Thanks to each of you who responded. I have created a table listing possible
contributing factors and recommended diagnostic actions. Comments would be
appreciated.
Contributing Factor Diagnostic Action
Antenna Factors Are antennas calibrated? Calibrated at same lab as last
time? Same standard procedure?
Antenna balance Does reading vary significantly when antenna is rotated 180
degrees? Rotate both antennas one at a time.
Currents on Tx cable Add ferrites on cable near antenna
VSWR Add 6-10 dB pad at both antennas. A 3 dB pad may work.
Dipole length Verify length is set per manufacturer's instructions
Damaged elements Check for loose, bent or corroded elements
Mutual coupling factors Verify that factors are applicable to your antenna
and distance
Antenna spacing Verify dimension. For other than dipoles, verify where
measurement point should be on antenna.
Antenna height Verify height is per ANSI C63.4
Receiver and signal generator calibration Verify calibration date.
Perform calibration self-check if available. Calibrate tracking when using
tracking generator.
Internal attenuator and scale tolerance Verify manufacturer's specified
tolerance is no more than 1 dB for scale(s) used.
Drift Allow sufficient warm-up time per manufacturer's specifications.
Check direct connection reading before and after test.
Size of ground screen Oversized pad is recommended especially when
elevated or a 3m pad. Consider increasing size. Also reference "Ground
screen edge transition" below.
Bonding of ground screen Check solder joints for cold joints. Check
bolted/pressure connections for tightness, weathering and corrosion.
Ground screen edge transition Extra screen may be required for undersized
pads. This may be in the form a buried section of screen.
Earthing of ground screen Add salt-water solution at ground points if
weather is dry. Check solder joints for cold joints. Check bolted/pressure
connections for tightness, weathering and corrosion.
Flatness Has screen stretched such that flatness is out of spec?
RF conductivity from table to screen Check solder joints for cold joints.
Check bolted/pressure connections for tightness, weathering and corrosion.
Check brushes.
Reflecting objects within ellipse Wash down mast, covers and other
"non-conductive" objects.
Reflecting objects outside ellipse Trim trees and shrubs. Park cars
away from site. Check for new fences. Move location of measuring equipment,
if possible.
Human factors Verify all steps in test procedure. Have second person
perform setup, tests, readings and calculations.
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