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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