How many repeaters does a site actually need

It is the question every radio project starts with and the one that cannot be answered on the phone. The reason is simple: repeater count is determined by what is between your radios, not by how many of them you have.

The misconception worth clearing first

More users do not require more repeaters. More users require more channels, which is a different problem with a different solution. A single repeater can serve a large fleet if the coverage works and the traffic fits; forty radios in one warehouse may need none at all, because they can talk directly to each other.

What requires a repeater is distance and obstruction. Two radios that cannot hear each other need something in between that can hear both.

What actually drives the count

Coverage area and shape

A compact site is easier than a long thin one of the same area. A corridor, a pipeline, a road or a perimeter fence needs coverage along its length, and length is what costs repeaters.

Terrain

Hills, escarpments and valley floors block signal. In much of Kenya this is the dominant factor, and it is why coverage that works on a map fails on the ground. A single well-sited repeater on high ground can replace three badly sited ones.

Vegetation

Dense tree cover attenuates signal significantly, and seasonally. A system commissioned in the dry season can degrade noticeably once the canopy fills in, which is a genuine and frequently missed cause of complaints months after handover.

Building structure

Reinforced concrete, metal cladding, lift shafts, basements and multi-storey layouts all attenuate. Indoor coverage in a large building can need in-building infrastructure regardless of what the site coverage looks like outside.

Antenna height and siting

Frequently more consequential than the repeater itself. Raising an antenna is often the cheapest coverage improvement available, and it is regularly overlooked in favour of adding equipment.

What it costs to get wrong, in both directions

Under-building leaves black spots. The system appears to work during commissioning, because commissioning happens in the accessible parts of the site, and fails in the places nobody walks, which are the places where somebody alone needs to call for help.

Over-building costs money three times: capital on equipment, installation on sites and masts, and recurring licence fees on frequencies you did not need. In Kenya each repeater generally means a frequency pair licensed from the Communications Authority, so redundant repeaters carry a recurring cost, not just a one-off one.

How it is actually established

By survey. Somebody walks or drives the site with test equipment, transmits from candidate locations and measures what is receivable where. Predictive modelling is useful for narrowing candidates, but terrain data does not capture a metal-clad warehouse or a stand of trees, and those are usually what decides it.

The survey also answers questions that determine cost more than repeater count does: whether existing masts can be used, whether power and backhaul exist at candidate sites, and whether site access is practical for maintenance.

What a survey report should give you

If you commission a survey, these are the things that make it worth what you paid.

• Coverage maps showing measured signal, not only predicted, with the measurement points marked
• Named candidate sites with the reasoning for and against each, including the rejected ones
• Antenna heights and types assumed, because changing those changes everything downstream
• Identified black spots and what it would cost to cover them, so you can decide whether to
• Power, backhaul and access notes for each site, which frequently decide feasibility
• The frequency requirement, so licensing can start rather than waiting for delivery

A survey that produces only a repeater count and a price has not told you anything you can check.

Designing for the site you will have

Sites grow. A compound adds a wing, a conservancy extends a patrol sector, a plant adds a yard. Repeater siting that was optimal for the original footprint is frequently poor for the extended one, and the cost of moving infrastructure exceeds the cost of having sited it with room to grow.

Ask the survey to consider the expansion you already know about. It rarely changes the recommendation, and when it does, it changes it cheaply.

When the answer is none

Small compact sites with clear line of sight frequently need no repeater at all. Radios operating direct (simplex) are cheaper, simpler and have no infrastructure to fail. Any supplier who proposes a repeater without establishing whether simplex would work is selling equipment rather than solving a problem.

Questions to ask anyone proposing a count

If a supplier gives you a repeater number, these four questions establish whether it is engineering or a guess.

• What is that based on: a measured survey, a predictive model, or experience of similar sites?
• Which parts of the site does this design not cover, and what would covering them cost?
• What antenna height does the design assume, and what happens to coverage if we cannot achieve it?
• How many frequency pairs does this require, and what is the recurring licence cost?

The last one catches most inflated proposals, because it converts an extra repeater from a one-off line item into a recurring cost the customer will see every year.

When the answer is not a repeater at all

If the operation is spread across a city or region rather than concentrated on a site, repeater coverage becomes disproportionate; cost rises with area while benefit does not. Broadband push-to-talk over a mobile network is usually better economics at that point, and it removes both the infrastructure and the recurring spectrum licence.

The right question is never 'how many repeaters'. It is 'what does coverage have to achieve, and what is the cheapest way to achieve it'.

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