Symptom
The received signal at one or both ends is within twenty decibels of the receiver sensitivity; the path drops in rain, fog, or summer; retries are high on a path that the study said was adequate; a new site cannot be made reliable at any data rate.
The short answer
Poor Signal Margin
Signal margin, or fade margin, is the difference between the received signal level and the level at which the receiver stops decoding, and it is what a path spends when rain, foliage, and atmospheric conditions reduce the signal. It is computed from the link budget: transmitter power plus the antenna gains at both ends, minus the coax and connector losses at both ends, minus the free-space path loss for the distance and frequency, minus any obstruction loss, compared with the receiver sensitivity; and it is measured by reading the received signal strength at each end. A margin below about twenty decibels is a path that will fail in bad weather, and the causes of a margin lower than designed are antennas mounted lower than the study assumed, coax longer or lossier than assumed, water or a bad connector in the line, an antenna pointed off the far end, terrain or trees in the Fresnel zone, or a receiver desensitized by a nearby transmitter. Margin is added with height, antenna gain, better coax, a repeater, or a clean receiver.
Key points
- Margin is received signal minus receiver threshold; under twenty decibels is a path that fails in weather.
- Compute the link budget with the installed heights, coax, and antennas, not the ones in the study.
- Sixty percent of the first Fresnel zone must be clear of terrain and trees, or the path loss exceeds free space.
- Coax loss at the frequencies telemetry uses is large; a long thin cable can eat the whole margin.
- Margin is added with height, gain, coax, or a repeater; transmit power adds little.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Antenna height lower than the path study | The installed mast against the study; the Fresnel zone at the installed height against the terrain profile. Raise the antenna. |
| Coax longer, thinner, or lossier than assumed | The coax type and length against the loss table at the operating frequency; a hundred feet of small coax at 900 megahertz loses many decibels. Use larger, lower-loss coax, or move the radio to the antenna. |
| Water in the coax or a bad connector | Return loss or standing wave ratio worse than the record; a distance-to-fault sweep. Replace. |
| Antenna misaligned or the wrong pattern | A directional antenna pointed off the far end, or an omni where a directional was needed. Peak the alignment on the received signal. |
| Terrain or trees in the Fresnel zone | The path profile at the installed heights; a tree line or a ridge within the zone adds loss beyond free space. Height, or a repeater. |
| Receiver desensitized by a nearby transmitter | Good signal reading but poor decoding; a cellular site or a paging transmitter near the antenna. A bandpass or cavity filter; antenna separation. |
| Transmit power reduced | The radio configuration shows a power setting below the licensed level after a reset or a change. Restore it. |
| Wrong antenna gain or a damaged antenna | The antenna model against the design; physical damage. Replace with the specified antenna. |
| Path longer than the radio was chosen for | The distance against the radio class; an unlicensed radio at the edge of its range. A repeater, or a licensed radio. |
The link budget
Margin = P_tx + G_tx − L_tx + G_rx − L_rx − L_path − L_obstruction − S_rx
- P_tx = transmitter output power in dBm
- G_tx, G_rx = antenna gains at each end in dBi
- L_tx, L_rx = coax, connector, and arrestor losses at each end in dB
- L_path = free-space path loss for the distance and frequency in dB
- L_obstruction = additional loss from terrain, foliage, or partial Fresnel zone blockage in dB
- S_rx = receiver sensitivity at the data rate in use, in dBm, a negative number
L_path = 20 log₁₀(d_km) + 20 log₁₀(f_MHz) + 32.44
- d_km = path length in kilometers
- f_MHz = frequency in megahertz
- A 20 km path at 900 MHz has a free-space loss of about 118 dB; at 450 MHz about 112 dB
A radio at 30 dBm with a 6 dBi antenna through 3 dB of coax, to a 9 dBi antenna through 2 dB of coax, over a 20 kilometer path at 900 megahertz with 118 dB of free-space loss, delivers about minus 78 dBm to a receiver whose sensitivity at the chosen data rate is minus 105 dBm: a margin of 27 dB with a clear path. Put a tree line into the Fresnel zone and add 10 dB of obstruction loss, use a hundred feet of small coax that loses 8 dB instead of 3, and the margin is 12 dB, which fails in the first heavy rain. The arithmetic is done with the installed values, and it is checked by reading the received signal at each end.
The Fresnel zone
A radio path is not a line; it is an ellipse around the line whose width depends on the frequency and the distance, and the signal that arrives depends on that ellipse being clear. The rule is that sixty percent of the first Fresnel zone radius must be free of terrain, trees, and buildings, or the path loss exceeds the free-space figure, sometimes by a great deal. At the midpoint of a 20 kilometer path at 900 megahertz the first zone radius is around 40 meters, so a ridge or a tree line within about 25 meters of the line of sight at midpath costs signal even though the far antenna is visible. The path profile from the study shows the clearance at the installed heights; if the installed heights differ, the profile is redone.
Measuring
- 1
Read the received signal at each end
From the radio diagnostics, with the far end transmitting, over a few minutes; note the average and the spread.
- 2
Read the receiver sensitivity
From the radio manual, at the data rate configured; higher data rates need stronger signals.
- 3
Compute the margin
Received signal minus sensitivity, at each end. The smaller number is the path margin.
- 4
Compare with the budget
The measured signal against the computed signal. A measured value several decibels below the computation is a loss that was not in the budget: coax, connector, alignment, or obstruction.
- 5
Find the loss
Return loss test on the antenna system; alignment check; the path profile; the coax type and length.
Adding margin
| Change | Typical gain in margin | Note |
|---|---|---|
| Raise the antenna to clear the Fresnel zone | Up to 10 dB or more | The largest and most common fix |
| Higher gain antenna | 3 to 6 dB per end | Directional antennas also reject interference |
| Larger, lower-loss coax, or a shorter run | 3 to 8 dB per end | Often the cheapest; or mount the radio at the antenna |
| Replace wet coax and bad connectors | Whatever they were losing | A return loss test tells |
| Lower the data rate | 3 to 10 dB | The receiver is more sensitive at lower rates; the polls take longer |
| A repeater | Turns one long path into two short ones | A site with power and a good location |
| Increase transmit power | 1 to 3 dB | Within the license; rarely the answer |
Frequently asked questions
- The far antenna is visible from this one. Why is the margin poor?
- Visibility is the line of sight; the signal needs the Fresnel zone, which is wide, clear. A path that grazes a ridge or a tree line that the eye sees as just below the line is obstructed by radio standards. Raise the antennas or run the profile.
- Does rain really matter at these frequencies?
- At the frequencies used for utility telemetry, rain itself attenuates little; what rain brings is wet foliage, which absorbs a great deal, and atmospheric conditions that bend the path. A path that fails in rain has trees in its zone or too little margin for the conditions.
- How do I get a path profile?
- Path study software with terrain data, which most radio vendors and integrators run, or a survey with a GPS and the antenna heights entered. The profile shows the terrain along the path and the Fresnel zone at the antenna heights, and it is the document to keep with the site record.
- Is a repeater better than a taller mast?
- A repeater adds a site to maintain and a hop that halves the channel capacity for the sites beyond it; a taller mast is a structure with its own cost. For one bad path, the mast; for several sites behind the same ridge, the repeater. The path study prices both.
Related topics
- Intermittent Radio PathA site that drops and returns by the hour, season, or weather: thin fade margin, foliage, a new obstruction, a loosened antenna, coax taking water, a corroding connector, interference, or a marginal supply. How to trend signal against time and weather.
- 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.
- Network DrawingsThe two drawings every control network needs: a logical drawing of zones, subnets, VLANs, firewalls, and conduits, and a physical drawing of switches, ports, cables, fiber, radios, and sites. What each shows and why they are sensitive.
Direct contact
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