An air carrier crew states that their transponder antenna is shielded by the fuselage.

Attributes
ACN	274628
Time
Date	199406
Day	Fri
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : eyw
State Reference	FL
Altitude	agl bound lower : 0 agl bound upper : 0
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : nqx
Operator	common carrier : air carrier
Make Model Name	Mallard
Operating Under FAR Part	Part 135
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 200 flight time total : 7700 flight time type : 4200
ASRS Report	274628
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
ASRS Report	274289
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : detected after the fact
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Key west naval air station departure advised us our transponder was malfunctioning. Every attempt was made to locate the problem. This aircraft is a seaplane and the antenna is located underneath the wing which causes, at times, a malfunction for the ATC center to receive depending on the angle they receive our transponder. The only corrective action that I suggested to our company is installing an antenna on each wing.

Original NASA ASRS Text

Title: AN ACR CREW STATES THAT THEIR XPONDER ANTENNA IS SHIELDED BY THE FUSELAGE.

Narrative: KEY WEST NAVAL AIR STATION DEP ADVISED US OUR XPONDER WAS MALFUNCTIONING. EVERY ATTEMPT WAS MADE TO LOCATE THE PROB. THIS ACFT IS A SEAPLANE AND THE ANTENNA IS LOCATED UNDERNEATH THE WING WHICH CAUSES, AT TIMES, A MALFUNCTION FOR THE ATC CTR TO RECEIVE DEPENDING ON THE ANGLE THEY RECEIVE OUR XPONDER. THE ONLY CORRECTIVE ACTION THAT I SUGGESTED TO OUR COMPANY IS INSTALLING AN ANTENNA ON EACH WING.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.