[BC] Rf exposure testing

[John Lyles]

L3, i. e., Narda is still the dominant provider of this instrumentation in the USA. They have dropped a lot of the analog meter lines, such as the old 8600 and 8700 meters. But they still service and calibrate them. They do offer replacements that are functionally equivalent if you ask them or look on their website. Today i used both an ancient (but just recalibrated) 8616 meter with E and H probes and a new NBM-550 wideband digital portable meter that they now sell. The new one is made by Narda in Germany, so a lot different than the old warhorses. It has standards built in such as the IEEE C95.1 and the FCC worker recommendations so can indicate in % of std or also in the usual field units. I took both meters because I didn't trust the digital at first. I have used the 8600s for years and know their tricks, such as pressing the zero button often, out of the field region. 

I measured at the base of the folded unipole with 1 kW today. Both meters agreed within what I would expect, and the fields were about as expected by the FCC bulletin OET-65, as calculated for 1/4 wave stick with 1 kW using MININEC. In other words, stay 1 meter away from the base and the ATU. The highest fields were around the ATU and of course the copper strap going out of it up to the skirt. And H field was dominant as far as exposure went. At 1 foot away from things, magnetic field varied from 30 to 250 mW/cm2 depending on azimuth around the base. The highest reading was on one of the legs of the ATU, about a foot above the ground, at 350. This station has poor grounding conductors (not enough) so there appeared to be quite a bit of RF current on that side. 

Table I of FCC OET bulletin 56, the Q&As of RF exposure monitoring suggest 100 mW/cm2 equiv plane wave power density for 1.5 MHz. At one foot from the base, I was seeing hot spots that were about 2.5 times the limit, but spatially averaged over the whole torso they would be in compliance, unless standing just beside the ATU, but then you might suffer a burn if you moved your arm and hit the copper anyhow. 

Anyway, find yourself a Narda, they should be able to provide, rent, sell. 

John 

> 
> Message: 19
> From: "Larry Lamoray" <lists at systemsstore.com>
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> Any suggestions for what meter to use for RF Exposure testing, and where to rent one?
> 
> It seems that the Halliday HI-3000, Narda 8700, and IFI RFH - as suggested in the NAB Engineering Handbook - are no longer available,