Wireless Antenna Questions

Discussion in 'Wireless & RF Design' started by nizee, May 8, 2008.

  1. nizee

    Thread Starter Member

    Apr 29, 2008
    I need some antenna education.

    Assuming same gain (db), is there a different say in 2.4Ghz Antenna (from those of wireless router) compare to say 900Mhz or 800Mhz Antenna (from those of home lighting/automation - eg. ZWave, X10)? Can i swap antenna from either systems - what are the concerns?

    How does an Antenna increase its gain (without power)? Is it by increasing the inductor value?
  2. joe.08


    Jan 30, 2007
    As far as concerns,

    Its been forever now since Ive dealt with antenna theory but the thing that sticks out in my head is that antenna elements physical size are dependent on the specific frequency (or wavelength).

    so swapping between 900 Mhz and 2.4Ghz will probably show some dB loss
  3. nizee

    Thread Starter Member

    Apr 29, 2008
    Does the dB loss changing from 2.4Ghz to 900Mhz or vise-versa?
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    Both ways. Antenna dimensions are critical. The antenna has to be a resonant structure at the frequency it's receiving. You will lose a lot of signal operating that far off the designed frequency.
  5. DC_Kid

    AAC Fanatic!

    Feb 25, 2008
    a low powered rf device feeding a omni is much different than feeding same power into a yagi. the yagi, using same low power rf, may/can easily exceed the directional power restrictions of the fcc. as a visual, place a piece of paper in the sun, nothing really happens with the incident energy hitting the paper. but then take a magnifying glass and focus some energy to a point on the paper....

    transmitting and receiving antenna may be designed completely different. feeding into wrong antenna can be death to the rf amp stages. VSWR should be noted. there's always a compromise somewhere......

    what exactly are you trying to accomplish ??
  6. roddefig

    Active Member

    Apr 29, 2008
    Realistically, no. As has already been said by beenthere and joe, antennas are designed to operate at specific frequencies. At frequencies other than the designed frequency they tend to reflect most of your power, leading to the VSWR problem that DC Kid mentioned. Instead of the antenna transmitting your power into space, it sends it back to the transmit amplifier, which can cause nasty problems...

    As far as your second question, my guess is that adding an inductor would be used in matching the cable and antenna, but someone else can probably answer that question better. To transfer the maximum amount of power to the antenna, the resistances (impedances, really) of the cable and antenna should be matched. One way to do this is a matching circuit made up of discrete components (e.g. inductors and capacitors).
  7. Caveman

    Senior Member

    Apr 15, 2008
    First of all, note that any antenna gain is specific to a certain range of frequencies, so a 2.4GHz antenna gain is typically spec'd to be at 2.4GHz. If you use it at 900MHz, the gain is not the same. It is typically much less.

    An antenna increases its gain by either being more efficient at the particular frequency or by being more directional.

    The first is why the size of the antenna matters. If an antenna is not size to be resonant at that frequency, it loses efficiency at that frequency and has lower gain.

    The second one basically comes from conservation of energy. An omnidirectional antenna designed for a specific frequency will have a lower gain than a yagi. This is simply because the omnidirectional is spreading the energy out more.
  8. nizee

    Thread Starter Member

    Apr 29, 2008
    I'm trying to replace a Z-wave antenna that is broken with a 2.4Ghz anternna i have laying around.

    From the manufacturer on the Z-Wave radio specifications: -
    - Bandwidth: 9,600 bit/s or 40 Kbit/s, fully interoperable
    - Modulation: GFSK (Gaussian Frequency-Shift Keying)
    - Range: Approximately 100 feet (or 30 meters) assuming "open air" conditions, with reduced range indoors depending on building materials, etc.

    - Frequency band: The Z-Wave Radio uses the 900 MHz ISM band: 908.42MHz (USA)

    I did the swap and did notice it works, but i can't compare the distance for the original broken antenna. I do not know the setback or consequences with result to this swap, the reason for this posting. So, i guess i'm putting a strains to the z-wave antenna driver circuit by swaping to 2.4Ghx antenna?

    the second question is more to my education.