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Coax suggestions for external GPS antenna

Mark_WS7M
Mark_WS7M Member ✭✭✭
I bought a TRAM external GPS antenna I want to mount up on the roof.  I'll need to run about 50-60 feet of coax to get to where the back of the radio is to feed the GPSDO.

The TRAM has like 20 feet of RG58/U.   

Any suggestions on what best to continue the run with for another 40 feet or so?

Answers

  • KD0RC
    KD0RC Broomfield, COMember, Super Elmer Moderator
    edited April 2020
    I think you need to look at the losses for 60 ft compared to the gain of the antenna.  You might be worse off with the ant that far away and high up vs. closer and lower.  RG58 is pretty lossy at these frequencies.  RG8 or even lower loss big stuff gets pretty unwieldy (tail wagging the dog problem).  An LNA mounted at the antenna might help if you go with smaller coax..

    73,
    Len, KD0RC
  • Andrew Rodland
    Andrew Rodland Member ✭✭
    edited June 2020
    I don't think you need to worry that much, actually. If it's the TRAM GPS antenna I'm thinking of, it already has a bit more LNA gain than most pucks, and the loss on ordinary coax is still <10dB/100ft at 1500 MHz. 40 feet of hardware store RG6 shouldn't provide more than 3 or 4 dB of degradation, so that's exactly what I would buy. If it doesn't work, you didn't spend too much :)
  • Steven WA8Y
    Steven WA8Y Member ✭✭
    edited April 2020
    Go to Home Depot, buy a 1000 foot roll of RG6 Quad braid, for $35. RG6 (F connectors to PL 259 ) are about $4.
  • Mark_WS7M
    Mark_WS7M Member ✭✭✭
    edited April 2020
    Thanks Len,

    That's why I asked!
  • Stan VA7NF
    Stan VA7NF President Surrey Amateur Radio Communications Member ✭✭
    edited April 2020
    I personally like the LMR-240 as a balance between loss and physical size,
    and LMR-400 for minimum loss before hardline.  The charts also show RG-6 as an excellent cost/loss solution

    Some numbers at 1500 except as noted:
    58/U (Cannot find anything above 1000Mhz) 11db @1000, ~15db @ 1500
    58  19db
    6A   12.5db
    240  10db
    and
    LMR-400 5.1db

  • KD0RC
    KD0RC Broomfield, COMember, Super Elmer Moderator
    edited April 2020
    Good call on the RG-6!  As a ham, I always think in terms of 50 ohm coax.  For this application, the infinitely cheaper and reasonably low loss 75 ohm RG-6 makes perfect sense.
    Len
  • Martin AA6E
    Martin AA6E Member ✭✭
    edited April 2020
    Maybe it's obvious, but you might want to test your cabling on the ground before pushing for the sky. I've found that the supplied GPS antenna works pretty well when set next to a window with a fair amount of open (southern) sky. I usually track 7 sats out of 11 visible with a good lock. I have always wondered how much better an antenna like yours would work, but I'm too lazy to try one. Sky coverage is what you want, not height. 73 + GL, Martin AA6E
  • Mark_WS7M
    Mark_WS7M Member ✭✭✭
    edited April 2020
    Thanks for all the input.  It's worth $35 to give it a try.

    My radio is in a shed, under a deck and the normal length coax supplied with most GPS antennas barely gets it to the edge of the deck.  

    Then right there is a 70 foot cotton wood tree which obscures a lot of the sky.  

    So my goal is not so much height but to get it to a location where it can see more of the sky.
  • HCampbell  WB4IVF
    HCampbell WB4IVF Member ✭✭
    edited April 2020
    Some time ago I asked a Jackson Labs engineer the same question.  His answer:

    The GPSDO units can use either RG-6 75 Ohms, or RG-58 50 Ohms cables. RG-58 is probably the worst choice you can make, as its insulation lets through multi-path, and the loss at 1.6GHz is significant over long runs. Quad shielded RG-6 cables as come with our antenna kit will have near perfect shielding, extremely low loss at 1.6GHz, and the impedance mismatch will only cause a negligible loss in signal strength.


    Howard

  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited April 2020
    Get the quad shielded cable.  It isn't that much more.
  • John KB4DU
    John KB4DU Member ✭✭✭
    edited April 2020
    True. I use RG-6 Quad for everything except jumpers in  the shack. Good connectors are weather-resistant and easy to install. The losses on HF+6 are nearly insignificant when measured in S-units. At 6dB per S-unit, 100ft of generic RG-6 is about 1/2 S-unit, imperceptible to most uses. 
  • Patrick
    Patrick WH6HI KauaiMember ✭✭✭
    edited April 2020
    I use LMR 195 about 25 feet.  Good for 4 to 5 db loss, with N at the antenna end and SMA at a splitter.  Divers to the Flex 6500, GPS NTP receiver and ADSB RX....antenna gain is 40 db with internal LNA.  Plenty of gain per the line, splitter and connector losses.  

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