Today's Message Index:
----------------------
1. 08:12 AM - Re: GNS 430W Antenna Requirements (Hopperdhh@aol.com)
2. 08:32 AM - Re: GNS 430W Antenna Requirements (John Rippengal)
3. 08:40 AM - Re: GNS 430W Antenna Requirements (John Rippengal)
4. 10:14 AM - Re: GNS 430W Antenna Requirements (Ron Quillin)
5. 10:40 AM - Re: GNS 430W Antenna Requirements (Tim Olson)
6. 01:06 PM - Re: GNS 430W Antenna Requirements (Ron Quillin)
7. 01:07 PM - Re: GNS 430W Antenna Requirements (Hopperdhh@aol.com)
8. 01:58 PM - Re: GNS 430W Antenna Requirements (John Rippengal)
9. 06:00 PM - Re: GNS 430W Antenna Requirements (Noel Loveys)
10. 10:16 PM - Re: GNS 430W Antenna Requirements (Deems Herring)
Message 1
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Subject: | Re: GNS 430W Antenna Requirements |
In a message dated 4/25/2007 1:57:37 AM Eastern Standard Time,
j.rippengal@cytanet.com.cy writes:
<j.rippengal@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.
Message 2
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Subject: | Re: 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@aol.com>
>
> In a message dated 4/25/2007 1:57:37 AM Eastern Standard Time,
> j.rippengal@cytanet.com.cy writes:
> <j.rippengal@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
Message 3
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Subject: | Re: 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@aol.com>
> j.rippengal@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
Message 4
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Subject: | Re: GNS 430W Antenna Requirements |
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
Message 5
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Subject: | Re: 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:
>
> 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
>
Message 6
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Subject: | Re: GNS 430W Antenna Requirements |
At 10:39 4/27/2007, you wrote:
>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.
Message 7
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Subject: | Re: 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@gmail.com) >
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.
<_j.rippengal@cytanet.com.cy_ (mailto:j.rippengal@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@cytanet.com.cy writes:
<j.rippengal@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.
Message 8
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Subject: | Re: 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@aol.com>
>
> 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
>
>
Message 9
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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@yahoo.ca
> -----Original Message-----
> From: owner-avionics-list-server@matronics.com
> [mailto:owner-avionics-list-server@matronics.com] On Behalf
> Of Tim Olson
> Sent: Friday, April 27, 2007 3:10 PM
> To: avionics-list@matronics.com
> Subject: Re: Avionics-List: 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:
> <rjquillin@gmail.com>
> >
> > 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
> >
>
>
>
>
>
Message 10
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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 mee
t with their coax requirements. Decide for yourself what their explination
is worth.
Deems
> From: noelloveys@yahoo.ca> To: avionics-list@matronics.com> Subject: RE:
Avionics-List: GNS 430W Antenna Requirements> Date: Fri, 27 Apr 2007 22:28:
yahoo.ca>> > I agree with you... The whys and wherefores are interesting re
ading but when> it comes to the actual installation do what the documentati
on says!
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