near field probe

Keith Nishihara

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

Steve W6SDM

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

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

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

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

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

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

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

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

Howard is younger than me. We had corn cobs and Sears and Sears & Roebuck catalogs.

KY6LA_Howard

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

Andrew VK5CV

James Del Principe

KY6LA_Howard

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

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

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

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

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

Kevin Va3KGS

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

Great paper. Very in-depth.    Shows how a simple subject/idea can be so sophisticated.          Jim

k0eoo

Yes, great paper, thanks for the link....