[time-nuts] GPS Antenna Cable Splicing

Didier Juges didier at cox.net
Sun Mar 2 09:17:30 EST 2008


This issue has been addressed in previous threads, partly as a result of
Trimble designing the Thunderbolt with F connector, intended for 75 ohm
cable.

Here is an excerpt from the Thunderbolt manual Page 39, section 3-5):

<quote>
Note – RG-59 is a 75 ohm coaxial cable. The ThunderBolt and the Bullet
antenna are 
compatible with either 50-ohm or 75-ohm cable. Compared to most 50 ohm
cable, 75 
ohm cable provides superior transmissibility for the 1.5 GHz GPS signal and
a 
better quality cable for the price. Mismatched impedance is not a problem.
Note – The input impedance of the ThunderBolt RF input & its antenna is 50
ohms.
</quote>

So even though the antenna and receiver are 50 ohm, Trimble recommends using
75 ohm cable because the reduction in insertion loss more than offsets the
mismatch loss as long as the cable is more than a few feet long.

I have used my Thunderbolt with good quality 50 ohm cable (with BNC
connectors and F adapters) and good quality (it was expensive) 75 ohm cable
and did not see observable difference. Interestingly, at the time, I bought
two 50 feet rolls of 75 ohm cable at the local Home Depot. One was about
twice the price of the other and was dubbed "high quality for high
definition signals" made by RCA and the other was run-off-the-mill no-name
brand. The cheap cable actually had slightly lower insertion loss at 1.5 GHz
than the more expensive one. YMMV. I remember comparing the leakage, but I
do not remember the results :-( I need to run that test again.

I would be concerned that the slight mismatch might cause reflections, which
would be equivalent to multipath, but if the cable is long enough, these
will be absorbed in the loss and have no practical consequence.

RG-59 cable is intended to be crimped (can you find a solder-type F
connector? I can't :-), so solderability was not a requirement.

Didier KO4BB


> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Robert Vassar
> Sent: Sunday, March 02, 2008 7:16 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GPS Antenna Cable Splicing
> 
> 
> On Mar 1, 2008, at 4:56 PM, Matthew Smith wrote:
> 
> >
> >
> >> The small diameter captive cable used by some GPS antennas 
> tends to 
> >> be somewhat lossy and it may be better to extend the 
> length by using 
> >> a lower loss cable.
> > Which is what I've got.  I have a part-reel of satellite TV 
> cable, so 
> > will make up an extension with that and see how it behaves.
> >
> 
> 
> TV cable here in the US is usually 75 ohm.  You'll likely 
> need some kind of match.
> 
> Also... I'm not sure what the braid is plated with but it 
> also seems to actually repel solder...  :(
> 
> 
> 
> Rob
> KC6OOM/5

No virus found in this outgoing message.
Checked by AVG Free Edition. 
Version: 7.5.516 / Virus Database: 269.21.2/1305 - Release Date: 2/29/2008
6:32 PM
 




More information about the time-nuts mailing list