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Garmin GNS430 Nav board repaired twice

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andre.beusch(at)bluewin.c
Guest

Posted: Sun Dec 10, 2006 1:45 pm Post subject: 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

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j.rippengal(at)cytanet.co
Guest

Posted: Sun Dec 10, 2006 9:56 pm Post subject: Garmin GNS430 Nav board repaired twice

Bite the bullet Andre and change the position of the Nav antenna.

John



From: "André Beusch"



<andre.beusch(at)bluewin.ch>

 	  Quote: 	
		  

 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

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deruiteraircraftservices(
Guest

Posted: Tue Dec 12, 2006 1:36 am Post subject: 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



---

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bakerocb

Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA

Posted: Tue Dec 12, 2006 5:00 am Post 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



 	  Quote: 	
		   Time: 01:45:56 PM PST US

 From: =?ISO-8859-1?Q?Andr=E9_Beusch?= <andre.beusch(at)bluewin.ch>

 Subject: 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

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j.rippengal(at)cytanet.co
Guest

Posted: Tue Dec 12, 2006 10:48 am Post subject: 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: "André Beusch"



 	  Quote: 	
		   

 <andre.beusch(at)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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