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


--
Eliminate "ns" for email address.

Do not do it. They both output a voltage (i belive 5VDC).

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)
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

Put it out of your mind and mount both antennas. GPS receivers
usually have the microwave part of the receiver up inside the radome
under the antenna because it's more sensitive that way. Very few of
them are compatible, as usual with proprietary marine crap. What
comes down the cable is IF (intermediate frequency RF) with DC power
going up to the electronics in the dome. Don't plug A's antenna into
B's receiver or you may blow it!



On Sun, 11 Jan 2004 13:04:42 -0500, "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


--
Eliminate "ns" for email address.

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

--
Eliminate "ns" for email address.
"Ian Malcolm" wrote in message
...
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)
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

In article ,
Ian Malcolm wrote:

N.B. they are only
officially rated to 2 GHz and GPS works at 2.4 GHz but they WILL work in
your application.

Bzzzzt, Nice try, Would you like to try again, for what's behind Cutain
No.3? GPS does NOT run and 2.4Ghz, it uses the 1500 Mhz band for
all the datastreams, Commercial and Miltary.

Bruce in alaska
--
add a 2 before @

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

Bruce in Alaska wrote:
In article ,
Ian Malcolm wrote:


N.B. they are only
officially rated to 2 GHz and GPS works at 2.4 GHz but they WILL work in
your application.


Bzzzzt, Nice try, Would you like to try again, for what's behind Cutain
No.3? GPS does NOT run and 2.4Ghz, it uses the 1500 Mhz band for
all the datastreams, Commercial and Miltary.

Bruce in alaska
Thank you for the correction, I was of course mistaken when I thought
it was 2.4 GHz. Where I got that number from, I don't know! It isn't
what you know you don't know that catches you out, but what you think
you know but don't.

I was inspired to dig out my copy of the
ADMIRALTY LIST OF RADIO SIGNALS
SATELLITE NAVIGATION SYSTEMS
NP288 VOL 8 (2001/2002)
for an authoritative reference.

Ahh, Bzzzzt yourself, It does *NOT* use the 1500 Mhz band for *all*
data steams! (Not even if you take the 1500 Mhz band to be 1400 to
1599.9.... Mhz. The above reference says L band, nominally between 1 and
2 GHz) I freely admit you are MUCH MUCH closer to the correct
frequencies than I was though. I see from other threads that you have a
STRONG professional background in marine electronics, while my
proffesional background in electronics is NOT in the marine field.
Perhaps you were oversimplifying for the end user . . .

Now for the prize behind curtain No.3, here is the fourth paragraph from
page 11 of NP288 VOL 8 (2001/2002).

"Each satellite transmits data on two frequencies in the L-band; L1 =
1575.42 Mhz and L2 = 1227.60 MHz. Both frequencies are integer multiples
of the basic 10.23 MHz clock frequency. Dual-channel receivers will
be able to use two frequencies to correct for the effects of ionospheric
refraction. Data transmitted on the L1 and L2 frequencies is encoded
by a Pseudo Random Noise (PRN) modulation. The L5 frequency will be
1176.45 MHz."

To summerize:

Freq/MHz Modulation codes Use & Status (as of 25 Jan 2001)
L1 1575.42 C/A ,P,M civil & military
L2 1227.60 P, M? military *
L5 1176.45 ? civil to be introduced from 2005

* Precise Positioning Service (PPS) is derived from the P code and has
specialist civil applications. L2 will also carry the second civil C/A
signal in the future.

Now then, what's the procedure for claiming the prize from behind
curtain No.3? If I have to sit through a timeshare presentation first,
or flights, batteries or whatever are not included, I decline it and
would prefer you to give it to charity :-)

Seriously, thanks for the heads up on my error, I am fortunate that it
was an error of fact not of principle and did not significantly affect
the validity of my argument and advice.

Whats the weather like with you at the moment and how long is your
boating season? We are very much out of season here, with the London
Boat Show in progress, though some hardy souls including a few members
of my Dinghy club are still racing. Its raining, Its windy and its
fairly cold.
--
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

In article ,
Ian Malcolm wrote:

Now then, what's the procedure for claiming the prize from behind
curtain No.3? If I have to sit through a timeshare presentation first,
or flights, batteries or whatever are not included, I decline it and
would prefer you to give it to charity :-)

Seriously, thanks for the heads up on my error, I am fortunate that it
was an error of fact not of principle and did not significantly affect
the validity of my argument and advice.

Whats the weather like with you at the moment and how long is your
boating season? We are very much out of season here, with the London
Boat Show in progress, though some hardy souls including a few members
of my Dinghy club are still racing. Its raining, Its windy and its
fairly cold.
--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)

Well Ian, the prize isn't really worth all that much for Curtain No. 3
this time. "Free cup of alaskan brewed coffee at the Trading Post's
Pickle Barrel Stove checker table." (www.99850.net) We have a few
folks steaming about around here but mostly they are waiting for Tanner
Crab Season to open in about three weeks. Season usually lasts about 10
days, but I get a "Healthy Crab (King Crab) Tax" for letting some of the
guys stage their pots on the Cannery Dock, prior to Season Openings.
Now that makes for good eating, when the crab are fresh out of the
seawater, and hot out of the steamcooker.

Not a big error, just a friendly correction, and your absolutly right
that it didn't effect the comments you made in any practical way.

If you ever get over here to alaska, on Holiday or whatever and happen
to stop by, the coffe will be waiting, and maybe a 5# baggie of Frozen
Crab, if thewre is any left after my brother stops by.

--
Bruce (semiretired powderman & exFCC Field Inspector for Southeastern Alaska)
add a 2 before @
Bruce Gordon * Debora Gordon R.N. Bruce's Trading Post
P.O. Box EXI Excursion Inlet South
Juneau, Alaska 99850 Excursion Inlet, Alaska 99850
www.btpost.net www.99850.net

Len,

If you really want to try this then I think you could minimize the risk
by using satellite distribution components. Connect the antenna to it's
correct receiver. Spilt the signal with and appropriate Diplexer (sat.
jargon for splitter). Block the DC from the second receiver with a
blocking diode. There is still a risk that the signal strength of the
ant. is too high as to damage the input of the second receiver but I
believe this to be minimal. All of this would be done ideally under the
supervision of someone who know electronics.

After saying all that. I believe Larry's advice to use two antennas is
the best cause you then have two entirely redundant systems.

Regards, Ron

I don't recieve e-mail at this address because of spam. E-mail me
at crtsrATmsnDOTcom.