Do reflections up/down the antenna cable cause a problem with GPS?
People state it is desirable to have a GPS antenna well clear of
obstructions, which I believe is to stop reflections. But there is another
source of reflections which I suspect could be just as problematic.
Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
but even on the same real of cable.The output of the pre-amp is most
unlikely to have a 50 Ohm source impedance. In fact, the output impedance
might be close to 0 Ohms, as it may be driven by a voltage source, without
any 50 Ohm resistor.
Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.
I just looked on my HP 8720D VNA, and see I can reduce the output power to
-70 dBm, which would should not do any damage. It will be interesting to
see just what the input impedance of the GPS receiver is. I'm tied up with
doing my accounts over the next few days, but later I will look.
If reflections on the antenna/cable/receiver are a problem, then
attenuators can improve the match, but of course they reduce the signal
level too. A more intelligent, but more difficult solution, is to build a
matching network. For that one would need a VNA to measure the impedance in
the first place.
Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
In message CANX10hAs4q4=i4F1dO_a5u4_9bEp89v+yjX_zFLtc9bLgwAujQ@mail.gmail.com, "Dr. David Kirkby (Kirkby Microwave Ltd)" writes:
Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
I think the installation manual for Trimbles timing products say you
can use either 75 or 50 Ohm cable...
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
I believe that reflections within the cable doesn't matter regarding
the GPS measurements, unlike the reflections coming from outside the
antenna. The measurements are made from the differences in the arrival
times of the different satellite signals to the antenna and delays after
that basically does not affect them. Well, the cable delay affects to
the phase of the PPS but this is accounted for in a parameter.
Anyway, I don't mean that a good matching is something that can be
totally disregarded and if the mismatch is very large it will affect to
the signal lever at the receiver input but not to the measurement
mechanism. BTW some GPSDO's, the Thunderbolt being an example, are
supposed to be feed with 75 Ohm antenna cable with F connectors.
I may be be totally wrong, but in that case I'll appreciate comments
from more knowledgeable people.
Best regards,
Ignacio, EB4APL
El 21/11/2016 a las 14:45, Dr. David Kirkby (Kirkby Microwave Ltd) escribió:
People state it is desirable to have a GPS antenna well clear of
obstructions, which I believe is to stop reflections. But there is another
source of reflections which I suspect could be just as problematic.
Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
but even on the same real of cable.The output of the pre-amp is most
unlikely to have a 50 Ohm source impedance. In fact, the output impedance
might be close to 0 Ohms, as it may be driven by a voltage source, without
any 50 Ohm resistor.
Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.
I just looked on my HP 8720D VNA, and see I can reduce the output power to
-70 dBm, which would should not do any damage. It will be interesting to
see just what the input impedance of the GPS receiver is. I'm tied up with
doing my accounts over the next few days, but later I will look.
If reflections on the antenna/cable/receiver are a problem, then
attenuators can improve the match, but of course they reduce the signal
level too. A more intelligent, but more difficult solution, is to build a
matching network. For that one would need a VNA to measure the impedance in
the first place.
Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
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I believe that reflections inside the cable (that is, after the antenna)
are very different from reflections before the antenna. GPS receivers do
their calculations based in the different arrival times of the
satellites signals to the antenna center, so delays caused from
different paths caused by reflections on external objects really
matters. Reflections inside the cable, where phase relationships between
the signals is already fixed, does not matter, or at least, not very much.
I don mean to disregard matching, which can cause big signal level
losses if it is very bad. BTW, GPSDO's like the popular Thunderbolt,
are meant to use 75 Ohm cable and connectors but 50 Ohm cable can be
used without much difference. The PPS phase with respect to UTC, witch
is affected by the cable delay, must be adjusted with the corresponding
parameter. I think that a cable delay measurement could be more useful
in this regard.
Anyway, I can be wrong and if it is the case, I would like comments from
the experts here.
Best regards,
Ignacio, EB4APL
El 21/11/2016 a las 14:45, Dr. David Kirkby (Kirkby Microwave Ltd) escribió:
People state it is desirable to have a GPS antenna well clear of
obstructions, which I believe is to stop reflections. But there is another
source of reflections which I suspect could be just as problematic.
Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
but even on the same real of cable.The output of the pre-amp is most
unlikely to have a 50 Ohm source impedance. In fact, the output impedance
might be close to 0 Ohms, as it may be driven by a voltage source, without
any 50 Ohm resistor.
Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.
I just looked on my HP 8720D VNA, and see I can reduce the output power to
-70 dBm, which would should not do any damage. It will be interesting to
see just what the input impedance of the GPS receiver is. I'm tied up with
doing my accounts over the next few days, but later I will look.
If reflections on the antenna/cable/receiver are a problem, then
attenuators can improve the match, but of course they reduce the signal
level too. A more intelligent, but more difficult solution, is to build a
matching network. For that one would need a VNA to measure the impedance in
the first place.
Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
El software de antivirus Avast ha analizado este correo electrónico en busca de virus.
https://www.avast.com/antivirus
Hi
Based on extensive testing of the line mismatch issue, the answer turns out to be “it does not matter”.
The reflection issue ahead of the antenna is a reflection of the signal from a single satellite. The multipath
reflection makes that satellite appear to be further away than it really is. In the case that the reflected
signal is stronger than the desired signal, the multipath reflection “captures” the receiver and the net
solution is messed up.
In the case of a mismatched cable, there is no “single satellite” issue. Everything is impacted by the mismatch.
Even if the mismatch is pretty bad, the “primary” wave is the one that will dominate at the receiver end. The
reflections will always be lower in amplitude. That effectively guarantees that you don’t have a multipath
issue from the coax.
Yes, there is more to it than this simple explanation. The conclusion is still correct. There is no significant impact switching
coax from 50 ohms to 75 ohms and having both ends of the cable at an impedance that is not equal to the cable’s
characteristic impedance.
Bob
On Nov 21, 2016, at 8:45 AM, Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby@kirkbymicrowave.co.uk wrote:
People state it is desirable to have a GPS antenna well clear of
obstructions, which I believe is to stop reflections. But there is another
source of reflections which I suspect could be just as problematic.
Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
but even on the same real of cable.The output of the pre-amp is most
unlikely to have a 50 Ohm source impedance. In fact, the output impedance
might be close to 0 Ohms, as it may be driven by a voltage source, without
any 50 Ohm resistor.
Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.
I just looked on my HP 8720D VNA, and see I can reduce the output power to
-70 dBm, which would should not do any damage. It will be interesting to
see just what the input impedance of the GPS receiver is. I'm tied up with
doing my accounts over the next few days, but later I will look.
If reflections on the antenna/cable/receiver are a problem, then
attenuators can improve the match, but of course they reduce the signal
level too. A more intelligent, but more difficult solution, is to build a
matching network. For that one would need a VNA to measure the impedance in
the first place.
Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
time-nuts mailing list -- time-nuts@febo.com
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and follow the instructions there.
On Mon, 21 Nov 2016 14:36:49 -0500
Bob Camp kb8tq@n1k.org wrote:
The reflection issue ahead of the antenna is a reflection of the signal from
a single satellite. The multipath
reflection makes that satellite appear to be further away than it really is.
In the case that the reflected
signal is stronger than the desired signal, the multipath reflection
“captures” the receiver and the net
solution is messed up.
Even a weaker reflected signal can cause significant change of the
correlation peak and thus of the apparent distance of the satellite.
The multipath error envolope diagrams are usually for multipath to
direct path ratios of between 1:2 to 1:10 (mostly depending on what
the author wants to show or how much he wants to cheat).
In the case of a mismatched cable, there is no “single satellite” issue.
Everything is impacted by the mismatch.
Even if the mismatch is pretty bad, the “primary” wave is the one that will
dominate at the receiver end. The
reflections will always be lower in amplitude. That effectively guarantees
that you don’t have a multipath
issue from the coax.
As above, weaker signals can still cause quite a bit of change in the
correlation peak, but in this case it will not matter because the
reflection acts the same on all signals. I.e. the net result is a small
time offset (but no position offset). Unfortunately, there is one big
assumption in here that does not hold true: for all signals to be affected
the same way by the reflection, the receiver must be exactly linear.
But we know that many of the components in the signal path of the receiver
are distinctly non-linear functions. So there is a slight change of the
position (and thus time) due to reflections in the cable. But, as we are
usually dealing with minute differences in impedance, the reflected signals
are heavily attenuated. Assuming we have a nominally 50Ω
--
Malek's Law:
Any simple idea will be worded in the most complicated way.
Sorry... I pressed the wrong button while editing the Mail and cut short...
Continuing where I left off
In the case of a mismatched cable, there is no “single satellite” issue.
Everything is impacted by the mismatch.
Even if the mismatch is pretty bad, the “primary” wave is the one that will
dominate at the receiver end. The
reflections will always be lower in amplitude. That effectively guarantees
that you don’t have a multipath
issue from the coax.
As above, weaker signals can still cause quite a bit of change in the
correlation peak, but in this case it will not matter because the
reflection acts the same on all signals. I.e. the net result is a small
time offset (but no position offset). Unfortunately, there is one big
assumption in here that does not hold true: for all signals to be affected
the same way by the reflection, the receiver must be exactly linear.
But we know that many of the components in the signal path of the receiver
are distinctly non-linear functions. So there is a slight change of the
position (and thus time) due to reflections in the cable. But, as we are
usually dealing with minute differences in impedance, the reflected signals
are heavily attenuated. Assuming we have a nominally 50Ω system and have
a variation of +/-2Ω, then the worst case 48Ω vs 52Ω would give a reflection
loss of 27dB. Even assuming the back reflection has no loss (which isn't
true) would mean that the cable reflection "multipath" is almost 30dB dampend.
More likely to be something in the order of 54dB (2*27dB). Even with a 50Ω
to 75Ω impedance missmatch the reflection loss is 14dB and the multipath
should be in the order of 28dB. So waaaay below the signal and thus distortion
should be very much limited and receiver non-linearities should be negligible.
Attila Kinali
--
Malek's Law:
Any simple idea will be worded in the most complicated way.
On 11/21/16 3:11 PM, Attila Kinali wrote:
On Mon, 21 Nov 2016 14:36:49 -0500
Bob Camp kb8tq@n1k.org wrote:
The reflection issue ahead of the antenna is a reflection of the signal from
a single satellite. The multipath
reflection makes that satellite appear to be further away than it really is.
In the case that the reflected
signal is stronger than the desired signal, the multipath reflection
“captures” the receiver and the net
solution is messed up.
Even a weaker reflected signal can cause significant change of the
correlation peak and thus of the apparent distance of the satellite.
Only if the multipath is less than a chip away.. if it's more than a
chip away, it doesn't change the timing of the correlation peak for the
primary path.
Actually, since the "timing"of the recovered code is averaged over many
chips/code periods, "close by" multipath might have an effect because it
affects the "shape" of the peak - it's not nice and triangular.
1 chip = 1 microsecond = 300 meters.
So multipath <300 meters away (probably a specular reflection off
something) - here the geometry helps as you average over multiple code
periods - the multipath timing, relative to the true path, is not fixed
- the reflected path may just disappear and reappear (specular
reflections from something "far-ish" away - 10 meters)
That's why, if you can get your receiver antenna up high above the
surroundings, multipath is less of a problem: the antenna has poor gain
below the horizon, and the enforced distance between antenna and
"nearest possible reflector" is greater, which makes the temporal
movement of the spurious signal that much bigger.
A pathological case would be an antenna next to a vertical wall. There
will likely be two signals with identical strength and very small
differential time that doesn't change very much.
The multipath error envolope diagrams are usually for multipath to
direct path ratios of between 1:2 to 1:10 (mostly depending on what
the author wants to show or how much he wants to cheat).
I haven't used one personally but a spirent gps simulator would let you do
a try it and see.
It will be interesting to see if out of the growing sdr community an open
source gps simulator emerges.
On Mon, Nov 21, 2016 at 1:01 PM Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby@kirkbymicrowave.co.uk> wrote:
People state it is desirable to have a GPS antenna well clear of
obstructions, which I believe is to stop reflections. But there is another
source of reflections which I suspect could be just as problematic.
Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
but even on the same real of cable.The output of the pre-amp is most
unlikely to have a 50 Ohm source impedance. In fact, the output impedance
might be close to 0 Ohms, as it may be driven by a voltage source, without
any 50 Ohm resistor.
Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.
I just looked on my HP 8720D VNA, and see I can reduce the output power to
-70 dBm, which would should not do any damage. It will be interesting to
see just what the input impedance of the GPS receiver is. I'm tied up with
doing my accounts over the next few days, but later I will look.
If reflections on the antenna/cable/receiver are a problem, then
attenuators can improve the match, but of course they reduce the signal
level too. A more intelligent, but more difficult solution, is to build a
matching network. For that one would need a VNA to measure the impedance in
the first place.
Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Hi
There are packages you can put on a fairly standard HP signal generator that will
do the mismatch stuff without spending all the money Sperient wants for one of
their machines. One suggestion: If you do go with the HP solution, running the
attenuator up and down to simulate fast fades (think urban canyon) will fry the generator
in about 30 days … I have empirical data ….
Bob
On Nov 21, 2016, at 8:53 PM, Scott Stobbe scott.j.stobbe@gmail.com wrote:
I haven't used one personally but a spirent gps simulator would let you do
a try it and see.
It will be interesting to see if out of the growing sdr community an open
source gps simulator emerges.
On Mon, Nov 21, 2016 at 1:01 PM Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby@kirkbymicrowave.co.uk> wrote:
People state it is desirable to have a GPS antenna well clear of
obstructions, which I believe is to stop reflections. But there is another
source of reflections which I suspect could be just as problematic.
Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
but even on the same real of cable.The output of the pre-amp is most
unlikely to have a 50 Ohm source impedance. In fact, the output impedance
might be close to 0 Ohms, as it may be driven by a voltage source, without
any 50 Ohm resistor.
Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.
I just looked on my HP 8720D VNA, and see I can reduce the output power to
-70 dBm, which would should not do any damage. It will be interesting to
see just what the input impedance of the GPS receiver is. I'm tied up with
doing my accounts over the next few days, but later I will look.
If reflections on the antenna/cable/receiver are a problem, then
attenuators can improve the match, but of course they reduce the signal
level too. A more intelligent, but more difficult solution, is to build a
matching network. For that one would need a VNA to measure the impedance in
the first place.
Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
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