Symptom
A site alternates between online and offline over minutes or hours; retries climb before it drops; the received signal strength wanders well below the commissioning value; the drops track rain, fog, morning hours, summer, or a time of day; the site is fine after a visit and degrades again.
The short answer
Intermittent Radio Path
An intermittent radio path is a path whose margin has shrunk until ordinary variation pushes it below the threshold: rain, fog, and temperature inversions fade the signal; leaves in summer absorb it; a new building or a grown tree line blocks part of the Fresnel zone; an antenna turned by wind or loosened on its mount points away from the far end; water working into the coax or a connector adds loss that grows with time; interference on the channel raises the noise floor at certain hours; and a site power supply that sags drops the radio output. The diagnosis is a trend of received signal strength and retry counts at both ends against time of day, weather, and season, compared with the commissioning record, and the fix follows the pattern: restore margin with height, gain, and low-loss coax, replace the wet coax or the corroded connector, realign the antenna, clear or route around the obstruction, or filter the interference.
Key points
- Trend the signal and the retries against time, weather, and season; the pattern names the cause.
- A path that was marginal at commissioning fails first; check the margin against the record.
- Water in coax and corroded connectors add loss slowly; a return loss test finds them.
- Foliage, new construction, and antenna movement change the path; look along it.
- Interference that comes and goes shows as a raised noise floor at those hours.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Insufficient fade margin | The received signal at commissioning against the receiver sensitivity: a margin under about 20 decibels fails in rain and inversions. Compute the link budget. |
| Seasonal foliage | Drops in summer and recovers in winter; trees in the path. Raise the antennas above the tree line or reroute through a repeater. |
| New obstruction | A step change in signal after construction, a new tank, or a tree that grew into the Fresnel zone. Look along the path from each antenna. |
| Antenna moved or loose | Signal changes with wind; the mount is loose or the antenna has turned. Realign to the far end and secure. |
| Water in the coax or a connector | Signal degrading slowly over months, worse after rain; a return loss or standing wave ratio test shows high reflection. Replace the coax and weatherproof the connectors. |
| Corroded connector or arrestor | Intermittent with vibration or temperature; visible corrosion. Clean or replace and weatherproof. |
| Intermittent interference | The noise floor at the receiver rises at certain hours; retries climb without a signal drop. Listen with the transmitter off; find the source. |
| Site power supply marginal | The radio output drops when the supply sags, on generator or with a weak battery. Measure the radio supply during transmit. |
| Temperature inversion or ducting | Drops in the early morning in certain seasons on long paths over flat terrain or water. More margin or a different path. |
| Repeater or master antenna problem | Several sites intermittent at once; the shared end. Check the master or repeater antenna system. |
Trend it
Most telemetry radios report received signal strength and retry or error counts, and the SCADA can historize them per site like any tag. A week of that trend against the weather and the clock is the diagnosis: a signal that sags at dawn and recovers by mid-morning is an inversion; one that is lower every day in July than in January is foliage; one that stepped down on a date is an obstruction or a moved antenna; one that drifts down over months is water in the coax; retries that climb at 9 a.m. and 5 p.m. with the signal steady are interference from something that keeps business hours. The commissioning record gives the baseline; the trend gives the departure.
| Pattern | Likely cause | Fix |
|---|---|---|
| Sags in rain, fog, or at dawn | Fade margin too small | Height, gain, low-loss coax, a repeater |
| Worse every summer | Foliage | Antennas above the trees; a different path |
| Stepped down on a date | New obstruction or antenna moved | Look along the path; realign |
| Drifts down over months | Water in the coax or a connector | Return loss test; replace and weatherproof |
| Retries up, signal steady, at certain hours | Interference | Noise floor measurement; filter or change channel |
| Drops when the site is on generator or the battery is low | Power supply | Fix the supply; a UPS for the radio |
| Several sites at once | The master or the repeater | The shared antenna system |
Checking the antenna system
- 1
Inspect
Connectors for water and corrosion; the weatherproofing; the arrestor; the coax for kinks, cuts, and crushed sections; the antenna for damage and alignment; the mount for looseness.
- 2
Test the coax and antenna
A standing wave ratio or return loss measurement at the radio end, compared with the commissioning value; a distance-to-fault sweep if the analyzer supports it finds where the water is.
- 3
Measure the signal at both ends
The site received signal from the master, and the master received signal from the site, compared with the record. A drop at one end only is that end antenna system or transmitter.
- 4
Realign
Directional antennas peaked on the far end with a signal reading, then secured.
- 5
Replace
Coax that has taken water is replaced, not dried; connectors are replaced and weatherproofed properly.
Interference
A path that loses data while the signal stays strong is being interfered with. The radio noise floor, measured with the far end silent, rises when the interferer is on; a spectrum analyzer or the radio diagnostic shows the frequency. Sources include another system on the same frequency, a nearby transmitter overloading the receiver, a drive or a lighting system radiating broadband noise, and a radio somewhere with a stuck transmitter. The fixes are a bandpass or cavity filter, a directional antenna that nulls the source, a channel change or a different hop set, and, for a licensed frequency, the coordination process.
Frequently asked questions
- The path study said the link was fine. Why is it intermittent?
- Path studies model a clear path with assumed antenna heights and coax; the installed antenna may be lower, the coax longer and lossier, the trees taller, and the margin smaller than modeled. Measure the actual received signal at both ends and compare with the study; the difference is where the margin went.
- Can I just increase the transmit power?
- Within the license or the unlicensed limit, a few decibels, which rarely closes the gap and can worsen interference to others. Antenna gain and height, and lower coax loss, add margin at both ends and are the usual answer.
- How much fade margin is enough?
- Twenty decibels is a common minimum for telemetry paths, thirty for paths over water or flat terrain prone to ducting, and more for long paths. The number is set in the path study against the availability the utility wants.
- The site is fine every time I visit.
- It is intermittent, and the visit is not at the bad time. The trend is the tool: leave the signal and retry counts historized and look at them against the times it fails. Then visit at that time, or with the evidence of what changed.
Related topics
- 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.
- High Retry CountPolls that succeed on the second or third try: a weak or noisy path, interference, collisions from a second master, a timeout shorter than the radio turnaround, a data rate the path cannot carry, or a slow controller. Reading retry statistics per site.
- Interference on ChannelData lost on a strong-signal path: another system on the frequency, a nearby transmitter overloading the receiver, intermodulation on a shared tower, drive noise, or a stuck transmitter. How to measure the noise floor, find the source, and cure it.
- Remote Site Stops CommunicatingOne radio site silent while its neighbors answer: site power, the radio, antenna and coax after a storm, a reset configuration, the controller behind it, or a changed path. What the master diagnostics say before anyone drives, and the order of checks on 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.
- TrendingUsing historian trends to understand a process and diagnose a problem: pens that belong together, scaling and time ranges, raw versus averaged data and what compression hides, overlays and cursors, event markers, and the patterns that point at each cause.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.