[time-nuts] ground plane for cheap helix antennas?

Bob Camp kb8tq at n1k.org
Fri Jul 24 16:08:35 EDT 2015


Hi

> On Jul 24, 2015, at 11:54 AM, Attila Kinali <attila at kinali.ch> wrote:
> 
> On Thu, 23 Jul 2015 17:38:00 +0000
> Alan Ambrose <alan.ambrose at anagram.net> wrote:
> 
>> Hi, is there any benefit in adding a ground plane to these simple
>> timing antennas? e.g. GPS-TMG-26N
>> 
>> http://www.antenna.com/apg_products.cgi?id_num=11740
>> 
>> In which case, how big? And I'm imagining immediately underneath the
>> antenna? This ublox note suggests probably not?
> 
> That highly depends on the inner structure of the antenna.
> There are two helical antenna designs, one that works with an
> anular ground ring/ground plane and one that works without.
> 
> In the former case, the antenna will benefit from a ground plane
> and you should use an as large as possible. In the latter case,
> a ground plane will not really help, respectively might actually
> decrease performance. What you want in that case is backplane of
> absorbtive material that minimizes reflections.
> (which is not the same as choke ring, btw, but a choke ring can
> substitute for absorbtive material, if used correctly)
> 
>> http://www.u-blox.com/images/downloads/Product_Docs/GPS_Antennas_ApplicationNote%28GPS-X-08014%29.pdf
>> 
>> The 58503A sitting on this atm gets '4.3us/initial 24 hours' prediction
>> although I have seen it around 2.2us. Also max ~110 'SS'.
>> 
>> Also is it helpful to increase the elevation mask?
> 
> Depends on your situation, ie on the antenna position and its surrounding.
> Generally speaking, you do want satellites as low on the horizon as
> possible, because they give you the most accurate x/y position solution.
> But at the same time, the lower a satellite is, the more multipath you
> will get. Hence this becomes a tradeoff that is defined mostly by the
> multipath generating structures around your antenna and their position.

You *probably* will do a survey of your location for some period (a couple days maybe) and
then put the device into position hold mode. You will lock it to the single surveyed
position and leave it there. Once you go into “locked” mode, the low angle satellites do not 
help you a lot for timing. 

There are (of course) a lot of “that depends on *exactly* what your setup and location are sort
of things. IF you are near either pole, there are not going to be any high angle sats.   If you are in some sort of cave, so
your only view of he sky is 35 degrees wide and from 0 up to 35 degrees vertical, then a mask
may chop a lot of sats.

Long before you start fiddling with antenna tweaks, the thing to do is to get a good 
sky view. You can look up what the GPS tracks are relative to your location online. The
idea is to get a location where you can see at least 2/3 of those tracks when they are above 
about 20 degrees to the horizon. If you start to have obstructions at 10 degrees, set your
mask to 15 or 20. If you are mostly clear at 20 degrees set the mask at 25. Past that point, 
I would suggest moving the antenna or simply accepting some degradation.

How much degradation in terms of estimated holdover?

I’ve seen the HP boxes estimate 100 ns of holdover in a setup with a really lousy 
antenna after running for about 2 days. I’ve seen them estimate 2 to 3 us of holdover
with excellent antennas after running for a month or more. (Hint: the holdover estimator
does not handle warmup as well as it should). 

 Bob


> 
> 				Attila Kinali
> 
> -- 
> I must not become metastable. 
> Metastability is the mind-killer.
> Metastability is the little-death that brings total obliteration.
> I will face my metastability. 
> I will permit it to pass over me and through me. 
> And when it has gone past I will turn the inner eye to see its path. 
> Where the metastability has gone there will be nothing. Only I will remain.
> 
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