Re: [time-nuts] How can I measure GPS Antenna quality?

[Charles Steinmetz]

Mark wrote:


The tricky bit is interpolating signal levels between logged points (there is a 
display options for showing the raw signal level data).   Heather interpolates 
between adjacent azimuth points at each elevation angle.


I always wondered about the "filled in" (interpolated) plot.  To my 
observation, the orbital tracks of the satellites do not wander anywhere 
near +/- 22.5 degrees.  It is unclear to me that interpolating the data 
to areas of the sky where satellites never appear has any utility.


I suppose it makes trends somewhat easier to spot -- but that could also 
be done just by making the dots on the raw chart a bit larger, without 
suggesting a continuity of reception-space that doesn't exist.


Or am I missing something?

Charles


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Re: [time-nuts] Best replacement for Trimble Bullet antenna

[Nick Sayer via time-nuts]

I bought a Gilsson marine antenna and an 8 port amplified splitter on eBay. The 
antenna is mounted on the roof of my garage, where it has visibility easily 
down to 20 degrees except to the North where there is an obstruction (but that 
doesn’t matter). The coax is 10 meters of whatever came with the antenna. My 
reception with this setup is as close to ideal as I’ve ever heard of. At the 
moment, gpsmon on my NTP server shows 5 satellites with 50+ SNR, another 5 with 
40+ and one at 22 (7 degrees elevation).



> On Nov 22, 2016, at 10:58 AM, Russ Ramirez  wrote:
> 
> I have a used version of the subject antenna, which is still quite
> operational. However, I recently needed either a way to tie into reception
> from this antenna or purchase additional antennas. Given the cost of active
> patch antennas, I tried one from Adafruit.
> 
> While comparing reception on a Trimble Thunderbolt to an integrated GPS
> module (used for time and PPS on a WSPR transmitter) is not apples to
> apples, I did notice how many more fixes the latter receiver obtains. For
> example, when the Thunderbolt has fixes on 6 birds, the YIC51612 (MediaTek)
> receiver module will have 10. I am sure the criteria is not the same for
> what constitutes a fix in both cases, but I also observed the following.
> 
> By simply using a 0.1 uF SMD MLCC cap to couple into the Trimble Bullet,
> i.e. the Thunderbolt still powers the amp with +5v, but the signal has a
> path to my other GPS receiver, the bullet antenna underperformed by a
> significant amount. What's possibly worse is that the patch antenna is on a
> window sill, whereas the bullet antenna is on the roof with a clear view of
> the sky.
> 
> I have not tried the Adafruit patch with the Thunderbolt yet as I will have
> to reduce the LNA voltage from 5 to 3.3 before trying the second test.
> 
> The general question I wanted to ask though is what others use/like as roof
> mounted antennas? Are some of the 'Marine' antennas better?
> 
> Russ
> K0WFS
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[time-nuts] How can I measure GPS Antenna quality?

[Mark Sims]

The colored signal level vs az/el plot  follows that convention...  outer edge 
is the horizon, center point is zenith (90 degrees above the horizon).   Color 
represents the average signal level seen at a given point in the sky.  

The tricky bit is interpolating signal levels between logged points (there is a 
display options for showing the raw signal level data).   Heather interpolates 
between adjacent azimuth points at each elevation angle.  The interpolation 
extends for a maximum of 22.5 degrees azimuth from each point that had signals. 
 Handling things like the "hole" at northern azimuth angles (for the northern 
hemisphere) complicates things (once you get far enough north, the horseshoe 
shaped signal hole becomes a closed circle).  Heather builds a "clipping" 
structure based on the min and max elevation angles that has signals for each 
azimuth angle.

I'd like to try something like a Voronoi tessellation,  but that gets rather 
nasty to implement...

---

> The usual "flat" plot for a 3 D pattern of a GPS antenna is to have the 
radius = 90-elevation angle (so horizon is outer border), angle is 
azimuth looking down on antenna, and color be power. 
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Re: [time-nuts] Fwd: Best replacement for Trimble Bullet antenna

[Chris Albertson]

With roof mounted antenna practical issuers are important.

1) Do birds like to tech on it, they will it then can find a way to stand
on top of it.  They poop on whatever they perch on.
2) Same with snow in some places
3) Can you route the lead wire down the mast?   Some antenna mount to a
pipe flange allowing coax to be completely inside a metal cover.   The wire
will last a LONG time if it is not exposed to the elements
4) easy way to attach a ground clamp.
5) with a pole mount one can extend the height of the antenna as far up as
you like.

It is not hard at all to find a high gain GPS antenna that is inside a
pointed radome.   Mine is a helix type but I think a good modern patch
inside a pointed radome would work as well.   Practical issues dominate
unless you want to go up on the roof now and then.   I put the antenna up
and figure it is good for the life of the building, decades of not thinking
about it I hope.

On Tue, Nov 22, 2016 at 10:58 AM, Russ Ramirez 
wrote:

> I have a used version of the subject antenna, which is still quite
> operational. However, I recently needed either a way to tie into reception
> from this antenna or purchase additional antennas. Given the cost of active
> patch antennas, I tried one from Adafruit.
>
> While comparing reception on a Trimble Thunderbolt to an integrated GPS
> module (used for time and PPS on a WSPR transmitter) is not apples to
> apples, I did notice how many more fixes the latter receiver obtains. For
> example, when the Thunderbolt has fixes on 6 birds, the YIC51612 (MediaTek)
> receiver module will have 10. I am sure the criteria is not the same for
> what constitutes a fix in both cases, but I also observed the following.
>
> By simply using a 0.1 uF SMD MLCC cap to couple into the Trimble Bullet,
> i.e. the Thunderbolt still powers the amp with +5v, but the signal has a
> path to my other GPS receiver, the bullet antenna underperformed by a
> significant amount. What's possibly worse is that the patch antenna is on a
> window sill, whereas the bullet antenna is on the roof with a clear view of
> the sky.
>
> I have not tried the Adafruit patch with the Thunderbolt yet as I will have
> to reduce the LNA voltage from 5 to 3.3 before trying the second test.
>
> The general question I wanted to ask though is what others use/like as roof
> mounted antennas? Are some of the 'Marine' antennas better?
>
> Russ
> K0WFS
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>



-- 

Chris Albertson
Redondo Beach, California
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Re: [time-nuts] Re; Do reflections up/down the antenna cable cause a problem with GPS

[jimlux]

On 11/22/16 4:36 AM, John Ponsonby wrote:

Remember that for a microwave LNA the condition for optimum noise figure is not 
the same as the condition for the input to be matched. Thus one doesn't expect 
the input of a good LNA to be matched to 50Ω. (Dr John Ponsonby)


this is also true for virtually any amplifier.
For lower frequency (say, <100MHz) you might find a current noise and 
voltage noise spec, from which one could figure out an optimum source 
resistance.  But when it comes to the reactive component? Good luck.


It's also something that doesn't seem to show up on data sheets for RF 
components - you *might* get a "input Z for lowest noise" curve on a 
Smith chart, but you'd be lucky.




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Re: [time-nuts] How can I measure GPS Antenna quality?

[jimlux]

On 11/22/16 10:13 AM, Mark Sims wrote:

Attached is a screen shot from Lady Heather showing various antenna signal 
displays.  The antenna was a cheap GPS/GLONASS patch antenna (mounted on a 3 
foot ground level tripod ) connected to a rather nice NVS-08 receiver tracking 
GPS, SBAS, and GLONASS satellites (typically around 22 sats).



very cool.

The usual "flat" plot for a 3 D pattern of a GPS antenna is to have the 
radius = 90-elevation angle (so horizon is outer border), angle is 
azimuth looking down on antenna, and color be power.  Somewhere I saw 
one that also integrated the multiple satellite tracks.


Another strategy is to plot it in u,v space.  Color is power, but u = 
cos(az)*cos(el) and v is sin(az)*cos(el)   (or sin(theta) where theta is 
the angle off boresight)



http://ww2.nearfield.com/amta/AMTA07-0092-GFM_SFG.pdf has a bewildering 
variety to choose from

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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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[time-nuts] Commercial avionics GPS bruohaha

[Artek Manuals]

http://gpsworld.com/faa-suspends-approval-of-certain-navworx-ads-b-units/


--
Dave
manu...@artekmanuals.com
www.ArtekManuals.com

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Re: [time-nuts] How phase stable is rg59 or alternate coax

[David G. McGaw]

Note that partial air core (9913) and foam dielectric is better than 
solid polyethylene.


David N1HAC


On 11/21/16 5:39 PM, Mark Spencer wrote:

At one point I contemplated running Andrews "Heliax" for my GPS antenna.   Part 
of the rationale was due to the data presented in page 2 of the following paper.

http://ivs.nict.go.jp/mirror/meetings/v2c_wm1/phase_stability.pdf

I subsequently decided to stay with my existing run of plenum rated RG58.  The 
bulk of my cable run is indoors where the temperature is fairly stable.

Regards
Mark Spencer




On Nov 21, 2016, at 12:59 PM, Scott Stobbe  wrote:

When I first took a look at some of the coax datasheets I couldn't find
anything. I was able to find the following paper "phase stability of
typical navy radio frequency coaxial cables"
http://www.dtic.mil/dtic/tr/fulltext/u2/628682.pdf I attached the table
from the last page. They estimate RG59 to have a tempCo of -330 PPM/degC
for electrical length. They also estimated RG-58 at -480 PPM/degC.


On Mon, Nov 21, 2016 at 2:44 PM, John Ackermann N8UR  wrote:

I can't find the data right now, but will keep digging.  There's also a
short paper from the early 2000s from Haystack on their measurement of
LMR400 in an environmental chamber.  They came to the same conclusion, but
I can't find that paper either. :-


John, many thanks for the Haystack tip! That is a wonderful paper, I
believe the one you are quoting is "Dispersion and temperature effects in
coax cables" http://www.haystack.mit.edu/tech/vlbi/mark5/mark5_memos/067.pdf

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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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>> 
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[time-nuts] How can I measure GPS Antenna quality?

[Mark Sims]

That screen is one of the features in the next (5.0) release.  If you click the 
mouse on one of the signal level maps in that display,  it will zoom that one 
to full screen.

Besides the normal "zoom" keyboard commands,  with the 5.0 release you can also 
zoom the clock/watch/sat map/signal level map shown on the main screen  to full 
screen by clicking on it.


-

> Was that screen invoked from the LH application or via command line?  I 
don't see a mention of something similar on the LH help pop-up.
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[time-nuts] Fwd: Best replacement for Trimble Bullet antenna

[Russ Ramirez]

I have a used version of the subject antenna, which is still quite
operational. However, I recently needed either a way to tie into reception
from this antenna or purchase additional antennas. Given the cost of active
patch antennas, I tried one from Adafruit.

While comparing reception on a Trimble Thunderbolt to an integrated GPS
module (used for time and PPS on a WSPR transmitter) is not apples to
apples, I did notice how many more fixes the latter receiver obtains. For
example, when the Thunderbolt has fixes on 6 birds, the YIC51612 (MediaTek)
receiver module will have 10. I am sure the criteria is not the same for
what constitutes a fix in both cases, but I also observed the following.

By simply using a 0.1 uF SMD MLCC cap to couple into the Trimble Bullet,
i.e. the Thunderbolt still powers the amp with +5v, but the signal has a
path to my other GPS receiver, the bullet antenna underperformed by a
significant amount. What's possibly worse is that the patch antenna is on a
window sill, whereas the bullet antenna is on the roof with a clear view of
the sky.

I have not tried the Adafruit patch with the Thunderbolt yet as I will have
to reduce the LNA voltage from 5 to 3.3 before trying the second test.

The general question I wanted to ask though is what others use/like as roof
mounted antennas? Are some of the 'Marine' antennas better?

Russ
K0WFS
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Re: [time-nuts] How can I measure GPS Antenna quality?

[Jeff AC0C]

Mark,

Was that screen invoked from the LH application or via command line?  I 
don't see a mention of something similar on the LH help pop-up.


73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie

-Original Message- 
From: Mark Sims

Sent: Tuesday, November 22, 2016 12:13 PM
To: time-nuts@febo.com
Subject: [time-nuts] How can I measure GPS Antenna quality?

Attached is a screen shot from Lady Heather showing various antenna signal 
displays.  The antenna was a cheap GPS/GLONASS patch antenna (mounted on a 3 
foot ground level tripod ) connected to a rather nice NVS-08 receiver 
tracking GPS, SBAS, and GLONASS satellites (typically around 22 sats).


The signal strength vs elevation mask plot shows a typical antenna 
characteristic where the signal strength drops off rapidly as the satellite 
elevation angle decreases.  The yellow marker tick is where the signal 
strength is 80% of the peak strength.   Swapping antennas can cause this 
angle to change by up to 15 degrees.  When Heather does an "autotune" 
function,  it uses this angle to decide where to set the antenna elevation 
mask angle.


The relative signal strength vs azimuth plot can be a bit deceiving... it 
shows great signal strength to the north,  but this is mainly due to the 
fact that the only times sats are visible there is when they are at high 
elevation angles.  Weighting the signal strengths by 1/ELevation angle does 
a better job of showing signal obstructions... There is a two-strory house 
25 feet to the west of the antenna.


The signal level vs az/el map gives the best information on the antenna sky 
view characteristics...  My best view is to the south east.   Lots of trees 
to the north,  house to the west,  lots of other ground level obstructions 
everywhere.









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[time-nuts] How can I measure GPS Antenna quality?

[Mark Sims]

Attached is a screen shot from Lady Heather showing various antenna signal 
displays.  The antenna was a cheap GPS/GLONASS patch antenna (mounted on a 3 
foot ground level tripod ) connected to a rather nice NVS-08 receiver tracking 
GPS, SBAS, and GLONASS satellites (typically around 22 sats).

The signal strength vs elevation mask plot shows a typical antenna 
characteristic where the signal strength drops off rapidly as the satellite 
elevation angle decreases.  The yellow marker tick is where the signal strength 
is 80% of the peak strength.   Swapping antennas can cause this angle to change 
by up to 15 degrees.  When Heather does an "autotune" function,  it uses this 
angle to decide where to set the antenna elevation mask angle.

The relative signal strength vs azimuth plot can be a bit deceiving... it shows 
great signal strength to the north,  but this is mainly due to the fact that 
the only times sats are visible there is when they are at high elevation 
angles.  Weighting the signal strengths by 1/ELevation angle does a better job 
of showing signal obstructions... There is a two-strory house 25 feet to the 
west of the antenna.

The signal level vs az/el map gives the best information on the antenna sky 
view characteristics...  My best view is to the south east.   Lots of trees to 
the north,  house to the west,  lots of other ground level obstructions 
everywhere.


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Re: [time-nuts] 53132 replacement fan

[Chris Albertson]

Is the original fan controlled by a temperature sensor?   That is the best
way to control noise because the fan runs at lower RPM most of the time and
high speed only when needed.  So the airflow and noise is only as much as
is required,

They make retrofit kits, fans with sensors most for use in old PCs but you
can use them anyplace.  Typically there is a pot you adjust to set to
desired internal use tempature.   Almost all new design hardware is like
this.  Notice the newer fans use three wires.  One of then is a tachometer
sensor to provide RPM feedback to the fan controller.   It costs only a
little to retrofit one of these.  Look were they sell computer parts.


On Mon, Nov 21, 2016 at 9:57 AM, jimlux  wrote:

> On 11/21/16 6:39 AM, Charles Steinmetz wrote:
>
>> Tom wrote:
>>
>> EFB0412MD
>>> Airflow 7.17 CFM
>>> 6300 RPM
>>> Noise 24 dBA
>>>
>>> FBK04F12U
>>> Same exact form factor.
>>> Air Flow 9.2 CFM
>>> 9500 RPM
>>> Noise 42 dB(A)
>>>
>>
>> Note the 18dB greater noise (that's a HUGE difference).  Even with bad
>> bearings in the original fan, it is probably considerably quieter (by
>> 10dB or more) than the proposed replacement.  On the other hand, the
>> replacement moves 28% more air, which may be a good thing.
>>
>>
> That's a 40mm fan, which is a standard size, I'll bet you can find a
> slower turning/quieter fan.
>
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-- 

Chris Albertson
Redondo Beach, California
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Re: [time-nuts] Logging SPAD pulses on synced devices (was: Linux PPS clues?)

[Ilia Platone]

Hi,
Do a SPI bus do this job correctly? I'd have the clock on one line and 
the pulses logged on a second line. Like CLK for clock pulses and 
MOSI/MISO for serial logging of pulses. I'd avoid I2C because of 
addressing packets.


Best Regards,
Ilia


On 11/05/16 04:48, Casey L. Jones wrote:
Yes, you might need a separate dedicated chip to take in the serial 
input steadily. Although you may not. Many serial ports have a small 
buffer to prevent missed serial input when the operating system gets 
distracted with something other than processing serial data.

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--
Ilia Platone
via Ferrara 54
47841
Cattolica (RN), Italy
Cell +39 349 1075999

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Re: [time-nuts] 53132 replacement fan

[cfo]

On Sun, 20 Nov 2016 14:14:02 -0500, John Ackermann N8UR wrote:

> Does anyone have a part number for the 53132 fan (or equivalent)?  Mine
> is getting pretty noisy.
> 
> Thanks!
> John

I got this one for my '132
http://www.ebay.com/itm/111317257709

Sunon 
HA40201V4-999 Axial-Lüfter 40x40x20mm 12V= 9m³/h von Sunon


Haven't mounted it yet though.

/CFO

-- 
E-mail:xne...@luna.dyndns.dk

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

[John Ponsonby]

Remember that for a microwave LNA the condition for optimum noise figure is not 
the same as the condition for the input to be matched. Thus one doesn't expect 
the input of a good LNA to be matched to 50Ω. (Dr John Ponsonby)
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Re: [time-nuts] Do reflections up/down the antenna cable cause aproblem with GPS?

[David J Taylor]

From: Dr. David Kirkby (Kirkby Microwave Ltd)

Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.
=

Only if the sending impedance of the LNA in the antenna is well off the 
nominal cable impedance, and in any case the reflection caused by mismatch 
at the GPS receiver input would be attenuated twice by the cable run.  For 
timing applications I don't believe it's an issue, and for location 
applications buildings and shielding are a far greater problem.


David GM8ARV
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 


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Re: [time-nuts] 53132 replacement fan

[David G. McGaw]

I will point out that I have rejuvenated many a fan by peeling off the 
round label covering the bearing and adding a small amount of oil.


David N1HAC


On 11/21/16 12:57 PM, jimlux wrote:

On 11/21/16 6:39 AM, Charles Steinmetz wrote:

Tom wrote:


EFB0412MD
Airflow 7.17 CFM
6300 RPM
Noise 24 dBA

FBK04F12U
Same exact form factor.
Air Flow 9.2 CFM
9500 RPM
Noise 42 dB(A)


Note the 18dB greater noise (that's a HUGE difference).  Even with bad
bearings in the original fan, it is probably considerably quieter (by
10dB or more) than the proposed replacement.  On the other hand, the
replacement moves 28% more air, which may be a good thing.



That's a 40mm fan, which is a standard size, I'll bet you can find a 
slower turning/quieter fan.


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