T7: Practical Circuits

Start with the two big jobs: hearing and talking

Strip away all the fancy parts and every radio station really does just two things. It listens to signals coming out of the air, and it talks by sending signals back out into the air. That is it. Everything on the bench exists to do one of those two jobs, or to help one of those two jobs go better.

The box that does the listening is called a receiver. An easy way to remember it: think of "reception," like a phone getting good reception, or a football player receiving a pass thrown to him. A receiver catches signals out of the air. The box that does the talking is called a transmitter. To "transmit" means to send something on its way — the way a TV station transmits a show to your home. A transmitter sends your signal out.

The transceiver: two boxes squeezed into one

Long ago, a ham needed a separate receiver box AND a separate transmitter box sitting side by side on the desk. That was clumsy and expensive. Today almost everyone uses a single box that does both jobs. We call that box a transceiver. The name itself is a clue if you say it slowly: it is "trans-" borrowed from transmitter, glued onto "-ceiver" borrowed from receiver. So in exam words, a transceiver is a device that combines a receiver and a transmitter in one unit. (T7A02) It is the grown-up cousin of the walkie-talkie you played with as a kid: one handheld thing that can both hear and speak.

Tuning the dial: the VFO

Radio signals do not all pile up in one place. They live at different frequencies. A good way to picture a frequency is to imagine a giant highway with many separate lanes, or a TV with many separate channels. Each signal rides in its own lane. To choose which lane you are listening to, you turn a tuning knob on the radio.

Behind that knob is a circuit called the VFO, which stands for Variable Frequency Oscillator. Let us unpack those words: "variable" means changeable, and "frequency" is the lane or channel. So a VFO is the changeable-channel maker. Its whole job is to set the receive and transmit frequency of your transceiver. (T7A11) When you spin the dial, you are gently nudging the VFO higher or lower along the band, so the radio hears exactly the channel you want — and, when you push the button to talk, speaks on that same channel.

Pushing the button to talk: PTT

A transceiver cannot listen and talk at the very same instant — just like you cannot really hear someone while you are shouting at the top of your lungs. So the radio needs a switch that flips it between "listening mode" and "talking mode." That switch is the PTT, short for Push-To-Talk. It is the button on the side of the microphone that you hold down while you speak.

Here is the part the exam cares about. Inside the radio there is a special wire (the PTT input). Most of the time that wire is "floating," doing nothing, and the radio just listens. When you press the button, it connects that wire to ground — "ground" is simply the radio's electrical zero, its home base, the level everything else is measured against. The instant the wire touches ground, the radio jumps into talk mode. So the rule to remember is: the PTT input switches the transceiver from receive to transmit when it is grounded. (T7A07) Press the button, the wire grounds, you talk. Let go, the wire floats again, and the radio snaps right back to listening. No grounding, no talking.

Making your signal louder and stronger: the RF power amplifier

Sometimes your radio's signal is simply not strong enough to reach a faraway friend. The cure is to add a separate box that boosts the signal: an RF power amplifier. "Amplify" means to make bigger, so an amplifier makes a signal bigger. "RF" is short for Radio Frequency — just a fancy label for the kind of fast electrical signal that radios use. Put the two together: an RF power amplifier is added to the output of a transceiver to increase the transmitted output power. (T7A10) Picture strapping a megaphone onto your mouth. You still make the words yourself, but they go out louder and travel much farther. The amplifier does that for your radio signal.

A big amplifier often has a little selector switch with labels like SSB / CW / FM. Those three are different modes — different ways a radio wraps up your voice or signal before sending it (we will meet modes again in T7B). The switch does not magically change your radio's mode for you. Instead, its job is to set the amplifier for proper operation in the selected mode. (T7A09) In plain words, you are telling the amplifier "here is the kind of signal I am about to send you," so it can handle that signal correctly — the same way you might warn a friend "careful, this box is heavy and awkward" so they grab it the right way.

Borrowing a radio for a different band: the transverter

Radios are usually built to work on a certain band — a band is just a chunk of that radio highway, a range of nearby lanes set aside for a purpose. But what if your radio only works on one band and you really want to try a different one without buying a whole new radio? You add a transverter.

A transverter is a device that converts the RF input and output of a transceiver to another band. (T7A06) Think of it as a travel adapter, like the plug adapter you use to charge a phone in another country. It takes the signal your radio makes and shifts it up or down to the new band on the way out, and shifts incoming signals from that new band back to where your radio expects them on the way in. One radio, suddenly able to play on a band it was never built for. ("Transverter," by the way, is a blend of transmit, convert, and er — a converter for transmitting.)

The little workers inside every radio

Now let us peek inside the boxes. A radio is built from many small circuits, and each small circuit has one simple job, like workers on an assembly line. Three of these workers show up on the exam, so let us meet them one at a time.

• The oscillator. An oscillator is a circuit that generates a signal at a specific frequency. (T7A05) The word "oscillate" means to swing back and forth — like a swing on a playground, or a pendulum in a clock. An oscillator makes electricity swing back and forth very, very fast and very, very steadily, and that steady swinging is exactly what a clean radio tone is. Almost every other circuit in the radio needs the oscillator's steady beat to do its own job, so the oscillator is like the heartbeat of the whole radio.
• The mixer. A mixer is used to convert a signal from one frequency to another. (T7A03) A mixer takes two signals, blends them together, and out the other side comes the signal moved to a brand-new frequency — either higher or lower than before. It works a little like a kitchen blender that combines two ingredients into something new, except here the "something new" is your signal now sitting in a different lane. Mixers are how a radio slides signals around inside itself so the rest of the circuits can deal with them more easily.
• Modulation. This one is a trick, not a box. By itself, a plain radio wave (called the carrier) carries no message at all — it is like an empty delivery truck driving down the road with nothing in the back. Modulation is the process of loading your message onto that truck. In exam words, modulation describes combining speech (your audio) with an RF carrier signal. (T7A08) Without modulation, the carrier would just be a silent, useless tone. Modulation is what actually puts your voice on the air so it can be delivered to the person listening.

How good is a receiver? Two important words

When hams describe how well a radio hears, they use two words that sound almost the same but mean very different things. The exam loves to test whether you can tell them apart, so slow down here for one easy point.

• Sensitivity is the ability of a receiver to detect the presence of a signal. (T7A01) A sensitive receiver can hear very weak, very faraway, whisper-quiet signals that a worse radio would miss completely. Think of sensitivity as having amazing ears: even a tiny sound far across a field, you still notice it.
• Selectivity is the ability of a receiver to discriminate between (separate) multiple signals. (T7A04) "Discriminate" here just means "tell apart." When a band is crowded and many stations are crammed close together, a selective receiver can lock onto the one you want and shove all the others aside. Think of selectivity as being able to follow one friend's voice in a loud, crowded cafeteria full of chatter.

Here is a memory trick that sticks: Sensitivity is about sensing that a signal is even there. Selectivity is about selecting one signal out of the crowd. "Sense it exists" versus "select the one you want."

One more helper: the RF preamplifier

We met the RF power amplifier above, which helps the talking side by boosting your outgoing signal. There is also a matching helper for the listening side: the RF preamplifier. The prefix "pre-" means "before," so a preamplifier sits before the receiver and gently boosts very weak incoming signals so the radio has an easier time hearing them. Do not mix the two up: the power amplifier makes your outgoing signal stronger, while the preamplifier makes faint incoming signals easier to hear. Same word "amplifier," opposite ends of the job.

When your voice is too loud for the radio: over-deviation

An FM radio sends your voice by making its signal "swing" a little bit as you talk — louder sounds make a bigger swing. The amount of that swing has a name: deviation. The radio is built to handle a certain amount of swing and no more. If you talk too loudly, or jam the microphone right up against your lips, you push the swing past the safe limit. That is called over-deviating, and it makes your voice come out the other end loud, fuzzy, harsh, and hard to understand.

The cure is wonderfully simple. If someone tells you your FM handheld or mobile radio is over-deviating, just talk farther away from the microphone. (T7B01) Back the mic off a few inches and speak in a normal, calm voice. It works instantly — the same way stepping back from a microphone at a school assembly stops that painful blasting, screeching sound. Closer and louder is not better; a little distance gives clean, clear audio.

More reasons your audio might sound bad on a repeater

Quick reminder of one word: a repeater is a helpful automatic station placed up high — on a tall tower, a tall building, or a hilltop. It listens to your signal and immediately re-sends it from way up there, so your little radio can reach much, much farther than it ever could on its own. Most local ham chatter goes through repeaters.

Now suppose someone listening through a repeater tells you that your audio is distorted or "unintelligible" (which just means they cannot make out your words). What is wrong? There are a few common culprits, and the exam wants you to know it could be any of them (T7B10). You might be slightly off frequency — not tuned exactly onto the repeater's input channel. You might be talking too loudly or too close to the mic — that is the over-deviation we just covered. Or you might simply be in a bad location, where your signal is weak or bouncing off buildings and hills before it arrives. So when a bad report comes back, run down the checklist: check your tuning, check your mic distance and volume, and check whether moving to a better spot helps.

RF feedback: when your own signal bites you back

Here is a sneaky problem. When you transmit, your own strong radio signal is floating in the air all around your equipment, not just out at the antenna. Sometimes a little of that signal crawls back into the radio through one of the cables — very often the microphone cable — and scrambles your audio from the inside. This is called RF feedback. You have probably heard ordinary audio feedback before: that awful screech when a microphone gets too close to its own speaker and the sound loops around and around. RF feedback is the radio version of that loop, where the transmitted signal sneaks back into the very transmitter that made it.

The classic, cheap, easy fix is a small part called a ferrite choke. "Ferrite" is a special dark, hard, magnetic material. A "choke" is a part that chokes off — blocks — unwanted radio energy trying to travel along a wire, while letting the normal audio pass through just fine. So to eliminate distorted voice transmissions caused by RF feedback, add a clip-on ferrite "choke" to the microphone cable so the stray transmitted signal cannot feed back into the transmitter. (T7B11) It literally clips on around the cable like a little snap-shut bead — no cutting, no soldering — and it quietly blocks the radio energy from sneaking up the wire. A favorite, very real-world fix.

What "interference" means and where it comes from

Interference is the general word for one signal messing up another. A common example: your transmission accidentally showing up in a neighbor's TV, stereo, or telephone. Radio frequency interference can come from more than one source, and the exam answer is that it can be caused by fundamental overload, harmonics, AND spurious emissions — all of these. (T7B03) Here are plain-English definitions so the words are not scary:

• Fundamental overload means your normal, perfectly legal, strong signal simply overwhelms a nearby device, the way a bright flashlight shined right at someone makes it hard for them to see anything else.
• Harmonics are faint extra copies of your signal that appear at higher frequencies — like echoes of your real signal showing up where they should not.
• Spurious emissions are stray junk signals that a transmitter should not be making at all, a sign something is not quite right with the gear.

You do not have to become an expert on each one for the exam. Just remember the headline: any of those three can cause interference, so the correct choice is "all of these."

Why a plain AM or FM radio might pick you up

Suppose your transmission shows up on a neighbor's ordinary broadcast AM or FM radio, even though you are operating nowhere near those broadcast channels. Whose fault is it? Usually it is the cheap receiver's fault, not yours. The exam answer: the receiver is unable to reject strong signals from outside the AM or FM band. (T7B02) Remember "selectivity" from T7A — the ability to separate signals and keep unwanted ones out? An inexpensive radio has weak selectivity, so when your strong nearby signal comes along, the cheap radio cannot keep it out and your signal barges right in.

How to cure interference to a neighbor's electronics

Since the trouble is so often that the victim device is letting your signal in, the cure is usually applied right at that device. The main tool is a filter — a part that lets the wanted signals through while blocking the unwanted ones, like a screen door that lets the breeze in but keeps bugs out. Here are the three exam cures:

• To reduce interference to someone's non-amateur over-the-air receiver (an antenna TV or radio), block the amateur signal with a filter at the antenna input of the affected receiver. (T7B05) You put the filter on the device that is being bothered, right where its antenna plugs in, so your signal is stopped before it can even get inside.
• What if the shoe is on the other foot, and a powerful nearby commercial FM broadcast station is bothering your 2-meter radio? Then you install a band-reject filter. (T7B07) A "band-reject" filter is built to reject — block — one particular band of frequencies (here, the loud FM station's band) while letting your own signals pass through normally.
• If your transmission shows up on a neighbor's cable TV (the regular copper-cable kind), the very first thing to check is the simplest, cheapest thing: be sure all the TV feed line coaxial connectors are installed properly. (T7B09) Loose, corroded, or sloppy cable connectors leave tiny gaps where outside signals can leak in. Tightening and reseating those connectors is step one — always try the easy fix before reaching for anything fancier.

Being a good radio neighbor

Hams are expected to be kind and responsible members of the community. Two exam questions are really lessons in good manners more than electronics:

• If a neighbor complains that you are interfering with their radio or TV, your very first move is to make sure your own 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. (T7B06) In other words, test it at home first. If your station is clean and does not even bother your own devices, then the problem is probably inside your neighbor's equipment, and you can help them work it out instead of feeling blamed.
• And if something in your neighbor's home is interfering with your station, the right response covers all of these (T7B08): work together politely to track down the noisy device, calmly explain the FCC rules about interference, and make sure your own station is following good amateur practice. The whole theme is teamwork and courtesy, never blame.

When your radio's own output suddenly gets weak

One last troubleshooting fact belongs here. If a modern (solid-state) transceiver suddenly puts out low power, a very common cause is high SWR. (T7B04) SWR is the antenna-matching number we meet in the next section. The short version: when the antenna and feed line are not matched well, the radio senses the problem and deliberately turns its own power down to protect itself from damage. So if your output goes weak, the message is usually "go check your antenna and feed line," not "the radio is broken." We will see exactly why this happens in T7C.

First, what is a feed line?

Your radio sits inside on the desk; your antenna usually lives outside or up high in the air. The cable that carries the signal between them is called the feed line, because it "feeds" power from the radio out to the antenna. The most common feed line is coax (say it "KO-ax," short for coaxial cable). Coax is a round cable built in layers: a single wire in the very center, wrapped in insulation, then a braided metal shield around that, then a tough outer jacket on the outside. That layered design lets it carry radio signals from one end to the other without leaking them all over the place.

SWR: the antenna's report card

For power to flow nicely from the radio, through the feed line, and out the antenna, all three pieces have to "agree" electrically. Engineers call this being matched — and for ham gear the standard agreement is 50 ohms (an ohm is the unit of electrical resistance; do not worry about the number, just know 50 is the target). When the radio, feed line, and antenna are all matched, every bit of power sails right out the antenna and into the air. When they are mismatched, some of the power bounces back toward the radio instead of leaving — picture waves sloshing back at you in a bathtub instead of flowing away.

We measure how good that match is with a single friendly number called SWR, which stands for Standing Wave Ratio. ("Standing waves" are those sloshing-back-and-forth patterns the bounced power makes; the "ratio" is just a comparison between the power going out and the power coming back.) SWR is written as two numbers with a colon between them. The best possible reading is 1:1, which indicates a perfect impedance match between the antenna and the feed line. (T7C04) At 1:1, nothing bounces back at all — everything goes out, a perfect score. As the match gets worse, that first number climbs higher. For example, an SWR reading of 4:1 indicates an impedance mismatch (T7C06) — the pieces are not agreeing well, and a real chunk of power is bouncing back instead of radiating. ("Impedance" is just the electrical word for how the pieces match up.) So burn this into memory: lower is better, and 1:1 is a perfect score; bigger first numbers are worse.

How to measure SWR and how to check an antenna

• A directional wattmeter can be used to determine SWR. (T7C08) The key word is "directional": this meter can separately measure the power flowing out toward the antenna and the power bouncing back toward the radio, then compare those two amounts to work out your SWR. ("Watt" is the unit of power, so a wattmeter measures power.)
• To check whether an antenna is resonant — meaning it is naturally happy and efficient at the exact frequency you want to use — you use an antenna analyzer. (T7C02) It is a small test box you connect in place of the radio. It quietly sweeps across a range of frequencies and tells you where the antenna works best, so you can trim it or adjust it to land right where you want.

Why high SWR makes power drop

Back in T7B we said high SWR can cause low output power. Here is the reason behind it. When power bounces back from a badly matched antenna, it travels back down the feed line and returns into the radio's delicate output transistors — the tiny parts inside that actually generate the transmit power. Too much bounced-back power can overheat or even ruin them. To stay safe, most solid-state transmitters reduce output power as SWR rises beyond a certain level, in order to protect the RF output amplifier transistors. (T7C05) ("Solid-state" just means built from modern chips and transistors rather than old glass tubes.) So when the power drops, the radio is not broken — it is protecting itself, the same way you yank your hand back from something too hot. Fix the match, and full power comes right back.

Where lost power goes, and why coax wears out

• No feed line is perfect; a little signal is always lost on the trip from radio to antenna. Where does that lost power go? It is converted into heat. (T7C07) The cable warms up ever so slightly, and that tiny warmth is energy that never reached the antenna — simply wasted. Longer cable, or cheaper cable, wastes more.
• The number-one enemy of coax is water. Moisture contamination causes failure of coaxial cables. (T7C09) If water sneaks inside the cable, it wrecks the cable's ability to carry signals and the losses shoot way up.
• That is exactly why the cable's outer jacket has to survive years of sunshine. Ultraviolet (UV) light from the sun can damage the jacket and allow water to enter the cable. (T7C10) UV is the invisible part of sunlight that fades, dries out, and cracks plastic over time. A UV-resistant jacket stays sealed and keeps the rain out, which keeps the cable healthy for years.
• Coax comes in different qualities depending on what is inside. A handy upgrade is foam-dielectric coax. The "dielectric" is the insulation around that center wire; making it out of foam instead of solid plastic gives the cable less loss per foot (T7C11) — so more of your power survives the trip and reaches the antenna, which is exactly what you want.

The dummy load: practice talking without going on the air

Sometimes you want to test or tune your transmitter without actually putting a signal out over the air, where it might bother other operators. For that you use a dummy load. The name says it all: it is a "dummy," meaning fake, antenna. It fools the radio into thinking a real, perfectly matched antenna is plugged in — but instead of broadcasting the power into the air, it quietly soaks all of it up. In exam words, the primary purpose of a dummy load is to prevent transmitting signals over the air when making tests. (T7C01)

So what is actually inside a dummy load? A typical RF dummy load is a 50-ohm non-inductive resistor mounted on a heat sink. (T7C03) Let us take that apart piece by piece, because every part is there for a reason:

• 50-ohm — because 50 ohms is the magic matching number all ham gear is built around, so the radio sees a perfect 1:1 SWR and is completely happy and safe.
• non-inductive resistor — a resistor is a part that turns electrical power into heat. "Non-inductive" means it is built to behave like a clean, plain resistance even at radio frequencies, so it mimics a real, well-matched antenna instead of misbehaving.
• heat sink — all that soaked-up power has to go somewhere, and it turns into heat (just like the lost feed-line power did above). A heat sink is a chunk of metal with cooling fins that carries the heat away so nothing inside melts or burns up.

In short, a dummy load lets you safely run your radio at full power right on the workbench while staying completely off the air — perfect for testing and tuning without bothering anyone.

Three things we measure in electricity

To check whether a circuit is healthy, we usually measure three things, and there is a dedicated tool for each one. A great way to picture all three is to imagine water flowing through pipes. Voltage is the push or pressure behind the water. Current is how much water is actually flowing past a point. Resistance is how tightly the pipe squeezes and slows the water down. Hold that water picture in your head and the three meters below make perfect sense.

The voltmeter — measuring the push

A voltmeter measures electric potential, which is voltage (T7D01) — the electrical "push." Here is the part the exam tests: you connect a voltmeter in parallel with the part you are testing. (T7D02) "In parallel" means you touch the two probes across the part, one on each side, without unplugging or cutting anything apart — like holding a ruler up beside a doorway to measure its height. You are comparing the electrical push on one side of the part to the push on the other.

The ammeter — measuring the flow

An ammeter measures electric current (T7D04) — how much electricity is actually flowing. (The name comes from the "amp," short for ampere, which is the unit of current.) To measure flow, the electricity has to pass right through the meter, so you connect an ammeter in series with the part. (T7D03) "In series" means you break the circuit open at one spot and drop the meter into the gap, so every bit of current is forced to flow through the meter on its way around — exactly like putting a flow meter inside a water pipe so every drop has to pass through and get counted.

Here is a memory hook that really sticks: Voltage goes in parallel (across); current goes in series (through). The last letters even match up.

The ohmmeter — measuring the squeeze

An ohmmeter measures resistance (named after the "ohm," the unit of resistance). But it has a clever, sneaky way of doing it. It cannot measure the squeeze directly, so instead it measures resistance by applying a small current and measuring the resulting voltage. (T7D05) In plain words, the meter pushes a tiny, known amount of electricity through the part, watches how much "push" that took, and from those two numbers it figures out the resistance for you. You never have to do any math yourself — the meter handles it and shows you the answer.

The multimeter — three tools in one

Carrying three separate meters around would be a hassle, so most hams use a single multimeter that does all three jobs. "Multi" means many, and a multimeter measures voltage and resistance (and current too) — you just turn a dial to pick which one you want. (T7D07) But that one-tool-does-everything power comes with a few important safety rules you must respect:

• Always turn the dial to the right job first, before you touch the probes to anything. You can damage a multimeter by trying to measure voltage when it is set to the resistance setting. (T7D06) On the resistance setting, the meter expects to supply its own tiny test current — it does not expect outside voltage to come shoving in, so feeding it voltage can hurt it. Set the dial first, then probe.
• When you measure the resistance of something that is still part of a larger circuit, ensure the circuit is not powered first. (T7D11) Turn it off and unplug it. An ohmmeter supplies its own tiny test current, so if outside power is also present, that outside voltage will give you wrong readings and may even damage the meter. The rule is simple: no power, then measure.
• A fun one to remember: if you connect an ohmmeter across a large, fully discharged capacitor (a capacitor is a part that stores up electric charge, a bit like a tiny rechargeable battery), the reading shows increasing resistance with time. (T7D10) That happens because the meter's little test current slowly charges the capacitor up. As the capacitor fills, less and less current can flow, so the meter reads a higher and higher resistance. Watching that number climb is actually a handy way to tell that a capacitor is alive and storing charge the way it should.

Soldering: joining wires together with melted metal

Hams often connect parts by soldering. To solder, you melt a special soft metal (called solder) so it flows around two pieces of metal, and when it cools and hardens, it locks them together — both physically and electrically, so current can pass across the joint. Two facts about soldering show up on the exam, and both are easy.

• Use the right solder. Acid-core solder should NOT be used for radio and electronic work. (T7D08) Acid-core solder has acid hidden inside it, and over time that acid slowly eats away at electronic connections and ruins them. For electronics you use rosin-core solder instead, which is gentle and safe for circuits. (Acid-core solder is meant for plumbing pipes, not radios — keep them separate.)
• Tell a good joint from a bad one. A good solder joint — made when everything got hot enough and the solder flowed smoothly into place — comes out shiny, smooth, and clean. A cold solder joint is one where the metal did not get hot enough for the solder to flow properly, and it has a rough or lumpy surface. (T7D09) A cold joint looks dull, grainy, or blobby, and it is weak and unreliable, so if you spot one, reheat it and let the solder flow correctly. The whole rule fits in one line: shiny and smooth means good; rough and lumpy means do it again.