[time-nuts] Better Tbolt [was Choke Ring Design for L1]
warrensjmail-one at yahoo.com
Sun Mar 14 16:09:51 UTC 2010
As is often the case, this discussion has spun off into several different
I'm not trying to make a choke ring antenna, because I do not think it is
necessary for a Tbolt.
I'm just suggesting a way to get better performance from a Tbolt GPSDO if on
a limited budget, using things readily available.
Initially I put my Indoor antenna on a large cookie sheet, which did help
But I found that moving it around a few inches one way or the other could
cause the Tbolt to loose lock.
Anyone that has a Tbolt, knows it is not good for it to go into holdover.
Solution is: move the antenna outdoors OR put a turned up pie pan under the
Now my PiePan indoor antenna works about as good as a non PiePan cheap
And of course using the PiePan antenna out doors works even better,
But that is no longer necessary to get reasonable performance from the
This is the test results I've received, At my location, with my cheap
gee who would of guessed that a pie pan antenna was not perfect.
But then again, maybe it does not have to be 100% perfect to help enough TO
MAKE A BIG DIFFERENCE.
Maybe its just luck and I have found a Magic pie pan that does not care that
it has not been tested in an microwave anechoic chamber.
or Maybe its cause I try ten different things, using Lady Heather to test
them, and pick the ones that work the best.
If you have a Tbolt on a cheap antenna that you want to make better, try it.
If you don't, then you can still criticize it and discuss why it should or
should not work.
Peter Vince wrote:
> As I understand it, the GPS signals are circularly polarised, and so
> surely reflections will reverse the sense of that polarisation such
> that the antenna will be insensitive to them?
Yes, as a first degree analysis. However, cancellation will not be 100%
since neither the wave, the reflection and the antenna is completely
according to that theory.
> Maybe Warren's simple
> pie dish is working by shielding the antenna from the true MULTI-path
> reflections, and any direct SINGLE reflections it produces are ignored
> due to the polarisation reversal?
It at least provides the function of the ground plane. Ground-plane only
antennas have been widely used in survey applications.
There are many ways to improve the situation for a patch antenna. A
ground plane is certainly one of the ways. Recall that wavelength is
about 19 cm so anything near optimal antenna should be expected to have
Oh, the choke ring antennas not only suppress reflections from below,
but also provides spatial nulls towards the horizon, such that low
elevation reflections is also suppressed. The more nulls, the wider lobe
and thus lower elevation angle gets can be accepted. For traditional
choke rings 3 or 4 is used. I have three rings in mine.
> But the incident wave is diffracted at the edge of a finite ground plane
> creating a surface wave that the patch antenna element does respond to.
> Unless one has a microwave anechoic chamber one has to calculate the
> antenna response with an EM simulation program.
> Its not possible to infer an antenna's response to multipath by merely
> measuring its apparent response pattern to a GPS SV except perhaps when
> one has a detailed and accurate EM model of the antenna environment.
> Peter Vince wrote:
>> As I understand it, the GPS signals are circularly polarised, and so
>> surely reflections will reverse the sense of that polarisation such
>> that the antenna will be insensitive to them? Maybe Warren's simple
>> pie dish is working by shielding the antenna from the true MULTI-path
>> reflections, and any direct SINGLE reflections it produces are ignored
>> due to the polarisation reversal?
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