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Author Topic:   GPS-Sounder Combo Interference From Own Antenna
Mudkap1 posted 03-14-2007 07:40 PM ET (US)   Profile for Mudkap1   Send Email to Mudkap1  
I just purchased a Lowrance GPS and Sounder unit with an external antenna for my Montauk. I would like to mount the antenna on the dash instead of on the rail of the center console. The distance from the unit would be only 2-feet. Will I get interference being too close? Lowrance says mininum of 3-feet; electronics dealer says "don't worry, it will be fine." Any thoughts?
gss036 posted 03-14-2007 07:48 PM ET (US)     Profile for gss036  Send Email to gss036     
I can't see where you would interference for the GPS as most handheld have the antenna built in or plug-in like my Garmin 176C. Maybe they were referring to sonar interference? I know if I get too close to some boats while fishing I get all kinds of interference on my screen, must be too close on the Megahertz for the transducers. I will be using a new Humminbird tomorrow and I will just have the small GPS antenna laying on the dash, just to see how it works. I also run two different sonar units and transducers. One is new so I don't think there will be any problem as I was using an Eagle depth sounder before [without] any problems.
jimh posted 03-14-2007 10:29 PM ET (US)     Profile for jimh  Send Email to jimh     
I recommend you follow the manufacturer's instructions.

gss036 writes:

must be too close on the Megahertz for the transducers.

SONAR devices do not operate in the Megahertz frequency range. Most SONAR transducers operate around 50-kilohertz to 200-kilohertz. If you pick up the transmissions from other boat's SONAR transducers, it is probably because they are on the same frequency or just a few kilohertz away, not Megahertz away.

In contrast to the very low frequency of the SONAR, the GPS antenna is trying to receive signals which are in the L1 frequency range of 1,575.42 MHz.

The suggestion to locate the GPS antenna at least three feet from the GPS unit itself may be related to the design of the GPS unit being insufficiently shielded. The unit may be emitting some radio frequency noise which could interfere with its own antenna. The SONAR transmitter generally excites its transducer with a sharply rising pulse of current which may contain a broad radio frequency component.

The best thing to do is to try the antenna location on a temporary basis and see how it works out.

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