Re: [Gumstix-users] Starting the GPS

[Bruce F. <bfo...@br...>]

"MadAlexGumstix" wrote:

> The primary advantage of an active antenna is that the LNA can provide

> enough gain to drive a fairly long cable (a _verrrrrrry_ long cable if

> it's nice and thick and stiff and thus verrrrry low loss). The primary

> disadvantage(s) of the LNA in an active antenna is TANSTAAFL - it will

> add some [hopefully] small amount of noise to the signal, which may=20
> degrade the receiver's performance by some small amount, and it will=20
> constantly suck several milliamps of power.

> The reason some of us squawked when the first version of the GPSstix=20
> came out configured for passive antennas and without a simple mod to=20
> enable active operation, is that passive antennas are generally harder

> to find at retail. Otherwise, for most people, the GPSstix will=20
> probably work pretty well with a decent passive antenna (I've been=20
> using a passive antenna, but it's not one most Gumstix/GPSstix people=20
> will be interested in! ;).

The RF performance is determined primarily by the noise figure of the
first gain stage seen by the antenna.  The NEO-4S LNA has a noise figure
of 1.6 dB, and the ANN-MS active antenna is 1.5 dB, so, all other things
being equal, the active antenna will give slightly better performance
than a passive antenna connected to the goliath-gps.

"All other things being equal" means that the passive antenna must be
connected with no cable to the goliath board - any cable losses here
will add directly to the noise figure of the LNA.  Cable losses after
the first LNA - i.e. the output of an active antenna, have almost no
effect on the system noise figure.

A bigger problem is that the gumstix motherboards are known sources of
GPS interference - an unshielded basix will degrade the GPS performance
by about 10 dB for an antenna (active or passive) that is close to the
basix board.  I haven't tested a verdex, but I expect it to be
considerably worse due to the 13 MHz clock - the 121st harmonic of 13
MHz is 1573 MHz and falls very close to the GPS L1 band centered at
1575.42 MHz.

So the real advantage of using an active antenna is that it allows you
to get the GPS antenna away from the gumstix 'jammer'.  If you want to
use an antenna placed close to the gumstix, an RF shield over the
gumstix motherboard is required for good GPS performance.

--Bruce