Hopefully this replies to the group!
As regards groundplanes, I have not tried alternatives, but the groundplane
does not have to be flat. Locally ining the inside of the fuselage with
heavy duty tinfoil should work provided it is well connected to the antenna
connector outer (i.e. be careful not to let sealants or other non-conductive
pastes spoil the connection, or corrosion for that matter). Antenna for
pressurised aircraft are often sold with special conductive gaskets to
achieve this.
You can share one GPS antenna between two recievers provided you use a
proper RF splitter/divider and dc-block. The antenna has a pre-amplifier
inside it, and this must be powered by one receiver. The other reciever must
be connected to the splitter output using a dc block.This stops the 5 volts
(or whatever it produces) connecting to the other receiver. Try a web search
for the company 'Narda' who make such things. (They are very small, and
usually have those tiny little SMA type connectors). You need one that works
at around 1.2GHz, the civilian GPS frequency. Trimble used to make one with
built in dc-blocks on all but the primary connection. Don't worry if you can
only find ones with 3 or 4 outputs, as the unused outputs cause minimal loss
and do not have to be terminated (but you could put 50 Ohm terminations on
unused outputs if you want to prevent crud getting into the connectors).
As regards antennae spacing, the general rule is to have them no closer than
half a wavelength. I can't recall how to convert frequency (1.2E+9) into
wavelength, but it involves the constant 'c', the speed of light. From
memory it is only a few inches for GPS. However, this only applies to
passive aerials. If the aerial has a built in pre-amplifier (and most have),
then this can actually radiate noise that can be picked up by another
aerial, so spoiling it's performance. With these aerials, about 18 inches
(0.5m) of separation should be considered a minimum.
I'm not saying that any other separation will not work, but really you owe
it to yourself and your aeroplane to get the best performance that you can.
Ian.
G BVVP
----- Original Message -----
From: "Curtis Jaussi" <jaussi@easilink.com>
Subject:
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?
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/
3. If two GPS antennas are needed how close can may they be to each other
without interference problems.
Hope you avionics people out ther can hel.
I am a new builder ( since Dec.) but have been watching this forum since
1997 and feel almost like family. Many thanks to all who contribute. This
is the first time I have not been able to find an answer in the archives. I
doubt i could build this aircraft successfully with out the forum.
Curtis Jaussi Kit 216 N4270G
jaussi@easilink.com
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