RF Propagation Planning Tools

Several free tools can help you model coverage and plan repeater placement before deploying hardware. Using these tools can save wasted trips and help you choose between candidate sites.

HeyWhatsThat (heywhatsthat.com)

The fastest tool for estimating radio horizon from a specific point.

• Enter a location (address, coordinates, or click on map)
• Set the antenna height
• Get a visualization of the radio horizon: which areas have line-of-sight from that point

How to use for repeater site selection:

1. Go to heywhatsthat.com
2. Click on your candidate repeater site on the map
3. Set height to your intended antenna mounting height
4. Click "Submit" and examine the color-coded visibility map
5. Compare multiple candidate sites by opening each in a new tab

Limitations: HeyWhatsThat uses SRTM elevation data, which is a Digital Surface Model, not bare earth - it partially captures building rooftops and tall vegetation (HeyWhatsThat's own documentation notes you may see "shadowy bumps and gaps" in cities because the elevation data includes rooftops). The real caveat is that SRTM is too coarse (~30 m) to model individual buildings or trees reliably, so treat the visibility output as terrain-dominated: actual coverage will still be lower than predicted in areas with tall buildings or dense forest that the coarse data cannot resolve.

Radio Mobile Online (radiomobileonline.pe1mew.nl)

More sophisticated link analysis tool with full path profile and link budget integration. The online version is hosted at radiomobileonline.pe1mew.nl.

• Enter transmitter and receiver coordinates, heights, antenna gain, TX power, and frequency
• Generates a path profile showing terrain elevation along the path
• Calculates predicted received signal level
• Shows Fresnel zone clearance along the path

Radio Mobile is best for detailed analysis of specific point-to-point paths, not area coverage visualization.

CloudRF (cloudrf.com)

Professional-grade coverage prediction with a free tier. Features:

• SRTM + LIDAR terrain data (more accurate than basic tools)
• Can include clutter data (buildings, vegetation) for urban environments
• Overlay predicted coverage on Google Maps or OpenStreetMap
• Free tier: limited calculations per month; paid plans for heavy use

Best for: precise coverage maps for presentations, permitting, or professional deployments. Overkill for casual site selection.

Splat! (free, offline)

Open-source RF propagation tool that runs locally on Linux/Mac. Uses SRTM terrain data. This is an advanced tool - it is not a copy-paste one-liner. Before you can run it you must (1) create .qth site-location files describing each transmitter/receiver (location, height, name), (2) download SRTM elevation data and convert it into SPLAT's own .sdf format using the srtm2sdf utility (SPLAT! does not read raw SRTM directly), and (3) set the operating frequency and other path parameters in an .lrp file - SPLAT! has no -f frequency command-line option (in SPLAT! the -f flag relates to Fresnel-zone clearance, not center frequency).

# Install on Debian/Ubuntu (package availability varies by release;
# you may need to build from source per the SPLAT! project site)
sudo apt install splat

# Download SRTM data for your region from dwtkns.com/srtm or usgs.gov,
# then convert it to SPLAT's .sdf format:
srtm2sdf N40W106.hgt

# Create your tx.qth / rx.qth site files and a tx.lrp parameter file
# (frequency, ERP, etc. go in the .lrp file, NOT on the command line).
# Then generate a point-to-point path analysis:
splat -t tx.qth -r rx.qth -d /path/to/sdf/data

Splat! is overkill for most community mesh deployments but valuable when building professional-grade coverage documentation or integrating with GIS workflows. Note that the 0.5 W ERP figures used in such examples are not buildable on stock LoRa radios - the SX1262 chip maxes out at +22 dBm and an external PA would be required.

Practical planning workflow

For most community mesh network planning, this workflow is sufficient:

1. Identify candidate sites
   Use topographic maps (USGS topo viewer, CalTopo) to identify hilltops, ridgelines, water towers, and tall buildings in your coverage area.
2. Quick horizon check
   For each candidate site, run HeyWhatsThat at the proposed antenna height. Immediately discard sites with poor visibility to your target coverage area.
3. Link budget for remaining candidates
   For the 2 - 3 best candidates, calculate link budgets to your target coverage area edges (farthest points). Compare margin values.
4. On-site test before permanent install
   Set up a temporary antenna and node at the candidate site. Walk/drive your coverage area while monitoring RSSI/SNR. Real-world testing always beats prediction tools.
5. Document the result
   Record the actual coverage performance after deployment. This data helps plan future expansion nodes.

Interpreting RSSI and SNR from field tests

The bands below are approximate field heuristics, not hard cutoffs - the actual decode limit depends on the modem preset (spreading factor). LoRa decodes below the noise floor: the relevant SNR threshold is the per-spreading-factor demodulator floor from the SX1262 datasheet - roughly −7.5 dB at SF7, −12.5 dB at SF9, −17.5 dB at SF11 (LongFast), and −20 dB at SF12 (Long Slow). A link that looks "weak" at SF7 may still decode reliably at SF11/SF12.

RSSI	SNR	Link quality assessment
>−90 dBm	>+10 dB	Excellent - reliable at all data rates
−90 to −105 dBm	+5 to +10 dB	Good - reliable for all normal use
−105 to −115 dBm	0 to +5 dB	Marginal - may see occasional packet loss
−115 to −120 dBm	−5 to 0 dB	Weak - intermittent; not suitable for infrastructure
<−120 dBm	<−5 dB	Very weak - expect frequent failure (though SF11/SF12 can still decode down to roughly −17.5/−20 dB SNR)

Key insight: In the noise-limited regime, what determines whether a packet decodes is whether the SNR exceeds LoRa's per-spreading-factor decode floor (e.g. −17.5 dB at SF11) - RSSI alone does not. Both metrics matter, though: when the signal is well above the noise floor, absolute received power (RSSI) becomes the limiting factor. A signal at −125 dBm with +5 dB SNR is more reliable than one at −100 dBm with −5 dB SNR (which is being swamped by noise). When diagnosing a noise-limited marginal link, prioritize improving SNR; when the link is power-limited, work on RSSI.