[time-nuts] Non-impedance matched antenna cables

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jun 13 05:01:22 EDT 2008

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

Before you do this find out what the reflection coefficient is for the 
Thunderbolt input and the active antenna output when using a 50 ohm cable.
The active antenna may not be that well matched to 50 ohm cable.

Then adjust the cable length to an odd multiple of half the signal 
wavelength in the cable.
If done well this should reduce the effect of such reflections by more 
than 30dB.
This of course only works with a single frequency antenna.


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