Every radio project starts with the same question: how far will it reach? It is also the question that causes the most disappointment after delivery, because the range printed on a datasheet describes a laboratory condition — one transmitter, one receiver, clear line of sight, no buildings, no interference and nobody standing in the way.
Real coverage is decided by the path between two antennas: how high each one sits, how much power reaches the receiver, and how much the environment removes or reflects on the way. Once those levers are clear, most “we need more power” problems turn out to be antenna problems.
How far can a two-way radio actually reach?
Over open ground with a clear path, the limit is the horizon rather than the power. A useful approximation is that the radio horizon in kilometres is about 3.57 times the square root of the antenna height in metres, so two handhelds held at 1.5 m can “see” one another for roughly 8–9 km across open water — and beyond that nothing is visible at all.
Real sites are never open water. Terrain, buildings, trees and the way people hold a radio cut that figure down quickly. Rules of thumb that hold up in the field:
- handheld to handheld outdoors, open or low-density suburban terrain: roughly 1–3 km;
- handheld to handheld indoors, across floors: often one floor or less;
- handheld to a repeater with a high antenna: kilometres across a site or a town;
- mobile to mobile outdoors: several kilometres, governed mainly by terrain.
The numbers differ by an order of magnitude, and what moves them is mainly antenna height and the path, not watts. Doubling transmit power adds 3 dB, which in a built-up area typically buys about 10–20% more range — worth having, but no substitute for antenna height and three concrete walls’ worth of path loss.
| What changes coverage | Why it matters | What you can change in the design |
|---|---|---|
| Antenna height | Dominant factor. Line-of-sight distance grows with the square root of height, so raising an antenna from 5 m to 20 m roughly doubles the horizon distance | Mast, roof position, which floor the antenna sits on, portable mast |
| Antenna gain and pattern (dBi) | 3 dB of gain doubles effective radiated power, but only in the direction the pattern points; high-gain antennas flatten the pattern and can leave a hole close under the mast | Antenna type, downtilt, orientation |
| Transmit power | 3 dB more power gives roughly 10–20% more range in built-up areas — real, but modest | Handheld, mobile or repeater class, within the power limit on the licence |
| Frequency band | VHF reaches further over open ground and through vegetation; UHF behaves better between and inside buildings | VHF or UHF version of radio, repeater and antenna |
| Terrain and clutter | One ridge, embankment or dense tree line can block a path completely | Site selection, an extra repeater or a link path |
| Building materials | Every boundary removes energy, and concrete and metal are far worse than plasterboard | Repeater position, distributed antennas |
| Feedline and connectors | Every 3 dB lost in cable is half the power that never reaches the antenna | Cable type, shorter runs, good connectors |
| Interference and noise floor | A raised noise floor increases the signal the wanted transmission must beat, shrinking usable range | Frequency selection, filtering, a proper survey |
Why does handheld-to-handheld range collapse inside buildings?
Three effects stack up indoors and explain most of the gap between the site plan and site reality.
Material loss. Every surface between two radios removes energy. Typical planning values: roughly 3–6 dB through a plasterboard partition, 8–15 dB per brick or block wall, 15–30 dB through reinforced concrete, and near-total blockage from metal cladding, metal-lined doors or a lift shaft.
Body loss. A handheld worn on a belt or held against the chest loses 3–6 dB or more compared with the same radio on a tripod. A walk test with the radio lying on a desk therefore looks far better than one with the radio being worn, so the test must be run the way the radio will be used.
Multipath fading. At UHF one wavelength is around two-thirds of a metre, and reflected copies of the signal arrive out of phase and cancel; 10–20 dB of variation over a fraction of a wavelength is normal. Coverage is statistical, not binary: a location is covered to a stated audio quality for a stated percentage of the time, not simply “covered”.
In practice:
- Floors are the hardest boundary. A reinforced slab is a severe barrier, and coverage below the transmitting antenna is usually worse than above it.
- Corridors, stairwells, lift lobbies, plant rooms and basements are the classic dead spots.
- The direction that fails first is normally talk-in — handheld to repeater — even when the repeater sounds clear, because the handheld is the weakest transmitter in the link and the one buried against a body.
- Design in at least 10–15 dB of margin over the minimum usable signal instead of designing to the threshold. Fading and a loaded building will find the weak spots.
What does a repeater actually do for coverage?
A repeater is a receiver and a transmitter working as one unit: it listens on one frequency while simultaneously transmitting on another. That is duplex operation, and the spacing between the two frequencies — the duplex separation, for example 5 MHz in UHF commercial plans or 600 kHz in VHF — has to match the pair your licence specifies. A duplexer, essentially a set of cavity filters, combines both onto one antenna and keeps transmit power out of the receiver.
Three things change once a repeater is in the link:
- Height. Its antennas sit on a mast or roof, so the system gains 20–30 m of height at one end instead of the 1.5 m of a handheld.
- Topology. Every radio talks to the repeater and the repeater repeats, so two handhelds that cannot hear each other directly can both hear the repeater. A mesh of unreliable links becomes a star with one strong hub.
- Power and receiver quality. A fixed repeater can feed more power into a higher-gain antenna than a handheld can, from a higher and quieter location.
Coverage through a repeater is asymmetric. The downlink is usually strong; the uplink is limited by the handheld’s power and antenna, and that is the direction that must be tested. Talk-in performance is ultimately set by the handheld the team carries, which is one reason a digital handheld such as the BF-TD516 is specified together with the repeater rather than after it.
Channel capacity is the repeater’s second job. An analog channel — 25 kHz wide, or 12.5 kHz narrowband — carries one conversation at a time. DMR and PDT Tier II change the arithmetic: a 12.5 kHz carrier is divided into two TDMA time slots, so two conversations run at once on one frequency. Two talk groups with different traffic patterns can share a carrier; two that both peak in the same busy hour cannot. Size channels on busy-hour traffic — the standard approach is an Erlang calculation against a target grade of service — not on the total number of radios on site.
Which repeater type fits your site?
The choice is driven less by brand than by where the antenna can go, how permanent the installation is, and how many sites must work as one system.
| Repeater type | Coverage profile | Typical use | Planning notes |
|---|---|---|---|
| Fixed rack repeater, such as the R2000 | Building, campus or town, set almost entirely by antenna height | Permanent single-site systems: hotels, factories, industrial parks, security teams | Rack space, mains power with backup, feedline with acceptable loss, lightning protection, duplexer tuned to your licensed split |
| High-power professional repeater, such as the HR1060 | Wider single-site or two-site coverage, heavier duty cycle | Larger industrial and municipal systems, utilities, transport depots | Confirm continuous-duty rating, cooling and backup power for expected traffic, not an average day |
| Portable or backpack repeater | Local and temporary; footprint limited by where you can carry it | Events, construction, tunnels and basements, search and rescue, cover during repairs | Battery capacity against duty cycle, field charging, enclosure protection, a duplex split that matches your licence |
| IP-linked multi-site network | Several sites behaving as one system | Campuses with many buildings, hotel groups, regional fleets | Local channels first, then backhaul; plan latency, time synchronisation and fallback |
| Trunking system, such as the BF-9300/9500 | City or region scale | Large fleets and several user departments with high channel demand | Channel pooling, dispatch integration, licensing at every site before rollout |
The first question is boring and decisive: where does the antenna go? A modest repeater on a good roof beats a powerful one in a plant room, and cable loss, mast cost, grounding and maintenance access all follow from that decision.
When does a portable or backpack repeater make sense?
A portable repeater exists for situations where permanent infrastructure is unavailable, unwanted or not built yet:
- temporary events, construction sites and seasonal operations;
- tunnels, basements and shielded buildings where a fixed installation is impractical;
- search, rescue and emergency work where the repeater must be carried to high ground;
- short-term coverage while a permanent system is installed or repaired.
The engineering does not change because the box is small, and two factors decide whether a portable repeater works.
Where the antenna goes. A portable repeater left on the ground floor of a concrete building is a repeater with a bad antenna site. Carrying it up two or three floors, onto a roof, or to the highest ground within reach is usually worth more than any power difference between models.
The duplex split. The unit has to work on the frequency pair your licence allocates. Compact repeaters — the BF-TR8500 and BR680 class, for example — are built for defined splits, so confirm your licensed pair before you order, not after the units arrive.
Then the practical questions: battery capacity against the duty cycle you expect, field charging, whether the case is protected for outdoor use (an IP rating to IEC 60529 — IP54 covers dust ingress and splashing water, IP66 dust-tightness and powerful water jets), how the antenna is carried, and who switches the unit on. Check the licensing position too: a portable repeater still occupies two frequencies and still needs authorisation.
How do you link sites that are kilometres apart?
Above a certain distance, or between sites with no usable radio path, you stop covering everything from one antenna and start linking systems:
- RF link. A dedicated UHF or VHF path between two repeaters, useful where terrain allows a clear shot and no network is available. It needs its own licensed frequency, antenna and mast.
- IP linking and radio over IP. Each site keeps its local repeaters and a gateway carries traffic across a WAN, VPN or leased line — the usual choice for campuses, hotel groups and multi-building sites. Watch latency and jitter on the link (tens of milliseconds are unremarkable, several hundred are audible), encryption across the network as well as over the air, talk-group mapping, and time synchronisation.
- Simulcast or voted multi-site. A voted system lets each site keep its own channel and selects the strongest receive path; simulcast transmits one frequency from several sites at once and needs accurate timing references. Simulcast gives the smoothest experience and carries the heaviest engineering and licensing burden.
- Trunking. For large fleets and several departments, a trunking system pools channels and allocates them on demand — more spectrum-efficient than fixed conventional channels, but a larger project in every respect.
Whichever option you choose, write down the failure mode: if the link to head office goes down, each site should keep working locally rather than going silent.
How should you plan the site survey and acceptance test?
Coverage claims are only useful when measured against something agreed.
1. Define the requirement. Which areas must work, indoors and outdoors; who must hear whom; which direction matters more; what counts as covered. A widely used convention from the TIA TSB-88 methodology is the DAQ score (delivered audio quality), where about DAQ 3.4 — speech understandable with only slight effort and repetition rarely required — is a common acceptance threshold, alongside a coverage target such as 95% of locations.
2. Do the desk study. Site plans, floor plans, terrain, building materials, the height available for an antenna, cable routes and where the rack can go. Predict coverage before spending money on hardware, and identify the buildings that will need help.
3. Measure the noise floor. Before choosing frequencies, scan the channels you intend to use. An existing carrier or a strong intermodulation product raises the noise floor and shrinks real range; finding it at survey stage is far cheaper than finding it after installation.
4. Walk or drive the test. Use a calibrated receiver, or radios in a service mode that reports signal level and bit error rate, at realistic body-worn height, on the final frequencies, with fully charged batteries. Record signal level and audio quality on a defined grid of points, log where each point is so the result can be reproduced, and test talk-out and talk-in separately.
5. Put the result in writing. A coverage map, the scored point list, the dead zones, the interference findings and the proposed fixes. This document, not a marketing leaflet, is the basis of the acceptance test.
6. Accept the finished system. Test the final programming and final antenna positions, in front of the people who will sign it off, against the thresholds agreed at step one, with a re-test clause for any location improved later. Keep the results as your baseline: any later change to antennas, power, programming or the building makes the earlier record obsolete.
Coverage is a design decision and a measurement, not a specification on a box. Decide early whether the site needs a repeater, where the antenna can go, and how the result will be verified — those three answers drive the rest of the bill of materials, the installation cost and the day-one user experience.