B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 219
Aircraft Equipment Problem Less Severe
General None Reported / Taken
Aircraft; Environment - Non Weather Related
The past four times I have flown an approach to Runway 28L at PDX; the aircraft with autopilot engaged; on glideslope; in approach mode with glideslope captured; at 3.5 DME at approximately 1000 feet AGL; the aircraft pitches nose down with a corresponding GS showing 1/4 dot high. The aircraft recovers; then continues normally on the approach.The first time it was day IMC several months ago. The last three times; it was day VMC on a visual approach. At first I thought it was just a 'glitch.' Now I think there may be an anomaly on the approach.  There is an antenna on a ridge at the 10 o'clock position; from the aircraft perspective; that is nearly level to the aircraft on the rising ridgeline south of the airport. Perhaps it interferes with the ILS signal?
A B737-700 First Officer believes that a radio tower south of the ILS 28L at PDX may be interfering with the glideslope signal causing a bump at about 1000 feet.