[time-nuts] Non-impedance matched antenna cables

[Javier Herrero]

Then, if the input is 50 ohm, at least the reflections in the other 
extreme of the Thunderbolt + cable (i.e., at the antenna side) should 
not be so bad... I'm afraid that since the cable is alredy routed using 
the inside-building tubes (in fact it uses the tubing destinated for the 
TV antenna) is not so easy for me to adjust the cable length :)

Yes, I was also surprised by the use of F connectors... and that made me 
suspect (with surprise) that the GPS input were 75ohm, but I must 
confess that I did not read more ;).

The antenna I'm using is a Motorola mainly intended for vehicle 
mounting. Surely not the optimum antenna... but not bad results with the 
Thuerbolt nor the M12's. I've another antenna, but is a cheap one with 
magnetic base, only used for a nixie clock :)

Regards,

Javier


Björn Gabrielsson escribió:
> 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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Javier Herrero                            EMAIL: jherrero at hvsistemas.com
HV Sistemas S.L.                          PHONE:         +34 949 336 806
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