Antenna Patterns and perceived "sensitivity" of GPS receivers

     Sometimes   people  do  side-by-side  comparisons   of   GPS
receivers  for  sensitivity and come  to  incorrect  conclusions.  
This  can  happen for several reasons already  discussed  on  the 
newsgroup.

1)   Some  GPS receivers emit small amounts of  Electro  Magnetic 
Interference  (EMI) which can desense the receivers of other  GPS 
receivers a few feet away.  One example of this is that the Eagle 
Explorer EMI causes desense of the receiver in the Garmin G-12XL,  
but not the other way around.  Thus,  if you put the two side  by 
side  you will always find that the EE receives better.   On  the 
other  hand,  if you separate them by 5 feet,  things are  pretty 
equal.  

2)   Antenna  patterns  are another cause  of  wrong  conclusions 
about  "which  GPS  has  a more  sensitive  receiver.   Many  GPS 
receivers  use the patch antenna which has the antenna gain  fall 
off as you near the horizon.  (Examples:  G-12XL,  EE,   M-4000).  
Others  use  the  wrapped "helix" style antenna  which  has  good 
coverage almost all the way to the horizon.  Outside in the  open 
spaces,   both perform very well.  The patch antenna is  conceiv-
ably  superior since it tends to reject signals very low  on  the 
horizon and thus is somewhat less sensitive to multipath  errors.  
On the other hand,  if you are indoors (or in a car),  the  lower 
pattern  coverage angles of the helix style antenna may  pick  up 
SVs  lower  on the horizon (or multipath  signals)  through  side 
windows  and  stay locked where you might not with the  patch. If  
multipath  signals are received and processed,  you  may  see your 
GPS position move hundreds and even thousands of meters away from 
your actual position.  

    In actual practice,  I find little difference in "real world" 
performance between the two antenna types under normal clear view 
of the sky situations.

The  questionable results come when someone takes his  GPS  units 
INDOORS and does a comparison test for receiver sensitivity.   In 
such  cases,  almost all of the direct signals from overhead  SVs 
are  blocked   and signals from off to the side  through  windows 
and/or walls may be stronger than any of the overhead SV signals.  
In such cases,  the helix type antennas will almost always  prove 
'better'  due  to  the fact that they have  the  ability  to  see 
signals  LOW  ON THE HORIZON and so can look out  windows  better 
than  the patch antenna equipped units.  However!  The fact  that 
the helix style antennas can look off to the side better does not 
indicate  that  the GPS they are attached to will (or  will  not) 
perform  better  in  a normal environment  with  an  unobstructed 
overhead view.

Joe Mehaffey