Antenna Connection - Garmin GPS 25 LP Technical Specification

3.4 Antenna Connection

The GPS 25LP sensor boards make their antenna connections via a 50 ohm MCX style
connector attached directly to the sensor board (+Vin-0.4V @ 15 mA power is supplied
on the center conductor for the antenna).
Coaxial cable supplies +Vin-0.4V @15 mA to antenna/preamp. 1.57542 GHz signal
returns to the sensor board.
The recommended GARMIN remote antenna for the GPS 25LP series is the GA27. It
has eight feet of RG-174A/U type cable with a MCX connector installed for attachment
to the sensor board. An extension cable with up to -5dB additional attenuation at 1.5
GHz may be used with the GA27 (either 50 Ohm or 75 Ohm impedance cable is
permitted).
A passive antenna may be used with the GPS 25LP series sensor boards, provided no
more than -2dB or -3dB cable loss is inserted between the antenna and the sensor.
Higher gain competitors' antennas may be used provided there is adequate cable
attenuation to prevent overloading the sensitive GPS 25LP receiver. 15dB excessive
antenna system gain (LNA Gain - Cable Loss) should be considered a maximum.
Check to insure that the antenna in question will operate properly with the antenna bias
voltage that is supplied by the GPS 25LP, when powered by the desired Vin voltage.
The GPS 25LP will current limit for loads above 50mA to prevent damage, should the
antenna cable become accidentally shorted.
The following table may be used to estimate the typical attenuation of various cable
types at 1.5 GHz. For example, a 15 foot length of RG-188 may have 4.5 dB
attenuation. The attenuation of different styles of the same cable type may vary. Refer
to the specifications of the actual cable that will be used.
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