[time-nuts] GPS Antenna Voltage-Dropping

WarrenS warrensjmail-one at yahoo.com
Thu Sep 3 15:58:57 UTC 2009


Seems like a good idea, a lot simpler and cleaner than the other suggestions.
Sounds like a data problem, I'd suspect your VSWR meter or cabling first. 
Maybe your VSWR meter does not like diodes especially if it is not running current thru them.
And if you are running current thru them to approximate the GPS antenna, 
whatever you're doing to run the current thru them is making the reading unhappy.
Did you try just the BNC adapter with just a short inside?  And with just the cap inside?  Better cap?
And the finial test does it work OK despite the VSWR reading?

What I have found works good with the Tbolt for the less than ideal antenna setups,
is to set the Trimble signal level threshold (AMU) lower and the GDOP threshold higher, 
and set the Elevation Mask low (0 to 5) for survey mode and high (around 35) for hold mode. 

I had a Tbolt setup that would hardly do a survey, 
and changing both AMU & GDOP 2 to 1 lets it do the 2000 sample survey "good enough" in about 1/2 hour. 
Then by changing the survey sample size to 20K+, it gives good answers in 1/2 day. 

I also found that Raising the elevation mask, and using a low signal threshold was the best way I found to keep weak sat signals 
from causing freq step noise, otherwise they cause noise when they switch in and out because the signal level hysteresis is not high enough.

----- Original Message ----- 
From: <Brucekareen at aol.com>
To: <time-nuts at febo.com>
Sent: Wednesday, September 02, 2009 6:04 PM
Subject: [time-nuts] GPS Antenna Voltage-Dropping

>I bought a 3.6-V Trimble Bullet GPS antenna on ePay and wish to use it with 
> my T-bolt.  Rather than try to internally modify the T-bolt to provide a  
> 3.6-V antenna feed, I decided to try to build an in-line dropping adapter.   
> I seriesed two Si diodes inside a 100 pf tubular ceramic capacitor and 
> installed  the shrink-wrapped assembly inside a salvaged BNC-M to BNC-F coaxial  
> assembly.  Unfortunately the completed assembly exhibits about a 4-to-1  
> VSWR when terminated in a 50 ohm load.  Has anyone else tackled this  
> challenge?
> The 3.6-V Trimble antenna has less gain than the 5-V version which  makes 
> my planned antenna rcable run on the edge even without the high  VSWR..
> Bruce Hunter 

More information about the time-nuts mailing list