[aprssig] GPS Intgrated Engine
Stephen H. Smith
wa8lmf2 at aol.com
Tue Mar 28 10:15:05 CST 2006
AA3JY at Winlink.org wrote:
>> What I don't like about integrated GPS receivers is that you still have to
>> connect an external antenna...
> Look at Garmin's Rino 520 and 530....
> Clay AA3JY
> (via WL2K
But that's a *hand held* where the radio is going to probably be
outdoors. It would be useless to do that with a mobile rig like a D700
where the radio chassis is inside the trunk or under a seat.
The external GPS antennas used with built-in GPS receivers normally
contain 15-20 dB gain preamps to offset the horrendous loss of
small-gauge coax at 1575 MHz where GPS operates. In turn, there is
virtually no selectivity in front of these high-gain amplifiers -- the
antenna element is a very broadband patch that is not sharply
resonant. These amplifiers often overload and block every time you get
within a few blocks of a 1900 MHz PCS cell site, causing the GPS
reception to drop out every time you get near one.
The combined receiver/antenna puck-type GPS devices don't use active
pre-amplified antennas because the ant is connected directly to the
receiver with no coax at all.
In real life testing in RF-saturated areas like Los Angeles or Chicago,
I have seen the inexpensive GPS18 or Deluo GPSes sitting on the
dashboard inside a car far outperform expensive two-piece "professional"
units with roof-mounted active antennas.
Further, a root-mounted GPS active GPS antenna will often overload,
block, and knock the GPS out of lock every time your own 25-50W two
meter or UHF rig keys up, if the antennas aren't separated far enough.
This usually isn't a problem on a ship or aircraft where the antennas
can be farther apart, but is often a problem on cars.
The "hockey puck" all-in-one GPS sitting on the dash UNDER the roof of
the car will experience far less RF from your other transmitters.
Stephen H. Smith wa8lmf (at) aol.com
EchoLink Node: 14400 [Think bottom of the 2M band]
Home Page: http://wa8lmf.com
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