Symptom
The modem signal reading is near the bottom of its range and fluctuates; data rates are low; sessions drop; the site is worse in rain or in summer; a phone at the site shows one bar or none; the modem antenna is inside the panel.
The short answer
Weak Cellular Signal
A cellular modem reports its signal as a received power and a quality figure, and a site at the edge of coverage shows a received power near the threshold and a quality that swings, which produces drops, retries, and low data rates. The usual causes are an antenna inside a metal enclosure or below grade in a vault, a stub antenna where an external one was needed, a modem on a band with poor coverage at the site, an antenna or coax damaged or lossy, a tower that the carrier decommissioned or re-oriented, or a site that was always marginal. The fix is almost always the antenna: an external antenna on a mast above the enclosure, a directional antenna aimed at the serving tower, low-loss coax kept short, the band the carrier serves well at the site, and, where none of that suffices, another carrier, a repeater, or a different medium for the site.
Key points
- Received power and quality together describe the signal; a strong power with poor quality is interference or a busy cell.
- An antenna inside a metal enclosure is inside a shield; the first fix is to get it outside and up.
- A directional antenna aimed at the serving tower adds gain and rejects the rest.
- Coax loss at cellular frequencies is large; short and low-loss, or the modem near the antenna.
- Bands differ in reach; the modem may be on the wrong one for the site.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Antenna inside a metal enclosure or below grade | The antenna location; the reading with the door open versus closed. An external antenna through a bulkhead on the enclosure top or a mast. |
| Stub antenna where an external antenna was needed | A short antenna on the modem in a marginal area. An external antenna with gain. |
| Modem on a band with poor coverage at the site | The serving band on the status page; the reading with the band forced to alternatives. Prefer the band that serves the site; higher frequency bands reach less. |
| Damaged antenna, connector, or lossy coax | A step drop in signal; water in the connector; a long run of small coax. Inspect, test, and replace with low-loss coax kept short. |
| Tower decommissioned, re-oriented, or carrier network change | Signal dropped on a date; the serving cell changed. The carrier; a directional antenna toward the new serving tower; another carrier. |
| Site always marginal | The commissioning reading was poor and the site has been intermittent since. A mast, a directional antenna, or another medium. |
| Interference or a congested cell | Good received power with poor quality; worse at busy hours. A directional antenna; another band or carrier. |
| Obstruction: a new building, foliage, terrain | Seasonal or step changes; the path to the tower. Height; a directional antenna; relocation of the antenna. |
| Wrong antenna for the bands or a multi-element antenna with one element open | An antenna rated for other bands, or one of two elements disconnected on a modem that uses both. Match the antenna to the modem and connect every element. |
Reading the signal
| Reading | What it is | Rough interpretation |
|---|---|---|
| Received power | The strength of the reference signal from the serving cell, in dBm, a negative number | Better than about minus 90 is good; minus 90 to minus 105 is usable; below about minus 110 is marginal and drops |
| Received quality | How clean the reference signal is relative to the noise and interference, in dB | Near minus 10 is good; below about minus 15 is poor and rates fall |
| Signal to noise | The ratio of the wanted signal to everything else, in dB | Above 10 is good; near zero the modem struggles regardless of power |
| Serving cell and band | Which tower and which frequency band the modem is using | A band change or a cell change can explain a change in readings |
The two readings together tell the story. Low power with good quality is a distant tower: gain and height help. Good power with poor quality is interference or a congested cell: a directional antenna or another band helps. Both poor is a site at the edge with everything against it. The readings are recorded at commissioning and trended where the modem can report them to the SCADA, so that a change is seen as a change.
Fixes in order
- 1
Get the antenna outside and up
An external antenna through a bulkhead connector on the enclosure top, or on a short mast, above the panel and clear of metal. This fixes most marginal sites by itself.
- 2
Choose the band
Read the signal on each band the carrier serves at the site; prefer the one with the best combination of power and quality; set a preference rather than a hard lock.
- 3
Go directional
A directional antenna aimed at the serving tower, found by rotating for the best reading or from the carrier tower data, adds gain and rejects the other cells.
- 4
Fix the coax
Low-loss coax, as short as the installation allows; the modem moved near the antenna with the Ethernet run instead where the coax would be long. Connectors weatherproofed.
- 5
Multi-element antennas connected
Modems with two antenna ports need both connected to a matched antenna; one element open halves the performance.
- 6
Change the carrier or the medium
A carrier with a closer tower; a dual-carrier modem; a radio or a repeater for a site that no cellular fix reaches.
Vaults and enclosures
A modem in a metal control panel with its stub antenna inside is inside a shield; the reading with the door open is what it could have, and the door will be closed. A modem in a below-grade vault has earth and a lid between it and the tower. In both cases the antenna goes outside: a bulkhead-mounted antenna on the top of the enclosure, a small mast beside the vault, or a pole-mounted antenna with the coax through a sealed entry. The antenna outside is also a lightning path, and it gets a surge arrestor at the entry bonded to the site ground.
Frequently asked questions
- My phone works at the site. Why does the modem not?
- The phone is outside the panel with a better antenna system and more bands; the modem is inside a metal box with a stub. Put the modem antenna where the phone was and the modem will do as well or better.
- How much gain do I need?
- Enough to bring the received power into the usable range with margin, which a directional antenna of moderate gain on a short low-loss coax provides in most marginal sites. More gain narrows the beam and makes aiming critical; the modem reading with the antenna in place is the test.
- The signal was fine for years and dropped last month.
- The carrier changed something: a tower re-oriented, a band moved, a cell decommissioned; or the antenna system failed: water in the connector, a cracked coax, a corroded element. Check the serving cell and band against the record, then the antenna system.
- Is a higher band worse?
- Higher frequency bands carry more data and reach less, and the modem may prefer them for speed when the lower band would be more reliable at a distant site. Preferring the lower band at a marginal site trades speed the telemetry does not need for reach it does.
Related topics
- Frequent ReconnectsA cellular site that connects, drops, and reconnects: a weak signal at the cell edge, tower handoffs, idle session teardown, a NAT timeout that needs a keepalive, a firmware fault, a sagging supply, or an antenna problem. Settings that keep sessions up.
- Cellular Modem Will Not RegisterA modem that never attaches to the network: a module not activated, provisioned, or suspended; unsupported bands or a network shutdown; no antenna or coverage; a wrong access point name; or a reset modem. Reading the registration state and asking the carrier.
- Poor Signal MarginA path with too little signal above receiver threshold to survive rain, leaves, and inversions: the link budget from transmitter to receiver, the Fresnel zone, and why the margin is smaller than the study said. How to compute, measure, and add to it.
- Segmenting a Remote SiteA lift station, a well, or a tank site as its own zone: the site firewall and what it permits, the modem as transport with no public address, cameras and extras off the control segment, engineering access through the plant, and local control without the link.
- Surge Protection for Signal CircuitsProtecting 4-20 mA loops, discrete inputs, network cables, and radio feeds from lightning and switching surges: where surges enter, the device types and how they clamp, the grounding that makes them work, the resistance and capacitance they add, and where to put them at a remote site.
- SCADA CommunicationsHow SCADA servers talk to controllers and the field: polling and report by exception, the protocols and what each is for, media from plant Ethernet to radio and cellular, poll rate arithmetic, timeouts and quality, time sync, store and forward, and security.
Direct contact
Have a controls question?
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