Help, need simple probe for 14 GHz EMF Measurements 1

[KL7FB]

Does anyone have some specs I can use to build a simple probe with known gain and bandwidth characteristics for 14.0 GHz? I need one to investigate the level of EMF from a 14 GHz transmitter to assure the amount of radiated microwave energy is below 10 mw per cm.

I am looking for a simple 1/4 wave monopole design using a SMA connector and ground plane if anyone has that info or can direct me to it, I've googled for two days without success.

Chris KL7FB

 

[Higgler]

2 options, cut cable or Pentel Pencil

1) cut a cable, tune for VSWR, call it 0 dBi gain.
Just strip a semirigid cable outer conductor back and expose about 1/2 inch, then cut the center conductor shorter while looking at VSWR, when you get around 1/4 inch length of the center conductor you'll get a good VSWR point.

You could make a dipole too if needed, but a monopole is easier.

2) find or buy a Pentel Pencil 0.5 mm with metallic head (part where the lead comes out), unscrew the metal cap of the pencil and stick it in an sma-f connector (fits nicely)and use that as a probe, it's good from 3-40 ghz typically. You might want to cut the height of the pentel part down to make it shorter.

kch

 

[KL7FB]

Thank you, those are some great recommendations. I could use more help because I'm lacking test gear here. I have an HP signal generator good to 18 GHz as well as a nice spectrum analyzer but do not have anything more. I might be able to find a directional coupler, I'm hoping I can use that to tune to minimum return signal using a spectrum analyzer. Does that seem like a reasonable way to do so? My main concern is that the probe has a known gain or loss at 14 GHz so I can calculate the EMF density per square cm at Ku-Band. I can build any of the three probes you suggested if I can find a way to tune to minimum VSWR.

I found this link for a COUPLED SECTORIAL LOOP ANTENNA UWB antenna/probe

http://www.eecs.ucf.edu/~behdad/papers/Allerton_2004.pdf

which was interesting for future use along with plotted characteristics but it requires more materials and time to build so I like your suggestions for a quick and easy way to produce a probe. I have attached a white paper on that probe for others to look at but your suggestion looks like the way to go if I can find a way to tune it.

 

[Higgler]

You could use the sig gen and the spectrum analyzer, make two identical probes, place them apart by 1.5 inches and trim the probes to maximize the gain. Have the probes sitting on foam, or away from other objects. You'll get about -27 dB coupling between them at 1.5 inch spacing if the probes are 0 dBi gain. here's a link to space loss calculation, input 14 ghz and 1.5 inches and it gives the loss

http://www.comsearch.com/broadband/tools_fsl.jsp

(most of these calculators use miles for distance since it's for radar, but converting 1.5 inches to miles works fine).

Knowing the "spaceloss", if you measure exactly that value, your probes are zero dBi. If you measure 2 dB less than that value, your probes are +1 dBi each, etc.

It's a legal way to determine antenna gains that's used industry wide in the antenna world. It'll help convince people if you do the calibration, then do the measurement at 1.5", 2", 2.5" and 3" and use the space loss equation to calculate all the results and see how close they are to each other.

The orientation of the cables of both probes can change results a bit (+/- ?1,2 dB). If you get the probes too close to each other, they'll hurt the accuracy. You may be able to be 3/4 inch apart and still have good results.

If you are really in need of accuracy, make 3 probes. It's called a "3 antenna test method", you can determine the exact value of each probe that way. 3 coupling measurements 1-2,1-3,2-3 and 3 variables gain1,gain2,gain3, hence 3 equations, 3 unknowns is solvable for the individual gain of each probe. It'll look more impressive to the management. Either way will work though.

kch

 

[KL7FB]

Thank you, this is neat stuff, I love it. I will be working with some 1/4 inch Heliax for the coax cable. Great feedback!

You have been great help!

Chris

 

[KL7FB]

Note: This might be of interest to someone, I searched Yahoo by entering "convert 1.5 inch to miles" and at the top of the search results page the answer was:

1.5 Inches = 2.3674242E-5 Miles

Entering .000023674 in the online path loss calcuator at your link to

http://www.comsearch.com/broadband/tools_fsl.jsp

the answer was 27 dB of path loss at 14 GHz.

Very nice!

 

[zappedagain]

Higgler, I like your pencil idea! Do you have a photo?

 

[Higgler]

Not handy, take an sma-f to Staples or KMart or Walmart and find a pentel pencil that fits in it.
Either make sure the pentel unit is metal, or buy some metal spray paint to metalize it better. You can buy small quantities from Chomerics, etc. that aren't too costly.

Many companies have gone to plastic tips, though some still use metal.

Any wedge shape will work. You could probably find alternative items, like any pin that fits in an sma and wrap metal tape around it to flare it out to increase it's bandwidth, making it more useful.

Or trim a semi-rigid cable and solder a copper tape triangle on it, that'd work too. Make it an inch long, look at VSWR and trim it if needed.

kch