Re: [time-nuts] Do reflections up/down the antenna cable cause a problem, with GPS? (Bob Camp)

[John Haine]

Hello all, I'm a recent recruit to this list, it looks very interesting 
and I hope I can make a contribution.  Though now retired I'm an 
electronics & radio engineer and until last March worked for u-blox 
(though not on the GNSS side of the business).


Looking at their product integration guides they don't mention antenna 
match and cable length.  They do suggest inserting a SAW filter between 
the antenna and module especially with an active antenna, and of course 
the group delay of a SAW filter makes it equivalent to quite a long 
length of cable - at least 30 cm a nanosecond.  Multiple reflections 
will be attenuated at each reflection - if the return loss is say 6 dB 
then you lose that much at each reflection, plus the attenuation in the 
cable itself.  So the effect of the cable plus mismatches will be to add 
an extra channel impulse response that will be quite short as the 
various echoes decay.  Given that the system is working with wanted 
signals well below thermal anyway, a couple of reflections plus a dB or 
two of cable loss would make the largest echo ~15 dB below the wanted 
signal.  I suspect that this will cause rather small error in the 
device's estimate of pseudo-range  but this will be the same for each 
satellite.  Overall I doubt that the effect will be serious either for 
positioning or timing - if it were I think (knowing u-blox) they would 
make recommendations in the guide.  By the way they don't even mention 
it in the context of their high-precision devices.


(And I've just found an application note where they do discuss matching 
but only in the context of maximising SNR, i.e. power transfer.)


But if one is really concerned then the obvious thing to do is mount the 
module/chip right on the back of the antenna so there is no cable.




On 23/11/2016 12:00, time-nuts-requ...@febo.com wrote:

Re: Do reflections up/down the antenna cable cause a problem
   with GPS? (Bob Camp)


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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[Magnus Danielson]

Hi,

I agree.

In general, you have one 50=>75 transmission loss, cable damping, one 
75=>50 reflection, cable damping, a 75=>50 reflection, cable damping, 
75=>50 transmission for the direct path of 50=>75 transmission, cable 
damping 75=>50 transmission. Thus, the reflection will be two cable 
damping and two 75=>50 reflections below the original signal.


The reflection will be close, so it cares, but the amplitude will be so 
small that the shift is not significant. For the L1 C/A code-receivers 
the offset will be fairly drowned in the noise and offsets. Probably 
below 1 ns.


So, for all practical matters for the type of receiver, no, no real impact.

If you are into carrier phase and maybe dual or more bands, then it 
cares more because you have less noise and offsets.


Cheers,
Magnus

On 11/21/2016 08:36 PM, Bob Camp wrote:

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) 
 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)
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[Bob Camp]

Hi

Relay based attenuator. Oddly enough I have data on more than one generator :)

(ouch indeed if it’s not being paid for on somebody else’s credit card)

Bob

> On Nov 22, 2016, at 10:50 AM, Scott Stobbe  wrote:
> 
> Ouch! Relay stepped attenuator? Or solid state components?
> 
> On Mon, Nov 21, 2016 at 11:00 PM Bob Camp  wrote:
> 
>> 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 
>> 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) <
>>> drkir...@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
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>>> ___
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[Scott Stobbe]

Ouch! Relay stepped attenuator? Or solid state components?

On Mon, Nov 21, 2016 at 11:00 PM Bob Camp  wrote:

> 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 
> 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) <
> > drkir...@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.
> >>
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[Scott Stobbe]

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) <
drkir...@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.
>
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[jimlux]

On 11/21/16 3:11 PM, Attila Kinali wrote:

On Mon, 21 Nov 2016 14:36:49 -0500
Bob Camp  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).



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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[Hal Murray]

p...@phk.freebsd.dk said:
> I think the installation manual for Trimbles timing products say you can use
> either 75 or 50 Ohm cable... 

I think they suggest using RG-6, the classic cable TV and/or satellite dish 
cable.  It's widely available at low cost.

The loss due to impedance mismatch is much less than the gain from the 
reduced attenuation when compared to equivalent size 50 ohm cable.


-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[Attila Kinali]

On Mon, 21 Nov 2016 14:36:49 -0500
Bob Camp  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.
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[Attila Kinali]

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 wy below the signal and thus distortion
should be very much limited and receiver non-linearities should be negligible.

Attila Kinali


-- 
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[EB4APL]

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)



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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[Poul-Henning Kamp]

In message 

Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[EB4APL]

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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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[Bob Camp]

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) 
>  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)
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> 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.

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[time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

[Dr. David Kirkby (Kirkby Microwave Ltd)]

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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