[time-nuts] GPS from a window seat

[jmfranke]

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.

John  WA4WDL

--------------------------------------------------
From: "Brian Kirby" <kilodelta4foxmike at gmail.com>
Sent: Thursday, October 01, 2009 7:57 PM
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Subject: Re: [time-nuts] GPS from a window seat

> In order to fake out some Garmin units, when using them via splitters on a 
> external antenna, we put 220 ohm resistors from the center of the coax to 
> the sheild.  The splitters we used were capacitive coupled and this work 
> fine for the Garmins.
>
> The Garmin units needed to see some sort of DC load, is you wanted to use 
> the external antenna ports.  If you did not pull any current, the unit 
> stayed on its internal antenna.
>
> Brian KD4FM
>
> Magnus Danielson wrote:
>> Jim,
>>
>> Jim Palfreyman wrote:
>>> I've done this with two separate GPS units. One was a basic unit with
>>> no maps - more designed for bushwalking, boating and other direct
>>> navigation. It worked really well.
>>
>> Mmm. Bushwalking is one of your local specialities I gather...
>>
>>> Just recently (a few days ago) flying to Perth I used my car-designed
>>> Navman. It locked easily and I chuckled as it rapidly swept across
>>> roads and intersection on the ground at 777 km/hr telling me "Go to
>>> nearest road".
>>
>> That must have been one jumpy ride... 777 km/hr offroad.
>>
>> Once again I have been forced to invent a DC fake load to handle the case 
>> where the current sensing of the GPS receiver is set higher than the 
>> hooked in GPS antenna consumes. The shot-from-the-hip solution involves a 
>> T-connector, a 10 uH SMD coil and a 470 Ohm SMD resistor. Not ideal in 
>> any sense, but hopefull pulls enought (additional 10 mA) while not 
>> mocking too much with the signal. A similar approach has worked before.
>>
>> Cheers,
>> Magnus
>>
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