Analog communications
All long-distance communication uses the same scarce resource: the electromagnetic spectrum. The frequency you choose determines how far the signal reaches, what interrupts it, what antenna is needed and how much information fits.
01The communication system
It is always the same blocks: an information source, a transmitter that adapts it to the channel, the channel —which adds noise and attenuation— and a receiver that recovers it.
- Antenna size. An efficient antenna is on the order of half a wavelength long. For the human voice —300 to 3400 Hz— that would mean hundreds of kilometers of antenna. Raising the signal to 1 MHz, the antenna drops to about 150 m; at 100 MHz, to 1.5 m.
- Sharing the spectrum. If everyone transmitted in baseband, all the signals would overlap. With modulation, each one takes its own place in the spectrum.
- Choosing the channel. Each band propagates differently: with the frequency you choose how the signal travels.
02The electromagnetic spectrum
| Band | Frequency | λ | Used for |
|---|---|---|---|
| VLF · LF | 3 – 300 kHz | 100 – 1 km | Communication with submarines, time signals. Penetrates water and terrain. |
| MF | 300 kHz – 3 MHz | 1000 – 100 m | AM radio (535 – 1705 kHz). Ground wave by day, ionospheric wave at night. |
| HF | 3 – 30 MHz | 100 – 10 m | Amateur radio, marine, long-distance aviation: bounces off the ionosphere and travels around the world. |
| VHF | 30 – 300 MHz | 10 – 1 m | FM radio (88 – 108 MHz), low-band TV, aviation, handheld radios. |
| UHF | 300 MHz – 3 GHz | 1 m – 10 cm | Digital TV, cellular telephony, 2.4 GHz WiFi, GPS. |
| SHF | 3 – 30 GHz | 10 – 1 cm | Microwave links, satellites, 5 GHz WiFi, radar. |
| EHF | 30 – 300 GHz | 10 – 1 mm | Millimeter-wave 5G, automotive radar, radio astronomy. |
Nobody transmits wherever they like: in Argentina the spectrum is managed by ENACOM (the national communications regulator) and worldwide by the ITU (International Telecommunication Union). There are license-free bands —the ISM bands: 433 MHz, 915 MHz, 2.4 GHz, 5.8 GHz— where you can transmit without a license as long as you respect power limits; that is where WiFi, Bluetooth and the radio modules used in projects live. Everything else requires authorization, and transmitting outside the rules —too much power, the wrong band, harmonics— is an offense with real consequences.
03How the wave travels
| Mode | Bands | Range | What it depends on |
|---|---|---|---|
| Ground wave | VLF to MF | Hundreds of km | Follows the curvature of the terrain. Improves with moist, conductive soil; gets worse as frequency rises. |
| Ionospheric wave | HF | Thousands of km | Bounces off the E and F layers. Depends on the time of day, the season and solar activity: at night the D layer disappears and AM is heard very far away. |
| Line of sight | VHF and above | Tens of km | Practically straight: it only reaches the radio horizon, plus a little by diffraction. |
| Tropospheric scatter | UHF · SHF | Up to 500 km | Takes advantage of irregularities in the atmosphere. Requires high power and large antennas. |
The radio horizon is somewhat farther than the optical one, because the atmosphere bends the wave slightly:
- FM antenna at 100 m and receiver at 2 m: d = 4.12 × (10 + 1.41) = 47 km.
- Raising the antenna to 200 m: d = 4.12 × (14.14 + 1.41) = 64 km.
- Doubling the height gives 36% more range; doubling the power, much less. That is why antennas are placed on tall towers and on top of hills.
The signal arrives by several paths —direct, reflected off the ground, off a building, off a layer of the atmosphere— and those copies add up with different phases. If they arrive in antiphase, they cancel: that is fading. It explains why a car radio cuts out at one particular spot and why WiFi works fine a meter farther on. The usual solutions are diversity —two antennas spaced apart— and digital techniques that tolerate echo.
04Decibels and link budget
In radio communications everything is expressed in dB, because the ratios are enormous and because that way multiplications become additions.
| Unit | What it measures | Reference |
|---|---|---|
| dB | Ratio between two powers | 10·log(P₂/P₁). Double the power = 3 dB; ten times = 10 dB. |
| dBm | Absolute power | 0 dBm = 1 mW · 30 dBm = 1 W · −100 dBm = 0.1 pW |
| dBW | Absolute power | 0 dBW = 1 W = 30 dBm |
| dBi | Antenna gain | Relative to an ideal isotropic antenna |
A 10 km link at 2400 MHz, with a 20 dBm transmitter and 12 dBi antennas at each end. The receiver needs at least −85 dBm.
- FSPL = 32.44 + 20·log(2400) + 20·log(10) = 32.44 + 67.60 + 20 = 120.0 dB.
- Received power = 20 + 12 + 12 − 120 = −76 dBm.
- Margin = −76 − (−85) = 9 dB. It is enough, but it is a tight margin: rain, vegetation and misalignment eat up those 9 dB quickly.
With 24 dBi antennas instead of 12 the margin rises to 33 dB, and then the link is reliable. In radio links, antenna gain is almost always cheaper than power.
05In the lab
With an AM/FM receiver and, if one is available, a software-defined radio receiver (RTL-SDR), scan the bands and note what you hear in each: local stations, services, noise. Compare day and night on the AM band: the difference in the number of distant stations is the ionosphere at work.
Calculate λ for five frequencies of real services and build a half-wave antenna out of wire for the FM band (1.5 m). Check with a receiver that the signal improves compared with a wire of arbitrary length. It is the practical check of the formula.
With two VHF or UHF handheld radios, measure how far clear communication holds: in a clear straight line, with a building in between, and from the highest point available. Note the distances and compare them with the radio horizon calculation.
Calculate the budget of a WiFi link between two buildings in the city, using real data for distance and commercial equipment. Determine whether the antenna needs to be changed, and by how much the margin improves. Then check the real measurement with the equipment's diagnostic tool.
06Common mistakes
| Symptom | Usual cause |
|---|---|
| Expecting HF range on a UHF band | Above VHF there is no ionospheric bounce: the range is line of sight. |
| The link works at night but not by day | In HF, the D layer absorbs by day. In AM, it is the everyday phenomenon of hearing distant stations at nightfall. |
| The signal cuts out at one spot and is fine a meter away | Multipath fading: two copies arrive in antiphase. |
| The power was doubled and the range barely changed | Doubling the power is 3 dB; the loss grows 6 dB for every doubling of distance. The antenna does much more. |
| The equipment transmits but nobody receives it | Poorly matched antenna, lossy cable or badly assembled connector: the power is dissipated in the line. |
| Interference with other services | Transmitter harmonics or an unauthorized band. An output filter is needed, and the allocation must be respected. |
| Comparing dBm with dB | dBm is absolute power and dB is a ratio. Adding dB to dBm gives dBm; adding dBm to dBm means nothing. |
07Self-assessment
What is the wavelength of a 150 MHz signal?
λ = 300/150 = 2 m. A half-wave antenna would measure 1 m.
Why is it impossible to transmit voice without modulating?
Because at 1 kHz the wavelength is 300 km: an efficient antenna would be unfeasible. Besides, all the signals would overlap in the same band.
Why is AM radio heard much farther away at night?
Because at night the D layer of the ionosphere disappears, and by day it absorbs the waves of that band. Without it, the signal bounces off the upper layers and returns to Earth hundreds or thousands of kilometers away.
Antennas at 50 m and 10 m height: what is the line-of-sight range?
d = 4.12 × (√50 + √10) = 4.12 × (7.07 + 3.16) = approximately 42 km.
How much loss does doubling the distance of a link add?
6 dB, because free-space loss goes as 20·log(d). Doubling the frequency adds the same amount.
Calculate the free-space loss for 5 km at 900 MHz.
FSPL = 32.44 + 20·log(900) + 20·log(5) = 32.44 + 59.08 + 13.98 = 105.5 dB.
What is the difference between dB, dBm and dBi?
dB is a ratio between two powers; dBm is absolute power referenced to 1 mW; dBi is antenna gain referenced to an ideal isotropic antenna.
A 1 W transmitter: how many dBm is that?
30 dBm. And 10 W would be 40 dBm: each factor of ten adds 10 dB.
What is multipath fading?
The partial cancellation that occurs when the signal arrives by several paths and the copies add up in antiphase. It is fought with antenna diversity or with digital techniques that tolerate echo.
What are the ISM bands and what limits do they have?
License-free bands —433 MHz, 915 MHz, 2.4 GHz, 5.8 GHz— subject to limits on power and bandwidth. You must accept interference from other users and not cause harmful interference.