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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