Thanks for the full explanation Ian -- very helpful!
Len

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"Ian Malcolm" wrote in message
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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.

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Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
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Len Krauss wrote:

Thanks for the full explanation Ian -- very helpful!
Len


Len, Sorry it took so long to get back to you, I was more than a little
busy with yacht club duties.

I made a NUMERICAL error (as you may have noticed Bruce in Alaska
pointing out to me). The great thing about USENet is that if one does
goof, someone will usually point out ones slip-up. This, in general, is
NOT true of the WWW as, for each site, the author usually controls the
medium. My reply to him was more than a little technical and I hope it
doesen't cause you any confusion.

The relevent part for you is the CORRECT frequency is approx 1575 Mhz
(1.575b GHz). This gives a wavelength of 18.9 cm in free space. This
is a *little* more forgiving than the higher frequency and I wouldnt
worry about terminating unused outputs on the splitter that DO NOT have
any cable or adaptor conected at all! (as the F socket is shorter than 2
cm) Also the splitter is being used WITHIN its specified bandwidth, so
it should perform to specification. The rest of my advice and
explanation stands as written.

Larry W4WSC has some very good points. There IS a risk of damage
connecting the wrong antenna to a GPS. The antenna may be 3.3V only and
the GPS outputs 5V or the antenna may require more current than the GPS
is able to supply without damage. I have the facilities to test an
antenna to see if it will be compatible BEFORE connecting it. I doubt
you are able to do so. His advice to install two antennae for two
recivers also has the advantage of redundancy which could be further
improved by running them off seperate batteries.

I am probably preaching to the choir, but as long as GPS has a millitary
application, it must not be your SOLE means of navigation.

To summerise, I belive your PRIMARY GPS should be wired direct to a
dedicated antenna especially if its your main meens of navigation, but
you may choose to use a single antenna for multiple backup or auxillary
GPSes as I originally described.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at]=@, [dash]=- &
[dot]=.
*Warning* SPAM TRAP set in header, Use email address in sig. if you must.
'Stingo' Albacore #1554 - 15' Uffa Fox designed, All varnished hot
moulded wooden racing dinghy circa. 1961