12XL requires active external antenna :-(
m...@smallworks.com
Although I have not found this stated in the manual or confirmed it
with Garmin, I have heard and believe that the 12XL will not function
well (or at all) with an external passive antenna.
This is due to the fact that the internal antenna is passive and there
is a pin diode which must see the load of an external active antenna
to (electrically) "disconnect" the internal passive patch antenna.
This obviously was not an issue with the GPS 45, 45XL, II as there you
were replacing the existing external antenna but the same situation
should have existed with the GPS 40.
Pity, since my original thought was to use the antenna off of my '45
with a short length of coax. Now it looks like I'd have to build a
small preamp to do that. May not bother since the 12XL works so well
off of the internal antenna although an antenna that I could stuff
between the shad and window on commercial jets would be nice.
Flames cheerfully ignored although I'd like to "corrected" by a
knowledgeable source.
Mike
PS Reasonable offers on a '45 in mint cond (used but never abused)
with all original docs/packaging considered. Otherwise someone I know
is going to get a slightly outdated but still pretty neat toy as a
gift.
Dave Martindale
m...@smallworks.com writes:
>Although I have not found this stated in the manual or confirmed it
>with Garmin, I have heard and believe that the 12XL will not function
>well (or at all) with an external passive antenna.
>
>This is due to the fact that the internal antenna is passive and there
>is a pin diode which must see the load of an external active antenna
>to (electrically) "disconnect" the internal passive patch antenna.
Well, it's an easy experiment. I'll try it sometime.
If it turns out to be true, you could *still* use an external passive
antenna by drawing enough current to switch the diode. About a 300
ohm resistor in parallel with the antenna will draw about as much
current as the Garmin GA-26 antenna, which obviously must be enough
for switching. Yet 300 ohms in parallel with the 50 ohm RF impedance
won't load the signal too much. And with some experimenting, you
might find that far less than 15 mA is enough to perform the switching,
saving some battery power.
>PS Reasonable offers on a '45 in mint cond (used but never abused)
>with all original docs/packaging considered. Otherwise someone I know
>is going to get a slightly outdated but still pretty neat toy as a
>gift.
I'm in the same situation - have both a 45XL and 12XL. I'll probably
keep both, since I actually prefer the 45XL BNC connector to the tiny
MCX on the 12XL for any place that needs an external antenna (e.g.
car), and the 45XL works fine if the antenna has a good view of the
sky. Plus battery life is up to twice as long at the 12XL. And I
figure the resale value of a 45XL just dropped a lot with the
introduction of the 12XL, so selling it wouldn't get me much anyway.
Dave
Bob
m...@smallworks.com wrote:
>
> Although I have not found this stated in the manual or confirmed it
> with Garmin, I have heard and believe that the 12XL will not function
> well (or at all) with an external passive antenna.
>
> This is due to the fact that the internal antenna is passive and there
> is a pin diode which must see the load of an external active antenna
> to (electrically) "disconnect" the internal passive patch antenna.
>
> were replacing the existing external antenna but the same situation
> should have existed with the GPS 40.
>
> Pity, since my original thought was to use the antenna off of my '45
> with a short length of coax. Now it looks like I'd have to build a
> small preamp to do that. May not bother since the 12XL works so well
> off of the internal antenna although an antenna that I could stuff
> between the shad and window on commercial jets would be nice.
>
> Flames cheerfully ignored although I'd like to "corrected" by a
> knowledgeable source.
>
> Mike
>
> with all original docs/packaging considered. Otherwise someone I know
> is going to get a slightly outdated but still pretty neat toy as a
> gift.
Maybe you could lie to it with a fairly high value resistor - ~470 - 1K
ohms - across the center conductor to shield. I wouldn't think that would
load the receive signal all that much considering it's expecting a 50 ohm
impedence.
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Dave Martindale
m...@smallworks.com writes:
>Although I have not found this stated in the manual or confirmed it
>with Garmin, I have heard and believe that the 12XL will not function
>well (or at all) with an external passive antenna.
>
>This is due to the fact that the internal antenna is passive and there
>is a pin diode which must see the load of an external active antenna
>to (electrically) "disconnect" the internal passive patch antenna.
I tested it, and you're quite right. The 12XL accepts GPS signals from
only one antenna at a time, either internal or external but not both.
And it requires current to be drawn from the external antenna jack to
switch to the external antenna. (See below for how I tested this.)
>Pity, since my original thought was to use the antenna off of my '45
>with a short length of coax. Now it looks like I'd have to build a
>small preamp to do that. May not bother since the 12XL works so well
>off of the internal antenna although an antenna that I could stuff
>between the shade and window on commercial jets would be nice.
You don't need an active antenna, you just need to draw some current.
I tried connecting a resistor in parallel with the W1GE passive patch
antenna and then seeing whether the 12XL was using the internal or
external antenna by shielding one and then the other. A 1 kohm
resistor did *not* draw sufficient current - the 12XL was still using
the internal antenna. But a 560 ohm resistor was sufficient to cause
the 12XL to switch to the external signal. So the switching point is
somewhere between 5 and 9 mA.
I didn't try it, but this should work with the '45 antenna too - just
build some sort of adaptor that lets you put a 500 ohm resistor in
parallel with the signal at some point between the antenna and 12XL.
This is a minor hassle when trying to use an external passive antenna
on the 12XL, but no problem with external active antenna. And it's a
good thing in general: disconnecting the internal antenna means that it
does not act as a load on the external signal, and (perhaps more
important) you can never get the signal from two antennas in two
different places summing, which can cause the signals to cancel.
I'll praise Garmin for putting in the components that do the switching,
and criticize them for saying nothing about it in the manual.
Dave
Test method: The Trimble "miniature OEM" antenna produces nice strong
GPS signals. When connected directly to either a GPS 45XL or a 12XL,
the signal quality bars are noticeably higher than when using the
standard antenna (external passive for 45XL, internal for 12XL).
When the Trimble is connected to the 12XL, shielding the Trimble
antenna so it doesn't see any satellites causes the 12XL to lose all
signal, even though the 12XL's internal antenna is not blocked.
Unplugging the Trimble causes the 12XL to start tracking again. This
demonstrates that when an external antenna is drawing current the
internal antenna is ignored.
I then connected the Trimble to a small circuit that independently
provides +5 V to the antenna through an RF choke, and passes the RF
signals to its output jack through a 100 pF capacitor that blocks DC.
When this is connected to a GPS unit, a strong RF signal is present,
but no current is drawn from the GPS's antenna jack. Essentially, it
looks like a *really* good passive antenna is connected to the GPS.
When this circuit is connected to a 45XL, it sees nice strong signals,
as strong as when the Trimble is connected directly to the antenna
jack. But when this is connected to the 12XL, it completely ignores
the strong GPS signals on its external antenna input, and uses only its
internal antenna (verified by blocking the internal and external
antenna alternately). So with no current drawn, the external antenna
is ignored, even when its signal is considerably stronger than the
internal antenna's signal.
Similarly, the W1GE antenna (a true open-circuit passive antenna) works
well on the 45XL but is ignored by the 12XL. Folding the leads of a
560 ohm resistor and stuffing it between the antenna and ground of the
W1GE draws enough current for the 12XL to switch to the external
antenna. (WARNING: if you try this yourself, disconnect the antenna
from the GPS, install the resistor, check it with an ohmmeter, and then
reconnect it to the GPS. If you cause a short when the antenna is
connected to the GPS, you might blow a fuse supplying power to the
antenna jack. I don't recommend finding out.)
Dave Martindale
"Greg Ottria" <Got...@Earthlink.net> writes:
>So the Trimble mini mag antenna will work ok, just need the coax adaptor?
The Trimble, the Garmin GA-26, and any other active antenna that draws
at least 9 mA at 5 V should work. (Perhaps even less than 9 mA; the
threshold is somewhere between 5 and 9).
Dave