[time-nuts] GPS Antenna Voltage-Dropping

phil fortime at bellsouth.net
Thu Sep 3 21:41:51 UTC 2009

> In a message dated 03/09/2009 02:05:20 GMT Daylight Time,
> Brucekareen at aol.com writes:
> 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..
> ----------------------------
> If you dont want to modify the T'bolt it would probably be easiest to
> derive 3.6v from the 5v supply external to the T'bolt and feed that into 
> the
> antenna line with a blocking T.
> In the long run though it might be cheaper just to cut your losses and 
> sell
> on the 3.6v version, buy a higher gain low cost 5v patch for now if you
> don't  already have one, and keep an eye out for a 5v bullet if that's 
> what
> you really  want.
> regards
> Nigel

I believe all the Trimble Bullet II and III's are rated 35 db gain. Most had 
"F" connectors and rated 5-volts. The "TNC" connector was the 3.3-volt 

All the Bullet II and III's (I've seen) are patch antennas in a "Bullet" 
housing. I have seen both versions use both a cast aluminum base with 4 
screws attaching base to dome and also a plastic base glued to the Bullet 

As have been discussed, you can use a bias-t and inject 3.3 from a separate 
supply or use a splitter such as the HP/Symmetricom 58535A and inject  3.3 
from a separate supply on port 1. Both the Bias T and the splitter options 
require an external 3.3-volt supply plus 50 to 100 bucks for either the T or 

By far the best solution would be to buy a 5-volt antenna.
If money is the problem, and you want a pole mountable outside antenna, 
search for a Marine GPS antenna. They are available in 25 to 37 db gain and 
can be had new for as little as 20 bucks. Most of the lower gain antennas 
seem to be marketed as jam resistant.

Not recommended, but you could also replace the patch in the bullet antenna 
if it is the "4-screw" version. There is a rubber o-ring seal that can be 
rather tight but that is the only thing other than the 4 external screws 
holding it together.


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