Allen Perrins tells us;

How to utilize the Lowe Active External Antenna with Lowrance GM 100 GPS.

Because the GM 100 does not supply voltage to support an active external antenna, it becomes necessary to provide the 5 VDC (typical) to the antenna via some sort of adapter using an external voltage source.

It was decided to construct a small box with the necessary hardware to work at 5V nominal.  This way, the source of power could be either a battery pack or a special cable assembly designed to operate from a vehicle, boat or other cigar lighter receptacle.Because the immediate need was for the latter, cable has been designed which consists of series resistance and a shunt Zener diode configured to be placed in the cable assembly itself. The circuit schematic and the components used are shown in the accompanying diagram. 

The first cable was built without the capacitor,  C1,  which is an afterthought to help suppress very short line drop-outs. The value of the capacitor could range up to much larger values if for a small diameter assembly which can be sleeved with shrink tubing to make the cable assembly of minimum bulk.The five volt mechanical adapter uses chassis mounted female connectors as shown for all external connections with the smallest aluminum "slip chassis" available from Radio Shack.  A series capacitor (axial leads preferred) is used to prevent coupling the DC voltage into the GM 100 which does NOT have a series capacitor internally.  The RF signal from the Lowe antenna is prevented from being shorted out by the power supply by the series inductor which couples the DC power to the Lowe antenna. The negative connection of the DC is made via the chassis.

A cable was purchased from:

                        CommSystems Intermountain

                        1107 Key Plaza,  Suite 164

                        Key West, FL  33040-4077

                         mailto:info@commsysin.com 

The cable has an SMB straight Male connector for the GM 100 and a straight BNC Male connector to feed the adapter box. My cable was too short (1 foot). I would opt for three feet another time.

 Operational results were excellent, with SNR numbers jumping from zero to 26 for one low lying satellite and typically from mid-twenties  to low fifties for most of the others.  I observed massive interference when the Lowe antenna was placed touching the GM 100. SNR's went to zero!  No problems were seen when the separation was increased to 3 or more inches. 

The next step will be to come up with a satisfactory battery pack design. It would appear that the best design here would be to use NiCad of NiMH cells (4) with a series germanium diode to drop the fresh battery voltage down to 5.25 volts which is the max. rated published value.  Lowe UK did not budge from this value when queried.  Lowe US indicated that operation has been successful down to 4.2 volts so that the above circuit should be able to get good operation from the NiCad technologies which are known to have rather flat discharge curves out to end of useful charge.    

(The diagram redrawn by  Carlos Puch . Thank you Carlos!)  

Notes from the GPS Nuts:

GPS Nuts have not tested the described set-up. We can not guarantee the accuracy or validity of the design. Any errors in the assembly may lead to damage to the receiver. Such damage would not be covered by the LEI warranty, and any persons using this design do so at their own risk.

DC CABLE

R1, R2, R3       (Resistors)        56Ω  ½ W

Z                      (ZENER)         5.1V 1W

C1                   (Capacitor)       10μF 50V

P1                    RCA male plug

5V ANTENNA ADAPTER

C2                   (Capacitor)       100 pF

L1                    (Coil)   15 turns, #30 wire

J1        RCA Female Receptacle

J2        BNC Female Receptacle

J3        BNC Female Receptacle