near field probe

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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
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Keith Nishihara

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Posted 3 years ago

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Steve W6SDM

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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.
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James Del Principe

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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
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KY6LA - Howard, Elmer

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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

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Andrew Russell

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A small one turn loop and 50ohm resistor in series on the end of the coax works well.
Andrew de vk5cv
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Keith Nishihara

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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 ;-)  
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James Del Principe

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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
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KY6LA - Howard, Elmer

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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


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KY6LA - Howard, Elmer

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

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James Del Principe

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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
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KY6LA - Howard, Elmer

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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.
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KY6LA - Howard, Elmer

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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....


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James Del Principe

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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
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Keith Nishihara

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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 ;-)  
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James Del Principe

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What???    You don't want a really high powered signal generator???    LOL      Good point. Jim
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Kevin Va3KGS

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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