EK
Erik Kaashoek
Sun, May 29, 2022 2:45 PM
During evaluation of a GPS module there was some concern over the
stability/correctness of the PPS and, as the NIST paper on PPS accuracy
mentions there could be substantial variation in PPS accuracy due to
atmospheric conditions, an experiment was set up to measure the PPS of 3
different GPS modules simultaneously.
The experiment uses one NEO-7M and two ATGM336H GPS modules
The PPS of each module is send to 3 separate timer capture inputs of one
timer running at 200 MHz providing 5 ns measurement accuracy.
The 200 MHz timer was clocked using a 10 MHz Rb output up converted to
200 MHz using a PLL.
A plot showing the timing variation is attached.
Vertical axis is in ns, horizontal axis is in seconds
All GPS modules had separate simple puck antenna all with almost free
sky view and all using at least 10 GPS satellites.
The differences between the PPS of the 3 modules was surprisingly large,
two modules where in average 10 ns apart and a 3rd module was in average
39 ns apart from the first two modules. More investigation is needed as
the HW paths between PPS output of the GPS modules and the timer capture
inputs of the 3 timers are not fully identical
One GPS module showed little variation in PPS timing versus the Rb
reference, only a slow drift of about 15 ns over 400 seconds. The other
modules showed a periodic variation of up to 20 ns. The worst variation
was observed from the NEO-7M module.
The measurement is at time of posting of this message still running to
see if over a longer period the differences reduce.
If the observed short term (< 400 s) PPS variation in the order of
10-20 ns are common and unavoidable it would make sense for the
requirement for phase stability of a GPSDO to be aligned with this level
of PPS phase uncertainty.
Erik.
During evaluation of a GPS module there was some concern over the
stability/correctness of the PPS and, as the NIST paper on PPS accuracy
mentions there could be substantial variation in PPS accuracy due to
atmospheric conditions, an experiment was set up to measure the PPS of 3
different GPS modules simultaneously.
The experiment uses one NEO-7M and two ATGM336H GPS modules
The PPS of each module is send to 3 separate timer capture inputs of one
timer running at 200 MHz providing 5 ns measurement accuracy.
The 200 MHz timer was clocked using a 10 MHz Rb output up converted to
200 MHz using a PLL.
A plot showing the timing variation is attached.
Vertical axis is in ns, horizontal axis is in seconds
All GPS modules had separate simple puck antenna all with almost free
sky view and all using at least 10 GPS satellites.
The differences between the PPS of the 3 modules was surprisingly large,
two modules where in average 10 ns apart and a 3rd module was in average
39 ns apart from the first two modules. More investigation is needed as
the HW paths between PPS output of the GPS modules and the timer capture
inputs of the 3 timers are not fully identical
One GPS module showed little variation in PPS timing versus the Rb
reference, only a slow drift of about 15 ns over 400 seconds. The other
modules showed a periodic variation of up to 20 ns. The worst variation
was observed from the NEO-7M module.
The measurement is at time of posting of this message still running to
see if over a longer period the differences reduce.
If the observed short term (< 400 s) PPS variation in the order of
10-20 ns are common and unavoidable it would make sense for the
requirement for phase stability of a GPSDO to be aligned with this level
of PPS phase uncertainty.
Erik.
EK
Erik Kaashoek
Mon, May 30, 2022 11:00 AM
Further evaluation did shown the time differences between the 3 GPS
modules was due to difference in the trigger level setting of the
timer/counter and difference in length of GPS antenna cables.
After removal of the phase drift due to Rb frequency offset the attached
image shows the phase differences of the 3 modules versus a Rb reference.
The two ATGM modules are very consistent over a 2.8 hours period. The
NEO-7M varies wildly with phase errors above 100 ns. Possibly due to a
somewhat less optimal antenna position.
It seems phase variations over time in the order of 10-20 ns are indeed
unavoidable, even with a good antenna.
Erik.
Further evaluation did shown the time differences between the 3 GPS
modules was due to difference in the trigger level setting of the
timer/counter and difference in length of GPS antenna cables.
After removal of the phase drift due to Rb frequency offset the attached
image shows the phase differences of the 3 modules versus a Rb reference.
The two ATGM modules are very consistent over a 2.8 hours period. The
NEO-7M varies wildly with phase errors above 100 ns. Possibly due to a
somewhat less optimal antenna position.
It seems phase variations over time in the order of 10-20 ns are indeed
unavoidable, even with a good antenna.
Erik.
CA
Carsten Andrich
Mon, May 30, 2022 3:24 PM
Hi Erik,
have you tried running all receivers off the same antenna via a power
splitter (make sure to dc block all but one receiver)? That should
remove the uncertainty due to antenna differences (location, RF
characteristics, etc.).
Also, are you using ground planes for your puck antennas? These types of
antennas typically require a ground plane for optimal performance [1].
Best regards,
Carsten
[1]
https://content.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_(UBX-15030289).pdf#page=16
On 30.05.22 13:00, Erik Kaashoek via time-nuts wrote:
Further evaluation did shown the time differences between the 3 GPS
modules was due to difference in the trigger level setting of the
timer/counter and difference in length of GPS antenna cables.
After removal of the phase drift due to Rb frequency offset the
attached image shows the phase differences of the 3 modules versus a
Rb reference.
The two ATGM modules are very consistent over a 2.8 hours period. The
NEO-7M varies wildly with phase errors above 100 ns. Possibly due to
a somewhat less optimal antenna position.
It seems phase variations over time in the order of 10-20 ns are
indeed unavoidable, even with a good antenna.
Erik.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Hi Erik,
have you tried running all receivers off the same antenna via a power
splitter (make sure to dc block all but one receiver)? That should
remove the uncertainty due to antenna differences (location, RF
characteristics, etc.).
Also, are you using ground planes for your puck antennas? These types of
antennas typically require a ground plane for optimal performance [1].
Best regards,
Carsten
[1]
https://content.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_(UBX-15030289).pdf#page=16
On 30.05.22 13:00, Erik Kaashoek via time-nuts wrote:
> Further evaluation did shown the time differences between the 3 GPS
> modules was due to difference in the trigger level setting of the
> timer/counter and difference in length of GPS antenna cables.
> After removal of the phase drift due to Rb frequency offset the
> attached image shows the phase differences of the 3 modules versus a
> Rb reference.
> The two ATGM modules are very consistent over a 2.8 hours period. The
> NEO-7M varies wildly with phase errors above 100 ns. Possibly due to
> a somewhat less optimal antenna position.
> It seems phase variations over time in the order of 10-20 ns are
> indeed unavoidable, even with a good antenna.
> Erik.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts@lists.febo.com
> To unsubscribe send an email to time-nuts-leave@lists.febo.com
BK
Bob kb8tq
Mon, May 30, 2022 3:42 PM
Hi
The variation you see is dependent on a number of things. One of them is space
weather. If you do your run during a very active period ( typically peak sun spots)
you may see some very dramatic swings on a single band device.
Bob
On May 30, 2022, at 3:00 AM, Erik Kaashoek via time-nuts time-nuts@lists.febo.com wrote:
Further evaluation did shown the time differences between the 3 GPS modules was due to difference in the trigger level setting of the timer/counter and difference in length of GPS antenna cables.
After removal of the phase drift due to Rb frequency offset the attached image shows the phase differences of the 3 modules versus a Rb reference.
The two ATGM modules are very consistent over a 2.8 hours period. The NEO-7M varies wildly with phase errors above 100 ns. Possibly due to a somewhat less optimal antenna position.
It seems phase variations over time in the order of 10-20 ns are indeed unavoidable, even with a good antenna.
Erik.
<3GPS_phase_difference.png>_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Hi
The variation you see is dependent on a number of things. One of them is space
weather. If you do your run during a very active period ( typically peak sun spots)
you may see some very dramatic swings on a single band device.
Bob
> On May 30, 2022, at 3:00 AM, Erik Kaashoek via time-nuts <time-nuts@lists.febo.com> wrote:
>
> Further evaluation did shown the time differences between the 3 GPS modules was due to difference in the trigger level setting of the timer/counter and difference in length of GPS antenna cables.
> After removal of the phase drift due to Rb frequency offset the attached image shows the phase differences of the 3 modules versus a Rb reference.
> The two ATGM modules are very consistent over a 2.8 hours period. The NEO-7M varies wildly with phase errors above 100 ns. Possibly due to a somewhat less optimal antenna position.
> It seems phase variations over time in the order of 10-20 ns are indeed unavoidable, even with a good antenna.
> Erik.
> <3GPS_phase_difference.png>_______________________________________________
> time-nuts mailing list -- time-nuts@lists.febo.com
> To unsubscribe send an email to time-nuts-leave@lists.febo.com
MD
Magnus Danielson
Mon, May 30, 2022 9:26 PM
Erik,
The NEO-7M may have sawtooth correction output, have you checked that
and made compensations?
Since the oscillator is not steered and free-floating, the
cycle-assignment of the PPS may be less than optimal so just measuring
that without the compensation can cause a wider range of PPS than the
actual receiver time stability represents.
In particular, check chapter 12 and the TIM-TP message of [1].
[1]
https://content.u-blox.com/sites/default/files/products/documents/u-blox7-V14_ReceiverDescriptionProtocolSpec_%28GPS.G7-SW-12001%29_Public.pdf
Do notice that the TIM-TP message is documented to be issued before the
(PPS) pulse it report on.
The variations you report is consistent with what the datasheet report
for the pulse assignment, which may not be representative of the
receivers performance.
Cheers,
Magnus
On 2022-05-30 13:00, Erik Kaashoek via time-nuts wrote:
Further evaluation did shown the time differences between the 3 GPS
modules was due to difference in the trigger level setting of the
timer/counter and difference in length of GPS antenna cables.
After removal of the phase drift due to Rb frequency offset the
attached image shows the phase differences of the 3 modules versus a
Rb reference.
The two ATGM modules are very consistent over a 2.8 hours period. The
NEO-7M varies wildly with phase errors above 100 ns. Possibly due to
a somewhat less optimal antenna position.
It seems phase variations over time in the order of 10-20 ns are
indeed unavoidable, even with a good antenna.
Erik.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Erik,
The NEO-7M may have sawtooth correction output, have you checked that
and made compensations?
Since the oscillator is not steered and free-floating, the
cycle-assignment of the PPS may be less than optimal so just measuring
that without the compensation can cause a wider range of PPS than the
actual receiver time stability represents.
In particular, check chapter 12 and the TIM-TP message of [1].
[1]
https://content.u-blox.com/sites/default/files/products/documents/u-blox7-V14_ReceiverDescriptionProtocolSpec_%28GPS.G7-SW-12001%29_Public.pdf
Do notice that the TIM-TP message is documented to be issued before the
(PPS) pulse it report on.
The variations you report is consistent with what the datasheet report
for the pulse assignment, which may not be representative of the
receivers performance.
Cheers,
Magnus
On 2022-05-30 13:00, Erik Kaashoek via time-nuts wrote:
> Further evaluation did shown the time differences between the 3 GPS
> modules was due to difference in the trigger level setting of the
> timer/counter and difference in length of GPS antenna cables.
> After removal of the phase drift due to Rb frequency offset the
> attached image shows the phase differences of the 3 modules versus a
> Rb reference.
> The two ATGM modules are very consistent over a 2.8 hours period. The
> NEO-7M varies wildly with phase errors above 100 ns. Possibly due to
> a somewhat less optimal antenna position.
> It seems phase variations over time in the order of 10-20 ns are
> indeed unavoidable, even with a good antenna.
> Erik.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts@lists.febo.com
> To unsubscribe send an email to time-nuts-leave@lists.febo.com
GE
glen english LIST
Mon, May 30, 2022 11:10 PM
Be aware not to confuse the antenna ground plane (the patch will always
have its own plane because the top metalization must be fed against a
plane or counterpoise - and a ground plane behind the antenna.
I can see the usefulness of the larger ground plane for any purchased
patch antenna to reduce the likelihood of interference underneath (if
the feed coax has a good RF contact with the plane), and if the plane is
coupled well, it may improve the low angle response .
The supplementary ground plane doesnt have to have a galvanic connection
if the gap between the underside of the patch is low- IE use purely a
capacitive coupling to tie the patch antenna ground to the large ground
sheet-
If we consider the patch area to be 10x10mm = 100uM^2, and the gap
being air (for simplicity sake) of 0.25mm, the capacitance is
Epsilon-nought times area, all divided by the distance between the two
plates
For the above example this is about 3.5pF or (-)j30. Really needs to be
< j5 .
That means reducing the gap to about 0.05mm OR increasing the area-
probably means using a bigger patch.
You might be able to sweat solder the patch antenna (bottom) to a sheet
of FR4- that would be my approach.
-glen
On 31/05/2022 1:24 am, Carsten Andrich via time-nuts wrote:
Hi Erik,
have you tried running all receivers off the same antenna via a power
splitter (make sure to dc block all but one receiver)? That should
remove the uncertainty due to antenna differences (location, RF
characteristics, etc.).
Also, are you using ground planes for your puck antennas? These types
of antennas typically require a ground plane for optimal performance [1].
Best regards,
Carsten
[1]
https://content.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_(UBX-15030289).pdf#page=16
On 30.05.22 13:00, Erik Kaashoek via time-nuts wrote:
Further evaluation did shown the time differences between the 3 GPS
modules was due to difference in the trigger level setting of the
timer/counter and difference in length of GPS antenna cables.
After removal of the phase drift due to Rb frequency offset the
attached image shows the phase differences of the 3 modules versus a
Rb reference.
The two ATGM modules are very consistent over a 2.8 hours period. The
NEO-7M varies wildly with phase errors above 100 ns. Possibly due to
a somewhat less optimal antenna position.
It seems phase variations over time in the order of 10-20 ns are
indeed unavoidable, even with a good antenna.
Erik.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
--
Glen English
RF Communications and Electronics Engineer
CORTEX RF
Pacific Media Technologies Pty Ltd trading as Cortex RF
ABN 40 075 532 008
PO Box 5231 Lyneham ACT 2602, Australia.
au mobile : +61 (0)418 975077
Be aware not to confuse the antenna ground plane (the patch will always
have its own plane because the top metalization must be fed against a
plane or counterpoise - and a ground plane behind the antenna.
I can see the usefulness of the larger ground plane for any purchased
patch antenna to reduce the likelihood of interference underneath (if
the feed coax has a good RF contact with the plane), and if the plane is
coupled well, it may improve the low angle response .
The supplementary ground plane doesnt have to have a galvanic connection
if the gap between the underside of the patch is low- IE use purely a
capacitive coupling to tie the patch antenna ground to the large ground
sheet-
If we consider the patch area to be 10x10mm = 100uM^2, and the gap
being air (for simplicity sake) of 0.25mm, the capacitance is
Epsilon-nought times area, all divided by the distance between the two
plates
For the above example this is about 3.5pF or (-)j30. Really needs to be
< j5 .
That means reducing the gap to about 0.05mm OR increasing the area-
probably means using a bigger patch.
You might be able to sweat solder the patch antenna (bottom) to a sheet
of FR4- that would be my approach.
-glen
On 31/05/2022 1:24 am, Carsten Andrich via time-nuts wrote:
> Hi Erik,
>
> have you tried running all receivers off the same antenna via a power
> splitter (make sure to dc block all but one receiver)? That should
> remove the uncertainty due to antenna differences (location, RF
> characteristics, etc.).
>
> Also, are you using ground planes for your puck antennas? These types
> of antennas typically require a ground plane for optimal performance [1].
>
> Best regards,
> Carsten
>
> [1]
> https://content.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_(UBX-15030289).pdf#page=16
>
> On 30.05.22 13:00, Erik Kaashoek via time-nuts wrote:
>> Further evaluation did shown the time differences between the 3 GPS
>> modules was due to difference in the trigger level setting of the
>> timer/counter and difference in length of GPS antenna cables.
>> After removal of the phase drift due to Rb frequency offset the
>> attached image shows the phase differences of the 3 modules versus a
>> Rb reference.
>> The two ATGM modules are very consistent over a 2.8 hours period. The
>> NEO-7M varies wildly with phase errors above 100 ns. Possibly due to
>> a somewhat less optimal antenna position.
>> It seems phase variations over time in the order of 10-20 ns are
>> indeed unavoidable, even with a good antenna.
>> Erik.
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts@lists.febo.com
>> To unsubscribe send an email to time-nuts-leave@lists.febo.com
> _______________________________________________
> time-nuts mailing list -- time-nuts@lists.febo.com
> To unsubscribe send an email to time-nuts-leave@lists.febo.com
--
Glen English
RF Communications and Electronics Engineer
CORTEX RF
Pacific Media Technologies Pty Ltd trading as Cortex RF
ABN 40 075 532 008
PO Box 5231 Lyneham ACT 2602, Australia.
au mobile : +61 (0)418 975077
CA
Carsten Andrich
Tue, May 31, 2022 10:23 AM
On 31.05.22 01:10, glen english LIST via time-nuts wrote:
Be aware not to confuse the antenna ground plane (the patch will
always have its own plane because the top metalization must be fed
against a plane or counterpoise - and a ground plane behind the antenna.
I can see the usefulness of the larger ground plane for any purchased
patch antenna to reduce the likelihood of interference underneath (if
the feed coax has a good RF contact with the plane), and if the plane
is coupled well, it may improve the low angle response .
The supplementary ground plane doesnt have to have a galvanic
connection if the gap between the underside of the patch is low- IE
use purely a capacitive coupling to tie the patch antenna ground to
the large ground sheet-
[...]
That means reducing the gap to about 0.05mm OR increasing the area-
probably means using a bigger patch.
Hi Glen,
thank you for the insight. I was referring to a ground plane behind the
antenna.
Gaps below 1~2 mm between a magnetic "puck"-type patch antenna with IP67
housing and an external ground plane seem practically challenging to me.
When it comes to stacked patch multi-band antennas like u-blox' ANN-MB
[1], the gap between the top patch and the external ground plane is
probably significantly higher. Yet, u-blox generally recommends the use
of a symmetric ground plane for the RTK applications [1,2]. From my
experience, the M8P and F9P RTK fix barely works without a ground plane
under the u-blox antennas.
While it's just an empirically educated guess, I'd assume that what is
required for RTK will not hurt for timing.
Could you share your expert opinion on this? My antenna expertise is
admittedly limited to reading data sheets and picking the right one for
the particular RF measurement requirements.
Thanks and best regards,
Carsten
[1]
https://content.u-blox.com/sites/default/files/ZED-F9P_IntegrationManual_UBX-18010802.pdf#page=114
[2]
https://content.u-blox.com/sites/default/files/ZED-F9P-MovingBase_AppNote_(UBX-19009093).pdf#page=8
On 31.05.22 01:10, glen english LIST via time-nuts wrote:
> Be aware not to confuse the antenna ground plane (the patch will
> always have its own plane because the top metalization must be fed
> against a plane or counterpoise - and a ground plane behind the antenna.
>
> I can see the usefulness of the larger ground plane for any purchased
> patch antenna to reduce the likelihood of interference underneath (if
> the feed coax has a good RF contact with the plane), and if the plane
> is coupled well, it may improve the low angle response .
>
> The supplementary ground plane doesnt have to have a galvanic
> connection if the gap between the underside of the patch is low- IE
> use purely a capacitive coupling to tie the patch antenna ground to
> the large ground sheet-
> [...]
>
> That means reducing the gap to about 0.05mm OR increasing the area-
> probably means using a bigger patch.
Hi Glen,
thank you for the insight. I was referring to a ground plane behind the
antenna.
Gaps below 1~2 mm between a magnetic "puck"-type patch antenna with IP67
housing and an external ground plane seem practically challenging to me.
When it comes to stacked patch multi-band antennas like u-blox' ANN-MB
[1], the gap between the top patch and the external ground plane is
probably significantly higher. Yet, u-blox generally recommends the use
of a symmetric ground plane for the RTK applications [1,2]. From my
experience, the M8P and F9P RTK fix barely works without a ground plane
under the u-blox antennas.
While it's just an empirically educated guess, I'd assume that what is
required for RTK will not hurt for timing.
Could you share your expert opinion on this? My antenna expertise is
admittedly limited to reading data sheets and picking the right one for
the particular RF measurement requirements.
Thanks and best regards,
Carsten
[1]
https://content.u-blox.com/sites/default/files/ZED-F9P_IntegrationManual_UBX-18010802.pdf#page=114
[2]
https://content.u-blox.com/sites/default/files/ZED-F9P-MovingBase_AppNote_(UBX-19009093).pdf#page=8
LJ
Lux, Jim
Tue, May 31, 2022 2:09 PM
On 5/31/22 3:23 AM, Carsten Andrich via time-nuts wrote:
On 31.05.22 01:10, glen english LIST via time-nuts wrote:
Be aware not to confuse the antenna ground plane (the patch will
always have its own plane because the top metalization must be fed
against a plane or counterpoise - and a ground plane behind the
antenna.
I can see the usefulness of the larger ground plane for any purchased
patch antenna to reduce the likelihood of interference underneath (if
the feed coax has a good RF contact with the plane), and if the plane
is coupled well, it may improve the low angle response .
The supplementary ground plane doesnt have to have a galvanic
connection if the gap between the underside of the patch is low- IE
use purely a capacitive coupling to tie the patch antenna ground to
the large ground sheet-
[...]
That means reducing the gap to about 0.05mm OR increasing the area-
probably means using a bigger patch.
Hi Glen,
thank you for the insight. I was referring to a ground plane behind
the antenna.
Gaps below 1~2 mm between a magnetic "puck"-type patch antenna with
IP67 housing and an external ground plane seem practically challenging
to me. When it comes to stacked patch multi-band antennas like u-blox'
ANN-MB [1], the gap between the top patch and the external ground
plane is probably significantly higher. Yet, u-blox generally
recommends the use of a symmetric ground plane for the RTK
applications [1,2]. From my experience, the M8P and F9P RTK fix barely
works without a ground plane under the u-blox antennas.
While it's just an empirically educated guess, I'd assume that what is
required for RTK will not hurt for timing.
Could you share your expert opinion on this? My antenna expertise is
admittedly limited to reading data sheets and picking the right one
for the particular RF measurement requirements.
Thanks and best regards,
Carsten
I would think that the large grounded sheet below the antenna helps more
for making the pattern uniform, and, to a certain extent, suppressing
some multipath coming from "below" the plane of the sheet. - not as
good as a choke ring(s), but not bad.
That is, the sheet is not intended to couple to the antenna's ground
plane, but is there as a predictable surface (and, probably, to provide
a magnetic material for a puck to stick to).
As such, the distance from the antenna's ground plane is not
particularly critical.
On 5/31/22 3:23 AM, Carsten Andrich via time-nuts wrote:
> On 31.05.22 01:10, glen english LIST via time-nuts wrote:
>> Be aware not to confuse the antenna ground plane (the patch will
>> always have its own plane because the top metalization must be fed
>> against a plane or counterpoise - and a ground plane behind the
>> antenna.
>>
>> I can see the usefulness of the larger ground plane for any purchased
>> patch antenna to reduce the likelihood of interference underneath (if
>> the feed coax has a good RF contact with the plane), and if the plane
>> is coupled well, it may improve the low angle response .
>>
>> The supplementary ground plane doesnt have to have a galvanic
>> connection if the gap between the underside of the patch is low- IE
>> use purely a capacitive coupling to tie the patch antenna ground to
>> the large ground sheet-
>> [...]
>>
>> That means reducing the gap to about 0.05mm OR increasing the area-
>> probably means using a bigger patch.
>
> Hi Glen,
>
> thank you for the insight. I was referring to a ground plane behind
> the antenna.
>
> Gaps below 1~2 mm between a magnetic "puck"-type patch antenna with
> IP67 housing and an external ground plane seem practically challenging
> to me. When it comes to stacked patch multi-band antennas like u-blox'
> ANN-MB [1], the gap between the top patch and the external ground
> plane is probably significantly higher. Yet, u-blox generally
> recommends the use of a symmetric ground plane for the RTK
> applications [1,2]. From my experience, the M8P and F9P RTK fix barely
> works without a ground plane under the u-blox antennas.
> While it's just an empirically educated guess, I'd assume that what is
> required for RTK will not hurt for timing.
>
> Could you share your expert opinion on this? My antenna expertise is
> admittedly limited to reading data sheets and picking the right one
> for the particular RF measurement requirements.
>
> Thanks and best regards,
> Carsten
I would think that the large grounded sheet below the antenna helps more
for making the pattern uniform, and, to a certain extent, suppressing
some multipath coming from "below" the plane of the sheet. - not as
good as a choke ring(s), but not bad.
That is, the sheet is not intended to couple to the antenna's ground
plane, but is there as a predictable surface (and, probably, to provide
a magnetic material for a puck to stick to).
As such, the distance from the antenna's ground plane is not
particularly critical.
G
glenlist
Wed, Jun 1, 2022 12:00 AM
Carsten, around the edges of the patch are all fringing fields, so the
required coupling to the plane may be quite low, depending on the design.
The compromised ground plane may also be permitting some rear size
pickup - multipath !
I'll do some EM modelling later to try and put some numbers on the
requirements. Those numbers to date were for the ideal scenario.
If there are rear side pickup issues, a screen / plane at least a few
wavelengths (19cm) square would stop the patch seeing the reflections /
rear sensitivity without being tightly coupled or connected to the patch
ground.
Maybe that is what you are seeing when you mention that you've
experienced poor performance without an additional plane. I don't know
for sure.
I've only built patches that were high performance, no compromises- It's
likely the mfrs are intentionally throwing away low angle sensitivity
and axial ratio equality , and rear pickup rejection to get the size
down as it still suits the majority of users.
What you can try is turn the patch upside down...and under the patch at
least 10cm away, use a sea of steel wool (maybe 50x50cm ) to act as an
effective absorber (so the sky wont reflect back underneath) .
-glen
On 31/05/2022 20:23, Carsten Andrich via time-nuts wrote:
On 31.05.22 01:10, glen english LIST via time-nuts wrote:
Be aware not to confuse the antenna ground plane (the patch will
always have its own plane because the top metalization must be fed
against a plane or counterpoise - and a ground plane behind the
antenna.
I can see the usefulness of the larger ground plane for any purchased
patch antenna to reduce the likelihood of interference underneath (if
the feed coax has a good RF contact with the plane), and if the plane
is coupled well, it may improve the low angle response .
The supplementary ground plane doesnt have to have a galvanic
connection if the gap between the underside of the patch is low- IE
use purely a capacitive coupling to tie the patch antenna ground to
the large ground sheet-
[...]
Carsten, around the edges of the patch are all fringing fields, so the
required coupling to the plane may be quite low, depending on the design.
The compromised ground plane may also be permitting some rear size
pickup - multipath !
I'll do some EM modelling later to try and put some numbers on the
requirements. Those numbers to date were for the ideal scenario.
If there are rear side pickup issues, a screen / plane at least a few
wavelengths (19cm) square would stop the patch seeing the reflections /
rear sensitivity without being tightly coupled or connected to the patch
ground.
Maybe that is what you are seeing when you mention that you've
experienced poor performance without an additional plane. I don't know
for sure.
I've only built patches that were high performance, no compromises- It's
likely the mfrs are intentionally throwing away low angle sensitivity
and axial ratio equality , and rear pickup rejection to get the size
down as it still suits the majority of users.
What you can try is turn the patch upside down...and under the patch at
least 10cm away, use a sea of steel wool (maybe 50x50cm ) to act as an
effective absorber (so the sky wont reflect back underneath) .
-glen
On 31/05/2022 20:23, Carsten Andrich via time-nuts wrote:
> On 31.05.22 01:10, glen english LIST via time-nuts wrote:
>> Be aware not to confuse the antenna ground plane (the patch will
>> always have its own plane because the top metalization must be fed
>> against a plane or counterpoise - and a ground plane behind the
>> antenna.
>>
>> I can see the usefulness of the larger ground plane for any purchased
>> patch antenna to reduce the likelihood of interference underneath (if
>> the feed coax has a good RF contact with the plane), and if the plane
>> is coupled well, it may improve the low angle response .
>>
>> The supplementary ground plane doesnt have to have a galvanic
>> connection if the gap between the underside of the patch is low- IE
>> use purely a capacitive coupling to tie the patch antenna ground to
>> the large ground sheet-
>> [...]
