Where to Put the Repeater

Height, power, weatherproofing and solar - turning a dev board into infrastructure

A repeater is only as good as its view of the world. This lesson is about the unglamorous physical stuff that decides whether your mesh covers a street or a county.

Height is everything

At 868MHz you are working with something close to line of sight, softened a little by diffraction. The single biggest thing you control is how high the antenna is.

The rough rule of thumb for radio horizon:

distance in km ≈ 4.12 × (√height_1 + √height_2), with heights in metres

Real world results will be a lot less than this - buildings, trees and terrain all take their cut - but the shape of the table is right. Doubling your height buys you far more than doubling your transmit power ever will.

Ranking the locations

Best: An external mast or pole above the roofline, antenna vertical, clear of the building.

Very good: A loft. Warm, dry, and you only lose a few dB through tiles - unless you have foil-backed insulation or a slate roof, which are brutal at 868MHz.

Good: An upstairs window, antenna against the glass. Cheap, easy, surprisingly effective in a direction.

Poor: A shelf downstairs, indoors, surrounded by other electronics.

Worst: Inside a metal enclosure, next to a router, behind a fridge. You have built a very expensive Faraday cage.

Powering it

A repeater wants to run 24/7, and it is a low-power thing - a Heltec V3 idles at a few tens of milliamps and peaks around 120mA while transmitting.

Mains USB - a plain 5V phone charger is ideal. Cheapest and most reliable.

USB power bank - fine for a temporary or portable repeater, but be aware that many power banks shut down when the load drops too low, which is exactly what a mostly-idle repeater does. Look for one with a “low current” or “always on” mode.

LiPo battery - a 3.7V cell on the SH1.25 connector, charged over USB. A 2000mAh cell will typically run a Heltec V3 repeater for a day or two.

Solar - a 5-10W panel, a charge controller and a 18650 pack will keep a repeater alive indefinitely in a UK summer and will struggle in a UK December. If you are serious about solar, look at the Heltec T114 instead - it is nRF52 based and uses a fraction of the power of the ESP32-S3.

Weatherproofing

If it goes outside, it needs to survive outside.

• A cheap IP65 junction box from a DIY shop is perfect. Drill one hole for an SMA bulkhead connector and one for the cable gland
• Put the board inside the box and the antenna outside, connected through the bulkhead. Do not try to transmit from inside a sealed plastic box with the antenna inside it - it mostly works, and mostly is not good enough
• Add a silica gel sachet. Condensation is what actually kills outdoor electronics, not rain
• Face the gland downwards so water runs away from it, and put a drip loop in the cable
• UV matters. Cheap plastic boxes go brittle after a couple of summers. Black or grey UV-stable boxes last

Settings for a fixed repeater

Once it is in place, a few settings are worth revisiting from the console:

set tx 20
set advert.interval 60
set flood.advert.interval 12
set flood.max 3

Full transmit power makes sense for a mains powered fixed node. Modest advert intervals keep you a good neighbour.

If your repeater is battery powered, consider:

powersaving on
set tx 17

Being a good mesh citizen

• Do not run a repeater on a moving vehicle. Paths break constantly and force the network back to flooding
• Do not set aggressive advert intervals. Every flood advert is rebroadcast by every repeater in the region
• Do register your node with your local community so people know it exists and who owns it
• Do set owner.info so someone can find you if your node misbehaves:

set owner.info Kev - kevsrobots.com

Try it Yourself

1. Walk your client node away from the repeater until messages start failing. Note the distance. Now move the repeater one floor up and repeat.
2. Run stats-radio before and after moving the repeater. How much did the noise floor change?
3. Challenge: work out the radio horizon for the highest point you can reach, using the formula above.

Common Issues

Problem: My repeater keeps dropping off overnight.

Solution: Suspect the power supply, particularly if it is a USB power bank.

Why: Many power banks auto-shutdown below a current threshold, and an idle repeater draws very little.

Problem: Range got worse after I put it in a box outside.

Solution: Get the antenna outside the box on a bulkhead connector, and check the coax is short.

Why: Enclosures, especially damp ones, detune and absorb. Thin coax at 868MHz is lossy - roughly 1dB per metre for RG174.

Problem: The loft install performs badly.

Solution: Check for foil-backed insulation.

Why: Foil-backed insulation board is effectively an RF mirror and will kill 868MHz almost completely.

< Previous Next >

You can use the arrows  ← → on your keyboard to navigate between lessons.