how to calibrate the wind vane

[corvairbob]

i got one of the la crosse 79400 weather stations, so i set it up and then stuck it outside and the wind vane is all over the place. i have a home made vane i built years ago and it is placed up on a shed like 20' in the air. and i have another home made unit that is 4" off the ground. so i put this unit on a 10' pole and stuck it in the yard about 20' from the house and then out by the shed but the one close to the ground. and the vane still is holding steady. 

so i now have it inside clamped to a counter and a fan on it and after calibrating the vane to read north on the receiver by hand, i then placed the fan facing it from the south and the vane is pointing sse and bouncing back and forth to south. the fan is maybe 4' away but that should not matter as wind is not further or closer for the source.

  i figured if i got a factory made unit i would be getting a unit that was at least somewhat accurate.  as i write this the anemometer has been between 1 and 3 mph and the vane between s and sse,  

 

so has anyone had any luck modifying the vane so it holds steady and points in the actual direction the wind is coming from? i know this is not a 1,000 unit but it should be a bit more accurate than what it is.  i just stuck a pc of tape on the vertical fin and made it like a rudder and it now is showing south but still bounches. so it this a normal think for these small plastic wind vanes? and i just taped 2 quarters tot eh vertical fin to see if that weight will overcome the vane movement.  i took photos but they are to big to upload so i have to reduce the size if needed. 

 

thanks bp.