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Thread: Garmin GPSmap 396 - GXM30 antenna a power hog"?

  1. #1

    Garmin GPSmap 396 - GXM30 antenna a power hog"?

    Aviation Consumer is so impressed with the new Garmin GPSmap 396 that they did a special product review and distributed it online to subscribers.

    I haven't read the whole review yet but I looked at the pictures.

    In one of them they show all the accessories that come with the unit. One is the GXM30 antenna. I think that's to receive the satellite WX and radio. There's a comment that it "is small but it's a power hog. Battery-only operation is limited to twohours or less."

    I thought an antenna was a passive device.

    Would one of our avioinics wizzes please explain to me how an antenna consumes power.

    Cloyd
    President, BAC

  2. #2
    jerry at tr4.tr2.com
    Guest

    Garmin GPSmap 396 - GXM30 antenna a power hog"?

    cloydvanhook@imtt.com wrote:
    >
    >
    > In one of them they show all the accessories that come with the unit. One
    > is the GXM30 antenna. I think that's to receive the satellite WX and
    > radio. There's a comment that it "is small but it's a power hog.
    > Battery-only operation is limited to twohours or less."
    >
    > I thought an antenna was a passive device.
    >
    > Would one of our avioinics wizzes please explain to me how an antenna
    > consumes power.
    >
    *** GPS and other satellite signals are very very VERY weak. They are so
    weak that they would be below the noise for an ordinary radio. A length of
    coax would make them even weaker. At the high frequencies of the GPS
    system, ordinary coax has serious losses. In other words, less signal comes
    out than what you put in.

    So remote-mount satellite antennas have built-in amplifiers. And there might
    also be a down-converter to a frequency that the coax will handle better - I
    think that's standard for GPS antennas.

    It's a characteristic of very high frequency amplifiers that they tend to
    be power hogs, all else being equal. In other words, using the same
    technology, a 1gHz ( microwave ) amplifier will invariably suck more power
    than a 100MHz ( VHF ) amp.

    - Jerry Kaidor ( jerry@tr2.com )
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  3. #3
    i just recieved my 396 yesterday. i am very impressed. iplayed with it on the back porch after setting up the xm weather for about an hour and half and the battery charge was down to about 1/3. i have not been able to fly with it but hope to this weekend. it did pickup the bad weather we had yesterday in atlanta. i will couple it to my 430 as i did my 295. i did not setup the xm radio at this time but i can add it at a discount later. i also signed up for the Aviator lite and if it works good i will upgrade to the full optionwith the lighting and echo tops etc.

    mike dawson

  4. #4

    Garmin GPSmap 396 - GXM30 antenna a power hog"?

    Cloyd,

    The antenna element itself is passive. Remote-mounted GPS antennas usually
    contain a low-noise-amplifier (LNA) that boosts the signal from the antenna
    and sends it down the cable to the receiver. The performance of a radio
    receiver is a function of the overall noise figure. If there is a long
    cable from the antenna to an LNA in the receiver itself, the overall system
    noise figure is approximately the cable loss plus the receiver noise figure
    (it is actually worse, but this is approximate). At GPS L1, which is
    1575.42 MHz, the cable loss for the skinny RG-174/U coaxial cable is around
    3 dB for a 6-foot length (3 dB equates to 50%). The overall receiver
    performance is reduced because of the cable loss. To maintain a lower
    overall noise figure, active antennas have an LNA that is powered by a
    5-volt DC signal on the coaxial center conductor. The XM radio antennas
    have a similar LNA. Unlike terrestrial broadcasts, the received signal
    levels from satellites are relatively low, so the antenna-mounted LNA is
    needed to maintain system performance. The noise figure of a typical GPS
    LNA is 1.5 dB, and the gain is 15 to 25 dB. Power consumption is usually
    less than 100 milliwatts (0.1 watt).

    For the record, I do radio data link design and performance analysis for a
    living, and have taught classes on the subject of digital communications. I
    can stop here or keep going with examples and lots more math. I figure most
    people have stopped reading by now anyway, so I will stop here.

    Carl Foster Sundowner 9761L Tucson AZ

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  5. #5
    jerry at tr4.tr2.com
    Guest

    Garmin GPSmap 396 - GXM30 antenna a power hog"?

    Carl Foster wrote:
    >
    > Cloyd,
    >
    > LNA is 1.5 dB, and the gain is 15 to 25 dB. Power consumption is usually
    > less than 100 milliwatts (0.1 watt).
    >
    *** Which is negligible for an item that works ultimately off the aircraft
    power buss. But not for something handheld.

    - Jerry Kaidor ( jerry@tr2.com )

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