QRP Coax, Connectors

Discussion in 'QRP Corner' started by KG5KOG, Dec 16, 2015.

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  1. KG5KOG

    KG5KOG Ham Member QRZ Page

    The connector on the QRP rig (QRPp) is BNC, connector on the antenna is a SO-239.
    Coax I was wanting to use is LMR-400. Reason being its the same as I will be using on the 23cm rig.

    Seems like I have one h* of mix here.
    That is not going to work. LMR-400 is almost 0.5 in in dia.
    I could pigtail the BNC (insertion loss), but will wait to xmtr gets in and see if I can drill case hole bigger.

    Coax run ~40 feet.
    QRP running flat is 250mW. Can go higher with added driver transistors.

    Lets say I did go with the LMR-400 because of the lower loss at the bandwidths using.
    If I pigtailed to the transmitter and pigtailed at the antenna, would I still be better off getting rid of
    the pigtails and going with a smaller dia coax, even though it would have a higher loss?

    Thoughts on a different coax to use?

    Never mind i'll calculate myself.
    thoughts anyway?
    Last edited: Dec 16, 2015
  2. AA9SD

    AA9SD Ham Member QRZ Page

    The LMR400 is about the best in terms of loss. Why not just put a BNC on one end and a PL259 on the other? No need to use adapters.
  3. AG6QR

    AG6QR Premium Subscriber QRZ Page

    What band(s) are you thinking of using? Coax loss depends very much on frequency. For a 40 ft run on the lower HF bands, you can probably use a high quality smaller diameter coax without too much penalty. If you're on VHF, UHF, or above, that won't hold true.

    The BNC connector is pretty close to ideal. Unlike the PL259/SO239, it has a consistent 50 ohm impedance. It's also quick to connect/disconnect. About the only advantage of PL259/SO239 is that it will handle higher power, but that's not a concern with QRP. Personally, I use BNC pretty much everywhere I can, with no adapters.
  4. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    How much bandwidth are you using? On HF Spread Spectrum operations are prohibited by Part 97. SSB uses typically 3Khz and CW 500khz. Even the wider data modes are expected to not use more than that?

    -Use either adapters (which have minimal loss below UHF) or short jumpers. Do you need more than a few feet? Again, even at HF for a few feet Radio Shack RG-58 would have minimal loss! :)
  5. KG5KOG

    KG5KOG Ham Member QRZ Page

    The transmitter is a WSPR that does auto band changing on 10, 20, 30, 40, 80 and 160m.
    Small, light. heavy coax might break something.

    I saw where the LMR-400 could be fit with a BNC.
    Just wondering now about the weight of coax on the BNC in the plastic box.

    I have found LMR-240, which I could use with the QRP.
    Guess I could tie the QRP rig down along with the cable so nothing moves.
    It an't going nowhere once setup.

    Still thinking about what setup to use if I go to parts unknown with a QRP rig.
    Don't know that I will, but the thought just keeps coming back.
  6. KG5KOG

    KG5KOG Ham Member QRZ Page

  7. K7KBN

    K7KBN Ham Member QRZ Page

    Half a megahertz for CW?o_O
  8. KG5KOG

    KG5KOG Ham Member QRZ Page

    USB FSK data, is the way I understand it.
    2500 Hz bandwidth on the receive end.
  9. W7CJD

    W7CJD Ham Member QRZ Page

    That must be misread. kHz? decimal point?
  10. KG5KOG

    KG5KOG Ham Member QRZ Page

    That's straight from here http://physics.princeton.edu/pulsar/K1JT/wspr.html

    I have not been able to find on the transmitter web site what bandwidth it actually uses.
    Can't be much more than that. I don't think it is more that.

    Back to connectors, will go with LMR-400, leave the BNC direct to the LMR-400 and on the antenna side with the SO-239 that comes with it.

    Thanks y'all.
    I now know what to do.
    Last edited: Dec 16, 2015

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