[time-nuts] GPS from a window seat

Magnus Danielson magnus at rubidium.dyndns.org
Fri Oct 2 03:05:20 UTC 2009


Bruce Griffiths wrote:
> Magnus Danielson wrote:
>> jmfranke wrote:
>>> I used a bias tee with a capacitor block.  I varied the resistor
>>> until I could see signals coming from the external antenna, the built
>>> in patch was shielded with aluminum foil and the receiver verified
>>> that no signals were coming from the internal antenna.  Some
>>> receivers needed only a 10K resistor, some models needed 220 Ohms.
>> In my case I still want it to pass 5 VDC, but let the receiver see
>> some additional current, so I put a coil in series with the resistor,
>> to not load the RF too much.
>> Cheers,
>> Magnus
> You need to ensure that the coil/inductor doesn't have its first
> resonance below 1575.42MHz for an L1 receiver, otherwise it isn't very
> effective.
> There are some wideband chokes with first resonances well above 10GHz
> when mounted correctly.

As with decoupling capacitors, it is not the actual inductace which is 
important as much as adding sufficient of impedance to ensure that the 
impedence line does not get a significant reflection.

In my case, I don't get much worse than tossing the 470 Ohm directly 
onto the conductor, and that isn't that big deviation even if it is far 
from ideal. Shifting in a better inductor is certainly possible. I 
needed this one fast, and the system it is for is not into deep sub-ns 
timings anyway.


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