios are available for FM).
The previous version of the exam asked about converting between a 28 MHz SSB transceiver and 222 MHz, but has been made more general. Nevertheless, numbers in the description are useful: To receive the 222 signal, we need to use a "LO", or local oscillator at the frequency, being the the difference between the two, 194 MHz. The signal at or near 222 MHz is then mixed with the LO, in a "non-linear" device, such as one or more diodes. This creates a signal at 28 MHz, and at 250 MHz, this latter being filtered out. For transmitting, the 28 MHz signal is attenuated (reduced in level), and mixed with the LO, generating the 222 MHz signal, which is then amplified, and fed to the antenna. The LO would actually consist of a lower frequency oscillator, and several stages of multipliers. If we wished to operate at 222.5 MHz the transceiver would be tuned to 28.5 MHZ, and for 223, to 29 MHz.
By the way, a device which generates signal on a desired frequency is called an oscillator. A device which produces a signal on an undesired frequency is called an amplifier, a bit of a joke, but amplifiers which oscillate, and oscillators which refuse to, are two of the banes of the technician or radio constructor's life. Both are actually quite similar in design, with the positive feedback from the output to the input which causes the oscillation (just as a microphone placed near a speaker in a PA generates a squeal). In "free running" Radio Frequency oscillators most often the interaction of capacitor(s) and inductor(s) is used to set the frequency. A crystal, being a sliver of quartz, typically housed in a small metal case can be used to disciple the oscillation to a specific frequency. There are also tweakable crystal oscillators, allowing a shift of a few kilohertz.
The transverter process is similar to that used in "super-het" or super-heterodyne receivers, and in transmitters, using local oscillators and "mixers" to convert to intermediate frequencies, called the "IF", where filtering and other processing can be carried out, such as 10.7 MHz or 455 kHz. The better modern transceivers do everything from the IF stage is done in "digital signal processing", or DSP ICs. Modulation, putting intelligence, such as voice, onto a carrier, can be a form of mixing, especially AM, including DSB.
Some modulation generates three signals, using the example of a 1 kHz tone and a 1 MHz (1000 kHz) carrier; the 1000 kHz carrier, the 999 kHz lower side-band, and the 1001 kHz upper side-band. This is Double-sideband, full carrier, called AM. If only the two sideband frequencies generated are output, but not the carrier input, this is double side-band (DSB) without a carrier.
However, the voice mode most often used on HF is SSB, single-sideband. In this case, the emission would be at either 999 kHz or 1001 kHz only. For voice, a USB signal would be an envelope from 1000.3 kHz to around 1004 kHz, depending on the bandwidth.
So called "weak-signal" work on VHF and UHF, named for the fact that the received signal can be very low, also uses SSB, specifically upper sideband. Morse (CW) is also used this. A range of weak-signal digital modes are also used, with the radio set to either SSB, or a dedicated Digital or Data mode. A radio which include these modes on VHF, perhaps with FM and/or AM are called "multi-mode VHF transceiver". Old single-band USB-only radios are available, but these are not particularly sensitive. USB and CW modes normally use horizontal antennas, often directional antennas, such as yagis, mounted with the a elements horizontal. While horizontal omni-directional (meaning all-direction) antennas, such as the halo and the "big-wheel", can be used, it is NOT the answer sought, and does not have the gain of a yagi or quad, etc. A quad is an antenna with a series of square or rectangular loops on a boom, and a loop yagi is similar, but with round loops.
One measure of how good a receiver is is its sensitivity, how well it receives weak signals. For FM this is the level of signal, in microvolts needed to reduce the hash from the speaker by 12dB. For AM this relates to a 10dB reduction, also in μV. It can also be represented in dBm. This might be a 0.5 to 2 microvolts, or around -128 to -140 dBm
Another is selectivity, where a receiver can select a weaker (distant) signal between nearby loud-mouthed stations with their 1.5 kW+ amplifiers. This can be related to rejection of a signal in the next channel, with the more dB, the better.
As well as power amplifiers (linear amplifiers, or just "linears"), external pre-amplifiers can be used. These increase the level of the received signal. In some cases they are places at the mast-head, so that there is very little loss from the antenna to the amplifier. Some pre-amps contain relays, etc, which switch it out of line when the transmitter is operating.
Amateur radio transmission can interfere with the operation of other radio based equipment, including TV. There are a few ways which this can occur. If you signal is clean, but the TV is a cheap Chinese model* then the problem is "Fundamental overload", where the TV cannot handle the fundamental signal, outside its design receive coverage. This is the owner of the TV, and its distributor's problem to solve, so you can carry on operating. Interestingly transmitting on 2m upsets my Palsonic TV, but if watching via the metal-cased set-top box, connected via HDMI, it is fine.
* If you just buy 50,000 TVs to flog online, then you are buying what is basically made at the lowest possible cost, skipping any sort of filtering, safety, etc. This isn't to say that a company can't have their own design made there, under close supervision, and get a reasonable product.
However, if your radio, or your amplifier is dodgy, then there are two possible issues. The first is "Harmonics", where the transmitter is generating emissions at multiples of the intended (fundamental) frequency. These harmonics can be on the receive frequency, thereby interfering with the signal. The other is "Spurious emissions" which are basically a high level of noise from the final stage, and again, this can be on the frequency a neighbour is trying to receive.
One trick to deal with neighbours, is a few weeks before you are licensed, put up an antenna. Then, when they complain, point out that you haven't been operating, as you are not licensed yet...
Filters are useful in dealing with real interference problems. For HF transmitters affecting VHF or UHF TVs, a Low-pass filter after your transmitter or amplifier might be useful, as it blocks harmonic and spurious emissions from the transmitter. On the TV antenna input, a high-pass filter can block HF signal from entering the TV. Band-reject and band-pass filters can eliminate specific problems, such as high level FM broadcast signals affecting a TV.
Ferrite chokes are a tube of dark grey mounded ceramic material which reduces the level of radio frequency energy conducted along the cable. These can be used on power leads, audio and video connections, USB leads, etc. To avoid having to remove the connectors, "Snap-on ferrite chokes" are available, which are moulded in two halves, and held within a hinged plastic clip. The "Ferr-" component of the word indicates it is iron related, specifically, composed of Iron(III) oxide in combination with other metallic elements. A Chinese branded plasma TV a local church bought actually came with a number of these, to go on the input leads... Plasma TVs are nasty, as the are essentially about six million tiny neon lamps, which require switching high voltages on and off many times per second, so generate a lot of radio interference. They also generate several hundred watts of heat in your closed room, which then places more load on your air-conditioning, so as you can see, they are expensive to run. LCDs, including LED backlit ones, termed "LED"; and the fairly rare OLED TVs are a far better option.
This a a bit beyond what you need to know for the Technician exam, but is both useful, and required for other papers.
If you place two resistors in series across a voltage source, to ground; and measure between the junction of the resistors and ground, the value will be some fraction of the input, depending on the ratio.
|This shows a voltage divider in A, where is the two resistors are equal (say 1kΩ), the output is half the input. At B it is redrawn as an attenuator. If the top resistor is 1.5kΩ, and the lower 500Ω, the output voltage will be a quarter the input. This can be useful when interfacing speaker-level audio to a microphone input. C is a low-pass filter, attenuating high frequencies more than low one; and D a high-pass one, attenuating low frequencies more that high ones. Note that in each case, energy passes from left to right.|
When we use a voltage divider at audio or RF frequencies we call it an attenuator, and usually specify its attenuation in dB.
As we know, inductors, or coils of wire, have greater impedance at high frequencies; and capacitors have greater impedance at low frequency. Thus, if we arrange these components in a similar way to our voltage divider, then we have a voltage divider which has different level reduction at different frequencies.
An inductor before a capacitor means that low signals go through the inductor just fine, and then sees a high inductance capacitor, which little goes into, so loss here is low, great if this is your fundamental HF signal. But if it is a harmonic, at higher frequency it struggles to get through, and much of what does travels to ground via the low-Z capacitor, so TVs around the area are protected from having signals in the TV band interfere with the signal. This is a Low-pass filter.
However, the Chinese quality TV is still not happy... So, at its antenna terminal, or before any mast-head amplifier, we can insert a High-Pass filter. Here a capacitor passes the VHF and UHF signal, but the HF less-so. Then the inductor shunts the HF to ground, without bothering the VHF or UHF. This is a High-pass filter.
Filters can have several stages, to improve performance; and a filter may be more complex than shown, to made sure input and output impedances are correct.
Note that, as it is handling serious power, the LP filter on the transmitter might be the size of a house-brick, maybe bigger; but the TV receive HP filter maybe be the size of a Texta marker lid, or a matchbox.
Additional components can make a band-pass filter; or a band-reject filter, also called a notch filter. The latter might block strong FM broadcast signals from entering the TV.
Filters are named for the letters which they look like, such as the inverted L shown; the T-filter, shown in Figure T-3; and the Pi-filter, for the shape of π. Others are named for the inventor, such as the Butterworth filter.
|Low-lass Pi-filters are used to match the output of a valve (tube) output stage (high impedance) in a transmitter to the (low impedance) antenna feedline. It also reduces the level of harmonics. In a multi-band transmitter some parts would be adjustable. A higher value series capacitor however blocks DC from reaching the filter, and a high inductance RF choke to ground shorts the DC to ground if the cap fails, blowing the fuse, but would be high impedance at the transmit frequency.|
Audio filters often consist of capacitors and resistors, as inductors for these frequencies are large. That said, speaker cross-overs do use inductors. Some audio filters, including those in graphic equalisers, use operational amplifier ICs, and by putting a capacitor in the feed-back loop, a "gyrator", emulating an inductor, is formed.
Beyond the exam, but so you can see what a tube circuit looks like:
Note that there are both directly and indirectly heated valves. In the first case, the filament itself emits the electrons. In the latter it is inside a tube, and that emits the electrons. In the former case there are just the two pins for these, in the latter, there are the two heater pins, plus the cathode. There are a range of heater voltages, such as 6.3 or 12.6 volts.
|Inside a tube-based radio circuit, in this case a 6m receive converter (or transverter). It is held upside-down for this photo, thus showing the under-chassis components, with the lid made from copper plate. For their ground connection components are soldered to tabs under mounting screws, with most components soldered to the terminals on the various valve sockets, the valves themselves mounted above the panel, this called "point-to-point" wiring. The half-moon plates are part of adjustable capacitors, and there are carbon composition resistors, fixed capacitors, and various coils (inductors). Most parts were probably made in Australia!|
|Yes, Australia made its own valves, this being a Mullard 6AU6 mounted on the top of the chassis above. It is a indirectly heated pentode, and is about 19mm in diameter. The white ring is part of the ceramic socket. Some tubes are a little small, and many significantly larger. The most basic valve is the diode, where electrons flow from the heated Cathode, to the Anode, or "plate", these are/were used for rectifying higher voltages, conventional current flowing from plate to cathode. Next comes the Triode, with a grid which is used to control the flow of current, and can thus be used as am amplifier, as the gate on a FET can be. Extra grids are used in some valves, hence Tetrodes and Pentodes, 4, and 5 elements. There will be one or 2 further connection, as either the cathode is directly heated, or indirectly heated. Those used in super-heterodyne receivers have even more electrodes, such as the 7 element heptode, also called a pentragrid converter. There are also double trides, and similar arrangements. Another Aussie maker, located in Sydney, was Amalgamated Wireless Valve Co. Pty Ltd (AWV), part of AWA.|
One piece of valve gear which is pretty handy is the (analogue) cathode ray oscilloscope, or CRO. Some are "hybrid" meaning they contain both transistors and valves; examples being the Australian made BWD series, such as the 501, 502, etc. The later 509B version was all solid state (transistor), with the obvious exception of the display tube. CROs let you look at the voltage, period, waveform, and rise-time of signals, and so also determine their frequency.
If the time-base is set to 2mS per cm, and the waveform is 5.45cm long it has a period of 10.9μS, and thus a frequency of 1 / 0.0109 = 91.743 Hz, close enough to 91.5 Hz to know you have selected the correct CTCSS sub-tone. (These are used tones used to access repeaters).
Like a FET, valves have high input impedance, at least at audio and low RF frequencies, although, they do have some capacitance.
More modern valve circuits can be built on printed circuit boards, and PCB sockets are available.
As always, these are actual exam questions, from the published NCVEC Technician pool.
Which term describes the ability of a receiver to detect the presence of a signal?
D. Total Harmonic Distortion
This is sensitivity, answer B.
What is a transceiver?
A. A type of antenna switch
B. A unit combining the functions of a transmitter and a receiver
C. A component in a repeater which filters out unwanted interference
D. A type of antenna matching network
A combination of the words transmitter and a receiver, it is these in one box, answer B.
Which of the following is used to convert a radio signal from one frequency to another?
A. Phase splitter
This is the mixer, which adds and subtracts the input signal and the local oscillator, answer B. These are used both within radios, and in external devices called transverters, discussed below.
Which term describes the ability of a receiver to discriminate between multiple signals?
A. Discrimination ratio
D. Harmonic Distortion
This is selectivity, answer C.
What is the name of a circuit that generates a signal at a specific frequency?
A. Reactance modulator
B. Product detector
C. Low-pass filter
Oscillators generate signals, at specific frequencies, depending on the design and values its components, answer D.
What device converts the RF input and output of a transceiver to another band?
A. High-pass filter
B. Low-pass filter
D. Phase converter
This is a transverter, answer C.
What is meant by term “PTT”?
A. Pre-transmission tuning to reduce transmitter harmonic emission
B. Precise tone transmissions used to limit repeater access to only certain signals
C. A primary transformer tuner use to match antennas
D. The push to talk function which switches between receive and transmit
Modern transmitters, where changing from receive to transmit is achieved by pressing a single switch, it is called the "push-to-talk" button or switch. Often this is on the side of a microphone or a handheld radio. Other options are a foot-switch under the desk, or a switch Velcroed to the gear-stick, used with a headset microphone. These can be bi-directional, with a spring-return in one direction, and non-sprung (locking) in the other. Answer D.
Which of the following describes combining speech with an RF carrier signal?
A. Impedance matching
D. Low-pass filtering
Combining voice with a carrier is called modulation, answer C.
What is the function of the SSB/CW-FM switch on a VHF power amplifier? A. Change the mode of the transmitted signal B. Set the amplifier for proper operation in the selected mode C. Change the frequency range of the amplifier to operate in the proper portion of the band D. Reduce the received signal noise
When transmitting using SSB (USB) the signal level varies, including dropping to near zero between words, and CW involves swithing between transmit and nothing rapidly. WIth FM, once keyed, the output level is constant. If the amplifier is "carrier keyed", responding to the output signal, rather than a keying wire from a more complex transceiver, it needs to delay its return to receive mode to take into account the low-signal periods in the transmission. Also, the bias setting on the amplifier may be different for a varying signal, compared to a constant level one. It is answer is B.
What device increases the low-power output from a handheld transceiver?
A. A voltage divider
B. An RF power amplifier
C. An impedance network
D. All of these choices are correct
In the past a company called "Tokyo Hy-Power" made an amplifier about the size of a paper-back, which amplified the output of a hand-held to 30 watts, answer B. (Dick Smith re-branded this one, and there are a range of similar products, with various outputs). These require 12-14 volts at several amps. Modern LDMOS (transistor) kit amplifiers can take the power of an HT up to 1500 watts, requiring a supply of around 65 volt at 30 amps; replacing valve-based designs which need up to 2000 volts, and a higher drive level. The other two, at least as stand-alone devices, are not relevant to this task, making the final option also wrong.
Build or buy W6PQL's 1.5kW 2m Amplifier, or one at a more sensible level.
Where is an RF preamplifier installed?
A. Between the antenna and receiver
B. At the output of the transmitter's power amplifier
C. Between a transmitter and antenna tuner
D. At the receiver's audio output
Both B and C will result in the contents of said pre-amp becoming a puff of smoke... It is A, between the antenna and receiver; and ideally at the "mast-head", so there is no coax cable loss before it.
What can you do if you are told your FM handheld or mobile transceiver is over-deviating?
A. Talk louder into the microphone
B. Let the transceiver cool off
C. Change to a higher power level
D. Talk farther away from the microphone
Over-deviating is the result of too high a signal going into the modulator in an FM radio. While a good radio should manage this, talking farther from the microphone (or more quietly) into the microphone, answer D.
What would cause a broadcast AM or FM radio to receive an amateur radio transmission unintentionally?
A. The receiver is unable to reject strong signals outside the AM or FM band
B. The microphone gain of the transmitter is turned up too high
C. The audio amplifier of the transmitter is overloaded
D. The deviation of an FM transmitter is set too low
This is due to the receiver failing to reject the strong Ham signal, answer A. If you are using 100 watts, maybe the radio is to blame, but 1.5kW, maybe the blame is more in the excessive field strength is is trying to deal with. Those in Europe or Australian cities might try a DAB or DAB+ digital receiver, as the gap from HF to 200 MHz is helpful. US HD (digital) radio at AM frequencies may or more likely, may not help.
Which of the following may be a cause of radio frequency interference?
A. Fundamental overload
C. Spurious emissions
D. All of these choices are correct
A poor receiver will suffer overload from a clean signal outside the intended band, this being the fundamental overloading the receiver. The other two are emitted by a grotty transmitter, placing them on the band the receiver is tuned to, causing interference. Thus, all are correct, answer D.
Which of the following is a way to reduce or eliminate interference by an amateur transmitter to a nearby telephone?
A. Put a filter on the amateur transmitter
B. Reduce the microphone gain
C. Reduce the SWR on the transmitter transmission line
D. Put a RF filter on the telephone
The telephone is reacting to a clean Amateur signal, so it needs to have a filter placed at its input socket, or perhaps internally, answer D. This is discussing old-fashioned, but reliable, (POTS = plain old telephone service) cord-connected telephones. If the 'phone or VoIP 'phone is connected to a VDSL (fraudband) router, and the fraudband connection drops out due to RF overload, well, all you can do is suggest they get into a time-machine back to 2013, to vote Labor, so they got lovely, non-metallic, not affected by RF, fibre instead. Filtering on the VDSL line is not really going to help the connection, as it is also at HF radio frequencies. Try 5G wireless?
How can overload of a non-amateur radio or TV receiver by an amateur signal be reduced or eliminated?
A. Block the amateur signal with a filter at the antenna input of the affected receiver
B. Block the interfering signal with a filter on the amateur transmitter
C. Switch the transmitter from FM to SSB
D. Switch the transmitter to a narrow-band mode
The low quality receiver can't handle the clean Amateur signal, because the importer specified that adequate internal filtering not be used, saving a few cents; so we have to provide this externally, at the antenna input of the affected device, answer A.
Which of the following actions should you take if a neighbor tells you that your station’s transmissions are interfering with their radio or TV reception?
A. Make sure that your station is functioning properly and that it does not cause interference to your own radio or television when it is tuned to the same channel
B. Immediately turn off your transmitter and contact the nearest FCC office for assistance
C. Tell them that your license gives you the right to transmit and nothing can be done to reduce the interference
D. Install a harmonic doubler on the output of your transmitter and tune it until the interference is eliminated
Ensuring your station is working properly (your signal is clean), and that it is not upsetting your own radio or TV is useful, answer A.
Which of the following can reduce overload to a VHF transceiver from a nearby FM broadcast station?
A. RF preamplifier
B. Double-shielded coaxial cable
C. Using headphones instead of the speaker
D. Band-reject filter
A band-reject filter will reduce the received level on the frequency or frequencies of the station, and helo stop it overloading teh transceiver, answer D.
What should you do if something in a neighbor’s home is causing harmful interference to your amateur station?
A. Work with your neighbor to identify the offending device
B. Politely inform your neighbor about the rules that prohibit the use of devices which cause interference
C. Check your station and make sure it meets the standards of good amateur practice
D. All of these choices are correct
All answers are correct, answer D. Equipment, such as plasma TVs generate interference on the HF bands (they are also very wasteful of electrical power, and should be replaced with LED backlit LCD models). Some solar systems may need clip-on ferrite chokes on the connections; ditto on the leads for plasma TVs.
What is a Part 15 device?
A. An unlicensed device that may emit low powered radio signals on frequencies used by a licensed service
B. An amplifier that has been type-certified for amateur radio
C. A device for long distance communications using special codes sanctioned by the International Amateur Radio Union
D. A type of test set used to determine whether a transmitter is in compliance with FCC regulation 91.15
Part 15 allows devices to emit low level signals on a wide range of frequencies, answer A. This includes permitting the operation of computers, etc, despite the RF garbage they emit; as well as deliberate transmitters, such as wireless microphones, car alarm fobs, garage door openers, and kids' walkie-talkies. The Part 15 compliance on Ham gear sold in the US relates only to the local oscillator for the super-heterodyne receiver circuit, which emits a small amount of RF.
What might be the problem if you receive a report that your audio signal through the repeater is distorted or unintelligible?
A. Your transmitter may be slightly off frequency
B. Your batteries may be running low
C. You could be in a bad location
D. All of these choices are correct
If you are transmitting off frequency (more than just minor drift, say 5kHz), then your voice will be distorted. Low batteries also affect transmitted audio quality, as well as weakening the signal; while being in a location where your signal is blocked by terrain, metal fencing, or large sheds, then the signal into the repeater will be low, and hard to understand. Standing on a higher point, if safe, or otherwise moving may help. Yep, all are correct, answer D.
What is a symptom of RF feedback in a transmitter or transceiver?
A. Excessive SWR at the antenna connection
B. The transmitter will not stay on the desired frequency
C. Reports of garbled, distorted, or unintelligible transmissions
D. Frequent blowing of power supply fuses
RF from your transmitter getting into the microphone cable, etc, means that the transmitted audio with be affected, as in C. RF feedback can also upset processor based radios, causing some unusual behaviour.
What should be the first step to resolve cable TV interference from your ham radio transmission?
A. Add a low pass filter to the TV antenna input
B. Add a high pass filter to the TV antenna input
C. Add a preamplifier to the TV antenna input
D. Be sure all TV coaxial connectors are installed properly
Note that this is a problem with Cable TV, not off-air broadcast TV, and these are presumably connected to the TV via RCA composite video and audio connectors, or maybe HDMI. Even if it is via a modulator into the antenna input, the level is probably high enough not to easily suffer interference via this port. A pre-amplifier is definitely not a solution. However, making sure the cables from the wall to the set-top box are correctly fitted to the cable, including the shield being well connected (and these are properly screwed in), ditto the box to TV connections, answer D.
As an aside, within the pay TV cable a wide range of radio frequencies, from HF to UHF are used to carry the TV signals, including Amateur and commercial two-way bands. If the cable is damaged, RF can leak out of, and into, the cable.
On to: Operating Part 2
You can find links to lots more on the Learning Material page.
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Written by Julian Sortland, VK2YJS & AG6LE, December 2017.
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