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Re: LF: EMF sensitivity?

To: [email protected]
Subject: Re: LF: EMF sensitivity?
From: Marek SQ5BPM <[email protected]>
Date: Mon, 01 May 2006 23:17:35 +0200
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Hello John and thank you for the reply.

[email protected] wrote:
Marek, I am not aware of effects like yours being reported from exposure to much stronger LF fields at LORAN sites and other transmitting facilities. The magnetic field 2 meters from a loading coil handling 500 watts might be near or slightly above accepted limits for human exposure at low frequencies, but not greatly in excess.
That's what I thought about. Polish Radio 225 kHz transmitter had 2 MW of power and the staff has been working quite close. You probably know the story about it's 646m-high mast collapse in 1991 due to a human error during conservation.
However, as you surmise, you are not just being exposed to LF alone. The two middle items on your list are, far and away, the most likely sources. In addition to the normal high-frequency components of the switching waveform, it is possible for there to be surprising amounts of UHF energy from parasitic oscillations during the on/off transitions of the FETs. This depends a lot on circuit layout.
I will check waveforms inside the circuit, but there cannot be too much power generated at high frequencies, because the power balance would be wrong. But, probably, I am unlucky and there is some kind of resonance at a few hundreds MHz and most of the oscillations concentrate at that range.
Your symptoms sound very much like a story I heard in my younger days of an amateur radio operator whose unshielded high-power HF rig was emitting parasitic oscillations in the high UHF range. He, too, experienced a rash and other allergy or flu-like symptoms, and eventually developed eye problems. I would very much encourage you shield the rig adequately before further operation!
Yes, that is the general plan, to move the whole TX to a new, better-designed enclosure. The current arrangement leaves too little place for the airflow, that's the only reason why the top cover is removed...
I also heard a story about some jerk tuning an open 2m 1kW PA until a
blood started to flow from his nose... Fortunately, no bloody symptoms
here so far ;)
73! Marek SQ5BPM

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