[time-nuts] Choke Ring Design for L1

[WarrenS]

>Bruce wrote:
<ws response

> Only for your particular location and antenna.
<True, My location is under the sky indoors or outdoors, and I used severl 
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.

>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.)

>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.


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
>> 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: ws5b-Jan-2-10.gif
Type: image/gif
Size: 64922 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20100313/124eeec3/attachment-0001.gif>