Today's Message Index:
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1. 01:36 AM - Re: Garmin GNS430 Nav board repaired twice (RAS)
2. 05:00 AM - Garmin GNS430 Nav board repaired twice ()
3. 09:16 AM - Re: Garmin GNS430 Nav board repaired twice (=?windows-1252?Q?Andr=E9_Beusch?=)
4. 10:48 AM - Re: Re: Garmin GNS430 Nav board repaired twice (John Rippengal)
Message 1
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| Subject: | Re: Garmin GNS430 Nav board repaired twice |
Hi,
FWIW, there's a section in the GNS 530 manual about the location of the comm
antenna because of it's power output. It transmits at 16W. Perhaps you can
have a look at a updated manual for the 430.
Marcel
----- Original Message -----
From: "Andr Beusch" <andre.beusch@bluewin.ch>
Sent: Sunday, December 10, 2006 9:44 PM
Subject: Avionics-List: Garmin GNS430 Nav board repaired twice
> <andre.beusch@bluewin.ch>
>
> I have a Garmin GNS430 in my Glasair Super II.
> The comm is connected to a dipole antenna in the vertical stabilizer, the
> nav is connected directly (no splitter) to the dipole antenna in the
> horizontal stabilizer.
> Both are the antennas that were provided by Stoddard Hamilton, the kit
> manufacturer, and were installed as per the instruction manual. The
> antennas perform well. Both times, the VOR/LOC lost 30 dB of sensitivity,
> apparently the input stage was destroyed.
> The first repair cost me about $1000, the second was warranty.
> I wonder if someone had this problem with any kind of nav receiver.
> Garmin said that it could have been overloaded by the comm transmission
> because the antennas are to close to each other.
> (they don't say anything about this in the installation manual)
> Many people use a splitter for NAV/glideslope, so their receiver gets
> less signal and would perhaps not be exposed to this problem.
> As an electronics engineer, I'd like to understand this and will make a
> measurement of the power that actually gets in the Nav receiver. I have a
> 250 MHz oscilloscope for that.
> This nav receiver should pass RTCA DO-196, if someone has these documents
> handy, I'd be interested to see what the damage input power should be.
> I also consider putting a 6dB attenuator or an RF limitter on the Nav
> antenna input.
>
> Any opinions?
>
>
> Thanks, --Andre Beusch
>
>
>
Message 2
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| Subject: | Garmin GNS430 Nav board repaired twice |
12/12/2006
Hello Andre, I am trying to better understand your antenna connections.
You wrote: "The comm is connected to a dipole antenna in the vertical
stabilizer,
the nav is connected directly (no splitter) to the dipole antenna in the
horizontal stabilizer."
The GNS 430 box has two separate inputs for VHF nav and glide slope. As you
say this is frequently done by using one VHF nav antenna and then splitting
that input just before it enters the GNS 430 box.
When you say that your nav is connected directly (no splitter) to the
antenna does that mean that you are also feeding the glide slope input
directly from some separate glide slope antenna?
What is the status of the unit now? You have it back repaired and are not
using it for concern over damaging it again with comm transmissions or ---?
Comant industries has a wide array of couplers that you might consider using
to combine and or split inputs to your GNS 430 from nav / glide slope
antennas in order to protect the input stages from excessive comm
transmission inputs.
http://www.comant.com/home.cgi?ua=sgroup&crit=Couplers/Diplexers/Combiners
OC
> Time: 01:45:56 PM PST US
> From: =?ISO-8859-1?Q?Andr=E9_Beusch?= <andre.beusch@bluewin.ch>
> Subject: Avionics-List: Garmin GNS430 Nav board repaired twice
>
>
> I have a Garmin GNS430 in my Glasair Super II.
> The comm is connected to a dipole antenna in the vertical stabilizer,
> the nav is connected directly (no splitter) to the dipole antenna in the
> horizontal stabilizer.
> Both are the antennas that were provided by Stoddard Hamilton, the kit
> manufacturer, and were installed as per the instruction manual. The
> antennas perform well.
> Both times, the VOR/LOC lost 30 dB of sensitivity, apparently the input
> stage was destroyed.
> The first repair cost me about $1000, the second was warranty.
> I wonder if someone had this problem with any kind of nav receiver.
> Garmin said that it could have been overloaded by the comm transmission
> because the antennas are to close to each other.
> (they don't say anything about this in the installation manual)
> Many people use a splitter for NAV/glideslope, so their receiver gets
> less signal and would perhaps not be exposed to this problem.
> As an electronics engineer, I'd like to understand this and will make a
> measurement of the power that actually gets in the Nav receiver. I have
> a 250 MHz oscilloscope for that.
> This nav receiver should pass RTCA DO-196, if someone has these
> documents handy, I'd be interested to see what the damage input power
> should be.
> I also consider putting a 6dB attenuator or an RF limitter on the Nav
> antenna input.
>
> Any opinions?
>
>
> Thanks, --Andre Beusch
Message 3
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| Subject: | Re: Garmin GNS430 Nav board repaired twice |
>
> When you say that your nav is connected directly (no splitter) to the
> antenna does that mean that you are also feeding the glide slope input
> directly from some separate glide slope antenna?
Yes, one antenna for VOR in the horizontal stabilizer, and one for the
GS, on the floor behind the
baggage compartment.
>
> What is the status of the unit now? You have it back repaired and are
> not using it for concern over damaging it again with comm
> transmissions or ---?
I have disconnected the VOR antenna. I will install either a 6 dB
attenuator or an rf limiter and hope it will not happen again.
>
>
I have now measured the voltage across a 50 Ohms terminator at the VOR
antenna connector when transmitting with the GNS430 com (no modulation).
The maximum measured voltage in the com frequency range was about 3.5
Vpp (at 123 MHz), which equals to 1.24 Vrms.
That is 14.9 dBm, at 50 Ohms a power of 31mW.
The peak power at 70 % modulation would be 87 mW. (19.4 dBm)
I don't know the spec of the GNS430 nav receiver, but IMO, this level
should not be destructive.
--Andre
Message 4
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| Subject: | Re: Garmin GNS430 Nav board repaired twice |
Andre,
Remembering that transistors can be destroyed in microseconds with
overvoltages you should be thinking in terms of peak rather than RMS
voltages. Moreover measuring the voltge using a 50 ohm termination neglects
the most likely possibility that there is a standing wave on that line and
the termination looking into the NAV receiver is nothing like 50 ohms at the
comms frequency.
I repeat, change the antenna position; it should be easy enough with a
plastic aeroplane. Unless of course you want to make a case against Garmin
and recover your outrageous repair bill.
John
From: "Andr Beusch"
> <andre.beusch@bluewin.ch>
>
>
>>
>> When you say that your nav is connected directly (no splitter) to the
>> antenna does that mean that you are also feeding the glide slope input
>> directly from some separate glide slope antenna?
>
>
> Yes, one antenna for VOR in the horizontal stabilizer, and one for the GS,
> on the floor behind the
> baggage compartment.
>
>>
>> What is the status of the unit now? You have it back repaired and are not
>> using it for concern over damaging it again with comm transmissions
>> or ---?
>
>
> I have disconnected the VOR antenna. I will install either a 6 dB
> attenuator or an rf limiter and hope it will not happen again.
>
>>
>>
> I have now measured the voltage across a 50 Ohms terminator at the VOR
> antenna connector when transmitting with the GNS430 com (no modulation).
>
> The maximum measured voltage in the com frequency range was about 3.5 Vpp
> (at 123 MHz), which equals to 1.24 Vrms.
> That is 14.9 dBm, at 50 Ohms a power of 31mW.
> The peak power at 70 % modulation would be 87 mW. (19.4 dBm)
>
> I don't know the spec of the GNS430 nav receiver, but IMO, this level
> should not be destructive.
>
> --Andre
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