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near field probe

It seems like the Flex radios could be used as a spectrum analyzer with a near field probe to track down sources of RFI in the shack.   Can anyone recommend a dyi design for a probe that would work in the HF bands?  I've come across designs in the GHz range but nothing for HF yet.
Thanks -- Keith KH6FQR

Answers

  • Steve W6SDM
    Steve W6SDM Member ✭✭
    edited June 2016
    I set this thing up just to see how it worked and I got division by zero errors and a message saying that an outgoing call cannot be made since the application is dispatching an input-synchronous call.
  • James Del Principe
    James Del Principe Member ✭✭
    edited February 2017
    Why not keep it simple and just connect a length of coax (RG58 for example) to the rec port and a few turns of wire at the other end. Seems it should work.    73, Jim
  • Keith Nishihara
    edited June 2016
    Hi James,   Thanks, I'll try that.  I was just checking to see if anyone had any experience doing this already.  It seems like the Flex could be a useful tool for this sort of thing.  It is certainly capable of  "seeing" much more RFI than my other radios ;-)  
  • James Del Principe
    James Del Principe Member ✭✭
    edited June 2016
    Keith, I agree that is an interesting idea..One of those "Why didn't I think of that" ....just be sure not to over drive it.    Micro volts are fine. More is not.      73,, Jim
  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited June 2016

    Sometimes the simplest solution is the most BRILLIANT

    Just tried your suggestion myself with a 50 OHM resistor soldered across the end of a piece of coax.. worked ... was able to see RFI sources

    Now to design a small coil with 50 ohm impedance @14.2MHz   for even better sensitivity

  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited January 2017

    To save you guys doing the math:

    50 Ohm Coll @14.2MHz = 0.561 Micro Henries

    Using a 2" diameter toilet roll...

    4 turns x 2" diameter x 2" length = 0.552 Micro Henries


    So all you need to do to make a HF Probe is to take an empty toilet paper roll, wind 4 turns over 2" length and you have close to 50 ohm probe...


    Or use a small 1/4" Dowel (pencil)

    36 turns x 1/4" x 4" = 0.492 microhenries


  • James Del Principe
    James Del Principe Member ✭✭
    edited June 2016
    Howard, I love the use of a toilet paper roll... should be a standard for hams. I have wound many a project coil on them. Thanks for the info. 73, Jim
  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited June 2016

    Ironic... The first ever crystal radio I built 65 years ago used a toilet paper roll for the tuning coil.

    For you younger guys... we had toilet paper back then too....

  • Steve W6SDM
    Steve W6SDM Member ✭✭
    edited June 2016
    Howard is younger than me. We had corn cobs and Sears and Sears & Roebuck catalogs.
  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited June 2016

    We actually used newspaper until we could afford real toilet paper....from which I stole the empty rolls for inductor forms...

  • Andrew VK5CV
    Andrew VK5CV Member ✭✭
    edited June 2016
    A small one turn loop and 50ohm resistor in series on the end of the coax works well.
    Andrew de vk5cv
  • James Del Principe
    James Del Principe Member ✭✭
    edited June 2016
    That's what Sears catalogs were for.
    On a technical side, why would the pickup coil need to be 50 Ohms when we are only trying to couple a few microvolts into the rcvr?   we're not looking for maximum energy transfer by matching impedance.   73, Jim
  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited June 2016
    Depends on the signal you are trying to see. The 6000 Series is an instrument capable of accurately displaying signals down to a range of -140dBm (20 nV) across a 50 ohm load. So if your trying to maximize the laboratory capability of the device you should use a 50 ohm load. On the other hand, if you are just trying to roughly see what RFI spurs are around then you don't need a resonant circuit. It took me less than 5 minutes to build a resonant probe so no big effort required BTW. Thanks to everyone for the idea.
  • Keith Nishihara
    edited June 2016
    Yes thanks all for the information and interesting discussion!  One other note -- use the Rx input rather than the Tx/Rx connector -- you don't want to accidentally send 100 watts out to your probe ;-)  
  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited June 2016

    Just to be an absolute technonerd about this, I just spent the last hour playing with my vector network analyzer to tune the probe to exactly 50.0 ohms @14.200MHz.

    I guess I have too much time on my hands this morning (more likely Jet Lag effects)...


    Again... Thank you Keith for the idea and James for thinking of a simple probe  solution.. I have already used the probe to find an RFI source that was bugging me and my other test instruments could not see... 

    Now I plan to play around designing a few different size probes...to see if I can get them into hard to reach places....


  • James Del Principe
    James Del Principe Member ✭✭
    edited June 2016
    Now I know why I like this forum so much. Great ideas and shared information. Really stimulates my interest. What a super bunch of guys!!!     Please share your designs with us.... I can see many uses for this.      Jim
  • James Del Principe
    James Del Principe Member ✭✭
    edited June 2016
    What???    You don't want a really high powered signal generator???    LOL      Good point. Jim
  • Kevin Va3KGS
    Kevin Va3KGS Member ✭✭
    edited June 2017

    I have a probe that I made just as Paul suggests, I left the centre conductor wire insulated so that when I was probing around in the radios I would not short things out!!  Worked great for reading on the frequency counter.  Also made a few probes on a piece of 3/4" PVC pipe with an SO239 connector for change ability.

    Good Luck

    Kevin, Va3KGS

  • James Del Principe
    James Del Principe Member ✭✭
    edited July 2016
    Great paper. Very in-depth.    Shows how a simple subject/idea can be so sophisticated.          Jim
  • k0eoo
    k0eoo Member ✭✭
    edited December 2016
    Yes, great paper, thanks for the link....

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