Curtis;
I have the Apollo GX65 GPS installed in our aircraft. I installed the GPS
antenna
just as it came out of the box onto a fiberglass flange bonded behind the
baggage
compartment. There have been no problems with acquiring signals.
Jim Brown
Fred Fillinger wrote:
> Curtis Jaussi wrote:
> >
> > I have several questions on GPS antennas for panel mount.
> > 1. The Garmin instructions refer to a 15" radius ground plane. How is this
being done in > > Europa and is it necessary?
> >
>
> UPS-AT says 18" ground plane. It's weird, in that it's counter to the
> general rule for ground plane = 1/4 wavelength (less than pinky finger
> for GPS). A handheld GPS can't fake a ground plane that big, and my
> UPS-AT panel map-comm works fine inside the house w/o such. It may
> not be so much performance as mitigating effects of interference
> re-radiating from your comm or ELT antenna, in presence of strong
> fields, or your comm's xmit. Even there, VHF comm transmissions
> (except for ATIS) are short and thus no big deal.
>
> It can be made with alum foil, but I hate to keep adding weight, not
> just foil, but the darn glue. If desired, a 15", 4-wire radial and
> duct tape ground plane can be tried later, if there's a problem.
> Unlike a metal airplane, all we're doing is adding more radiating
> elements inside a small plastic plane so as to mess something else
> up.
>
> > 2. I have seen some indications of splitting and sharing antenna with two
> > nav
radios. Can this be done with comm radios and can the GPS antenna be shared
with tow GPS/
>
> I don't think you'll ever see a GPS splitter. The signals are too
> weak to suffer the mutual loss of signal. Also, the center wire feeds
> DC to the antenna, so we can't design a splitter that interconnects
> that DC also, and wait to see who calls back about why their $3,000
> GPS box mysteriously conked. Circuitry to prevent that causes further
> losses.
>
> There are splitters for comm, but they have to prevent xmit power from
> feeding back to the other comm. A relay design does this, but one
> catalog I have wants you to pay $409 for the privilege of a huge 3db
> loss.
>
> > 3. If two GPS antennas are needed how close can may they be to each other
> > without
interference problems.
>
> UPS-AT is also silent on this, but theoretically a few inches is
> enough, as they don't xmit. More than that would seem to not pose a
> layout prob on the fuselage top - later rectification method being
> unappealing?
>
> Best,
> Fred F.
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