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Desoldering F-connector for Battery Pack

  1. Jun 6, 2013 #1
    Recently I de-soldered an f-connector (coax) from a pre-made circuit board. This consisted of 3 soldering connections, 1 for the center wire and 2 for the braid which was electrically connected through a mounting plate. Then I attached the V+ from a 18V battery to the where the center wire was, and ground to just one of the braid connections. My circuit seems to have experienced some signal loss as a result, and I am at a loss to explain why connecting the 2nd ground would affect anything. As far as I can tell, there is nothing on the circuit board even connected to the ground I left open. Any ideas?

    Before this, the battery was connected to a normal coax cable I had stripped, V+ to the center wire and ground to the braid. I'm thinking perhaps coax has some transmission properties that aren't matched by the now present 22 AWG stranded wire. Is that the case?

    The signal is in the 950-1450 range. I can provide pics if needed.
     
  2. jcsd
  3. Jun 7, 2013 #2

    berkeman

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    Staff: Mentor

    Yes, pictures would help. Are you saying that you switched from coax to twisted pair? If so, what are the Zo values of the two different transmission line cables?
     
  4. Jun 7, 2013 #3
    I went from this (not my pic, but similar):

    fcon.jpg

    to this (my pic):

    newcon.jpg

    On the other side of the RF choke is about 4 inches of 22 AWG going straight to the positive of my battery (the other going to ground, not twisted). I don't know the Z0 values. This is for a radio telescope. My signal for this new setup isn't noisy, it just seems less sensitive when I wave my hand around the dish. I guess it could be anything really, I was just never sure about leaving that second ground open.
     
  5. Jun 7, 2013 #4

    nsaspook

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    Science Advisor

    Without a proper RF connector and grounding for your DC power inserter it's no wonder your signal power is down. The F connector provided a ground path across the void of the PC board. I have seen 'cheap' designs that use the connector shell as a DC and/or RF signal path. If you look at your picture the two solder pads on either side of the center pad were once connected by the RF connector. Try soldering a short braid or thin sheet of tinned copper between them (spaced away from the center pad) to see if the signal level increases.
     
  6. Jun 7, 2013 #5
    I just tried connecting the 2 grounds with some alligator clips and you are right, things are back to normal. I really have no idea why it matters though, the ground I left open isn't connected to anything on the circuit board, its just a pad with a ball of old solder on it.

    I am measuring the signal with an arduino. With only 1 ground connected I would get a change in signal of about 20, with 2 grounds I get a change in signal of about 50. Can you help me understand why the second ground is necessary?
     
  7. Jun 7, 2013 #6

    nsaspook

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    Science Advisor

    Without knowing the complete design of the circuit I can only guess but it's likely some sort of RF ground plane or impendace matching stub.

    http://www.maximintegrated.com/app-notes/index.mvp/id/5100
     
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