Impedance Matching

Discussion in 'Wireless & RF Design' started by almond, Oct 31, 2009.

  1. almond

    Thread Starter New Member

    Oct 31, 2009
    2
    0
    What is the best way to do the following impedance matching:

    - 50 ohm DGPS Antenna (receiving about 300kHz 1.6GHz signals) to 75 ohm co-axial cable (30m)

    -75 ohm co-axial cable (30m) to 50 ohm DGPS Receiver.

    This is for a DGPS system onboard a military vessel, hence performance is of utmost importance.

    Thanks.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Having seen cabling get replaced on the captain's orders, replacing the 75 ohm mismatched coax with 50 ohm characteristic coax would be the best solution. Simply eliminate the mismatch. Talk to the EMO and have him recommend the change. Labor's free if shipboard personnel do it.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,179
    1,800
    The real problem is that matching components at those frequencies are diificult to obtain and fabricate without going to considerable expense. My approach would be a PC board with microstripline inductors. The losses in garden variety coax will eat you alive so be careful to understand the losses that you will experience at the frequency of interest. Even LMR-400 is kinda marginal at 1296 MHz, we prefer at least 7/8" hardline.
     
  4. almond

    Thread Starter New Member

    Oct 31, 2009
    2
    0
    Am I able to use an impedance matching transformer at both ends of the cable?
     
  5. Xantor123

    New Member

    Nov 1, 2009
    4
    0
    Won't be much left at 1.6GHz, the 75 Ohm cable you have is probably not even rated for these frequencies, loss will be more then the antenna's gain. You may be better off just connecting the stuff up directly and live with the 1.5 SWR. Also don't try to get the antenna to the highest point, the satellites are 31,000 KM or so up in the sky, you won't get significantly closer to them by mounting the antenna up in the crows nest... All you need is a clear sky to look up to. Less cable = less loss.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,179
    1,800
    Like I said discrete transformers at that frequency are scarcer than hen's teeth. Think printed circuit board with MICROSTRIPLINE inductors. You can hardly design these by hand with a pencil and paper -- It takes specialized CAD tools. Then there is the equipment to analyze and evaluate your results. A minimally acceptable RF lab might run you a quarter of a million dollars.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    As -
    - the system sounds as if it is in place. How did this sail past NavShips?

    I saw some really kudgy stuff get installed, but someone is usually interested in the hardware making it back to port.

    Can the receiver be relocated to the antenna location, and the output from the GPS unit get sent to the bridge and CIC? Sidesteps the whole impedance mismatch.
     
Loading...