Small transmitting loops (STL) are often recommended for use inside buildings, on balconies and even hand held, but are they EMR safe?
EMR safety in Australia is governed by ARPANSA RPS3 (ARPANSA 2002). Similar standards exist in USA and Europe.
The following presents near field E and H field strength for an octagonal loop 1m between opposite sites, at 7MHz. The coordinate of the loop centre is 0,0,0, so the plotted area is from 0.1m above the top loop segment.The tuning capacitor is at the top of the loop, coordinate 0,0,0.5.
Above is the E field distribution at 100W radiated. (ARPANSA 2002) stipulates a maximum instantaneous E field strength of 799V/m at 7MHz, and nowhere on this chart is the E field below that value.
Above is the H field distribution at 100W radiated. (ARPANSA 2002) stipulates a maximum instantaneous H field strength of 2.5A/m at 7MHz, and nowhere on this chart is the H field below that value.
These charts show zones that are an absolute no-go area, not even for a moment.
There are pics on the ‘net of a hand held STL where the operators hand is in the centre of the loop. That point is not mapped above, but exceeds the instantaneous H field limit even at 50mW radiated.
The structure above is a similarly sized loop, albeit circular. The operator’s had is adjusting the tuning capacitor, and area of very high E field strength that is not safe for instantaneous exposure in RPS3 terms at power levels of even 1W radiated on 7MHz.
Above, a clip from an Alexloop promotional video. Note the hand on the tuning cap where E field is very intense, and he waves the loop side (very high H field) around near his head.
ARPANSA. 2002. ARPANSA Radiation Protection Standard (RPS3 2002) – Maximum exposure levels to radiofrequency fields – 3kHz to 300GHz (assuming plane wave conditions).