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Thread: Preferred Antenna Type...

  1. #1
    Join Date
    Jan 2007
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    North Little Rock
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    Preferred Antenna Type...

    I am researching GPS receivers and am narrowing the field down. One difference, that seems major, is the antenna type - "patch antenna" vs "quad helix antenna". I will be using the GPS receiver for geocaching, hiking, bicycling, driving and to mark points of interest I may want to visit later.

    Which do you prefer, particularly in wooded areas (Ozark Nat'l Forest, Ouachita Nat'l Forest, Burns Park, etc.)? Are there advantages to one over the other? Is it worth paying the extra $$ for the "quad helix antenna"?

    -------
    cupstack
    NLR, AR

  2. #2
    Join Date
    Jan 2004
    Location
    Maumelle, AR
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    211
    In general they are about the same, if you use them correctly.

    The magellan sportrak series has a helix antenna, and it what I have. For best reception you need to hold it upright, like a soda bottle. That is because the helix is similar to a spool of thread and is vertical above the display pointed out of the top of the unit. If you don't hold it upright then the signal quality drops, and you will likely lose a few satellites. The helix is a 360 degree around antenna, but if you hold it flat, only one small part of the antenna is pointed up, and the rest is pointed out to the sides or towards the ground.

    Now, the entry level garmin models (etrex, etc) use a patch antenna, which is a flat square about the size of a postage stamp. It is located just above the display behind the garmin logo, flat against the front of the unit. For this reason, you should hold these units flat, like a compass, or flashlight, with the garmin logo facing upwards at the sky.

    If you hold them correctly, they are pretty much effectively the same. The helix is supposed to be slightly better, but for the most part not enough to notice. The helix is a bit better at picking up satellites that are less directly above you (lower on the horizon) than a patch, but the patch does a better job of picking up those directly above. The concept behind the helix being better is that the satellites lower on the horizon are further away than ones directly above, and thus a design that improves their signal should be superior. It is a good theory, but the reality is that in a typical wooded area, or around buildings, there isn't a lot of practical difference. These low horizon signals are also more likely to be reflected or affected by ground cover and atmospheric conditions, making a patch antenna more immune to those. All in all it is a trade off. In my experience, WAAS helps more than the antenna type. (WAAS is two extra channels dedicated to signal correction data, and is sometimes called a 14 channel GPS. Technically there are only 12 GPS channels and 2 WAAS channels. The two (US) WAAS satellites are in orbit over the est and west coasts.

    The bottom line is, don't base your decision on just the antenna type, but on what other features you want.

    It is also true that if you have two GPS units in close proximity then both may lose accuracy due to interference with each other. This is because each unit leaks out a bit EMI or RF signal. This is normal for the type of receiver circuits that most GPS units have, but most people don't realize they can interfere with nearby units. This has led to lots of incorrect conclusions about GPS performance by people trying to compare two units side-by-side.

    There is lots of good info on these subjects on this site:
    http://gpsinformation.net/

    The main difference, at any rate, is that holding either type incorrectly for it's antenna orientation can cause a signal loss of 60% to 75% or more, and has far more effect than anything else. If you hold an etrex and a sportrak side beside flat, the etrex will be hands down better. If you hold them side beside vertically, the sportrak will look greatly superior. It is a simple thing, but most people don't know about it.

    Oh, and holding either type too close to your body will reduce signal, as will holding your hand too close to the antenna portion of the GPS.

  3. #3
    Join Date
    Jan 2007
    Location
    North Little Rock
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    Thank you so much for the informative lesson. I guess I was worrying too much. I am ready to make my decision and purchase my first GPS receiver.

    Thanks again.

    -------
    cupstack
    NLR, AR

  4. #4
    Join Date
    May 2004
    Location
    Conway, AR
    Posts
    1,392
    Great information, Bert! Mind if I post this as an article in the "Contents" section?

    Wayne
    I get my directions from above.
    View my profile

  5. #5
    Join Date
    Jul 2006
    Location
    Bee Branch
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    113
    Hmmm. Thanks...but.....
    How come trees and things bother reception but that little neoprene cover does not?

    Good info. I been holding my Garmin up in the air for naughta!
    Got peace like a river!

  6. #6
    Join Date
    Jan 2004
    Location
    Maumelle, AR
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    211
    Quote Originally Posted by Q
    How come trees and things bother reception but that little neoprene cover does not?
    That's actually an interesting question. In the case of trees it is primarily water content, grounding, and density. The frequencies that GPS signals use are very much like light, in that they travel in a straight line and are easily deflected or blocked. It is the mosture content in trees that dampens the signal, and the density of the trunks can also cause reflected or scattered signals. (Although building structures with large flat metal or reflective glass outer walls are the worst reflectors.)

    The dampening effect also applies to people. Wearing a GPS against your waist or in a backpack or pocket will also block signals from the side you are blocking, and also dampen the overall signal. Holding it as little as 6 inches out often makes a noticable difference. With my old eyesight, it isn't a problem for me, because I usually wind up holding it at arms length just to be able to read it.

    Sorry if I got a little long winded. I teach GPS usage to boy scouts, so I get a lot of questions and get to see all the usual (and unusual) problems operating one that you can imagine.

  7. #7
    Join Date
    Jan 2004
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    Maumelle, AR
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    Quote Originally Posted by Gaddiel
    Great information, Bert! Mind if I post this as an article in the "Contents" section?
    Not at all. You are also more than welcome to work it over and make it more user friendly. Sometimes I get too technical or carried away with details.

  8. #8
    Join Date
    Sep 2004
    Location
    Ridgway, CO
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    464
    Thank you very much for your information. This gives substance to my impression about the "attitude" of the GPS and its reception.

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