Len Krauss wrote:
Any problem having a single external GPS antenna, like Garmin's GA 29 pole
mount model, connected to two GPS receivers via a simple Y-type connector?
If one GPS is disconnected, any problem leaving one open connection on the Y
while using the other GPS?

Thanks,
Len


Seriously *BAD* idea. Each GPS provides power to the Antenna.
Typically 5 V dc but may be 3.3 V for some of the newer models.
Connect two GPSes supplying power at the same time and there is a risk
that one will blow its input circuit. With some mixtures of GPS models,
it isnt a risk, its a certainty.

There is however a way of doing what you want, but NOT with a standard Y
or T connector.

There is a purpose designed product for a similar function with
Satellite TV recivers to connect two to one LNB. It is a passive
splitter with blocking diodes for DC power pass. It prevents power from
one receiver port reaching the other one while letting either power the
LNB and splitting the signal from the LNB to both receivers. Not
commonly used as a multi output LNB is tecnically superior, but any
decent satellite shop should be able to get you one. N.B. they are only
officially rated to 2 GHz and GPS works at 2.4 GHz but they WILL work in
your application. Only fly in the ointment is they use 'F' type
connectors so you may need some adaptors as well. Also the insertion
loss is ~~4 dB so use low loss cable and keep runs short or you may need
an inline amplifier (bit specialist as the Sattelite ones expect 12V on
the line, A good radio HAM or savvy tech could open one up and convert
it to seperate power feed if you had to.)

Its a general rule of thumb that all stubs longer than wavelength/10
MUST be correctly terminated with a terminator matched to the cable
impedance to prevent trouble from reflected signal. The wavelength in
free space at GPS frequencies is 12.4 cm, that in the cable will be less
depending on the velocity factor of the cable. This means that you
CANNOT disconnect the cable at the GPS end without connecting a
terminator to replace the GPS or you will interfere with the other GPS.

If that seems too complicated, then if you NEVER need to run both GPSes
at the same time, (e.g. you take your expensive plotter below in rough
weather) there is a simple low cost workaround. You need to have an
exrension cable from the GPS mounting at the chart table up to the mount
on deck. Just disconnect the cable from the GPS below and connect it to
the end of the extension, take the GPS up top aand hook it up. Power
and data cables to the two positions can connect in parallel. You
*MUST* have some way of keeping the ends of the outside cables DRY when
they are not being used. One approach is to take them through
watertight glands into a small tupperware box. Pop the lid off and
uncoil the cables to use them and coil them back into the box and pop
the lid on when you disconnect. There may be no other way that will
work for some makes of power/data cable. If however I could get a
locking waterproof bulkhead connector that is for mixed signal use,
(i.e. has coax contacts for RF and ordinary contacts for power and data)
I'd use that, cap it off and take the GPS and its short hookup leads
below together.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
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