RF & Antenna Systems

Possible Causes

• •Water in connector
• •Cracked or kinked coax
• •Connector pulled partially off
• •Center conductor broken internally

Diagnostic Steps

1. 1.Inspect all connectors for moisture and corrosion
2. 2.Flex coax along its length while monitoring SWR meter — change in SWR indicates internal break
3. 3.Disconnect antenna and measure impedance of feedline looking into open end
4. 4.Replace suspect section of feedline

Possible Causes

• •High SWR causing high reflected power
• •Excessive power for cable rating
• •Poor connector causing arcing
• •Damaged dielectric

Diagnostic Steps

1. 1.Check SWR and reduce transmit power if high
2. 2.Verify cable is rated for transmit power level
3. 3.Inspect connectors for burn marks or carbon deposits
4. 4.Replace cable if dielectric is discolored or damaged

Possible Causes

• •Shield continuity problem
• •Broken braid strands in connector
• •Loose center conductor
• •Poor weatherproofing allowing moisture

Diagnostic Steps

1. 1.Rotate antenna and note if noise changes with orientation
2. 2.Inspect connector braid for broken strands or incomplete coverage
3. 3.Check center conductor for solid connection in connector body
4. 4.Re-weatherproof all outdoor connectors with self-amalgamating tape

Possible Causes

• •Poor antenna placement
• •Antenna obstruction
• •Damaged antenna cable
• •Cold start after long storage

Diagnostic Steps

1. 1.Ensure antenna has clear 360-degree view of sky
2. 2.Check antenna cable for damage or tight bends
3. 3.Allow up to 15 minutes for initial cold start fix
4. 4.Verify antenna is active type and receiving power from receiver

Possible Causes

• •High HDOP due to poor satellite geometry
• •Multipath reflections
• •Antenna near metal structures
• •Interference from other electronics

Diagnostic Steps

1. 1.Check satellite count and HDOP on receiver display
2. 2.Relocate antenna away from radar domes, masts, and metal surfaces
3. 3.Check for interference from VHF transmissions
4. 4.Compare position to known landmark or chart

Possible Causes

• •Incorrect baud rate
• •Wiring error
• •Wrong NMEA sentence type enabled
• •Ground loop

Diagnostic Steps

1. 1.Verify baud rate matches between GPS and chartplotter — default is 4800 baud for NMEA 0183
2. 2.Check TX from GPS connects to RX on chartplotter
3. 3.Verify GGA and RMC sentences are enabled in GPS output settings
4. 4.Use differential wiring (RS-422) to eliminate ground loops on long cable runs

Possible Causes

• •Antenna element damaged or missing
• •Feedline loss too high
• •Antenna connector corroded
• •Wrong antenna type for frequency

Diagnostic Steps

1. 1.Visually inspect antenna element for physical damage, corrosion, or missing radials
2. 2.Measure insertion loss of feedline — at VHF a 3 dB loss halves the signal
3. 3.Disconnect feedline at radio and antenna separately to isolate loss to feedline vs. connector
4. 4.Verify antenna is rated for the frequency in use — a VHF antenna used on UHF will have very high SWR and poor performance

Possible Causes

• •High SWR reducing radiated power
• •Radio reducing power due to SWR protection
• •Antenna gain lower than expected
• •Height too low for required range

Diagnostic Steps

1. 1.Measure SWR — most radios reduce output power when SWR exceeds 2:1
2. 2.Check antenna height — VHF range is line-of-sight, doubling height increases range by approximately 40%
3. 3.Verify antenna gain specification matches installation requirements
4. 4.Check for nearby metal structures that may be blocking radiation pattern

Possible Causes

• •Wind damage to element
• •Lightning strike damage
• •Ice loading causing element bend
• •UV degradation of radome

Diagnostic Steps

1. 1.Inspect element for bends, cracks, or missing sections
2. 2.Check feedline for burn marks or melted sections indicating lightning damage
3. 3.Measure SWR before returning to service — physical damage may not always be visible
4. 4.Inspect mounting hardware for corrosion or loosening from wind loading

Possible Causes

• •Water ingress into connector or coax
• •Corrosion at connector increasing with humidity
• •Antenna radome cracked allowing moisture entry
• •Ground plane affected by wet surroundings

Diagnostic Steps

1. 1.Inspect all outdoor connectors for water ingress — reseal with self-amalgamating tape
2. 2.Check inside the antenna base or connector housing for moisture or corrosion
3. 3.Replace antenna if radome is cracked — moisture inside the radome changes the electrical length of the element
4. 4.Compare SWR readings in dry vs. wet conditions to confirm moisture is the cause

Possible Causes

• •Switching power supply or inverter
• •Electric motor arcing (bilge pump, windlass)
• •Alternator noise
• •Poorly shielded electronics

Diagnostic Steps

1. 1.Turn off suspected noise sources one at a time while monitoring receiver to identify culprit
2. 2.Check alternator output for AC ripple — more than 100mV AC on the DC bus indicates a failing alternator diode
3. 3.Add ferrite cores to power cables of noisy equipment
4. 4.Ensure all electronics share a common ground bonded to the vessel's ground bus

Possible Causes

• •Another transmitter on or near the frequency
• •Harmonic from a nearby transmitter
• •Intermodulation product from two strong signals
• •Spurious emission from electronic equipment

Diagnostic Steps

1. 1.Use a spectrum analyzer or scanning receiver to identify the interfering signal
2. 2.Note the frequency and signal characteristics — continuous, intermittent, pulsed, or keyed with voice
3. 3.Check if the interference correlates with use of specific onboard equipment
4. 4.If the interfering signal is a harmonic, identify the fundamental frequency and locate the source

Possible Causes

• •VHF transmitter causing interference on GPS
• •Radar interference on other equipment
• •High-power transmitter near GPS L1 frequency (1575.42 MHz)
• •Poor bonding between equipment chassis

Diagnostic Steps

1. 1.Check GPS performance while transmitting on VHF — GPS is susceptible to strong nearby RF sources
2. 2.Add a bandpass filter on the GPS antenna feedline to reject out-of-band interference
3. 3.Verify chassis bonding between all equipment — a common ground eliminates many interference paths
4. 4.Separate GPS antenna from VHF and radar antennas by as much distance as possible

Possible Causes

• •Audio pickup of RF in microphone cable
• •Intermodulation in a shared antenna or splitter
• •RF entering equipment through power leads
• •Insufficient isolation between radios sharing an antenna

Diagnostic Steps

1. 1.Add ferrite cores to microphone cable near the radio — wrap cable through core 3–5 times
2. 2.Check antenna splitter isolation rating — minimum 30 dB isolation between ports is recommended
3. 3.Add ferrite cores to power leads of the affected equipment
4. 4.Verify antenna splitter is rated for the power levels being used

Possible Causes

• •SWR meter fault
• •Feedline shorted causing SWR meter to read incorrectly
• •Meter bypass or calibration error

Diagnostic Steps

1. 1.Disconnect antenna and verify SWR changes — a properly functioning meter should show infinite SWR with antenna disconnected
2. 2.Try a known-good dummy load — SWR should read 1:1 on a 50-ohm load
3. 3.Replace SWR meter if it reads 1:1 regardless of what is connected

Possible Causes

• •Connector arcing at high power
• •Dielectric breakdown in coax under high voltage
• •Antenna element heating and detuning at high power
• •Non-linear element in antenna system

Diagnostic Steps

1. 1.Inspect all connectors for signs of arcing — carbon deposits or discoloration
2. 2.Check coax manufacturer's power rating at the operating frequency
3. 3.Measure SWR at different power levels to confirm power-dependent behavior
4. 4.Inspect antenna feedpoint impedance matching network for heat damage

Possible Causes

• •Wrong antenna for frequency range
• •Feedline impedance mismatch
• •Antenna element incorrect length
• •Ground plane inadequate for vertical antenna

Diagnostic Steps

1. 1.Verify antenna is designed for the frequency in use — check manufacturer specifications
2. 2.Confirm feedline impedance is 50 ohms — mixing 50-ohm and 75-ohm components degrades SWR
3. 3.Measure antenna element length — quarter-wave verticals must be within a few percent of the correct length
4. 4.Check ground plane or counterpoise system — inadequate ground raises feedpoint impedance