RF 101: zero to hero

4 min

What a radio actually does

A radio turns your voice (or a few bytes of text) into a wave at one exact frequency and pushes it out of the antenna. Every radio in range tuned to that frequency hears it. There is no address, no account and no connection: whoever is listening on that channel gets it, including people you did not intend.

A channel is a frequency with a friendly name. FRS channel 1 is 462.5625 MHz; the name is for humans. Two radios talk only if they are on the same frequency, the same mode (FM for everything in this app) and, if tone squelch is set, the same tone.

Frequency sets the wavelength: 300 divided by the frequency in MHz gives meters. 146 MHz is the amateur 2 meter band; 462 MHz is about 65 cm (the 70 cm neighborhood); LoRa at 906.875 MHz is a hand-span. Antennas are cut to the wavelength, which is why a 2 m antenna does nothing useful at 900 MHz.

VHF (30 to 300 MHz) bends a little over hills and through trees. UHF (300 MHz to 3 GHz) gets into buildings and vehicles better and its antennas are shorter. Neither goes through a hill.

Simplex means both radios take turns on one frequency: press to talk, release to listen, one voice at a time. Everything RADIO 4DAT plans starts as simplex.

• Same frequency, same mode, same tone, or nothing is heard.
• Wavelength in meters = 300 ÷ MHz. Antennas are cut to it.
• Anyone in range on that channel hears you.

5 min

Who may transmit what

Transmitting needs two things to line up: the service (a block of frequencies with its own rules) and you. Some services need no license at all, but only on radios certified for them (FRS, GMRS or MURS). Others need a license held by a person: an amateur license (Exam and current FCC license (Technician or higher)), which brings an exam and a public call sign, or a GMRS license ($35, no exam, covers your household for 10 years).

The license is yours; the certification belongs to the radio. A cheap programmable handheld can be tuned to a license-free channel, but it is not certified for that service, so transmitting there is illegal for everyone, licensed or not. FCC enforcement notices say this plainly. That is why RADIO 4DAT shows Baofeng-class radios as listen-only unless you hold a current amateur license.

LoRa mesh (Meshtastic) sits in a third group: short-range devices under FCC Part 15.247. Anyone may use a certified module on 902–928 MHz at low power for text and positions, and encryption is allowed. That is the app's default data path when you hold no license.

The radio never checks any of this. It transmits whatever you program. You are the only safeguard, and the person named on the license, or the person holding the radio, is the one the regulator writes to.

• License-free services work only on radios certified for them.
• Amateur radio: license, exam, public call sign, no encryption.
• LoRa mesh: no license, text only, encryption allowed.

3 min

Listening is free

Receiving needs no license in any country. A $40 software-defined receiver (RTL-SDR) plugged into a laptop or Android phone shows every signal from the broadcast band to 1.7 GHz as a waterfall: who is on which channel, how strong, how busy the band is.

This is how you learn your local radio environment before you ever press transmit: which FRS, GMRS or MURS channels your neighbors use, where the amateur repeaters are, what the noise looks like at your house versus the field.

It is also RADIO 4DAT's sensor. The Survey screen watches your planned channels and flags any that rise more than 6 dB above their normal background, which is how you spot interference or jamming early. With a directional antenna, swinging it for the strongest reading gives a bearing.

Start with the APRS tutorial on the Guide screen: decode real position packets on 144.390 MHz without transmitting a thing.

• Receive anything, anywhere, with no license.
• Learn the local bands before you plan on them.
• A receiver is the planner's early warning for interference.

4 min

Your first net this weekend

You do not need a license, an exam or expensive gear to run a real backup net. Two things cover most groups: a pair of certified FRS, GMRS or MURS radios for voice within a kilometer or two, and two LoRa nodes paired to phones for text and positions across the property or the hike.

Set a check-in time (on the hour), a channel, and one fallback (if channel 1 is busy, move to channel 3). Write it down. That is a PACE card, and RADIO 4DAT prints one from your plan.

Walk the range. Send a text or make a call every 250 m and note where it stops working. Then raise the far antenna (a window, a mast, a hill) and walk it again. You will learn more from that afternoon than from any chart.

When you want more range or repeaters, that is the point to look at a license: a GMRS license ($35, no exam) unlocks 5 W handhelds, repeaters and detachable antennas for your whole household; an amateur license opens far more spectrum.

• License-free voice plus LoRa text covers most first nets.
• A written channel plan and a check-in time make it a net.
• Walk the range; let the test calibrate the app.

5 min

The other entry-level radios: CB, MURS, marine

CB (27 MHz, 40 channels, 4 W) is the oldest license-free service and still the trucker's net: channel 19 on the highway, channel 9 for emergencies. Its wavelength is 11 meters, so a handheld with a stubby antenna reaches a kilometer or two while the same radio on a vehicle whip reaches well beyond the horizon when the band is open. Voice only, no repeaters, no amplifiers, and a vehicle antenna that must be tuned (SWR) before you trust it.

MURS (151 to 154 MHz, 5 channels, 2 W) is the quiet workhorse for a property or a job site: no license, external antennas allowed, data allowed, VHF so it carries well over open ground and through trees. A MURS base with an antenna on the barn plus MURS handhelds is a legal, license-free net that FRS cannot match. No repeaters, and no connecting it to the phone network.

Marine VHF (156 to 162 MHz) needs no license on a recreational boat in US waters, which is why so many waterproof 6 W handhelds are already in tackle boxes. The catch is absolute: it is a ship station, legal only aboard a vessel. Carried ashore to the campsite it is an unlicensed transmitter on channels the Coast Guard monitors. Channel 16 is for calling and distress only; register a free MMSI so the DSC distress button works.

All three share the FRS rule: use radios certified for that service, within its limits. A programmable amateur handheld tuned to a MURS or marine frequency is still illegal. RADIO 4DAT lists these radios in the Gear guide with their catch on the card, and the Rules screen shows the services side by side.

• CB: no license, big antenna, voice only, highway and emergencies.
• MURS: no license, VHF, external antennas and data allowed; the property net.
• Marine VHF: no license on a boat, and only on a boat.

5 min

Decibels without the math

Radio people count in decibels because signals change by factors of millions. Two rules cover 90% of it: +3 dB doubles the power, +10 dB is ten times the power. So 0.5 W to 5 W is +10 dB; 5 W to 50 W is another +10 dB.

dBm is power compared with 1 milliwatt: 0 dBm is 1 mW, 30 dBm is 1 W, 37 dBm is 5 W. A handheld hears down to about −120 dBm. The gap between what you send and what the other radio can still hear is your link budget: about 157 dB for 5 W.

dBi is antenna gain compared with a perfect round radiator. A stock rubber duck is about −5 dBi (it wastes 5 dB); a well-placed J-pole is about +2 dBi. The 7 dB between them is more than the step from 1 W to 5 W.

Here is the catch with power: near the ground, signal falls off by roughly 12 dB each time distance doubles. So +10 dB of power (ten times the battery drain) buys only about 1.8 times the range. In open country with clear line of sight it is about 3 times. Either way, power is the expensive lever.

• +3 dB doubles power; +10 dB is ten times.
• 0.5 W to 5 W buys roughly 1.8× range near the ground.
• A 7 dB antenna change beats a 5× power change.

4 min

Height beats power

VHF and UHF signals travel in nearly straight lines, so the first limit is the horizon. From a handheld at 1.5 m the radio horizon is about 5 km. Raise one end to 6 m and the pair reaches about 10 km; at 10 m, about 13 km. No amount of power moves the horizon.

Below the horizon, height still pays twice. In the plane-earth model the app uses, doubling the height of one antenna adds 6 dB. Going from 1.5 m to 6 m is two doublings: +12 dB, the same as turning 0.5 W into 8 W, with no extra battery drain.

This is why the planner's first move for a link that will not close is a roll-up J-pole on a 6 m mast at the relay end, not more power. One high antenna at a base, a vehicle or a hilltop lifts every link that touches it.

In practice: a second-floor window beats a ground-floor one; a mast on the truck beats the dashboard; a hilltop relay node beats three more watts.

• Radio horizon ≈ 4.1 × √height(m) km from each end.
• Doubling one antenna's height ≈ +6 dB.
• Height is free range; power is paid for in batteries.

5 min

Antennas: the part that matters most

The antenna is the only part of the system that touches the air, and it is where cheap radios lose the most. A stock rubber duck on a handheld is a compromise: short, bendy, about −5 dBi. A better antenna, placed higher, is worth more than any power setting.

Keep it vertical. Almost all VHF/UHF voice and mesh traffic is vertically polarized; a radio lying flat on a table or a node clipped sideways can cost 10 to 20 dB. Keep the antenna clear of your body, metal and the ground.

Never transmit without an antenna connected, even for a second. The transmitter's output stage can burn out into an open connector. LoRa boards are especially fragile here: antenna first, power second.

Services with fixed antennas (FRS, PMR446) do not allow swapping the antenna; that is part of their certification. GMRS, MURS, amateur and LoRa allow detachable antennas, within the power limits.

The antenna guide on this screen shows what each one needs and how to set it up: stock duck, longer whip, roll-up J-pole on a mast, LoRa stub and outdoor antenna, SDR dipole, vehicle mag-mount.

• Vertical, high, clear of your body and metal.
• Never key a transmitter with no antenna.
• Fixed-antenna services stay fixed; the rest can be improved.

4 min

Terrain, buildings and trees

Hills block VHF and UHF outright. Trees, walls and vehicles do not block, they attenuate: the planner calls this clutter loss and uses 0 dB for open ground, 10 dB for forest and 15 dB for urban until your own range tests tell it better.

Wet leaves and summer foliage hurt more than bare winter trees; UHF suffers a little more in foliage, VHF a little more indoors. Steel buildings and vehicles are near-total shields; a radio on the dashboard talks through the windscreen, not the roof.

Line of sight is not a laser line. The signal needs a football-shaped clearance around the straight path (the Fresnel zone); a ridge that just grazes the line still takes several dB. Stand where you can see the other end's antenna, and get the antenna above the roofline or treeline when you can.

Measure instead of guess: do the LoRa range walk in the Guide, log distance and SNR, and set the clutter slider so the Link screen matches what you saw.

• Hills block; trees and walls cost 10 to 15 dB.
• Clear the straight line, and some space around it.
• Calibrate the clutter number with a real walk.

4 min

Talking on the air

Press, pause, speak. Most of the time someone did not hear you, the first word was clipped before the transmitter came up. Hold the radio a hand's width from your mouth, speak across the microphone rather than into it, and release when done.

Listen before you transmit. Only one station can talk at a time on simplex; two keying at once produce a squeal that neither hears. Keep calls under 15 seconds. Say who you are calling, then who you are: TRUCK 2, this is HIKER 1.

Use the phonetic alphabet for anything spelled (Alpha, Bravo, Charlie) and say numbers digit by digit. Agree on a few pro-words: OVER (your turn), OUT (done), SAY AGAIN (repeat), ROGER (understood).

Tone squelch (CTCSS or DCS, often sold as privacy codes) only mutes your speaker; everyone on the channel still hears you. On amateur frequencies you must give your call sign: Call sign at least every 10 minutes (97.119).

Schedule check-ins (top of the hour) and a silence rule: two missed check-ins means move to the fallback channel. The PACE card carries both.

• Press, pause, then speak; keep it under 15 seconds.
• Privacy codes are not privacy.
• Check-ins and a fallback channel make a group a net.

6 min

Growing your radio ability: licenses, filings and certifications

Every rung adds either power, antennas, channels, repeaters or data, and each has a filing behind it. The Rules screen's ladder table lists them with the cost and the form; this lesson is the story.

Rung one is free: FRS, MURS, CB, LoRa mesh, and marine VHF on a boat. No filing, just certified radios used within their limits. Most family, neighborhood and property nets never need more.

Rung two in the United States is GMRS: a $35 FCC license, no exam, ten years, and it covers your household. You create an FCC CORES account, get an FRN, file ULS Form 605 for GMRS, pay, and the grant usually arrives within days. It unlocks 5 W handhelds, 15 to 50 W mobiles, detachable antennas and repeaters.

Rung three is the amateur license (Exam and current FCC license (Technician or higher)). The Technician exam is 35 questions from a public pool (26 to pass), given by volunteer examiners for $0 to $15; then a $35 FCC application fee paid in CORES within ten days, and the call sign appears in the public database. General and Extra add HF (worldwide) bands. The price of the ticket: no encryption, no business traffic, and your call sign every ten minutes.

Rung four is for organizations. A business that wants its own channels files for a Part 90 private land mobile license through a frequency coordinator (FCC fee plus a coordination fee; itinerant channels cover temporary jobs). A company doing radio R&D on the air files in the FCC's Experimental Licensing System for an FCC experimental license (Part 5). Anyone who wants to sell or import a transmitter needs equipment certification through a Telecommunication Certification Body and an FCC ID. Outside the United States, FCC runs the equivalents.

After the amateur ticket, the volunteer programs are where the training is: ARES and RACES for emergency communications, SKYWARN for weather spotting, MARS (Army and Air Force) for military auxiliary traffic, and CERT for community response. They are also the legal way for a ham to help a unit or an agency.

• Free rung: certified gear within its limits, no filing.
• GMRS: CORES account, FRN, Form 605, $35, no exam, household.
• Amateur: exam plus $35; then ARES, RACES, SKYWARN, MARS, CERT.

5 min

Text when voice can't

Voice needs a continuous channel and a human at each end. Text and positions need a few dozen bytes, so they get through on links that are too weak, too busy or too quiet for speech. RADIO 4DAT packs a position into about 38 bytes.

LoRa mesh (Meshtastic) is the license-free way to move those bytes: about 1 kb/s, low power on 902–928 MHz under FCC Part 15.247, encrypted with a key you share by QR code, and each node relays for the others. Three nodes cover a property; a hilltop node covers a valley.

Airtime is the budget. A 38-byte position is about a third of a second on LoRa; with 8 people sending every minute the channel is already 5% busy. Keep the channel under 20% busy or messages start colliding, which is why the planner sets positions to every 5 minutes.

Over a voice radio, text rides as audio tones (AFSK 1200, the APRS standard) at 1,200 b/s. That path exists for amateur licensees; it goes out in the clear, and your call sign goes with it.

• Text needs far less link than voice.
• LoRa mesh: no license, encrypted, multi-hop, text and positions only.
• Keep channel busy time under 20%.

4 min

Watching the spectrum

Every band has a noise floor, the hiss the receiver hears when nobody transmits, around −112 dBm on a quiet channel. Signals are measured as rise above that floor. A channel that is usually quiet and suddenly sits 6 dB or more above its own floor has something on it.

RADIO 4DAT's rule is exactly that: a watched channel more than 6 dB over its normal background is flagged. A narrow, occasional rise is another user. A wide, constant rise across several channels is interference, or, if it appears the moment your net starts, jamming.

A bearing comes from a directional antenna: a small Yagi, or a handheld shielded by your body, swung slowly while watching signal strength. The strongest reading points at the source, within about 15°. Two bearings from two places give a location. A KrakenSDR does this with phase and no swinging.

The response is on the PACE card: two failed check-ins on the flagged channel means move to the fallback, and report the bearing.

• Measure rise over the channel's own noise floor.
• +6 dB over normal: flag it. Wide and constant: interference.
• Swing a directional antenna for a bearing; two bearings give a fix.

4 min

Routes and fallbacks

A message has several ways out of a phone: cell, Wi-Fi, phone-to-phone, LoRa mesh, a radio with a modem, a satellite terminal. Each has a reach, a speed, a legal status for you, and a signature (how easy you are to notice and locate). A plan names the order.

RADIO 4DAT's Network screen does this per message: an emergency floods every legal route at once; a photo waits for an IP route; a position takes the quietest route that reaches. Routes that are down or illegal for your license are skipped, and anything with no route is queued and sent when a route returns.

This is PACE applied to data: Primary, Alternate, Contingency, Emergency, decided in advance so nobody improvises under stress. Cell towers down? Text goes to LoRa. Mesh jammed? Phone-to-phone inside the vehicle still works.

A phone with two routes up can be a gateway between them, such as relaying mesh traffic to the internet. Whether that is legal depends on the routes: LoRa to internet is fine; an amateur radio to the internet needs a licensed control operator and no encryption.

• Rank routes by reach, legality and signature before you need them.
• Emergency floods; big files wait; small messages take the quiet route.
• Gateways are legal or not per pair of routes.

4 min

Batteries and power

Count in watt-hours. A 20,000 mAh USB bank at 3.7 V is about 74 Wh, and you get roughly 60 Wh out after conversion losses. A Raspberry Pi running a voice server draws 3.5 to 4.5 W: 13 to 17 hours. A LoRa node averages well under 0.2 W: days. A handheld's 1,800 mAh pack at 7.4 V is about 13 Wh and lasts a shift at 5-5-90 (5% transmit, 5% receive, 90% standby).

Transmit is the expensive part: a 5 W handheld pulls 1.5 to 2 A while keyed. Low power when the link allows it, and short transmissions, are the biggest savings, and they also keep you quieter on the air.

Cold cuts lithium capacity by a third or more; keep spares inside your jacket. Charge everything before, not during. A 20 to 40 W folding solar panel refills a bank in a day of sun and keeps a relay node alive indefinitely.

The Kit screen tracks each radio's battery; anything under 25% is flagged on the Plan before you leave.

• Wh = Ah × V; a 20 Ah bank gives about 60 usable Wh.
• Transmitting is what drains a radio; keep it short and low.
• Cold halves batteries; carry spares warm.

4 min

Testing without breaking the law

You can test transmitters, modems and audio paths with no license at all, as long as nothing radiates: radio to coax, through attenuators, into a dummy load or the other radio. The Guide's bench tutorial walks it, with about 60 dB of attenuation between two handhelds so the receiving radio sees a strong but safe signal.

Never connect a transmitter straight to a receiver or an SDR; even 0.5 W will destroy a receiver's front end. Attenuators must be rated for the transmitter's full power (10 W or more for a 5 W handheld); an SDR wants at least 90 dB in front of it.

On-air testing is where the license comes in: a current amateur license for amateur frequencies, the right certified radio for license-free services. A company testing its own product on the air needs an FCC experimental license (Part 5); amateur frequencies cannot be used for business.

A NanoVNA (about $60) checks an antenna's match (SWR) without transmitting at all, which is the right way to prove a new antenna before it goes on a radio.

• Cables, attenuators and a dummy load: full tests, zero radiation.
• Power attenuator first, rated for the radio; 90 dB before an SDR.
• On-air company tests need an experimental license.

6 min

Liability: what you answer for

The person who keys the radio, and the person whose license it runs under, are responsible for the transmission. In the amateur service the licensee is the control operator and answers for the station's emissions, including harmonics from a cheap radio they never measured. On license-free services, you answer for using a certified radio within its limits.

Penalties are real. In the United States the FCC's base forfeiture for unlicensed operation is $10,000 per violation, and continuing violations can run past $100,000; willful interference with any licensed communication is a separate offense, and equipment can be seized. Other regulators (FCC where you are) have their own schedules. Treat every figure here as one to verify, not legal advice.

Three traps catch good people: a lapsed amateur license (the radio still works; the authority to use it is gone), privacy or encryption on a service that forbids it, and running a company's traffic or tests on amateur frequencies. RADIO 4DAT blocks all three in the planner, but the planner is not on the radio with you.

Identification is a duty too: on amateur frequencies, Call sign at least every 10 minutes (97.119). Your call sign ties to a public record with your name and address, so think about who hears it before you use amateur radio for anything location-sensitive; LoRa carries no call sign.

Physical liability sits with you as well: a mast near power lines, a guy line across a path, lightning on an outdoor antenna, a 5 W antenna held against someone's eye. Keep antennas and masts at least their full height plus a few meters from any power line, lower them in storms, and keep transmitting antennas a hand's width from faces.

• You answer for every transmission from your radio.
• Lapsed license, forbidden encryption, business on amateur bands: the three traps.
• Masts, power lines and lightning are on you too.

4 min

Write the PACE card

PACE is four ways to reach the same people, in order: Primary (what you normally use), Alternate (works almost as well), Contingency (slower or clumsier but independent of the first two), Emergency (last resort, often not radio at all). Each line names the means, the channel or address, and when to switch.

RADIO 4DAT fills it from your plan: cell phones or your main radios as Primary; the voice radios and channel as Alternate with the power and antenna the planner chose; the TAK or text path as Contingency; and a rally point with a whistle signal as Emergency.

Add the rules that make it work: check-in times, the maximum transmission length, the fallback channel and the trigger (two missed check-ins), and the emissions rule (lowest power that closes the link; raise the antenna before the power).

Brief it before you leave, give everyone a copy (Copy as text, then print it or paste it into the group chat), and run one drill: turn the primary off and watch the group find the alternate.

• Four means, in order, each with a trigger to move on.
• Check-in time, max transmission, fallback channel, rally point.
• A card nobody has read is not a plan.

5 min

Extending range with relays

When one link cannot close, add a hop. The planner computes how many: a relay radio (a person on a hill or a vehicle midway who repeats traffic), a mesh node in a window or on a mast (Meshtastic relays automatically, up to several hops), or a repeater where the service allows one.

Human relays work on any service, including the license-free ones: the relay hears both ends and repeats. It is slow and takes a person, but it is legal everywhere. LoRa nodes relay on their own, which is why a solar-powered node on a high point is the cheapest range you can buy.

Repeaters (a receiver and transmitter on a tower that rebroadcast you) are allowed on GMRS and amateur bands with the right license, and forbidden on FRS, MURS and PMR446. A GMRS repeater pair uses a +5 MHz offset; amateur 2 m repeaters use ±600 kHz. Program the offset and the tone, or you hear the repeater and it never hears you.

Place the relay where it sees both ends. The planner's intercept estimate is a reminder that a high relay is also heard far away, so in a low-signature posture keep relay power low and let height do the work.

• One high relay lifts every link that touches it.
• Human relays are legal on any service; repeaters only on some.
• Mesh nodes relay automatically; put one up high on solar.