DMR Radio Systems
A dual-frequency repeater receives and transmits at the same time on a frequency pair, which needs a duplexer. A single-frequency repeater records a transmission and repeats it afterwards on the same frequency, halving throughput but removing the duplexer and the second licence.
Most repeaters work on a frequency pair. A minority work on one frequency. The second kind is offered when spectrum is scarce or budget is tight, and it is not a smaller version of the first — it behaves differently in a way users notice within a day.
A dual-frequency repeater receives on one frequency and simultaneously transmits on another. Handsets transmit on the input frequency and listen on the output. Because receive and transmit happen at once, the repeat is instantaneous: from a user's point of view the repeater is invisible and the system simply has better range.
Simultaneous transmit and receive on one antenna requires a duplexer — a set of tuned cavity filters that keeps the transmitter's energy out of the receiver. The duplexer is the reason a repeater cabinet is heavy, and it is the component that most often causes mysterious coverage complaints when it is wrong.
A single-frequency repeater cannot receive and transmit at the same time on the same frequency, so it does not try. It listens, stores what it hears, and repeats it once the transmission ends.
That removes two costs at once. No duplexer, so the equipment is smaller, lighter and cheaper. One frequency instead of a coordinated pair, so licensing is simpler and often faster.
It introduces one cost, and it is not small: delay. Anyone outside direct range hears the message only after the speaker has finished, and the repeater is occupied while it repeats. The channel effectively carries half the traffic it would otherwise, and conversation stops being conversational — you cannot interrupt, correct or acknowledge in the moment.
Tolerable:
Not tolerable:
DMR's two timeslots create a middle option. A DMR single-frequency repeater can receive on one timeslot and transmit on the other, alternating fast enough that the delay is not perceptible — no duplexer, one frequency, and no store-and-forward pause.
The price is the second slot. DMR's normal advantage is two simultaneous conversations through one repeater on one pair; in single-frequency mode that becomes one. You keep digital voice quality, error correction, encryption, text and location, and you give up the capacity doubling.
That is often a good trade for a small operation with a licensing constraint, and a poor one for an operation that chose DMR precisely for capacity.
If you buy a dual-frequency repeater, the duplexer deserves attention it rarely gets.
It is tuned to a specific frequency pair. Reusing a repeater at a new site with a new assignment, or changing frequencies after a licence revision, requires retuning. A mistuned duplexer produces desensitisation: the receiver's noise floor rises and weak signals are lost. Coverage measured near the site looks fine; coverage at the edge — which is the coverage you bought the repeater for — is gone.
The related failure is feeder loss. Cable attenuation rises with frequency and with length, and a long run of thin coax to a rooftop antenna can consume most of the transmitter's output. A repeater specified at 25 watts delivering a fraction of that to the antenna is a common and entirely avoidable finding.
Ask for measured figures at commissioning: transmit power at the antenna connector, receive sensitivity, duplexer tuning report, and VSWR. Without them, a coverage complaint six months later has no baseline to be diagnosed against.
Single frequency if spectrum is genuinely constrained, traffic is light, delay is acceptable, and the deployment is small, temporary or supplementary. In DMR, if you can accept one conversation instead of two.
Dual frequency in every other case, which is most of them. It is the standard because instantaneous repeat and full capacity are what an operational radio system is for.
And a scaling note worth deciding early: a system built on a single frequency has a lower ceiling. Growing past it means a new licence, a new duplexer, a new repeater and reprogramming every handset — a second project rather than an expansion. If growth is plausible, plan the architecture for it now.
Because it needs one frequency rather than a coordinated pair. In Türkiye the BTK assigns frequencies per organisation and site, and a pair takes more coordination than a single channel. Where spectrum is congested, the single frequency may also simply be available sooner.
Roughly the length of the transmission itself, because the repeater has to finish receiving before it can start sending. A five-second call is heard five seconds late by anyone outside direct range. For routine coordination that is tolerable; for an emergency call or a crane movement it is not.
A DMR single-frequency repeater uses one timeslot to receive and the other to transmit, so it works without a duplexer but gives up the second conversation. You keep the digital advantages and lose the capacity doubling that is usually the reason for choosing DMR.
The receiver goes deaf. The transmitter's own energy leaks into the receive path and raises the noise floor, so only strong signals get through. Coverage looks acceptable near the site and collapses at range — which is easily misread as an antenna or power problem and leads to the wrong repair.
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