Make Radio Concepts Click
Five everyday questions. Five clear pictures. Follow the signal, then learn the words that describe it.
Conceptual diagrams, not equipment instructions or coverage predictions.
Why can I hear them, but they cannot hear me?
Two lanes, one conversation.
Example only: receive 146.940 MHz with a −0.600 MHz offset means transmit on 146.340 MHz. This example repeater requires a 100.0 Hz transmit PL tone. Wrong offset: you miss its input frequency. Wrong tone: its access gate stays closed. The barrier is a visual analogy for tone-controlled access, not a physical gate or encryption.
Correct settings alone do not guarantee a contact; signal strength, range and repeater availability still matter.
Connect the picture to the terms
Your radio listens to the repeater output and transmits to its input. The offset moves between those frequencies. A PL tone can open the repeater’s receive gate; receive tone squelch controls what your own speaker lets through.
What to remember Check the published input, output and transmit tone separately. Hearing a repeater does not prove it can hear you.
Explore this topicHow can a signal reach beyond the horizon?
The sky can bend the path.
Daytime ionospheric regions are labeled D, E, F1 and F2. This example path bends back in F2. Heights are approximate, and the drawing is not to scale.
At night D largely fades, and F1 and F2 generally form one F region. E-region propagation also occurs; not every skywave path uses F2.
Connect the picture to the terms
HF skywave uses refraction in the ionosphere. The usable path depends on frequency, ionization and launch angle. Local VHF/UHF operation is more often limited by terrain and the radio horizon.
What to remember Before adding power, ask whether the frequency and antenna support the path you want.
Explore this topicDoes low SWR mean a great antenna?
A good fit is not the same as a useful result.
These are two separate setups, not a splitter or two simultaneous branches. A suitable RF dummy load replaces the antenna for testing, directly or through suitable coax. It converts power mostly to heat; use its specified frequency, power and duty-cycle limits.
Low SWR alone does not show how effectively an antenna radiates.
Connect the picture to the terms
Impedance matching, resonance and radiation efficiency answer different questions. Resonance means net reactance is zero at the measurement point; that resistance may still differ from the line impedance. SWR describes the match, not where all the power goes.
What does measure antenna performance?
- Radiation pattern: where does the signal go?
- A controlled antenna-range test measures signal level at different angles, at a specified frequency and polarization. Rotating the antenna and plotting those readings reveals its lobes and nulls.
- Radiation efficiency: how much accepted power becomes radiation?
- This is radiated power divided by power accepted at the antenna terminals. Calibrated gain and full-pattern measurements can establish it; specialized chamber methods are also used. Mismatch and feed-line losses must be accounted for separately.
- What can I check at home?
- An antenna analyzer or VNA can measure impedance and SWR at its measurement point. Those readings alone do not measure radiation pattern or efficiency. Controlled on-air comparisons can reveal useful differences, but changing propagation and surroundings prevent a casual signal report from proving an efficiency percentage.
What to remember Judge the match and the antenna’s ability to radiate separately. Keep cable losses in the picture, too.
Explore this topicWhy does twice the power not mean twice the range?
Small numbers can describe big ratios.
The horizontal scale is relative distance, not miles. With frequency, antenna gains and required received signal level held constant in ideal free space, distance scales with the square root of transmitter power. Thus 5 → 10 W gives about 1.41× distance, while 5 → 20 W gives 2×.
Terrain, obstructions, noise and the radio horizon can change or limit the result. This is not a coverage forecast.
Receiver gain control and FM limiting can keep voice loudness similar even when signal strength changes.
Connect the picture to the terms
A decibel expresses a ratio. Doubling power adds about 3 dB; doubling it again gives about 6 dB above the original. Antenna gain favors particular directions, and feed-line loss subtracts from the power delivered.
A contact needs a path in both directions
Increasing your transmitter power may help the other station hear you. It does not, by itself, improve your receiver’s ability to hear their reply. Your receive antenna, local noise and the other station’s signal still determine whether the return path works.
Check both directions before reaching for more power. Better antenna placement or lower noise may help where extra watts cannot. Use only the transmitter power needed for the intended communication; that is the US amateur-service requirement in 47 CFR § 97.313(a).
What to remember Once SWR is acceptable, antenna type, placement and height are often more useful improvements than extra transmitter power. For local VHF/UHF, height and a clear path can be especially important. For HF, choose the antenna and height for the intended path; higher is not always better for every contact. Check feed-line loss and local noise, too.
Explore this topicWhat are volts, amps and ohms actually doing?
Push, flow and resistance.
The resistor is the load in the first circuit: it dissipates 24 W as heat. The second circuit replaces it with a radio. The radio’s 2 A draw is an assumed operating point, not a fixed resistance or a specification.
In both loops conventional current returns to the negative terminal; charge is not consumed. The load converts electrical energy. The radio’s 24 W DC input is not its RF output.
Connect the picture to the terms
Voltage is potential difference; current is charge flow; resistance opposes current. For an ideal resistor, I = V/R. In this example 12 volts across 6 ohms gives 2 amps, and P = VI gives 24 watts.
What to remember With voltage held steady, doubling resistance halves current in this simple model. Try predicting the answer before calculating.
Explore this topicRadio concepts explained: common questions
Are these diagrams real radio coverage predictions?
No. They explain relationships and common misconceptions. Actual coverage depends on equipment, installation, terrain and changing conditions.
Can I learn these concepts without seeing the illustrations?
Yes. Each section includes the relationships in text, a takeaway and an explanation of the diagram’s limits. Images also have descriptive alternative text.