[FOSS-GPS] RTKLib - Constant offset between UBlox NEO-M8T receivers

David Kelley DavidKelley at ITSware.net
Tue Apr 14 09:22:09 PDT 2015

As general rule (first taught to me by my father an EE who was also a 
avoid Ham radio type, but seen myself over decades), you want to place 
any antenna roughly 10 wavelengths from other objects to get a far field 
effect.  But on a vehicle, where flat space is limited, this is not 
realistic, however you still want to has the most spacing you can get, 
certainly over a wavelength (hence >19cm). You can find in the 
literature both reports that say this matters and reports that say it 
does not (as well as ones where precise fractions of a wavelength were 
used for various bean forming or heading needs).

All these are general best practices, but what can really kill you is to 
have two identical devices (in this case uBlox) near each other where 
the IF down convert frequency is emitted by one device and received by 
another (regrettably the antenna is also at times an inadvertent 
transmitter).  Because they use the same design and frequency, they can 
beat against each other can cause disastrous results to the L band 
signal.  Sharpe filters may provide attenuation for this, but at times 
not, and the jamming signal may be far outside the spec for the SAW 
filter used..  (Aside: Typically a splitter has about 30db between each 
leg is enough to prevent this in shared antenna configurations)  Short 
of a lab and a spectrum analyzer there is no good way to "prove" it is 
occurring except that further separation distance removes the effect.    
Sorry for such a long winded reply.

Regards, David Kelley

On 4/13/2015 9:50 PM, Forrest Voight wrote:
> On Mon, Apr 13, 2015 at 12:40 PM, David Kelley <DavidKelley at itsware.net> wrote:
>> What was the antenna separation distance (one hopes >>19 cm)
> Why should it be more than 19cm (the wavelength of the L1 signal)?
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