[time-nuts] Non-impedance matched antenna cables

Björn Gabrielsson bg at lysator.liu.se
Fri Jun 13 05:38:45 EDT 2008


Javier,

According to Trimble the Thunderbolt IS 50 ohm at the antenna input.
(maybe the sole user of 50ohm F-connectors... ;-) )

See page 3-5, Section 3-4 "Antenna Cables" (page 39) in
http://trl.trimble.com/dscgi/ds.py/Get/File-10001/ThunderBoltBook2003.pdf

"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."

--

   Björn

On Fri, 2008-06-13 at 10:47 +0200, Javier Herrero wrote:
> I've seen that you're right, Thunderbolt is originally supplied with 
> 75ft of 75 ohm RG-59 cable, as indicated in the manual... So I've the 
> inverse problem: the GPS antennas we have in the roof both have 50ohm 
> cable :) So... would be good to change the impedance at the Thunderbolt 
> antenna port from 75 to 50 by means of some impedance converter? (of 
> course, not altering the DC supply to the antenna).
> 
> Regards,
> 
> Javier, EA1CRB
> 
> Magnus Danielson escribió:
> > Hi fellow time-nuts,
> >
> > For those of you that isn't aware of it, using 75 Ohm antenna cables rather
> > than 50 Ohm cables is a bad move, as the antenna cable itself will create a
> > multi-path system. This article elaborate on reflections i cables:
> >
> > http://tycho.usno.navy.mil/ptti/1998/Vol 30_39.pdf
> >
> > Nothing new, but needs to be said, especially with Thunderbolts getting more
> > popular, as the manual clearly recommends 75 Ohm cable as I recall it. Do not
> > listen to that.
> >
> > Cheers,
> > Magnus
> >
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> >
> 




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