[time-nuts] Better GPSDO (was Choke Ring Pictures)

WarrenS warrensjmail-one at yahoo.com
Mon Mar 15 14:37:53 UTC 2010


Something that I often find unasked is
a) How much difference does it make?
b) Are there simpler ways to get the same or better results.

Concerning the home made Choke ring antenna, Nice job for a survey nut
BUT for freq nuts
I'm wondering if placing an 8 inch PiePan with its lip up between the choke 
ring assumable and the Puck antenna would:
1) improve the performance when used as a GPSDO antenna
2) Simplify the construction by eliminating the cake pans


Short summery of why I think that may be true:
A survey antenna and a GPSDO antenna have different Jobs.
Two different set of compromises can be used to best do their job.

A choke ring antenna's job is to determine its unknown location with sub cm 
accuracy.
To do that, it need to pick up low horizon satellites signals but not pick 
up any multipath reflections that add a mm or so error.
One of the ways they accomplish this is to add a sharp cut off at somewhere 
around 10 deg above the horizon.
The goal is to reject signals below this, and pass signals that are above 
this without adding any reflection errors of their own.


A GPSDO on the other hand is told its fixed location and  its job is to pick 
up satellites signals above around 30 deg elevation.
It needs to do this by reducing multipath signals without adding more than a 
foot or so of reflected errors of its own.
What I have found is that a PiePan that serves as a ground plane / dish 
reflector / low horizon attenuator,
seems to work pretty good for this. (if you don't mind the 1ns or so of 
added phase noise it causes)

If you think a pie pan will not reduce a GPS reflected signal from below the 
antenna, try placing the pie pan above the antenna.
I'm sure there are other even better ways to get the job done and NOT add 
the 1 ns of noise.
Next on my list of things to try is a smaller open taco maker deep dish 
metal pan.

I've only experimented using Tbolts, so I can not say if the same results 
would be seen with other GPS units.
But
It is safe to think that I have more experimental experienced than anyone 
out there when it comes to ways to make a Tbolt perform better.
I would be interested in hearing from anyone that could challenge that 
assumsion.


PS. if the PiePan does not improve the cake pan antenna, try lowering the 
position of the antenna, in relation to the choke rings.
For a survey antenna you want near zero horizon signals to come thru but for 
a GPSDO a 30 deg lower limit seems to be the best compromise.

ws

*******************
----- Original Message ----- 
From: "Brian Kirby" <kilodelta4foxmike at gmail.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Sunday, March 14, 2010 5:49 PM
Subject: Re: [time-nuts] Choke Ring Pictures


> The home made choke ring was calculated and the pie/cake pans came close. 
> I think somebody all ready pointed at it, but Unavco or NASA had the 
> dimensions to about 6 or 8 common choke ring reflectors on line.
>
> They are aluminum cake pans - I bought them in a crafts store.  They are 
> one inch apart between the rings.   They are 2 inches deep at the rims. 
> The outside pan is 12 inches (measured inside).  Next is 10 inch,  then 8 
> inch, then 6 inch, and 4 inch.
>
> When making them, I found the center of each pan and then drilled them. 
> Then I used a bolt to hold them all together except for the very center 
> unit.  Then I drilled two holes about and inch from the center and used 
> thin long bolts to  attach them to a piece of square tubing on the bottom. 
> The alignment bolt was removed from the center hole and drilled out larger 
> to pass the antenna coax thru.    The very center pan, I used epoxies to 
> glue the antenna too and I used a clamp to hold it in place.   Then the 
> coax was pass thru a hole I drilled out thru the center holes of the other 
> pans.   Then I glued the antenna pan to the other pans and used several 
> blocks of wood to center the pan and a big piece of pipe and weights as a 
> clamp to hold it down while it was curing.   The square tubing was aligned 
> into a  laser level mount and epoxied and that allows the unit to be 
> attached to a tripod.  The mount has three adjusting screws that allows 
> the antenna to be leveled.   I used the units in pairs when surveying and 
> always aligned then north in  an attempt to bias out centering 
> differences.
>
> The first unit mounted on the house roof did have a drainage problem the 
> first time it rained.  I drilled 1/8 holes inside of each ring to let it 
> run out.  That was not the worst problem.  A bird decided it would make a 
> good home and started building a nest on it when I went on vacation.  I 
> bought a large plastic funnel and inverted it and glued it inside of the 
> outer ring to stop that.
>
> The antenna is the common Antenna97 from Motorola, the coax was cut at 
> about one foot (I think it was originally 18 feet long) and a connector 
> was attached.  I then used a very low loss coax (1/2 inch heliax) about 40 
> feet long to bring it into the basement.
>
> Brian - KD4FM
>
>
> 



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