On Thu, 9 Sep 2010 06:32:06 -0700 (PDT), Andy
wrote:

On Sep 1, 6:36 am, Meat Plow wrote:


I'm sure aviation and military GPS units are as robust as loran. Consumer
units might be helped by a better antenna.


Andy comments:

The only way the antenna can be "better" is to have more gain.

Ummm....
1. Better match (VSWR)
2. Better front to back ratio
3. Fewer side lobes
4. Better looking pattern.
5. Smaller, cheaper, lighter, easier to build, etc.
6. Grounded design to avoid static electricity buildup.
Gain is not the only thing that's important in antennas.

That
requires the antenna to be more directional.

True. Antennas only redirect RF. The don't create any. If you need
more signal in one direction, you have to steal it from another
direction.

Such is not a good idea
'in GPS since one needs a hemispherical pattern, to account for
where all the satellites are.

Not really. I've built a perfectly functional GPS antenna that has a
pattern similar to a donut (torus) elevated off the ground with no
gain directly overhead. For location purposes, overhead satellites
are nearly useless because there's no doppler shift.

It's true that some GPS units have a poor antenna, but that's a
matter
of cosmetics more than anything else.

Ahem... There are two basic types of GPS antennas. Patch antennas
and quadrifilar (QFA) antennas. Both have roughly a hemispherical
pattern. The QFA antenna is "better" for some applications because
the radiation angle extends closer to the horizon and below than does
the patch antenna. There's no benefit for 2D positioning, but the
lower radiation angle offers better altitude information. A patch
antenna becomes less circular polarized and more towards elliptical
polarization hear the horizon. Well, at the horizon, it's
horizontally polarized which is good for a -3dB gain loss. QFA
antenna remains fairly RHCP at the horizon.

If you look inside some of the higher end handheld and vehicle mounted
GPS antennas, you'll find a QFA antenna. Garmin GPS 76
http://i36.photobucket.com/albums/e8/KiwiGPS/Inside%2076CS/76CSInsideViews1Large.jpg
Kinda looks like it was designed to fit either a patch and QFA.

Many military units are
handheld
and therefore have the same problem. Any unit with a remotely
mounted antenna will perform about the same as any other unit,
whether military or commercial, since they are all designed to
have a hemispherical pattern.

Agreed. The differences are small but if you're trying to squeeze the
last bit of performance out of a handheld, it can be important.

Just feel like talking....

Andy in Eureka, Texas


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS