Ed's answer is right on as usual. I would add a few particulars. The effective height of a tuned dipole driving a matched load is its physical length divided by pi. The source impedance of a tuned dipole is 72 Ohms. Those two facts will allow you to calculate the power available at the antenna terminals. I really don't think that for the purpose you cite that it is necessary to worry about out-of-band characteristics. It sounds as if what the spec is saying is that any fortuitous conductor can be assumed to have the power-gathering efficiency of a tuned dipole of the same length. Then if you had some maximum power level that was assumed safe in an explosive atmosphere, you could work backwards and determine the allowable field intensity, and hence the appropriate distance from an emitter of known characteristics. My interpretation of what you said may be way off, but if it is correct, I am interested because I see a problem. The problem is in how you determine the maximum allowable power allowed in the explosive atmosphere. If it is simply heating, that is relatively straightforward but I don't think it can be heating alone. I think the limiting factor will be if the field intensity were high enough to strike an arc, and the energy associated with that spark is to be compared to what will ignite the explosive atmosphere. But I don't know how you use a dipole to calculate the energy of a spark induced by an intense electric field induced between metallic objects in the field. I did this by accident (and my microwave oven was never the same afterwards). Many years ago I bought a jar of peanut butter which had a metallized freshness or security seal. I didn't realize it was metallized. I stored it in the fridge and one day popped it in the microwave for a few seconds to soften it. What I didn't realize is that when I had previously torn off the seal I hadn't gotten every bit of it and metal strips still were affixed to the rim of the jar. It was quite a show - arcing and sparking. The microwave radiation frequency is - despite what any number of participants in this forum believe - 2.45 GHz and that means a wavelength is about 12 cm or 5 inches. A quarter wavelength would then be 3 cm or 1.2". I think the arcs were drawn over shorter distances than one quarter wavelength. It was about a 1 kW microwave oven. I honestly don't know if that is power input, or magnetron output, and in any case I don't know what the effective field intensity in the oven cavity was. But I do know that is a lot less power than most radars put out.
---------- From: "Price, Ed" <[email protected]> To: "'k3row'" <[email protected]>, [email protected] Subject: RE: Rf flammable atmosphere ignition and Halfwave Dipoles List-Post: [email protected] Date: Fri, Aug 2, 2002, 6:11 PM -----Original Message----- From: k3row [mailto:[email protected]] Sent: Friday, August 02, 2002 2:03 PM To: [email protected] Subject: Rf flammable atmosphere ignition and Halfwave Dipoles Can anyone help me? The overall context of this question is the extraction efficiency of a dipole representing a generic mechanical structure from the point of view of rf fields (> 30 MHz) and ignition of explosive and flammable atmospheres. A British standard I have been looking at assumes that the structure has the rf energy extraction efficiency of a half wave dipole (I am ignoring here the extra gain also assumed due to the potential for the structure to behave as an antenna with extra gain). The basic question is to do with the extraction efficiency of a dipole versus frequency, since, if the rf frequency is known and the structure is known then it need not be assumed that the structure will act as a half wave dipole (the frequency and structure dimensions may not be compatible) My specific questions are these: Assuming that what I am actually taking about is dipole gain (I am a bit of an ignoramus I'm afraid) Can anyone give me a basic approximate formula for the variation of gain with frequency for frequencies that are up to a factor of (say) 10 away (above and below) from the resonant frequency of a half wave dipole. Is the maximum gain cyclic (e.g is there a resonance at, say, a dipole length of 1.5, 2.5 etc wavelengths or does the gain just "disappear" when the frequency moves away from a half wave dipole condition?). If the gain is cyclic what would be an approximate formula for the gain at, and around, these cyclic frequencies?(Note that I am not interested in polar diagram directions, merely gain) I am not entirely sure that I have made these questions very clear - but I hope so. Has anyone got any formulae or does anyone know where I can get some? In hope Dave Palmer, UK Dave: A quick answer is that the efficiency of a dipole antenna is cyclic with frequency. Let's assume you have a center-fed dipole whose arms are each about 25 cm long. Connect a signal generator to a coax feeding the center of that dipole (let's not worry about impedance matching yet). You can now start a frequency sweep at 1 MHz, and continue through 1 GHz. If you had placed a dual directional coupler into the coax line, you would see periodic changes in the amount of RF power reflected from the antenna. Remember that reflected power is power not radiated, so reflected power is an indication of antenna efficiency (radiating ability). At certain frequencies, you would see a marked decrease in the reflected power. Minimum reflected power would be at about 300 MHz, but you would also have seen dips (indicative of resonances) at 150 MHz, 75 MHz, 37 MHz, etc. If you are concerned about "extraction" efficiency, you can remember that efficiency of an antenna is reciprocal. Once you get above the fundamental resonant frequency, you will also see the periodic resonances, at 2Fo, 3Fo, 4Fo, etc. For some good weekend reading, look at the ARRL Antennas Handbook, or the ARRL Radio Amateurs Handbook. The RSGB also has some fine (but slimmer) publications. Regards, Ed WB6WSN Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Is Our Specialty Shake-Bake-Shock - Metrology - Reliability Analysis
