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A station needs to be connected to an effective antenna, be this a horizontal dipole, or some sort of vertical. These are usually resonant antennas. However, any antenna is better than none, so electrically short antennas may be used, usually with an antenna tuning unit.
Lets say we want to operate on 40 metres, at 7.100 MHz, and we have a decent sized yard, and trees, so decide on a dipole. Interestingly, this happens to calculate to needing to be 20.04 metres overall, or 10.02 metres per leg. Some times the length can be somewhat different to a fraction of the nominal wavelength. If we want to talk on a local net, low is better, for DX, high is good. Electricians' wire is fine.
|This is the central feed-point of a dipole antenna. The red and green is silicone tape, which has either crumbled due to UV, or been attacked by birds. Under the yellow tap there is a PL-259 plug connecting to the SO-239 on the centre. It is a Budwig HQ-1. The hole in the top allows a variation of the dipole to be made, the Inverted-V, useful for when you have a single tree, or single pole or chimney in the middle of your property, and can use lower objects, such as the top of a fence as the ends. There are also centre units which connect to coax without the connector, it to ladder-line. More advanced versions contain a balun. Why 4 wires? The blue wires are for the 40m band, the heavier green and yellow for 20m. Provided there is some spacing, you can have several dipoles one one feeder.|
|This is on of the end insulators of the dipole. It is a high-strain plastic insulator designed for electric fence use. Just twisting the wire is often sufficient, it is is reasonably stiff. It would also be used on an inverted-L, Zepp, Carolina Windom, or other wire antenna. Small - medium sized ceramic versions are also suitable, as may the softer black plastic ones.|
We have hung it up, connected it to some 50 ohm coax, and now we need to know if it is resonant. There are two ways to do this, using the transmitter and an SWR meter; or using an antenna analyser, a box with a tuneable oscillator and fancy SWR analysing circuitry inside.
If an antenna is not the correct electrical length, then energy will be reflected back to the transmitter, which is not an ideal situation, and if this mismatch is bad enough, it may be damaged.
The ideal SWR = standing wave ratio is 1:1, with an increase in the number on the left meaning a poor match. This should be below 2:1. If using a radio without a linear amplifier, below around 1.5:1 is a case of "She'll be right". If using an analyser and the SWR is not good, it will tell you where the SWR is the lowest, either on a graph, or by tuning across the band; with a meter you will have to do a series of test to find whether it is above or below the desired frequency. If it is at a lower frequency you need to trim the antenna, or adjust the element to make it shorter. If it is too high, you may need to add wire, or adjust it to be longer. Some vertical antenna bases have hex screw clamps for the wire element, which allow a degree of adjustment. If cutting elements, be a little conservative, and trim in small increments.
Modern HF and multi-mode VHF radios have an SWR circuit, there the power output is folded back. This is often integrated with power control circuitry. (Apparently some 100 watt HF radios put out a pulse at something liKe 150 watts, before folding back to the selected power, even if this is 5 watts). Many also include a rudimentary SWR display, using information from this circuitry, meaning you may not need to buy a stand-alone meter. A few of the larger radios also include a basic antenna tuner.
Beyond the exam, but it is important to note that an antenna may be resonant, but not 50 ohms. An example is a full-wave delta loop, which has an impedance of 102 ohms. The dodgy method is to just use 50 ohm coax, and an antenna tuner; the right way is to use a quarter-wave length of 75 ohm coax (RG-59, etc) to match the antenna to 50 ohm coax. For some a "balun" or other transformer is needed. Balun means Balanced to Unbalanced (and tiny versions are used to connect balanced ribbon to the unbalanced coax input on TVs). Some are 1:1, others 1:3, 1:4 1:6, or 1:9, depending on the impedance of the dipole. Un-Uns also exist, for interfacing coax to things like a half-wave long wire, which has high impedance.
|A 6 metre Delta, named for the Greek upper-case delta Δ The timber crossbar is about 3 metres long. As the feedpoint is in the bottom the antenna is horizontally polarised. A bit hard to see, maybe, but at the point cable is taped to the tower, there is a joint between 75 and 50 Ω coax. This is soldered, and a bit bodgey; I should have used an F connector on the 75 ohm cable, an adapter, and a TNC or FME connector on the RG-58. RG-6 is used for pay-TV, satellite TV, and ordinary TV antennas. I think the 75 Ω is RG-59. (RG-6 is a little thicker, and RG-11 and RG-216 are larger again).|
If the SWR suddenly increases on a frequency it was good, this means either the coax has water in it, or more likely, something has broken.
The alternative way to measure antenna mismatch is using a Directional Watt-meter. This allows us to measure the power coming from the transmitter to the antenna; and to compare this to that coming back. If we found 400 watts is going out, and 5 watts is coming back, that is quite a good figure, the lower the return, the better.
This isn't a piece of test equipment, it is a rhythm used in poetry, consisting of five sets of unstressed and stressed syllables. Shakespeare used it extensively, for example, "Shall I compare thee to a summer's day?" in his Sonnet 18.
Did you notice the da-DUM da-DUM da-DUM da-DUM da-DUM rhythm?
The term "iambic" does apply to certain "side-swipe" keyers, where one key sends a dit, the other a dah; and squeezing them, alternative dits and dahs: dah-di-dah-di = C. These require additional electronics to generate the elements, but some radios include this, so just need the actual switching parts. Penta, meaning 5 is applied to those valves (tubes) with 5 elements - Anode, Cathode, and 3 grids (plus heater); or 5 grids, the pentagrid converter (Heptode, for 7 elements). A multi-function meter is a "multi-meter", below.
Note that using an ohmmeter or ohms range on a multi-meter to to measure an antenna will usually give one of two results, either a DC open, for antennas such as dipoles, and quarter-wave and five-eights verticals; or a DC short for folded dipoles and loop antennas. I can't think how a volt-meter is useful for testing an antenna.
An antenna tuner makes the radio see a 50 ohm impedance, but does not itself "tune" the antenna. It may also be called an automatic tuner (if automatic), an ATU (antenna tuning unit), or a "trans-match". Manual tuners typically consist of one or two large variable capacitors, and either a switch tapped inductor, or a roller inductor. These can be motor driven is some systems. Automatic tuners usually consist of a bank of capacitors, a bank of coils, selected using a bank of relays. Often another relay bypasses the tuner, and another moves the capacitor bank between input and output.
Off topic, but you need to know: A practice among those using small loop antennas is to use a vacuum variable capacitor to match the loop. These use a metal bellows, which is designed for infrequent (set-and-forget) adjustment in fixed-frequency systems. These bellows will fatigue over time if constantly adjusted, and tear!
For maybe the past 50 years, the most common way to connect a radio device to its antenna is via coaxial cable; the (US) RG cable numbering system started in WWII, standing for "Radio General". "Co-axial" means that the two conductors are around a common centre. This consists of an inner conductor mad from stranded or solid wire, copper clad aluminium or steel wire, or silver plated copper; or from a thin metal tube, sometimes corrugated. This is insulated from an outer metallic shield; this consisting of braided wire, foil, or solid metal, sometimes corrugated for a degree of flexibility. The "dielectric" between these can be solid or foam plastic, or teflon spacers in the case of solid cable, meaning the dielectric is air, or in broadcast systems, dry nitrogen. The ratio of the diameter of the outside of the inner, and the inside of the outer, in combination with the type of dielectric determines the characteristic impedance, such as 50 ohms (sometimes 52 Ω) for most transmitting systems, and 75 ohms (sometimes 72 Ω) for many receive only ones. The story goes, that in establishing and after WWII, in re-establishing TV in the UK, (shut down for fear it would be used for bombers to navigate), engineers found that two sizes of water pipe, which gave a 48 ohm impedance, meaning transmitters and antennas were built to this specification. When transmission started the day before the VE day anniversary, they simply continued the Mickey Mouse cartoon where it had been cut-off, apologised for the interruption to transmission, and carried on as normal.
50 ohms is a compromise between 30 ohms, which would provide maximum power transfer at high power (for transmitters), and 75 ohms, which has lower loss for reception.
Hams often use RG-58 for lower power systems, and RG-213 or RG-8 for larger, and suitable for VHF, UHF, and higher powered systems on all bands. Various LMR cables have also become popular. Heliax, made by Andrew, and related products are used for long runs, especially at VHF and UHF, and in repeater systems. H155 and H500 are (Eastern?) European types.
Plastics, such as used in coaxial cable consist on long chains of hydrocarbons, such as Polyvinyl chloride, and these long chains give the plastic strength. However, ionising radiation, such as ultraviolet light, smash into the plastic, and break the chains, thus making the plastic powdery and crumbly. Adding carbon to the plastic, making it black, can help reduce this activity. This is also important with cable-ties / zip-ties, when used outside, as black ones last much longer than natural (translucent white) plastic. Stainless steel is best. Light coloured cables should only be used indoors, or in temporary (field day) set-ups.
Velocity factor is the speed of signals in the cable, expressed as either a percentage, or decimal fraction of the speed of light. If you could get this to to exceed 1, you need to know Stockholm is in Sweden, as you will need to go there to collect your Nobel Prize for Physics, for getting signals to exceed the speed of light! Back to reality, and different dielectrics in coax affects the speed of signals in coax; a foam dielectric might had a VF of 0.85 (85%), a solid one around 0.66 (66%). If you are just connecting an antenna to your radio this does not matter, although some espouse a multiple of a half wave.
Coax also has an interesting property, shared by other feedlines, that, if an impedance greater (or less) than the cable impedance is places across one end, then a signal applied at the other end will see a lesser, (or greater) impedance, by the same difference. Thus you can make a matching section to interface a 102 ohm delta or other loop to a 50 ohm radio, by making initial link from the loop from 75 ohm coax, a quarter-wave long. This has to take the velocity factor into account. Thus, for 50.150 MHz, you will calculate a free-space quarter-wave is 1.493 m. If VF = 0.85, then you need a 1.269 m; and for 0.66, just 985 mm. THe loop will see the radio as a 100 ohm load.
Another use for coax is as a filter. A quarter-wave stub which is open-circuit at the far end appears as a short at the frequency in question, so it will suppress that frequency, perhaps useful if you live near a repeater, or a pager transmitter. If closed, it will pass only the relevant frequency, as it appears open at the connection point. In both cases VF must be taken into account.
Note that while a coax has a specific impedance, if you measure it with your multimeter, you will find the end-to end resistance to be near zero, as you find with any wire. You should find an open circuit between the inner and outer. It will have some capacitance, and were you to apply an insulation tester to a long length you may notice it charging, and if you don't discharge it, then touch the inner and outer, you may get a shock.
|These are a selection of Coaxial connectors terminated onto cable, and inter-series adapters.|
These are a PL-259 (UHF) plug to BNC socket; a BNC plug on RG-58C/U; a BNC plug to RP-SMA socket; an SMA plug on RG-223/U; an SO-239 (UHF) socket to SMA Socket; and N socket on the other end of the RG-223/U.
Across the bottom is RG-213 from the folded dipole.
An alternative is "ladder line" or "window line" a balanced cable consisting of two conductors in parallel, spaced by plastic spacers, or a plastic ribbon with cut-outs. Window-line, often 450 ohms, is about 30 mm wide, ladder-line, often 600 ohms, up to maybe 100 mm, more on high-power transmitter sites. These are similar to 300 ohm TV ribbon cable but bigger.
When testing a transmitter, so as not to cause interference to other Hams, connect the radio to a "dummy load", also called an "artificial antenna". This consists of a non-inductive resistor placed inside a heatsink which also provides shielding, so the signal does not radiate.
The Amateur will need to measure various electrical parameters, such as battery or power supply voltage, mains voltage, current used by equipment, and resistance of components, or connections.
Note that these are usually done using a hand-held or bench multimeter. These can have manually set ranges, or automatic ones. In the former case you select, say, the relevant voltage or current range, in the latter you just select the mode.
A voltage meter, (voltmeter), or multimeter in voltage mode is connected across the two points, say the positive and negative terminals of a battery, or a the radio's power connector. For this a DC range (say 20 volts if around 12 volts is expected), or DC voltage mode, must be selected. For the low voltage output of a transformer, an appropriate AC voltage range is selected, or the AC volts mode. For mains a quality meter, such as Category III must be used. It may be wise to initially set a range above 240 volts, even if 120 volts is expected, in case it is wired wrongly. Other terms for voltage are electromotive force (EMF), electrical potential, and potential difference (PD).
Off topic, but if you live in a country with large transformers serving multiple houses (like Australia), and are back from the street, so have a long power lead-in, and several large loads are on, the common earth-neutral connection can be raised above surrounding earth, meaning that if your radio earth is not bonded to the power earth, there can be an electrical potential between the two, enough for a mild shock, say if touching PC and radio! Hot water, oven, and heaters might draw 50 amps, and say 1 ohms resistance = 50 volts AC. Alternatively, a portion of the current will attempt to flow between the PC and Radio via the shield of the audio connection, or via a serial or USB connection - not good for either device!
For current measurements the circuit must be cut at the point at which the measurement is to be taken, and the meter inserted in series. Points such as fuse sockets, or power switches can be handy. An appropriate current mode and range must be selected. This is called an ammeter.
Measuring resistance uses the internal battery of the meter to push current through the circuit under test. The circuit being tested must be un-powered, and large capacitors discharged. To get an accurate reading on a component it may be necessary to disconnect one connection. This is termed an "ohmmeter". If you test a large capacitor it will appear as a low resistance device, with this increasing as the capacitor charges. One fault in capacitors is a high DC leakage. A large inductor will appear high resistance, with this dropping over time. Note that testing large inductors, including transformer windings can be dangerous, as when you remove the probes, the magnetic field you have generated collapses, and a high voltage pulse is generated, potentially enough to knock you over. The test, however deals with capacitors.
Precision mains powered equipment is sometimes left on, or needs to be turned on for around half an hour to come up to temperature, to have the best accuracy, but not doing this will not damage anything.
Original multimeters used an electromagnetic meter movement, where a needle moved across a scale. These only used batteries for the ohms range. The king of these meters is the AVO-8, make in Great Britain, and only just discontinued, but are available on ebay for as little as £20, a bit like being offered a vintage Rolls Royce for the price of a Kia. Then came the VTVM, or Vacuum Tube Volt Meter, which was usually mains powered, and used a valve amplifier to provide a high impedance input. These were replaced by FET (field effect transistor) based analogue meters. Digital multimeters (DMMs) have become the standard since the 1990s. Most use a dial to select the mode and ranges, but in the late 1980s side-mounted, mechanically interlocked buttons were a trendy method to select mode and range on some more expensive models.
|An analogue meter, indicating 405 volts AC. As the meter is set to 500 volts, we read from the main black scale, below the mirror, and read on the 0 to 5 range, multiplying by 100. The offset red scales are for lower AC voltage, and take into account voltage drop in the rectifier.|
Many meters are what are called 3.5 digits (3½ digits), as they display up to 2000 counts, with scales such as up to 19.99 volts, 1.999 mA, 199.9 ohms, marked as the 20 v, 2 mA, or 200 &2Omega; scales. Higher end hand-held meters are 4.5 digit, going up to 19999. Intermediate one are often called 3.75 or 3¾ digit meaning 4000 count or 5000 count; 3 digit referred to 3000 and 3200 count units. One thing which annoys me is that many cheap meters have a 200 mA range, then a 10 amp range, so reading at 1 amp is only 1.00, whereas a with a 2 Amp or similar range, it can read 1.000 amps, handy if setting up a current source for low-ohms testing.
Category marking is important for safety, as some power sources, including the mains is capable of delivering a LOT of energy, should a voltage spike cause an arc in the meter: CAT IV is outdoor power connections and switchboards; CAT III is industrial power, and domestic power close to switchboards, so my bedroom power socket, close to the meter-box is in this class; CAT II is further from the box, so my lounge-room, and mains within domestic equipment; and CAT I is electronic equipment, after the transformer, including perhaps lower power valve power supplies, internals of TVs, but not high-power linear amplifier supplies, or microwave ovens.
I recently bought a couple of meters, as sometimes it is handy to be able to read, say voltage and current simultaneously; and they have different ratings. The TENMA has 4, 40, and 250 volt AC ranges, the RS one only 200 and 600 VAC ranges, so the former is better for reading lower AC voltages. Both feature white LED back-lighting.
From Element 14 (Farnell / Newark / CPC) I got a TENMA 72-7765A, with auto-ranging, including a 4 amp range, and 4000 count. It is only CAT II, 250 volts, so has limited application for mains use. It is also CAT I-600 V, so can be used in electronics above 250 volts. The price is around A$30, including tax. See: E14 Part# 2499507
One of its big brothers, also 4000 count, with CAT IV-600 V and CAT III-1000 V safety, and more features is here: E14 Part# 2499514
The other one is from RS Components (Allied), the RS-PRO RS-12, with a CAT III-600 V & CAT II-1000 V ratings, and a battery test mode (a loaded voltmeter). It is 2000 count manual range unit, and including GST, is just under A$30. See: RS Part# 1231939
|On the 600 volt AC range, this meter is reading 427 VAC. The two power-boards are connected to different phases, so instead of 240 volts, there is a nominal 415 volts between phases. The phase to neutral voltage for this reading calculates to 246.5 volts, a fairly normal voltage. This is perhaps not something you should try at home, unless you know what you are doing. Power was switched off while making the connections, and this is a CAT II situation using a CAT III-600 V meter, so well within its ratings.|
Expensive hand-held meters may be 4¾ digit, 50,000 count; and bench-top include 5½, 6½ or 7½ digit options, and increasing prices.
Lower cost meters, including the ones I bought actually measure AC volts as an average, and display a calculated RMS figure based on the assumption that the signal is a sine wave. More expensive units read "root mean squared" or "True RMS", and display that. A figure of 240 RMS means that applying this to a heating element has the same effect as applying 240 volts DC. A 240 volt RMS sine-wave has a peak voltage of 339.4 volts, this being 1.4142 times the RMS. For audio and related signals we often discuss the peak to peak voltage, say "2v p-p", as this is easily read off a CRO screen.
Additional modes are frequency, usually, audio frequencies, but may not have good mains frequency measuring accuracy or resolution, some were pretty rough, as I think they did an analogue frequency to voltage conversion, before digitising this; temperature, using an external thermistor, usually more for high temperatures, than an accurate room temperature reading; transistor gain (HFE), and capacitance. A few invert the resistance reading to milliSiemens. Many have a diode-test, where a small voltage is applied to the probes, and this can be used to determine the forward voltage drop of a diode or other semiconductor junction, although not usually high enough for an LED; and if shorted in this mode, the meter beeps, allowing you to concentrate on placing the probes. There are specialist units for automotive use, displaying RPM and Dwell angle (essentially, duty cycle) from the 12-volt* side of the ignition circuit.
* Note that high voltage pulses, from back-EMF from the coil, do appear on the LV side, and can give a nasty shock.
Quadrature means at 90 degrees, such as a phase shift in data systems, or pilot tones for AM Stereo. "In phase" means two signal rise and fall at the same time, something which also applies to generators, so if you join the Navy, your ship will probably have a generators connected to the main engines, and smaller units, and when adding one into the system, you need to make sure they are in-phase when you close the breaker, or you could black-out the whole ship! Connecting a voltmeter in quadrature, or in phase are both silly concepts, they are connected in parallel. (A synchroscope will help you decide when the close the breaker, above).
Proper frequency counters are usually a desktop format, and many read hundreds or thousands of MHz with 1 Hz resolution, or better. Many can be connected to caesium standards (atomic clocks) or the clock signal of a GPS special receiver, for frequency accuracy.
Dedicated "L-C-R meters" are also available, which measure inductance, capacitance, and resistance over a wide range, and with good accuracy. In the old days several "bridge" methods were used to compare the unknown component with known ones, to measure them with high accuracy, but NOT the noise bridge.
While digital meter modules can be used when building power supplies, analogue meters are still used in some, and in many linear amplifiers to display parameters such as plate voltage and current; and in SWR meters and some antenna analysers. LED bargraphs were also used used on some amplifiers.
Outside the exams, test probes with neon lamps or LEDs can be used for mains testing, with US versions having 120, 240, 277, 480, and sometimes 600 volt lamps; or in Europe, something like 127, 220, and 380 volts.
|This shows the voltage is between 277 and 480 volts. This probe is designed for US use so has the more popular US industrial three phase voltages show, in addition to the domestic voltages of 120 and 240 volts.|
One of several methods of fitting connectors to cables is soldering. It is also used to mount components on printed circuit boards. This uses a low temperature melting point alloy, such as tin and lead, in the form of a wire with a rosin core. When heated, this rosin flux becomes liquid, and cleans the metal surface to be soldered, then the solder melts onto these surfaces, forming a metallurgic bond. A good joint is reasonably shiny; a bad joint dull or grainy (or forms a ball on the wire only, or bonds to the pad only). There are also rosin-cored lead-free solder, which is mostly tin, and requires a high tip temperature.
There are many online videos about soldering, but the basic method is to put the hot iron on the joint, melt a tiny bit of solder onto the iron to improve heat transfer from the iron to the joint, wait a second or so, then apply the solder to the joint, and when enough solder has melted, withdraw both the solder and the iron. The joint must not be disturbed for a few moments while it cools, or the joint can fracture. Note that it is OK to keep soldering other contacts on a PCB.
Acid-core solder is only used for non-electronic use, as the aggressive flux would cause the leads to corrode; and (lead-based) solder for aluminium has acrid smelling, very aggressive flux, although the term can also refer to an aluminium brazing rod, with a high melting point. Silver solder, used for jewellery and silver-smithing, contains a large amount of silver, and also has a high melting point. All these are usually used with a gas torch.
An alternative for cable termination, not on the exam, is crimping connections. This requires a range of specific crimp tools, or a tool, and interchangeable jaws. These deform the rear of the connector, so they compress the conductor, and form an air-tight metal-metal interface. Crimping applies to the inner and shield of coax of versions RF connectors designed for such termination. Examples include BNC, TNC, SMA, N, and "UHF", FME, and other styles. It also applies to various power connectors, such as 0.25" push-one spade terminals, bullet connectors, and eyelets, the latter used for grounding connectors, where a high fault current would cause a soldered joint to melt. Also crimped are the terminals in Anderson Powerpole connectors, adopted as a standard 12 volt connector by various emergency support groups such as RACES and WICEN; and many clubs. Ask first, but this often means if you want to bring a 23 cm radio (and antenna) along to a "field day" operating event, you should be able to plug it into a battery or other supply belonging to the group, or another member. RJ connectors, used for telephones, and Ethernet; and for microphones and remote head connections on many radios are crimped. Note that the 6P6C RJ12 connections on the Yaesu FT-857D, which allow the control head to be separated from the body of the radio are wired in reverse to the telephone "key system" terminations. A standard blue cable and coupler can be used to extend the microphone lead.
A TNC connector is a threaded, or screw connector, the name standing for "Threaded Neill–Concelman", after the inventors; it is a superior version of the popular bayonet BNC. TNC also stands for Terminal Node Controller, the modem and packet assembler (smart interface) between the PC and Radio in packet radio systems, some were quite fancy, including mail-boxes, saving having the PC on all the time. The TNC to PC connection was usually RS-232C serial cable. I think the question writer hopes you will conflate a coaxial connector type with the cable type used to connect a device with the same name. The threaded N connector, and the large bayonet (and rare) C connector are also named for the gentlemen above.
|A TNC plug on thinner Heliax, with heat-shrink. This shows the pin, shield contacts. On a BNC a bayonet coupler is used.|
This is a 50 ohm version, and maintains this through the connection, with little power reflection.
Another distracter is "Relaxation Oscillator", these are often simple oscillators, an example being a neon lamp flasher, consisting of a high resistor feeding a capacitor from a DC source of maybe 150 volts or more, with a neon across the capacitor. Once the cap reaches around 90 volts the neon with "strike", and discharge the capacitor to below the minimum voltage at which it can maintain current flow, so it extinguishes, allowing charging to recommence. These are quite cool, as no transistors, etc, are required. Using small CFL or LED lamps, such as in double or triple switched arrangements, where there are long lengths on cable can cause a current to flow into the lamp through capacitive coupling between the wires, charging the internal capacitor until the lamp attempts to start, at which point the capacitor discharges, and the lamp shuts down, thus causing it to flash every few seconds - an unintentional relaxation oscillator, of sorts.
Once again, these are actual exam questions, from the published NCVEC Technician pool.
What is the primary purpose of a dummy load?
A. To prevent transmitting signals over the air when making tests
B. To prevent over-modulation of a transmitter
C. To improve the efficiency of an antenna
D. To improve the signal to noise ratio of a receiver
A dummy load absorbs energy from the radio without radiating, or transmitting it "over-the-air" (it becomes heat), answer A.
Which of the following instruments can be used to determine if an antenna is resonant at the desired operating frequency?
A. A VTVM
B. An antenna analyzer
C. A Q meter
D. A frequency counter
To analyse an antenna's characteristics, including resonant frequency we just might use an antenna analyser, answer B.
What, in general terms, is standing wave ratio (SWR)?
A. A measure of how well a load is matched to a transmission line
B. The ratio of high to low impedance in a feed line
C. The transmitter efficiency ratio
D. An indication of the quality of your station’s ground connection
It is A, how well the a load (antenna, input to a linear amplifier, etc) is matched to the transmission line.
What reading on an SWR meter indicates a perfect impedance match between the antenna and the feed line?
A. 2 to 1
B. 1 to 3
C. 1 to 1
D. 10 to 1
The perfect match is a 1:1 ratio, C. The number on the left is always 1 or greater, with 1 on the right.
Why do most solid-state amateur radio transmitters reduce output power as SWR increases?
A. To protect the output amplifier transistors
B. To comply with FCC rules on spectral purity
C. Because power supplies cannot supply enough current at high SWR
D. To improve the impedance match to the feed line
Many MODERN radios start winding back power at perhaps 2:1, to protect the output transistors, answer A.
What does an SWR reading of 4:1 indicate?
A. Loss of -4dB
B. Good impedance match
C. Gain of +4dB
D. Impedance mismatch
4:1 is a poor match, or "Impedance mismatch", answer D. (The loss is a little less than 2dB.)
What happens to power lost in a feed line?
A. It increases the SWR
B. It comes back into your transmitter and could cause damage
C. It is converted into heat
D. It can cause distortion of your signal
In many systems lost power becomes heat, and feedline losses are no exception, answer C.
What instrument other than an SWR meter could you use to determine if a feed line and antenna are properly matched?
C. Iambic pentameter
D. Directional wattmeter
A Directional watt-meter does a similar job to an SWR meter, but gives its readings a absolute power levels, rather then a ratio. The answer is D.
Which of the following is the most common cause for failure of coaxial cables?
A. Moisture contamination
B. Gamma rays
C. The velocity factor exceeds 1.0
Moisture entering the cable, causes corrosion in the shield, and degrading the dielectric, answer A. Overloading a correctly specified cable is some what difficult, as a transmitter cannot suddenly produce multiple times its usual power (unlike shorting a battery). Even at 1500 watts into 50 ohms, the current is a fairly sensible 5.48 amps.
Why should the outer jacket of coaxial cable be resistant to ultraviolet light?
A. Ultraviolet resistant jackets prevent harmonic radiation
B. Ultraviolet light can increase losses in the cable’s jacket
C. Ultraviolet and RF signals can mix together, causing interference
D. Ultraviolet light can damage the jacket and allow water to enter the cable
UV light will can make plastic brittle or crumbly, allowing moisture to enter, damaging the inside of the cable, answer D.
What is a disadvantage of air core coaxial cable when compared to foam or solid dielectric types?
A. It has more loss per foot
B. It cannot be used for VHF or UHF antennas
C. It requires special techniques to prevent water absorption
D. It cannot be used at below freezing temperatures
Air cored, or rather, air-dielectric coax is low-loss, and great for VHF & UHF*, but you do need to follow the manufacturer's instructions to keep water out, answer C. Heliax cables require also specific connectors, supplied by Andrew. The rear of the connector is sealed with the hot-glue lined heat-shrink tubing supplied with the connector. All connectors used outdoors need appropriate protection, often in the form of a mastic or self-amalgamating tape, followed by black electrical tape, then light grey (which discourages the cockatoos, and it looks like the galvanised tower legs, which hurt their beaks when they try to chew them).
* Very large coaxial cable is unsuitable for higher UHF signals, as is starts to behave like waveguide, and the different speeds of signal propagation through the cable are a big problem.
What does a dummy load consist of?
A. A high-gain amplifier and a TR switch
B. A non-inductive resistor and a heat sink
C. A low voltage power supply and a DC relay
D. A 50 ohm reactance used to terminate a transmission line
A dummy load is a non-inductive resister, which converts the RF to heat, mounted in a tubular heatsink, which dissipates this to the air, answer B.
Which instrument would you use to measure electric potential or electromotive force?
A. An ammeter
B. A voltmeter
C. A wavemeter
D. An ohmmeter
These are terms for voltage, measured in volts, so a voltmeter, answer B.
What is the correct way to connect a voltmeter to a circuit?
A. In series with the circuit
B. In parallel with the circuit
C. In quadrature with the circuit
D. In phase with the circuit
The voltmeter is places in parallel with the circuit, answer B.
How is an ammeter usually connected to a circuit?
A. In series with the circuit
B. In parallel with the circuit
C. In quadrature with the circuit
D. In phase with the circuit
The ammeter is placed in series with the circuit, so that the current passing through the load also flows through the meter, answer A.
Which instrument is used to measure electric current?
A. An ohmmeter
B. A wavemeter
C. A voltmeter
D. An ammeter
Current in in Amps (Amperes), so it is the ammeter, answer A.
What instrument is used to measure resistance?
A. An oscilloscope
B. A spectrum analyzer
C. A noise bridge
D. An ohmmeter
Resistance, that's in ohm's, so ohmmeter, answer D.
Put some resisters on the stove, and sing, "Ohm, ohm on the range". Lame, or what?
Which of the following might damage a multimeter?
A. Measuring a voltage too small for the chosen scale
B. Leaving the meter in the milliamps position overnight
C. Attempting to measure voltage when using the resistance setting
D. Not allowing it to warm up properly
The first and last won't damage the meter, leaving it on any current range is a bad idea, as you'll stagger out of bed for an early-morning net, decide you need to test the voltage of a high-energy power source, and BANG!, or at least "futt", as the fuse blows; but this requires that second error. The actual answer is C, trying to measure voltage while in the resistance mode. Modern meters are however usually well protected.
Which of the following measurements are commonly made using a multimeter?
A. SWR and RF power
B. Signal strength and noise
C. Impedance and reactance
D. Voltage and resistance
A multimeter can measure voltage and resistance, answer D. The other measurements are Radio Frequency tests, some require an RF energy source.
Which of the following types of solder is best for radio and electronic use?
A. Acid-core solder
B. Silver solder
C. Rosin-core solder
D. Aluminum solder
Rosin-cored solder, C is the answer. The others have corrosive flux, and/or melt at very high temperatures.
What is the characteristic appearance of a cold solder joint?
A. Dark black spots
B. A bright or shiny surface
C. A grainy or dull surface
D. A greenish tint
A poorly heated joint is grainy or dull, answer C. Such joints are likely to fail.
What is probably happening when an ohmmeter, connected across an unpowered circuit, initially indicates a low resistance and then shows increasing resistance with time?
A. The ohmmeter is defective
B. The circuit contains a large capacitor
C. The circuit contains a large inductor
D. The circuit is a relaxation oscillator
The capacitor initially appears short circuit, then charges to the voltage in the meter's battery voltage (or its test voltage), appearing open, answer B.
Which of the following precautions should be taken when measuring circuit resistance with an ohmmeter?
A. Ensure that the applied voltages are correct
B. Ensure that the circuit is not powered
C. Ensure that the circuit is grounded
D. Ensure that the circuit is operating at the correct frequency
Ohmmeters should only be used on un-powered circuits, answer B.
Which of the following precautions should be taken when measuring high voltages with a voltmeter?
A. Ensure that the voltmeter has very low impedance
B. Ensure that the voltmeter and leads are rated for use at the voltages to be measured
C. Ensure that the circuit is grounded through the voltmeter
D. Ensure that the voltmeter is set to the correct frequency
You must ensure the meter, and leads, are rated for the voltage being measured, answer B.
On to: Antennas and Feedlines
You can find links to lots more on the Learning Material page.
This has taken a fair bit of work to write and update, so if you have found this useful, there is a "tip jar" below.
Written by Julian Sortland, VK2YJS & AG6LE, February 2018.
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