Need info on a homemade probe for 14 GHz measure EMF/EMI

[CCH1957]

emi homemade euiqpment

Hi, I'm in Afghanistan and working with the US Army where we have some concerns the EMF field intensity at Ku-band 14 GHz might be too high for some people working near a parabolic antenna. I have a good HP 4407B spectrum analyzer to work with but do not have a EMI or EMF probe to measure the field intensity levels at 14 GHz. Does anyone have the info or can point me to a good link for the fabrication of a simple monopole or horn antenna with known gain characteristics (or loss) which I can use as a quick home made probe to show if we are exceeding 10 mw per cm over a 6 minute period of time which is the OSHA limit at 14 GHz?

Any help appreciated, please steer me in the right direction on this.

Thanks in advance.

Chris Ku-Band SatCom Engineer (field).

 

[vfone]

coax for homade spectrum analyzer

I would recommend building a horn antenna. Is not very complicated to make and the gain (which is very important for your field calculation) is also very predictable.

Follow this document and the affiliate free software.
http://www.qsl.net/n1bwt/chap2.pdf
**broken link removed**

Good luck,

P.S.
Probably in that area is better to avoid 7.62mm than 14GHz

 

[VSWR]

homemade rf field

You can use a WR-75 waveguide to Coax Adapter for 10.0-15.0 GHz as a probe. See picture below (shows a WR-42 adapter as an example). A WR-75 adapter should be available in any US Army microwave tool box.

Connect it to your spectrum analyzer via a coaxial cable having suitable length but the analyzer must not be directly exposed to these strong fields. The WG adapter has a gain of about 7 dB when used as a "receiving antenna" at 14 GHz. A power density of 10 mW/cm2 at 14 GHz gives a received power level of about 13 dBm, using the formulas below. If the cable loss is 10 dB at 14 GHz, the spectrum analyzer reading is 3 dBm at a power density of 10 mW/cm2.

 

[CCH1957]

rf signal dbm to mw/cm2

Thank you very much, this information has been helpful. Can you also help me understand how you arrived at the level I should see for 10 mw per cm2? I would like to study this further. I have been in RF since high school, now over 50 years old but never had to calculate EMF fields per cm before.

Regards,

\Chris

 

[VSWR]

emf probe

See below.

 

[CCH1957]

how to measure decibels homemade

Thanks! Great help

 

[biff44]

spectrumanalyzer homemade

Something like this, where you are advocating just using an open ended single moded waveguide as a probe. Couldn't you just assume that the effective aperture was the area (width x height) of the waveguide opening? Would that be that far off?

 

[Flighttek]

Someone out there in control of PMEL (Air Force) or that calibrates test equipment or has radar or a high power microwave setup should have a microwave dosimeter that should tell you exactly what the exposure levels are. Not sure what band they cover but you may need to request a Ku band. Safety should be able to coordinate this if you don't have the resource. I know, the safety guy, but he needs a job too. Put him to work and earn his keep.

 

[CCH1957]

Thanks, we looked for someone who had a probe and ended up having someone from another area fly in to take the measurements using a broad band EMF probe which worked fine before leaving on their trip here but would not properly zero once they began using it here. Regardless, the Ku band RF levels were far below even the tightest standards for human exposure by a factor of well over one thousand. The EMF probe they brought couldn't measure that low but the 1/4 wave monopole (stub at 1/4 inch for 14 GHz) with an Agilent E4407B spectrum analyzer showed the level to be far below .000 mw anywhere I could measure, even right below the transmitting antenna.

Since I didn't have a three port coupler to test for return loss from the stub when using a signal generator I could not tune it or know how close to 14 GHz it was but building two identical quarter wavelength monopole antennas, one at the signal generator output and one feeding a spectrum analyzer I found that at approx. 1.5 inches apart the free space path loss was about 30 dB. It should have been 27 dB at 14 GHz for zero dB gain, according to the formula I used. For my measurements that was close enough, I called the accuracy plus or minus 2.5 dB and made the measurements.

I have a question I still don't have an understanding of yet. How can I convert what I measured into mw per square cm? Let's say I measure -36 dBm on the spectrum analyzer (after factoring out the cable loss from the probe/antenna) how can I know how many milliwatts there are of field intensity per square centimeter using a 1/4 wavelength monopole (vertical stub) with an approximate zero db gain?

 

[FvM]

VSWR has presented the said calculation. There may be doubts about the actual gain. A monopole without a "counterweight" doesn't exist.
A practical quarterwave monopole, e.g. a stub at a coax end can be expected to have still some gain in the range of a halfwave dipole (around 2 dB)

 

[CCH1957]

Thanks for your answer, seems the little "monopole' 1/4 inch piece of copper protruding out of the end of a length of coax was not close to resonant at 14 GHz because I measured two to three dB less signal level than there should have been at 1.5 inches away from the transmit antenna/stub. However, the signal generator I used has a possible error of 2 dB so there are too many unknowns to make a reasonable assumption of what I really had with the probe.

If I had more time to work with making the probe I would have built a small ground plane disk to fit on the end of the coax where the wire came out. Although far from a real reference antenna it was close enough to know if we were anywhere near the maximum field strength allowed in the Ku-Band, since the highest level I could find was -36 dBm, most areas from near -55 to -80 dBm or lower and covered by the noise floor of the instrument at the settings I had used.

Still looking for an answer to how I can convert what I measured with the small far from perfect quarter inch monopole to milliwatts per cm2, as a very rough estimate. I'm imaging that the area of the aperture of a feed horn could be used to calculate that number but what about a simple probe? How does that relate?

 

[FvM]

The calculation by VSWR is valid independ of the antenna type, you just have to adjust the gain. The effective antenna area or "aperture" of a 0 dB gain (or isotrope) antenna at 14 GHz is 0.35 sqcm. Thus 0 dBm corresponds to 2.85 mW/sqcm respectively -36 dBm to 0.7 uW/sqm.

A quarterwave monopole with a large ("infinite") ground plane has a theoretical gain of 5.1 dB due to it's higher directivity. Because a real microwave monopole is radiating to/receiving from the lower halfspace as well, it's rather theoretical.

 

[CCH1957]

Thanks again,

One thing I didn't do I could have done to determine the resonant frequency of the 1/4 wave stub is to have frequency sweeped the probe I built using the RF signal generator and watch the peak response on a spectrum analyzer a few inches away on max hold receiving the signal to see where it was most efficient at transmitting RF, that would have told me where it was best tuned to for 50 ohms if I had a flat receive antenna on the analyzer. I have a couple of ideas of how to do that, a dummy load on the analyzer which has some amount of leakage ought to be flat, or a length of coax with a dummy load on the end of it might allow enough signal to leak through from the transmiting test probe using a RF signal generator.

I have been searching for a relatively inexpensive EMF probe which could be used with a spectrum analyzer for field intensity measurements which is good from 3 GHz up through 18 or more GHz. The only thing I can find so far is a log periodic antenna made in Germany which comes with a calibration chart. It isn't perfectly flat but the calibration chart with each antenna allows adjustments to later be made to the measured levels. If anyone knows of something better, smaller and flatter which can be used with a spectrum analyzer please let me know.

Thanks!