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EraSeek

Detuning?

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I'm jumping into your topic here because I figure you guys know about antennas. In the following reference, is this ''detuning'' a permanate thing? or just refering to current reception?:

 

2) Helix type antennas tend to get "detuned" and have lower sensitivity if the antenna is placed close to a metal surface. Some detuning can result if the antenna is placed in a pocket or in a knapsack with metallic items or your body in close proximity. Patch type antennas suffer less and work better if you want your GPS to receive with the unit close to your body or in your pocket. NO GPS will perform properly in these conditions unless it has an external antenna out in the clear. GPS receivers are DESIGNED to operate properly WITH A CLEAR SKY VIEW. Any time you compromise with less than a full sky view, you are compromising the operation of the GPS and the accuracy of your position fix.

 

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This "detuning" refers to changes in antenna characteristics due to a nearby conductor.

 

It is not permanent; it only happens while the antenna is near the conductor. Move away and things go back to normal.

 

Think of the old jokes about TV rabbit ears, where people would notice changes when they were moving the antenna that would only work while they hold it and disappear when they let go.

 

Any good conductor over, under, or around an antenna will change the resonant frequency and directional pattern. Usually at the GHz+ frequencies used by GPS any change is for the worse...

 

There is also a phenomenom known as "receiver desensitization" that occurs due to overloading the receiver input by a nearby transmitter. Some times transmitting with a hand held transmitter (Ham, CB, FRS, whatever) can reduce the sensitivity of your GPSr--also a temporary effect.

 

Hope this helps,

 

Dave_W6DPS

Amateur Extra Class

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the "DETUNING" effect is only noticable if it is in the near-field of the antenna. Near field is a variable distance derrived by looking at the frequency of operaton, and mathamatically determining the distance of the wavelength. For a GPSr, it is only a matter of a few inches. Of more importance for us would be masking. A metal plate several feet above your GPSr will have more effect than a car chassis just below it. Hope this helps.

 

Mike. Desert_Warrior (aka KD9KC).

El Paso, Texas.

 

Citizens of this land may own guns. Not to threaten their neighbors, but to ensure themselves of liberty and freedom.

 

They are not assault weapons anymore... they are HOMELAND DEFENSE WEAPONS!

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