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myths about gps antennas

 
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kjlpdx



Joined: 18 Dec 2018
Posts: 4

PostPosted: Wed Dec 19, 2018 12:23 pm    Post subject: myths about gps antennas Reply with quote

I'm interested in hearing from an engineer who might know about what really goes on with my stratus esg gps antenna. they say to use RG400 cable, 10' minimum, and never paint the antenna. they also told me the strength ratings should run in the 30's. I have 3' of RG400 with a mechanical 90° elbow, I spray painted the antenna flat black and mounted it on the glareshield of my RV. my readings are all in the 40's. they told me this shows an overdriven condition.
so why use low impedance cable then need 10'? my iFly740 and Lowrance 1000 gps's have antennas with much smaller cables. the iFly shows WAAS grade accuracy, using an antenna 1/4 the size of the stratus RAMI antenna.
when does being overdriven prove a problem? is this a situation where I could add more mechanical connectors to increase impedance rather than more cable length?
I get the feeling things get written and then forever repeated, such as never paint an antenna, without asking any questions. could I use a different antenna than the RAMI brand? should I not worry about any of this and just go fly?


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gilles.thesee(at)free.fr
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PostPosted: Wed Dec 19, 2018 12:41 pm    Post subject: myths about gps antennas Reply with quote

Le 19/12/2018 à 21:24, kjlpdx a écrit :
Quote:
I get the feeling things get written and then forever repeated, such as never paint an antenna, without asking any questions.

Paint could be a problem with *some* conductive paints.

Quote:
could I use a different antenna than the RAMI brand?

Yes.
Quote:
should I not worry about any of this and just go fly?
I suggest you submit your questions to the Aeroelectric List at Matronics.

You'll get really knowledgeable info.
BTW, as you mentioned the 10' minimum coax length looks like some hangar
tale...
--
Best regards,
Gilles
http://contrails.free.fr
http://lapierre.skunkworks.free.fr


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ceengland7(at)gmail.com
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PostPosted: Wed Dec 19, 2018 2:02 pm    Post subject: myths about gps antennas Reply with quote

On 12/19/2018 2:24 PM, kjlpdx wrote:
Quote:


I'm interested in hearing from an engineer who might know about what really goes on with my stratus esg gps antenna. they say to use RG400 cable, 10' minimum, and never paint the antenna. they also told me the strength ratings should run in the 30's. I have 3' of RG400 with a mechanical 90° elbow, I spray painted the antenna flat black and mounted it on the glareshield of my RV. my readings are all in the 40's. they told me this shows an overdriven condition.
so why use low impedance cable then need 10'? my iFly740 and Lowrance 1000 gps's have antennas with much smaller cables. the iFly shows WAAS grade accuracy, using an antenna 1/4 the size of the stratus RAMI antenna.
when does being overdriven prove a problem? is this a situation where I could add more mechanical connectors to increase impedance rather than more cable length?
I get the feeling things get written and then forever repeated, such as never paint an antenna, without asking any questions. could I use a different antenna than the RAMI brand? should I not worry about any of this and just go fly?

disclaimer: I'm not an engineer (but I do play with airplanes with a

couple of them)

Like Gilles said, paint *can* cause problems in some situations. Not
just with reception, but also by getting on the connector(s). Trust me;
somebody will find a way to paint the entire thing. I'm pretty sure you
won't find any antenna mfgr that says it's ok to paint their antenna,
because the paint (and the painter) is outside their control. Some
antennas are black from the mfgr, so that's obviously ok for them. But
if it wasn't black to start with, do you know for a fact that converting
it to a 'black body' heat absorber won't affect its performance in the
sun? (Another potential reason to restrict painting.)

FWIW, the cable isn't 'low impedance'; it's low loss (that usually means
that it has lower capacitance per foot). Most aviation stuff is '50 ohm'
(impedance) cable, but poorer cables can have higher capacitance per
foot, meaning that their actual impedance  at different frequencies will
vary more over longer lengths than the higher quality, lower capacitance
cable. The mfgr knows their device, the frequencies over which it will
operate, the capacitance of their chosen cable, and whether everything
will stay within their needed limits at a given cable length for all
frequencies and signal levels (which can vary quite a bit). You might
get away with what you're doing with no issues whatsoever. But you might
not. And the mfgr isn't likely to tell you it's ok, because they likely
haven't tested the *system* configured the way you configured it. Think
about buying a new fuel injector for your Chevy. Would Porsche tell you
that it'll work fine in your 911?

Charlie

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