Toni wrote:

When talking about loop antennas people talk about "capture area".
Whatever that is, this seems to be what makes a ferrite bar antenna
more sensitive than the equivalent simple coil tuned to the same freq.
I know that by using a ferrite bar you are narrowing the pattern, but
I'd think that the main gain does not come from the pattern narrowing
but from "capture area increase" (again, please bear with my ignorance,
these are only my thoughts on what I've read on the Web)

Capture area is exactly the same thing as gain, but expressed in
different units. There are two ways of increasing the gain or capture
area of an antenna: increase its efficiency, or narrow its pattern. The
former increases the gain or capture area in all directions; the latter
increases it in some directions at the expense of others. A ferrite loop
antenna simply has better efficiency than a standard loop of the same
physical size. Hence it has better gain or capture area.

I know one can not have more than 0 dB with full omni, I just guess the
minimalistic antenna in pocketable gps is way below 0 dB and could
maybe be improved a little.


[RL]
0 dB relative to what?


An isotropic antenna; AFAIK a perfect isotropic antenna would have 0 dB
gain

dB is a ratio, in this case of gains, or field strengths with a given
power input, or capture areas. (All three ratios are the same for a
given pair of antennas.) So the reference always must be specified,
otherwise a statement of dB gain is meaningless. When using the gain of
a free space isotropic antenna as a reference, gain is expressed in dBi,
that is, dB relative to isotropic. The gain of a perfectly
omnidirectional antenna over an infinite ground plane is 3 dBi, since
the same power is concentrated in half the volume as it is for an
isotropic free-space radiator.



[RL]
Once you get the desired coverage angle, the only way to improve the
reception of the GPS is to improve the receiver signal/noise ratio. The
only way you can do that from outside the GPS is to use an external
antenna with a preamp having a lower noise figure than the GPS's receiver.


Please let me doubt that. For a given coverage angle you can't make
better than a perfect antenna, but you can certainly make worst (think
of a T2FD).

If by "perfect" you mean "perfectly efficient", I agree.


I'm not sure what the "high loss antenna" is. If you mean the GPS
antenna, it's not high loss at all, but is likely very efficient. If
it's a patch antenna, you can't model it at all with EZNEC. But even if
it's a quadrifilar helix, you can't model it with one segment.


I dont know how efficient they are, but I do know that normal
commercial patch antennas are noticeably less efficient than helical
ones, and then, for the helicals, I doubt that something aprox 1/10 wl
is anything close to efficient. If this was to be true I'd love to
build an equivalent 6.5 ft. helix to work on 20m!

I wasn't aware of that. So if typical GPS patch antennas are indeed
significantly inefficient, the reception could be improved without
narrowing the pattern. But I don't know if there would be any practical
way to do this externally.

The civilian GPS frequencies are about 1228 and 1575 MHz. The wavelength
in air of the lower frequency is about 9.6 inches. Patch antennas can be
made with a side equal to about 0.25 wavelength divided by the square
root of the dielectric constant of the material within the patch. So for
Teflon dielectric (er ~ 2.3), a patch would have sides of 1.6 inches. If
something like alumina is used (very low loss, dielectric constant ~
10), the side would be 0.76 inches. Some patches have sides twice this
long, but alumina would permit even one of those to be used in a typical
GPS receiver. The two versions of patches have different patterns, and I
don't know right off which would be preferable for GPS use. At any rate,
I believe that patches of those dimensions are quite efficient, assuming
the dielectic has low loss. Both Teflon and alumina, as well as others,
meet this criterion. Maybe someone with a greater knowledge of patch
antennas, and GPS antennas in particular, could provide some additional
information.

Roy Lewallen, W7EL