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Thread: GPS SAW filter

  1. #1
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    Default GPS SAW filter

    Donít really know where else to post this, so Iím putting it in the antenna section.

    I recently purchased a GPS SAW filter, to knock down overload I was getting on the front end of my GPS DO at my repeater site.

    The UHF is overloading the GPS DO and it loses sync, causing frequency drifting of my repeater.

    I purchased this SAW filter:

    https://rover.ebay.com/rover/0/0/0?m...2F323574268611

    Unknown if it works, because when I tested it today, apparently it doesnít work with a passive antenna (DC on the line), which is what most GPS antennas are. With it inline, I do not get any GPS signals.

    Just wanted to give a heads up to anyone else who tries implementing the same fix. Waste of money.


  2. #2
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    Quote Originally Posted by Mars View Post
    Donít really know where else to post this, so Iím putting it in the antenna section.

    I recently purchased a GPS SAW filter, to knock down overload I was getting on the front end of my GPS DO at my repeater site.

    The UHF is overloading the GPS DO and it loses sync, causing frequency drifting of my repeater.

    I purchased this SAW filter:

    https://rover.ebay.com/rover/0/0/0?m...2F323574268611

    Unknown if it works, because when I tested it today, apparently it doesnít work with a passive antenna (DC on the line), which is what most GPS antennas are. With it inline, I do not get any GPS signals.

    Just wanted to give a heads up to anyone else who tries implementing the same fix. Waste of money.
    I think you meant to say it does not work with an active antenna.

    What you could do is solder a pair of RF chokes from the center pins of each SMA and then solder them together with a chip capacitor to ground midway between. That way you would have a DC path for the bias, and also block any 450 Mhz signal from leaking past. This off course assumes the filter does not present a DC short to ground.

    However, your GPS antenna might be picking up a harmonic of your repeater TX, or the preamp in the antenna itself is saturated with UHF signal, in which case a different type active antenna might be more suitable.

    Sent from my SM-T350 using Tapatalk

  3. The Following 3 Users Say Thank You to RFI-EMI-GUY For This Useful Post:

    Bill_G (Jul 10, 2019),Mars (Jul 10, 2019),MattSR (Jul 10, 2019)

  4. #3
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    Quote Originally Posted by RFI-EMI-GUY View Post
    I think you meant to say it does not work with an active antenna.

    What you could do is solder a pair of RF chokes from the center pins of each SMA and then solder them together with a chip capacitor to ground midway between. That way you would have a DC path for the bias, and also block any 450 Mhz signal from leaking past. This off course assumes the filter does not present a DC short to ground.

    However, your GPS antenna might be picking up a harmonic of your repeater TX, or the preamp in the antenna itself is saturated with UHF signal, in which case a different type active antenna might be more suitable.

    Sent from my SM-T350 using Tapatalk
    You know, I'm really getting tired of Siri. I said ACTIVE antenna. The stupid thing thought I said "passive" antenna. It must be my Canadian voice. My fault for not proofreading, but I was tired.

    It doesn't pass the DC required to make the antenna work. Either way, two different GPS receivers will not function when the UHF keys up and wipes out the front end. Same with my VHF ham rig.

    The filter is essentially a passband device. It was my hope it would knock down the offending signals enough to the point where they weren't killing the GPS signal. I guess I was asking for too much. I'm sure the little filter works fine, but due to design, no DC.

    Just found this one: https://www.ebay.com/itm/BPF-GPS-DC-....c100008.m2219

    Price is good. Giving it a try...

  5. The Following User Says Thank You to Mars For This Useful Post:

    Bill_G (Jul 10, 2019)

  6. #4
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    Agreed. All GPS disciplined oscillators I've worked with have an active antenna requiring 5VDC up the cable in older models, and 3.75VDC in more recent models. So, any inline filters should be DC pass. That said, these same GPS receivers work with both an active and a passive antenna. If your cable is short enought to support a passive antenna, like well under 100ft of LDF4-50, 50ft of LMR400, or 20ft of RG58, you have the option of using a passive antenna.

    There were models of DO with the GPS receiver built into the antenna that sends serial data back to the oscillator, but they aren't great at busy sites. RF gets into the serial drop cable corrupting the data envelope.
    Bow wow wow yippie yo yippie yay

  7. The Following User Says Thank You to Bill_G For This Useful Post:

    Mars (Jul 10, 2019)