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How to measure the real GPS signal on NanoVNA?


Kevil 2021/12/16 10:31

What type of measurement to select on NanoVNA to measure the antenna received GPS 1.575 GHz signal on CH0 ? I measured the LogMAG without LNA = -13 dB and with LNA = -32.5 dB. I assume the LNA gain is 19.5 dB.

When the NanoVNA V2 SAA-2 has the sensitivity -90 db and I use the LNA with 40 dB gain do I have a chance to see real GPS signal level on NanoVNA? What type of measurement to select?

W0LEV 2021/12/16 18:55

A VNA, any VNA, is not the correct instrument for receiving weak signals,
or strong signals, for that matter. The Tiny SA, any SA covering the GPS
frequencies, is the appropriate instrument to use.

https://www.tinysa.org/wiki/

However, you will have to mix down into the range of the TinySA (spectrum
analyzer) as its max frequency is 950 MHz. If you need a more capable SA,
visit the Rigol or Siglent sites. Expect those to come in around $2K ±.
You'ss still need an LNA ( low noise figure, not necessarily high gain!!).

Dave - WØLEV

On Thu, Dec 16, 2021 at 6:31 PM Kevil via groups.io <tiputa=
yahoo.com@groups.io> wrote:

> What type of measurement to select on NanoVNA to measure the antenna
> received GPS 1.575 GHz signal on CH0 ? I measured the LogMAG without LNA =
> -13 dB and with LNA = -32.5 dB. I assume the LNA gain is 19.5 dB.
>
> When the NanoVNA V2 SAA-2 has the sensitivity -90 db and I use the LNA
> with 40 dB gain do I have a chance to see real GPS signal level on NanoVNA?
> What type of measurement to select?
>
>
>

--
*Dave - WØLEV*
*Just Let Darwin Work*

Jim Lux 2021/12/16 10:55

On 12/16/21 10:31 AM, Kevil via groups.io wrote:
> What type of measurement to select on NanoVNA to measure the antenna
> received GPS 1.575 GHz signal on CH0 ? I measured the LogMAG without
> LNA = -13 dB and with LNA = -32.5 dB. I assume the LNA gain is 19.5 dB.
>
> When the NanoVNA V2 SAA-2 has the sensitivity -90 db and I use the LNA
> with 40 dB gain do I have a chance to see real GPS signal level on
> NanoVNA? What type of measurement to select?
> _._,_._


The NanoVNA isn't a spectrum analyzer.  You can't measure the signal
strength of an off-the-air signal.  You *could* use the NanoVNA as part
of an improvised antenna range.  Connect CH0 to a radiating antenna, put
it some distance away, then look at what signal you get from your
Antenna Under Test (AUT) with CH1 - (i.e. make a S21 measurement). (or
your AUT with and without preamp).

Alan 2021/12/17 09:13

You won't see GPS signals very well on any VNA or Spectrum Analyser as
the signal is a spread spectrum signal, it is a weak signal spread over
2MHz bandwidth. The only sensible way of indicating GPS - or any other
GNNS signals- is to use an appropriate receiver module which will
de-spread the signal back to a much lower frequency.  Generally the
direct spread signal is just around the noise level, despreading should
produce a very much better Carrier to Noise ratio which is normally able
to be displayed via the software for the module being used.

However GNNS signals are not very stable for other reasons, All sats
transmit on the same frequency ( except Glonass) but have different
Doppler shifts. The signal levels depend on range and angle which are
all slowly changing while multipath reflections can mess levels up even
more. Jim's idea of using the Tracking Gen output is the much more
sensible way to go however  you will find out that  test ranges and
antennas  have their own issues!


Regards,

Alan G8LCO

Reinier Gerritsen 2021/12/17 16:47

GPS signals are received at around -125 dBm. Thermal noise however is
-110 dBm (in a 2 MHz bandwidth). No way you can see the GPS signals on a
spectrum analyzer, it is deeply burried in the background noise.
De-spreading the signal will give a signal processing gain of 43 dB and
now it is above the noise floor.


Op 17-12-2021 om 10:13 schreef Alan:

Kevil 2021/12/17 08:26

Thank you for all of your advice. I understand that GPS signal level is very low and NanoVNA is not a good piece of equipment to measure it.

It doesn't help probably to boost the received GPS signal by 40 dB LNA (-125 dBm + 40 db = - 85 dBm) which may be 5 dB above the NanoVNA sensitivity (-90 dB) measured on CH1 without the stimulation on CH0.

Is there any affordable device to directly measure the level of the received GPS signal from the antenna without a GPS receiver? I'm thinking something with an LNA and an analog or digital dBm meter similar to a satellite dish alignment meter. ( https://www.aliexpress.com/item/32837736749.html )

N2MS 2021/12/17 11:36

Why don't you use an app on a smartphone to measure GPS satellite signal strengths?

Mike N2MS

Dragan Milivojevic 2021/12/17 17:36

Might be of interest:
https://www.rtl-sdr.com/rtl-sdr-tutorial-gps-decoding-plotting/

On Fri, 17 Dec 2021 at 17:26, Kevil via groups.io <tiputa=
yahoo.com@groups.io> wrote:

Dragan Milivojevic 2021/12/17 17:40

Alternatively if you just need one number as a figure of merit, get a cheap
NEO gps module ...

On Fri, 17 Dec 2021 at 17:36, Dragan Milivojevic via groups.io
<d.milivojevic=gmail.com@groups.io> wrote:

Reinier Gerritsen 2021/12/17 17:42

A 40 dB LNA will also amplify the noise and add a little bit of its own
noise. You cannot see a GPS signal burried deeply in the noise. The only
way to see it is by signal processing (correlation with the spreading
signal). Most if not all GPS receiver modules provide signal strength
indicators per satellite. All you need is a serial interface with a
cheap microcontroller. I used my garmin bicycle computer for developing
a GPS antenna repeater, but any recent smartphone can do that as well. I
see no way how to measure signal strength without a GPS receiver.


Op 17-12-2021 om 17:26 schreef Kevil via groups.io:

N2MS 2021/12/17 11:50

Here is an example, GPS Fix:

<">https://play.google.com/store/apps/details?id=com.chartcross.gpstest&hl=en&gl=US> https://play.google.com/store/apps/details?id=com.chartcross.gpstest&hl=en&gl=US

Mike N2MS

Kevil 2021/12/17 13:54

@Dragan Milivojevic

Thank you, interesting link.

Actually I am working on Micro GPS/LoRa Tracker with GPS dipole wire antenna build into cat collar around the neck to achieve almost 360° coverage. I will use the u-Blox ZOE-M8Q GPS Receiver (4.5x4.5), SparkFun GPS Breakout ( https://www.sparkfun.com/products/15193 ) just for testing - I will use the bare chip, and my own 2x GPS LNA (39.6 dB). The PCB dimensions are 8x25x2 mm. I will measure the GPS position each hour.

I have fine tuned the GPS lambda*1.25 wire dipole antenna (theoretical 5.2 dB gain) for the best reception and would like to know what is the real output power from it to adjust LNA gain accordingly to get the strong GPS signal in the difficult conditions, wired dipole antenna around cat neck near the ground and to get the GPS position within 60 seconds to save battery power. When the cat will be hidden somewhere, still the MCU allows me to send dummy LoRa message and get approx. position by LoRa triangulation.

With the 1x GPS LNA (about 19 dB) I was able to receive signal from 11 GPS satellites.

On the picture GPS dipole wire antenna (in the free space), one arm length is 130 mm (total two arms):

Dragan Milivojevic 2021/12/17 23:32

This looks fine to me but I think that you are overengineering it.
There is nothing wrong with it, if the goal is to learn but if you just
want to get a working solution, any cheap ebay module with a ceramic
patch antenna will work, even indoors (within reason, window nearby etc).
GPS signals bounce around and after initial lock (can take a few minutes)
it will provide positioning with a reasonable accuracy. For power savings
look into how to setup the NEO, quite a few power saving options.
For ideas about GPS antenna design checkout ublox documentation (if you
haven't already).
A flexible PCB with integrated pcb antennas (ground plane helps) with a pvc
sleeve
if you want to overdo it ;)


On Fri, 17 Dec 2021 at 22:54, Kevil via groups.io <tiputa=
yahoo.com@groups.io> wrote:

Reinier Gerritsen 2021/12/18 10:39

A dipole antenna directly at the neck of a cat? That will detune the
antenna to a lower frequency. You'll have to trim the ends. You could
simulate your cat by using your own arm. Similar dielectric load of the
antenna.

Op 17-12-2021 om 22:54 schreef Kevil via groups.io:

Kevil 2021/12/18 03:17

@Reinier Gerritsen

I measured the antenna on the nylon cat collar placed on my thigh. The tuned arm length is 123 mm with VSWR 2.370 (see chart below).

@Dragan Milivojevic

I can't use the patch antenna. It is too big, directional and cannot provide 360° GPS signal reception when the collar is turned around the cat's neck.

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