Before this:What is a radio wave?Antennas 101
How signals travel
Key takeaways At the VHF/UHF frequencies most scanning uses, radio is essentially line-of-sight — it travels to the radio horizon and stops, blocked by hills, buildings, and the Earth’s curve. Reflections create multipath, which causes fading and decoding errors. Lower frequencies travel and penetrate better; higher ones are blocked more easily. The single biggest lever you control is antenna placement: height, a clear path, matched polarization, and distance from noise usually beat any radio upgrade.
You’ve got an antenna sized for your band. Where you put it, and what’s between it and the transmitter, decides whether a system comes in clean or not at all. This lesson explains why.
What is line-of-sight and the radio horizon?
At VHF and UHF, radio waves travel in nearly straight lines — line-of-sight. They don’t bend far around the Earth, so they reach a limit called the radio horizon, where the planet’s curvature gets in the way. It’s a bit beyond the visual horizon because the atmosphere refracts radio slightly.
The key consequence: height extends range. Raising either antenna pushes the horizon out. A transmitter on a tall tower or hilltop reaches far; a receiver up high with a clear view hears far. This is why repeaters live on towers and why getting your antenna up and outside helps so much.
A rough number. The radio horizon in kilometres is about 4.1 × √(height in metres). An antenna at 2 m reaches ~5.8 km to the horizon; lift it to 10 m and it reaches ~13 km; a repeater at 100 m reaches ~41 km. Two stations each see to their own horizon, so their ranges add — which is why a hilltop repeater can link two handhelds that could never hear each other directly. Every metre of height you add literally extends how far you can receive.
How do terrain and buildings weaken signals?
Anything in the path attenuates (weakens) the signal. Hills and large buildings can block it entirely (a “shadow”). Walls, foliage, and even rain take their toll — more so at higher frequencies. This is why a station only a few kilometres away can be unreachable if a ridge or tower block sits between you, while a more distant hilltop system comes in fine. Distance is only part of the story; the path matters more.
What are multipath and fading?
A signal rarely takes just one route to your antenna. It also bounces off buildings, terrain, and vehicles, so several copies arrive — slightly delayed relative to each other. That’s multipath. Because the copies are out of step, they can add up or partly cancel, making the signal level fade up and down, sometimes as things (like traffic) move.
For analog you hear this as flutter; for digital signals multipath smears the symbols and can push decoding past its error limit. Often the fix is simply to move the antenna a short distance — even tens of centimetres can swap a deep fade for a strong spot.
How does propagation change with frequency?
As a rule of thumb, lower frequencies travel and penetrate better:
| Band | Behaviour |
|---|---|
| HF | Can refract off the ionosphere — worldwide “skip” |
| VHF | Line-of-sight, bends slightly, decent building penetration |
| UHF | Line-of-sight, more easily blocked, but good in urban multipath |
| SHF | Very line-of-sight, blocked by almost anything |
The trunked systems GopherTrunk follows sit in VHF/UHF/700-800 MHz, so think line-of-sight with multipath — height and a clear path are your friends.
Practical placement tips
- Go up and out. Higher and outdoors beats low and indoors almost every time.
- Clear the path toward the systems you want; aim around big obstructions.
- Match polarization — vertical for most land-mobile (see antennas).
- Keep coax short to limit loss.
- Escape noise. Computers, USB hubs, chargers, and LED/CFL lighting raise your noise floor; distance from them improves SNR more than you’d expect.
Watch the effect live in GopherTrunk’s tuning meters: reposition the antenna and see SNR rise or fall in real time.
Quick check: a system 3 km away won't decode, but one 30 km away on a hilltop does. The likeliest reason?
Recap
- VHF/UHF radio is line-of-sight, limited by the radio horizon; height extends it.
- Terrain and buildings attenuate or block signals — path beats distance.
- Multipath causes fading and digital errors; moving the antenna often helps.
- Lower frequencies travel/penetrate better; the trunked bands are line-of-sight.
- Placement — height, clear path, polarization, low noise — is your biggest lever.
That wraps Module 1. Next module: how information is actually put onto these waves, starting with the anatomy of a signal on screen.
Frequently asked questions
Why can't I receive a station that isn't far away?
At VHF and UHF, radio is mostly line-of-sight. Hills, buildings, and even dense trees between you and the transmitter block or weaken the signal. A station only a few kilometres away can be unreachable if terrain is in the way, while a more distant one on a hilltop comes in clearly. Height and a clear path matter more than raw distance.
What is the radio horizon?
The radio horizon is the farthest point a line-of-sight signal reaches before the curvature of the Earth blocks it. It’s slightly farther than the visual horizon because the atmosphere bends radio waves a little. Raising the antenna extends the radio horizon, which is why height helps so much for VHF/UHF reception.
What is multipath?
Multipath is when a signal reaches your antenna by several paths at once — directly and via reflections off buildings, terrain, or vehicles. The copies arrive slightly out of step and can reinforce or cancel each other, causing fading and, for digital signals, decoding errors. Moving the antenna a little can change multipath dramatically.
How do I improve reception?
Get the antenna higher and outdoors with a clear view toward the systems you want, match its polarization (usually vertical), keep coax short, and move it away from sources of electrical noise like computers, chargers, and LED lighting. For VHF/UHF, placement and height usually beat buying a better radio.