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GNS 430W Antenna Requirements

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bakerocb

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

Posted: Fri Mar 09, 2007 5:13 am Post subject: GNS 430W Antenna Requirements

3/9/2007



Hello List Experts, It is fairly well documented that Garmin wants high 

quality antenna cable (RGU 142 or 400) to be used for the GNS 430W and it 

must be between 13 and 35 feet long.



I can understand a maximum limit. I cannot understand the physics or 

electronic principles behind the minimum length requirement.



Can someone please educate me? Thanks.



OC -- The best investment we can make is the effort to gather and understand 

knowledge.

reichec

Joined: 23 Apr 2007
Posts: 26

Posted: Mon Apr 23, 2007 6:26 pm Post subject: Re: GNS 430W Antenna Requirements

The input signal to the 430w must be at a maximum XdB (some number)   the antenna has a gain factor of like 25dB and therefore there must be some loss to get it down to XdB  thus 13 feet..  I beleive the magic number at the reciever is 20dB signal strength but not sure.

Got it?

Charles

flyboy.bob(at)gmail.com
Guest

Posted: Mon Apr 23, 2007 6:52 pm Post subject: GNS 430W Antenna Requirements

I doubt that it's to attenuate the signal as suggested?  More like

it's to avoid having the feedline (coax) be susceptible to picking

interference (acting like an antenna) at certain frequencies?  I

wouldn't worry so much why but I'd follow there suggestions.



Regards,

Bob in SE Iowa

RV-8 Finishing - Slowly



On 3/9/07, bakerocb(at)cox.net <bakerocb(at)cox.net> wrote:

 	  Quote: 	
		   



 3/9/2007



 Hello List Experts, It is fairly well documented that Garmin wants high

 quality antenna cable (RGU 142 or 400) to be used for the GNS 430W and it

 must be between 13 and 35 feet long.



 I can understand a maximum limit. I cannot understand the physics or

 electronic principles behind the minimum length requirement.



 Can someone please educate me? Thanks.



 OC -- The best investment we can make is the effort to gather and understand

 knowledge.

w_sweet(at)comcast.net
Guest

Posted: Mon Apr 23, 2007 7:59 pm Post subject: GNS 430W Antenna Requirements

I have my GNS430W installed with the antenna mounted on the aft of the 

canopy (BUSHBY MustangII); the RG400 coax is about 8 feet in length. No 

noticeable difference from the same location prior to the WAAS update.

BTW, I'm having a ball flying GPS approached with ILS like display. In fact 

the needles are much calmer than on a ILS approach, and no worry about false 

lobes.   ))  Only problem (small), from the approach plate can't tell if 

there will be an active GS till I get on the approach and get the LNAV+V 

indication. The GS comes active on the waypoint just outside the FAF.

Wayne



---

bakerocb

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

Posted: Tue Apr 24, 2007 2:20 am Post subject: GNS 430W Antenna Requirements

4/24/2007



Hello Wayne, Thanks for your input. One more question if I may.



I have read that the connector on the end of the coax cable that connects to 

the antenna must be a male TNC connector. Can you please confirm that?



Also I'd appreciate it if you can shed any light on what additional 

equipment, antenna, black box, or...........? would be needed if one wanted 

to have XM weather appear on their GNS 430W.



Thanks.



OC -- The best investment we can make is the effort to gather and understand 

knowledge.



Time: 08:59:22 PM PST US

From: "Wayne Sweet" <w_sweet(at)comcast.net>

Subject: Re: GNS 430W Antenna Requirements

I have my GNS430W installed with the antenna mounted on the aft of the

canopy (BUSHBY MustangII); the RG400 coax is about 8 feet in length. No

noticeable difference from the same location prior to the WAAS update.

BTW, I'm having a ball flying GPS approached with ILS like display. In fact

the needles are much calmer than on a ILS approach, and no worry about false

lobes.   ))  Only problem (small), from the approach plate can't tell if

there will be an active GS till I get on the approach and get the LNAV+V

indication. The GS comes active on the waypoint just outside the FAF.

Wayne

w_sweet(at)comcast.net
Guest

Posted: Tue Apr 24, 2007 5:39 am Post subject: GNS 430W Antenna Requirements

Yes, it does need a TNC connector at the antenna; the dealer that sent mine 

in to Garmin for the WAAS upgrade made a pigtail TNC to BNC for me ( $10.00 

)) ) . Don't know about what additional boxes needed for XM.

Wayne



---

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Tue Apr 24, 2007 3:23 pm Post subject: GNS 430W Antenna Requirements

The impedance the receiver wants to have is achieved with the stated cable

at the stated lengths.  Too short or too long and the sensitivity of the

receiver will suffer.



Noel



[quote] --

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

j.rippengal(at)cytanet.co
Guest

Posted: Tue Apr 24, 2007 9:56 pm Post subject: GNS 430W Antenna Requirements

The wavelength of the GPS signal is around 8 inches so the advice from 

Garmin to use coax feeder of anywhere between 13ft and 35 ft length can have 

nothing whatsoever to do with the impedance seen by the receiver. The 35ft 

is almost certainly the maximum to use without attenuating the signal too 

much. I don't understand the 8ft minimum like 'bakerocb' but it may possibly 

be to avoid overloading with too high a signal as he suggests but anyway 

it's surprising.



Also the minimum length secures the maximum freedom from pickup.



Those suggesting specific lengths aid impedance matching or minimising 

pickup are thinking in terms of HF radio techniques where it is not 

uncommon, at least in HAM circles to use coax feeder with large standing 

waves on them (high VSWR). It involves cutting the feeder to a fraction of a 

quarter wavelength. (in this case within - say a quarter of and inch). You 

wouldn't do that at 1500Mhz.



As has already been advised - if Garmin says so, just do it.



John Rippengal



From: "Noel Loveys" <noelloveys(at)yahoo.ca>

 	  Quote: 	
		   



 The impedance the receiver wants to have is achieved with the stated cable

 at the stated lengths.  Too short or too long and the sensitivity of the

 receiver will suffer.



 Noel



> From: owner-avionics-list-server(at)matronics.com

>

> 

>

> I doubt that it's to attenuate the signal as suggested?  More like

> it's to avoid having the feedline (coax) be susceptible to picking

> interference (acting like an antenna) at certain frequencies?  I

> wouldn't worry so much why but I'd follow there suggestions.

>

> Regards,

> Bob in SE Iowa

> RV-8 Finishing - Slowly

>

> On 3/9/07, bakerocb(at)cox.net <bakerocb(at)cox.net> wrote:

> > 

> >

> > 3/9/2007

> >

> > Hello List Experts, It is fairly well documented that

> Garmin wants high

> > quality antenna cable (RGU 142 or 400) to be used for the

> GNS 430W and it

> > must be between 13 and 35 feet long.

> >

> > I can understand a maximum limit. I cannot understand the physics or

> > electronic principles behind the minimum length requirement.

> >

> > Can someone please educate me? Thanks.

> >

> > OC -- The best investment we can make is the effort to

> gather and understand

> > knowledge.

Hopperdhh(at)aol.com
Guest

Posted: Fri Apr 27, 2007 7:12 am Post subject: GNS 430W Antenna Requirements

In a message dated 4/25/2007 1:57:37 AM Eastern Standard Time, 

j.rippengal(at)cytanet.com.cy writes:


<j.rippengal(at)cytanet.com.cy>



The wavelength of the GPS signal is around 8 inches so the advice from 

Garmin to use coax feeder of anywhere between 13ft and 35 ft length can have 

nothing whatsoever to do with the impedance seen by the receiver. The 35ft 

is almost certainly the maximum to use without attenuating the signal too 

much. I don't understand the 8ft minimum like 'bakerocb' but it may possibly 

be to avoid overloading with too high a signal as he suggests but anyway 

it's surprising.

 

 

 

 

 

 

The 13 ft minimum may be because the loss in the coax tends to make the 

impedance seen by the receiver closer to the characteristic impedance of the coax, 

regardless of the impedance of the antenna.  This has the effect of moderating 

the impedance variation due to the mismatch.  When the line is too short the 

loss is not enough to have this effect.

 

Dan Hopper

K9WEK

 



************************************** See what's free at http://www.aol.com.

j.rippengal(at)cytanet.co
Guest

Posted: Fri Apr 27, 2007 7:32 am Post subject: GNS 430W Antenna Requirements

I understand what you are saying Dan but the impedance the receiver sees is 

not a big deal unless it is quite violently wrong, like a near dead short 

for instance, especially in the case of a short feeder when the signal will 

be strong.  In any case I believe that pretty well all the antennas supplied 

are active and supplied with DC power via the feeder. I don't see it as any 

difficulty at all to design the amplifier with the right output impedance to 

as close a limit as necessary, so the length of the feeder will be 

immaterial so far as the impedance the receiver sees is concerned.

John

From: <Hopperdhh(at)aol.com>

 	  Quote: 	
		   



 In a message dated 4/25/2007 1:57:37 AM Eastern Standard Time,

 j.rippengal(at)cytanet.com.cy writes:

 

 <j.rippengal(at)cytanet.com.cy>



 The wavelength of the GPS signal is around 8 inches so the advice from

 Garmin to use coax feeder of anywhere between 13ft and 35 ft length can 

 have

 nothing whatsoever to do with the impedance seen by the receiver. The 35ft

 is almost certainly the maximum to use without attenuating the signal too

 much. I don't understand the 8ft minimum like 'bakerocb' but it may 

 possibly

 be to avoid overloading with too high a signal as he suggests but anyway

 it's surprising.

 The 13 ft minimum may be because the loss in the coax tends to make the

 impedance seen by the receiver closer to the characteristic impedance of 

 the coax,

 regardless of the impedance of the antenna.  This has the effect of 

 moderating

 the impedance variation due to the mismatch.  When the line is too short 

 the

 loss is not enough to have this effect.



 Dan Hopper

 K9WEK

j.rippengal(at)cytanet.co
Guest

Posted: Fri Apr 27, 2007 7:40 am Post subject: GNS 430W Antenna Requirements

On further thoughts, Dan, it seems you are saying that if you shorten the 

cable the signal can actually be weaker ie if you start with 8 ft and reduce 

it to 6ft then the signal is lower. I don't think so unless the impedance 

conditions are so extreme as to be very unlikely in any practical situation. 

Even then I have my doubts but have not worked it all through. It is not a 

simple problem.

John



From: <Hopperdhh(at)aol.com>

 	  Quote: 	
		   j.rippengal(at)cytanet.com.cy writes:



 The wavelength of the GPS signal is around 8 inches so the advice from

 Garmin to use coax feeder of anywhere between 13ft and 35 ft length can 

 have

 nothing whatsoever to do with the impedance seen by the receiver. The 35ft

 is almost certainly the maximum to use without attenuating the signal too

 much. I don't understand the 8ft minimum like 'bakerocb' but it may 

 possibly

 be to avoid overloading with too high a signal as he suggests but anyway

 it's surprising.

 The 13 ft minimum may be because the loss in the coax tends to make the

 impedance seen by the receiver closer to the characteristic impedance of 

 the coax,

 regardless of the impedance of the antenna.  This has the effect of 

 moderating

 the impedance variation due to the mismatch.  When the line is too short 

 the

 loss is not enough to have this effect.



 Dan Hopper

 K9WEK

rjquillin(at)gmail.com
Guest

Posted: Fri Apr 27, 2007 9:14 am Post subject: GNS 430W Antenna Requirements

At 08:11 4/27/2007, you wrote:

 	  Quote: 	
		  The wavelength of the GPS signal is around 8 inches so the advice from

Garmin to use coax feeder of anywhere between 13ft and 35 ft length can have

nothing whatsoever to do with the impedance seen by the receiver. The 35ft

is almost certainly the maximum to use without attenuating the signal too

much. I don't understand the 8ft minimum like 'bakerocb' but it may possibly

be to avoid overloading with too high a signal as he suggests but anyway

it's surprising.


	


As mentioned, 1.5gHz is just under 20 cm; ~ 8 inches.

While the length of the transmission line will have little to do with 

the input Z seen by the receiver, it's (the coax) impedance, and 

those of the connectors and terminations, most certainly will affect 

the overall performance of the system.  Were the coax and connectors 

not closely matched to the expected source and load impedances a 

mismatch will occur, possibly creating an unacceptably high signal 

loss.  Even with correct connectors and proper terminations, losses 

in excess of 1dB are to be expected.  Additionally standing waves, 

reflections, will occur in the line causing additional losses and 

time delays of reflected signals.



The antennas we use are active devices, they include a fixed gain 

amplifier, and they do receive their power via the coax.  The power 

for the unit will be DC coupled to the coax.  The signal output will 

be AC coupled, and likely designed for maximum (signal) power 

transfer; matched impedances.



*IF* the X30W boxes are at all similar to the CNX-80/GNS-480, the 

receiver front end is designed to accept a signal within certain 

limits.  The antenna/transmission line signal presented to the input 

of the receiver must be high enough to provide an acceptable S/N 

ratio for the receiver to acquire and track signals; however it 

cannot be so strong as to create additional unwanted signals in the 

front end due to overloads from excessive input level.  The IM for 

the 80/480 units provides input gain calibration to compensate for 

feed line length losses.



The specified minimum length may be to insure excessive signal does 

not reach the input stage of the receiver.  However, considering line 

reflections, it may also be to insure any reflections are 

sufficiently delayed by cable length to be outside some time 

windowing function in the receiver to reject obviously invalid 

signals.  For RG-400, with a propagation delay of 69.4%, that eight 

feet works out to about 23.5 ns or about 35 times longer than our 

signal of interest and pretty easy to ignore.



Just another possibility.



Ron Q.

NJ9W

Tim Olson

Joined: 25 Jan 2007
Posts: 2871

Posted: Fri Apr 27, 2007 9:40 am Post subject: GNS 430W Antenna Requirements

This situation has always left me with one other question:

It's easy for me with my GNS-480 to just do it "by the book"

and have some extra length there to meet the install documentation.

My question is though, in my WSI installation they make no notes

about this sort of thing.  Being another satellite based

signal of low power, I wondered if indeed the same situation

would apply to that installation as well, even though they don't

mention cable length requirements.

Comments?

Tim Olson

do not archive

Ron Quillin wrote:

 	  Quote: 	

 At 08:11 4/27/2007, you wrote:

> The wavelength of the GPS signal is around 8 inches so the advice from

> Garmin to use coax feeder of anywhere between 13ft and 35 ft length 

> can have

> nothing whatsoever to do with the impedance seen by the receiver. The 

> 35ft

> is almost certainly the maximum to use without attenuating the signal too

> much. I don't understand the 8ft minimum like 'bakerocb' but it may 

> possibly

> be to avoid overloading with too high a signal as he suggests but anyway

> it's surprising.

 As mentioned, 1.5gHz is just under 20 cm; ~ 8 inches.

 While the length of the transmission line will have little to do with 

 the input Z seen by the receiver, it's (the coax) impedance, and those 

 of the connectors and terminations, most certainly will affect the 

 overall performance of the system.  Were the coax and connectors not 

 closely matched to the expected source and load impedances a mismatch 

 will occur, possibly creating an unacceptably high signal loss.  Even 

 with correct connectors and proper terminations, losses in excess of 1dB 

 are to be expected.  Additionally standing waves, reflections, will 

 occur in the line causing additional losses and time delays of reflected 

 signals.

 The antennas we use are active devices, they include a fixed gain 

 amplifier, and they do receive their power via the coax.  The power for 

 the unit will be DC coupled to the coax.  The signal output will be AC 

 coupled, and likely designed for maximum (signal) power transfer; 

 matched impedances.

 *IF* the X30W boxes are at all similar to the CNX-80/GNS-480, the 

 receiver front end is designed to accept a signal within certain 

 limits.  The antenna/transmission line signal presented to the input of 

 the receiver must be high enough to provide an acceptable S/N ratio for 

 the receiver to acquire and track signals; however it cannot be so 

 strong as to create additional unwanted signals in the front end due to 

 overloads from excessive input level.  The IM for the 80/480 units 

 provides input gain calibration to compensate for feed line length losses.

 The specified minimum length may be to insure excessive signal does not 

 reach the input stage of the receiver.  However, considering line 

 reflections, it may also be to insure any reflections are sufficiently 

 delayed by cable length to be outside some time windowing function in 

 the receiver to reject obviously invalid signals.  For RG-400, with a 

 propagation delay of 69.4%, that eight feet works out to about 23.5 ns 

 or about 35 times longer than our signal of interest and pretty easy to 

 ignore.

 Just another possibility.

 Ron Q.

 NJ9W

rjquillin(at)gmail.com
Guest

Posted: Fri Apr 27, 2007 12:06 pm Post subject: GNS 430W Antenna Requirements

At 10:39 4/27/2007, you wrote:

 	  Quote: 	
		  I wondered if indeed the same situation

would apply to that installation as well, even though they don't

mention cable length requirements.


	


Actually there is mention of maximum, but no minimum, cable length, 

but only so the total loss does not exceed 10dB.



Garmin however with the GDL series receivers, unlike WSI and Sandia, 

is very specific with regard to antenna gain and cable loss to the 

receiver, and does use TSO'd antennas.



Purely a WAG here.



Satellite weather has little impact on immediate safety of flight, 

navigation, as far as the FAA is concerned.

Since weather is advisory only, and currently there are no TSO 

requirements, receivers don't have to meet the requirements of TSO 

C146a as do receivers used for navigation.  It may only be logical, 

lacking any guaranteed reception requirements, there may not be 

motivation from the manufacturer to insure or guarantee a specific 

quality level of reception and therefore require a minimum line 

length to meet those requirements.



Ron Q.

Hopperdhh(at)aol.com
Guest

Posted: Fri Apr 27, 2007 12:07 pm Post subject: GNS 430W Antenna Requirements

John,

 

No, I was not saying that the signal would be weaker if you shorten the 

cable.  I'm really not sure how important the match is.  But, Ron brings out a good 

point.  Any mismatch would cause reflections on the line and confuse the 

decoding of the data stream.  The time delay explanation is probably more correct. 

 The loss in 13 feet of RG-400 (from a quick search for the characteristics 

and some interpolation) comes out to about 3 dB, which could be necessay to 

weaken any reflections.  The reflections would travel down and back which would 

make them 6 dB weaker.  Reflections only occur if there is a mismatch, and I 

agree that there shouldn't be very much of a mismatch, but there will be some 

given the tolerance of production parts -- meaning the amplifier chips.  I'll 

admit that I'm in over my head here!  So, who knows the real answer?

 

Dan K9WEK

 

 


(mailto:rjquillin(at)gmail.com) >

 

At 08:11 4/27/2007, you wrote:

 	  Quote: 	
		  The wavelength of the GPS signal is around 8 inches so the advice from

Garmin to use coax feeder of anywhere between 13ft and 35 ft length can have

nothing whatsoever to do with the impedance seen by the receiver. The 35ft

is almost certainly the maximum to use without attenuating the signal too

much. I don't understand the 8ft minimum like 'bakerocb' but it may possibly

be to avoid overloading with too high a signal as he suggests but anyway

it's surprising.

 
	


As mentioned, 1.5gHz is just under 20 cm; ~ 8 inches.

While the length of the transmission line will have little to do with 

the input Z seen by the receiver, it's (the coax) impedance, and 

those of the connectors and terminations, most certainly will affect 

the overall performance of the system.  Were the coax and connectors 

not closely matched to the expected source and load impedances a 

mismatch will occur, possibly creating an unacceptably high signal 

loss.  Even with correct connectors and proper terminations, losses 

in excess of 1dB are to be expected.  Additionally standing waves, 

reflections, will occur in the line causing additional losses and 

time delays of reflected signals.

 

 


<_j.rippengal(at)cytanet.com.cy_ (mailto:j.rippengal(at)cytanet.com.cy) >

 

I understand what you are saying Dan but the impedance the receiver sees is 

not a big deal unless it is quite violently wrong, like a near dead short 

for instance, especially in the case of a short feeder when the signal will 

be strong.  In any case I believe that pretty well all the antennas supplied 

are active and supplied with DC power via the feeder. I don't see it as any 

difficulty at all to design the amplifier with the right output impedance to 

as close a limit as necessary, so the length of the feeder will be 

immaterial so far as the impedance the receiver sees is concerned.

John

 

 

 

In a message dated 4/27/2007 11:41:56 AM Eastern Standard Time, 

j.rippengal(at)cytanet.com.cy writes:


<j.rippengal(at)cytanet.com.cy>



On further thoughts, Dan, it seems you are saying that if you shorten the 

cable the signal can actually be weaker ie if you start with 8 ft and reduce 

it to 6ft then the signal is lower. I don't think so unless the impedance 

conditions are so extreme as to be very unlikely in any practical situation. 

Even then I have my doubts but have not worked it all through. It is not a 

simple problem.

John



************************************** See what's free at http://www.aol.com.

j.rippengal(at)cytanet.co
Guest

Posted: Fri Apr 27, 2007 12:58 pm Post subject: GNS 430W Antenna Requirements

Don't forget Dan that the level of any delayed signal that reaches the 

receiver due to coax relection/mismatch is reduced by the reflection 

coefficient at the receiver itself plus the attenuation of the coax plus the 

reflection coefficient at the antenna plus the attenuation of the cable 

again. Once again this would result in rather a low level unless the 

impedances were grossly wrong. Then again the effect of such a reflection on 

the rather complex spread spectrum coding which GPS uses is far beyond me to 

figure out. However such modulation methods do protect particularly against 

multipath reception.

John



From: <Hopperdhh(at)aol.com>



 	  Quote: 	
		  

 John,



 No, I was not saying that the signal would be weaker if you shorten the

 cable.  I'm really not sure how important the match is.  But, Ron brings 

 out a good

 point.  Any mismatch would cause reflections on the line and confuse the

 decoding of the data stream.  The time delay explanation is probably more 

 correct.

 The loss in 13 feet of RG-400 (from a quick search for the characteristics

 and some interpolation) comes out to about 3 dB, which could be necessay 

 to

 weaken any reflections.  The reflections would travel down and back which 

 would

 make them 6 dB weaker.  Reflections only occur if there is a mismatch, and 

 I

 agree that there shouldn't be very much of a mismatch, but there will be 

 some

 given the tolerance of production parts -- meaning the amplifier chips. 

 I'll

 admit that I'm in over my head here!  So, who knows the real answer?



 Dan K9WEK

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Fri Apr 27, 2007 5:00 pm Post subject: GNS 430W Antenna Requirements

I agree with you... The whys and wherefores are interesting reading but when

it comes to the actual installation do what the documentation says!  



Noel Loveys, RPP, AME intern

Campbellton, Newfoundland, Canada

Kitfox Mod III-A, 582, B box, Ivo IFA

Aerocet 1100s

noelloveys(at)yahoo.ca

 

[quote] --

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

dsleepy47(at)hotmail.com
Guest

Posted: Fri Apr 27, 2007 9:16 pm Post subject: GNS 430W Antenna Requirements

I am not an avionics guy but here: http://www.pennavionics.com/WAAS_UPGRADE.html is a place that gives an explination of what  Garmin is trying to meet with their coax requirements. Decide for yourself what their explination is worth.

  

 Deems





   

 > From: noelloveys(at)yahoo.ca

[quote] To: avionics-list(at)matronics.com

 Subject: RE: GNS 430W Antenna Requirements

 Date: Fri, 27 Apr 2007 22:28:52 -0230

 

 --> Avionics-List message posted by: "Noel Loveys" <noelloveys(at)yahoo.ca>

 

 I agree with you... The whys and wherefores are interesting reading but when

 it comes to the actual installation do what the documentation says! 

Discover the new Windows Vista Learn more!   	  Quote: 	
		  [b]

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