Bruce wrote
Those plots give little information about the antenna response to
variations in SV elevation directly as the signal strength from theSV also
varies with
elevation. No account is taken of the SV signal variation with elevation.
ws) Correct, but a really Silly comment. Of course for that you compare the
before and after plots.
BUT, In your zeal to find something to criticize no mater how small, you
missed an important point again.
These plots are examples that show the whole overall signal strength
picture, which can be used to set the Elevation mask among MANY
other things.
ws
*********************> WarrenS wrote:
Bruce wrote:
<ws response
Only for your particular location and antenna.
<True, My location is under the sky indoors or outdoors, and I used
several cheap car type antennas and a pole mounted timing antenna.
May not work as well in other locations or with other types of
antennas. (Like under the water or with choke antennas)
You have no actual idea of how the antenna response is affected by
the presence of the cake tin,
<Sure I do, You have not been keeping up with the latest Lady Heather
updates, Take a look at her new antenna plot capability.
Those plots give little information about the antenna response to
variations in SV elevation directly as the signal strength from the SV
also varies with elevation.
nor did you specify what sort of antenna you are using with it.
<Yes I did give a recommendation in the beginning for a timing antenna,
but I get the same kind of effect on all "car" type also.
(which is no surprise case they are made to be mounted on a metal body.)
The latter are all patch antennas which don't work at all well without a
ground plane (either an external one or a built in one).
Even the thickness of the ground plane makes a difference to the antenna
response:
http://www.emtalk.com/tut_1.htm
http://www.orbanmicrowave.com/The_Basics_Of_Patch_Antennas.pdf
http://www.specemc.com/patch.asp
http://piers.mit.edu/piersproceedings/download.php?file=cGllcnMyMDA5bW9zY293fDVQNV8xOTI2LnBkZnwwOTAzMTQwODAxMzA=
http://ctvr.ie/docs/RF%20Pubs/IEEEVTC_2007_Investigation%20on%20Miniature%20Wideband%20Patch%20Antenna%20.pdf
http://140.117.166.1/eehome/ISCOM2005/SubmitPaper/UploadPapers/ISCON05_00100.pdf
A patch antenna at the bottom of a cake tin may not have much gain
over the elevation range of interest.
<What I can say, It works good, and it is not likely it is the antenna
gain, cause by making a directly antenna out of it,
cause I get some of the same effects with the pie pan turned the other
way.
Your explanation of why this appears to work well is still nonsense.
<That is because I did not give one yet. MY guess is that it is
blocking out multipath reflections of which I have many, and these
reflections tend to case dips in the signal level due to cancellations
as the sat moves across the sky. Also adds some gain to some antennas.
The variation in antenna response with elevation needs to be
measured/calculated before one can safely conclude that such an antenna
is universally useful.
<READ answer AGAIN> That is exactly what I did with the new Lady
Heather, You may want to try it for yourself before your unfounded
criticizing or look at the attached.
No you didnt, no account is taken of the SV signal variation with
elevation.
ws
***************
WarrenS wrote:
Thanks, Nice set of references as always.
OK, so its not 100db, But you may of missed my point yet again.
If you think I said that a 10 cent pie pan is a good substitute for a
$1000 survey antenna in all applications,
I think we all know that would be Nonsense.
BUT what may not be known is:
that a turned up pie pan under my cheap patch/puck antenna,
works about as good as an expensive survey antenna for use with a
Tbolt GPSDO,
That's not Nonsense, It's a tested fact.
Only for your particular location and antenna.
Your explanation of why this appears to work well is still nonsense.
You have no actual idea of how the antenna response is affected by the
presence of the cake tin, nor did you specify what sort of antenna you
are using with it.
A patch antenna (like your puck) at the bottom of a cake tin that just
happens to be a quarter wave deep may not have much gain over the
elevation range of interest.
The variation in antenna response with elevation needs to be
measured/calculated before one can safely conclude that such an antenna
is universally useful.
So I'm not saying there is anything wrong with using a survey antenna
on your GPSDO,
if you have more money than cents, and you want the last sub ns of
performance.
But it is not really necessary, there are many many ways to get more
bang for the buck,
and they would still need to be done first to take advantage of the
survey antenna's small improvement when used for a GPSDO.
There appears to be a false impression that choke rings remove all
Multipath signals.
What they do is to remove multipath ground reflected signals from
below the antenna, not multipath signals from above it.
ws
Bruce
***************
Bruce wrote:
Nonsense, a finite ground plane by itself has little effect on waves
incident from underneath, unless of course its a resistive ground plane
with just the right surface resistivity.
A choke ring antenna doesn't have anything like 100db rejection of
multipath signals, the rejection varies with SV elevation.
http://www.javad.com/jns/index.html?/jns/technology/Choke%20Ring%20Theory.html
http://www.trimble.com/infrastructure/pdf/gnss-choke-ring-antenna_brochure.pdf
http://www.novatel.com/Documents/Papers/3D_choke_ring.pdf
http://www.weblab.dlr.de/rbrt/pdf/TN_0505.pdf
http://www.leica-geosystems.com/downloads123/zz/nrs/AR25/brochures/AR25_Brochure_en.pdf
Bruce
*******************
WarrenS wrote:
All true,
BUT
Just to keep from being mislead,
I said what a choke ring antenna does,
You have described how it is so good at its job.
The Main purpose of the choke ring's, hi impedance, 1/4 wave, tuned
thingies, around the antenna is to keep reflected Signals from BELOW
the antenna from sneaking around into the antenna. It does not effect
the signals above the 10 deg or so elevation point.
The choke rings are a necessary to get cm type survey accuracy,
but a pie pan shield /ground plane pretty much does the same thing at
the GPSDO 1ns (1ft) level,
where you don't need 100 db rejection for signals below the antenna.
What I've found; don't even need to be lucky.
Just put an 8 inch pie pan in the right place, facing the right
direction.
ws
***************
Hi there,
just to prevent from making mistakes ...
You can have two different accessories for GNSS antennas:
1) a ground plane
it prevents the antenna from receiving signals from beneath the
antenna (from the ground, roof or whatever is beneath)
... so this could be your pie pan normally
2) a choke ring
it creates an area of high impedance around your antenna;
because it is just a (set of) simple quarter wave waveguide(s)
with
short-circuit on one end, you can build it from a sheet of
metal by
yourself; you only need to calculate the sizes for your
waveguide on
the L1 frequency ... and if you are a lucky guy and your pie pan
fits
these sizes, you probably can use it for that ;-)
Best regards
Martin
****************
Is there a design for a choke ring to add to my existing L1 GPS
antenna ?
Yeah
If you are using it for GPSDO work where a ns is good enough and you
don't need cm accuracy,
It is called a pie pan with its Lip turned up.
A choke rings blocks reflected signals from below the antenna from
effecting the signals, a pie pan works pretty good at that also.
ws
***************
Hello The Net:
Is there a design for a choke ring to add to my existing L1 GPS
antenna ?
While there may be some variations to the exact dimensions of a choke
ring to a specific type of L1 antenna, either patch, helical, etc.,
I suspect there is some commonality to the design dimensions.
I am looking to start construction and I will be using a cheapie
patch/puck antenna.
Stan, W1LE
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