Questions are put to us almost daily as to which is the "best" type of GPS antenna. The answer is: There is NO best type of antenna for all GPS applications. The most popular type for consumer model receivers is the Quad Helix style. This style has several advantages and several disadvantages. The most popular alternative is the PATCH antenna. It too has advantages and disadvantages. Other GPS antenna configurations include:
Microstrip. (one form is the "patch" antenna.)
Planar rings (aka "choke ring"), and other multipath-resistant designs.
Some people suggest that the Quad Helix antenna is "best" consumer GPS antenna under tree cover. This is not categorically true. There is little difference in performance between patch and quad helix antennas which have the same effective APERTURE. Aperture is directly proportional to antenna size and antenna gain is proportional to antenna aperture. Note that the Garmin G-12xl, G-12map, and eMap (among others) have a patch antenna while the G-76 series and the Magellan Meridian series use quad helix antennas. These models all have similar performance and sensitivity. All GARMIN models except the GPS 12 and eTrex series can be used with an external antenna. Some recent Magellan models have external amplified antenna capability.
The more recent units do have slightly more sensitivity but this is due more to improvements in semiconductor technology than anything else. The trend of improvements in effective sensitivity in newer technology units is likely to continue for the forseeable future and is not necessarily related to antenna type.
Note also that the eTrex models are more compact units than the
others listed. As a result, they have slightly less antenna gain (on the
order of perhaps 2db
less) than the physically larger models and thus will have slightly less sensitivity which may be important under forest canopy.
Thus, as a rule-of-thumb, physically larger GPS receivers are
likely to have better forest canopy receive performance than more compact
units regardless of
Now to Specifics.
Many of the earlier Garmin units used a PATCH style antenna similar to the TOKO antenna. Note the antenna patterns and main lobe gain for this model.
An example of a Sarantel Ltd. commercial Quad Helix antenna can be found HERE.
Here is a good article on Quadrifilar Helix Antenna (QFHA) antenna design and construction.
Here is a link to a listing of GPS antennas of various designs which you may be able to construct yourself. (via the Helenav website)