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RF Exposure from Small Transmitting Loops
I am curious as to whether RF exposure concerns are greater for a
small transmitting loop [like the MFJ tuned loop] compared to a dipole radiating the same power. It would seem that close to the loop, the RF power density may be greater [than it would be at the same distance from the dipole apex] since the radiating volume is smaller. Can I just assume that the power is evenly distributed on the surface of a sphere having a radius equal to my distance from the loop antenna, calculate the power density on the sphere surface, and use that number for evaluation - or are there some near-field considerations not captured using this approach? Thanks, -JJ |
RF Exposure from Small Transmitting Loops
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RF Exposure from Small Transmitting Loops
"Roy Lewallen" wrote in message ... wrote: I am curious as to whether RF exposure concerns are greater for a small transmitting loop [like the MFJ tuned loop] compared to a dipole radiating the same power. It would seem that close to the loop, the RF power density may be greater [than it would be at the same distance from the dipole apex] since the radiating volume is smaller. Can I just assume that the power is evenly distributed on the surface of a sphere having a radius equal to my distance from the loop antenna, calculate the power density on the sphere surface, and use that number for evaluation - or are there some near-field considerations not captured using this approach? Thanks, -JJ The method you describe is valid only in the far field. There are higher order terms to the field strength (field relative to distance) in the near field, and they're strongly a function of the distance and the antenna geometry. Using the method you propose can produce very erroneous results close to the antenna. Roy Lewallen, W7EL A good question and an interesting, but not very helpful response. It seems to me that there are elements of truth in both the original proposition and in the response comment, but that each is only a partial truth. The essential aspect is surely the separation distance relative to the size of the loop antenna. However, the obvious comment is that the small physical size of a loop is likely to lead to use in a situation (for example, indoors and close to the operating point) that would/could lead to excessive levels of RF exposure. For an electrically small loop (the typical loaded loop less than 0.1 wavelength), then it is probably fair to assume that all of the input power is radiated through the sphere surrounding the loop provided that the separation is reasonably large, however, for a large loop (eg half-wave or larger) its probably best to approach the RF exposure issue as you would with any other antenna such as a dipole or vertical. Keith G Malcolm VK1ZKM 28 July 2007 |
RF Exposure from Small Transmitting Loops
In article , who knows
wrote: However, the obvious comment is that the small physical size of a loop is likely to lead to use in a situation (for example, indoors and close to the operating point) that would/could lead to excessive levels of RF exposure. Does it really matter? What are the odds of serious health consequences from RF exposure? If I quit smoking and avoid RF over-exposure will I live forever, or will I be dead as a doornail 100 years from now just like everyone else currently participating in this newsgroup? -- -30- |
RF Exposure from Small Transmitting Loops
Anonymous wrote:
In article , who knows wrote: However, the obvious comment is that the small physical size of a loop is likely to lead to use in a situation (for example, indoors and close to the operating point) that would/could lead to excessive levels of RF exposure. Does it really matter? What are the odds of serious health consequences from RF exposure? If I quit smoking and avoid RF over-exposure will I live forever, or will I be dead as a doornail 100 years from now just like everyone else currently participating in this newsgroup? Well, like exposure to anything, it depends on how much exposure you get. It is possible to die from drinking too much water for example. So, unless your plan is to live fast, die young, and leave a good looking corpse, it is probably a good idea to know how much RF is too much and avoid it at dangerous levels. -- Jim Pennino Remove .spam.sux to reply. |
RF Exposure from Small Transmitting Loops
Anonymous wrote:
... Does it really matter? What are the odds of serious health consequences from RF exposure? If I quit smoking and avoid RF over-exposure will I live forever, or will I be dead as a doornail 100 years from now just like everyone else currently participating in this newsgroup? I worry little about exposure to rf 30 Mhz and 1kw or less ... I am NOT aware of any malady/disease which strikes hams any more often than any other group ... which suggests the safeguards in place (simple plain common sense) is/are more than adequate. However, as frequency of the rf increases so does my concern ... everyone is aware microwaves can cook, maim and kill biological entities. Regards, JS |
RF Exposure from Small Transmitting Loops
Calculating, or even defining, power density in the near field is a bit
dicey to say the least. But the field strength (E or H, which aren't necessarily in phase or oriented at right angles in this vicinity) can readily be determined. Here are some values for the E field as a function of distance from the center of an octagonal loop about 3 feet in diameter at 7 MHz, with 100 watts of applied power, compared to the E field calculated using the proposed simple spherical method (and further assuming, incorrectly, that the wave impedance is 377 ohms resistive): Dist m E V/m E sph apx V/m 1 683 54.8 2 133 27.4 4 34.2 13.7 8 11.1 6.85 16 4.58 3.42 32 2.15 1.71 64 1.06 0.856 1000 0.0548 0.0673 As you can see, the approximation might be adequate at some distances and for some purposes but not for others. Roy Lewallen, W7EL |
RF Exposure from Small Transmitting Loops
On 28 jul, 00:33, "
wrote: I am curious as to whether RF exposure concerns are greater for a small transmitting loop [like the MFJ tuned loop] compared to a dipole radiating the same power. It would seem that close to the loop, the RF power density may be greater [than it would be at the same distance from the dipole apex] since the radiating volume is smaller. Can I just assume that the power is evenly distributed on the surface of a sphere having a radius equal to my distance from the loop antenna, calculate the power density on the sphere surface, and use that number for evaluation - or are there some near-field considerations not captured using this approach? Thanks, -JJ Hello, When you are close to the loop, let say less then 0.1 lambda, the exposure for the loop will be significantly higher with respect to the full size HW dipole. The reason for that is that at short distance the reactive fields dominate (that are the fields that obey "DC/lumped AC" calculus). While the radiation H field has 1/r relation, the reactive field has a relation between 1/r^2 to 1/r^3. So you cannot calculate the field strength (both H and E) based on the 1/r relation. Some years ago I did a calculation on the H field from a loop with D=3m, radiation efficiency 22%, input power 50W (so radiated power is just 11 W), 3.6 MHz. The H-field at 2m would be about 1.33A/m, while the ICNIRP reference level for the general public is 0.22A/m. At 4.5m from the loop, the field drops to 0.2A/m The reason for the strong local magnetic field is the high Q factor of the loop (about 1500), while a HW dipole will have a Q of about 12. The same radiated power for a HW fipole would result in a about 0.5A feed current. This would result in about 0.04A/m at 2 m distance from the center of the dipole. At the higher HF bands, the levels for a loop and HW dipole will come closer as the reactive fields vanish faster with respect to distance and (with same size of loop), the Q-factor decreases because of higher radiation resistance (hence lower circulating current in the loop). Best regards, Wim PA3DJS www.tetech.nl |
RF Exposure from Small Transmitting Loops
Thanks all.
Roy, I was not implying that I believe one can assume that the power is from a point source and one can consider the power density passing through a sphere to determine RF safety. I was looking for some guidance as to how to determine a "safe" distance from a small tuned loop assuming a particular frequency and power. It appears that the simple sphere approach works reasonably well beyond a wavelength or so, and may be an acceptable first-order approximation at 1/2 wavelength [from a small loop]. -JJ |
RF Exposure from Small Transmitting Loops
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RF Exposure from Small Transmitting Loops
Roy Lewallen wrote in news:13an54e9keccc59
@corp.supernews.com: wrote: Thanks all. There's no distinct boundary between the near and far field, but at a wavelength, or even a half wavelength, you're pretty much in the far field of a small antenna. So far field approximations such as the one involving power density on the surface of a sphere are quite reasonable at those distances. Some years ago, I implemented an online calculator based on the method proposed by our communications regulator (then, the ACA). The calculator includes several overseas SAR levels, including that later struck by our radiation regulator (ARPANSA). The key difference between the model used and todays regulatory environment in Australia is that the modelled results are not acceptable below 10MHz. If readers want to play with the model, it is at http://www.vk1od.net/tl/emrcc.php . (The model assumes the antenna is 100% efficient, it it isn't, then adjust the input power to the expected radiated power.) Is assessing the radiation hazard of the loop, the mode is very important to the outcome, and for reasons I don't understand, the FCC, then apparently the rest of the world, recommended a very high average/peak ratio for SSB telephony. If one was really concerned about the loop, a simple measurement instrument could be made from a small loop terminated in a resistive load and detector with a small battery powered LCD panel meter. The loop Antenna Factor can be determined from an NEC model, the detector can be calibrated on a signal generator, and the whole lot then calibrated in mV DC to Field Strength in dBuV/m. I have done this for a 0.6m square loop and the measurement results at locations in the induction and radiation near field areas around a 20m dipole reconciled reasonably with expectations based on the calculator above understanding that the calculator's method is conservative. Owen |
RF Exposure from Small Transmitting Loops
However, the obvious comment is that the small physical size of a loop is likely to lead to use in a situation (for example, indoors and close to the operating point) that would/could lead to excessive levels of RF exposure. For an electrically small loop (the typical loaded loop less than 0.1 wavelength), then it is probably fair to assume that all of the input power is radiated through the sphere surrounding the loop provided that the separation is reasonably large, separation being large - many loop diameters i.e. in the radiating far field. |
RF Exposure from Small Transmitting Loops
Anonymous wrote:
In article , who knows wrote: However, the obvious comment is that the small physical size of a loop is likely to lead to use in a situation (for example, indoors and close to the operating point) that would/could lead to excessive levels of RF exposure. Does it really matter? What are the odds of serious health consequences from RF exposure? If I quit smoking and avoid RF over-exposure will I live forever, or will I be dead as a doornail 100 years from now just like everyone else currently participating in this newsgroup? It's probably not the health consequences you have to worry about (although you should...). It's losing your license for not following the basic RF exposure safety rules. Or, it's the legal exposure for violating those rules. Lots of hams rely on the "safe harbor" limits to avoid the need for a routine evaluation of RF safety hazards. How many realize that if they're holding a cell phone, or use an HT at the same time as their HF rig, the safe harbor doesn't apply (the multiple transmitter rule)? The last thing you want is your HOA or other busybodies being able to shut you down for operating in an "unsafe" manner. |
RF Exposure from Small Transmitting Loops
John Smith I wrote:
Anonymous wrote: ... Does it really matter? What are the odds of serious health consequences from RF exposure? If I quit smoking and avoid RF over-exposure will I live forever, or will I be dead as a doornail 100 years from now just like everyone else currently participating in this newsgroup? I worry little about exposure to rf 30 Mhz and 1kw or less ... I am NOT aware of any malady/disease which strikes hams any more often than any other group ... which suggests the safeguards in place (simple plain common sense) is/are more than adequate. Not to be too hostile here, but that suggests ignorance rather than knowledge. a) one of the early studies that triggered the whole "currents of death" fiasco happened to be one that showed that radio amateurs (among some other groups) happened to have higher incidence of some forms of cancer. Later shown to be statistically insigificant and confounded by other factors, but there it is. b) There is ample evidence of adverse effects of RF exposure in this frequency and power range. One of the more interesting is the ankle and wrist pain experienced by workers on the flight line in a high RF field environment. The eventual analysis was that the pain was likely due to RF currents flowing through the body from hand (on or near airplane) to feet(then to ground). There's also some interesting cases of things like people on transmitting towers when the transmitter was turned back on (or the power turned on), but that's not necessarily a credible situation for a ham. Common sense is a good start (don't look into the waveguide with your remaining good eye)(don't turn on the transmitter when someone is working on the antenna), but it actually takes a bit more thought to figure out the RF exposure hazards in a off-nominal situation. A dipole 30 feet up in the trees is easy, so is a 3 element yagi on a 100ft tower. But something like a flagpole vertical in your yard, or an attic dipole, or a compact loop on a picnic table is a much trickier situation. However, as frequency of the rf increases so does my concern ... everyone is aware microwaves can cook, maim and kill biological entities. And so can MF, HF, VHF, and UHF... Jim |
RF Exposure from Small Transmitting Loops
Roy Lewallen wrote:
wrote: Thanks all. Roy, I was not implying that I believe one can assume that the power is from a point source and one can consider the power density passing through a sphere to determine RF safety. I was looking for some guidance as to how to determine a "safe" distance from a small tuned loop assuming a particular frequency and power. It appears that the simple sphere approach works reasonably well beyond a wavelength or so, and may be an acceptable first-order approximation at 1/2 wavelength [from a small loop]. There's no distinct boundary between the near and far field, but at a wavelength, or even a half wavelength, you're pretty much in the far field of a small antenna. So far field approximations such as the one involving power density on the surface of a sphere are quite reasonable at those distances. Somehow, though, I suspect that many people will operate within a half wavelength of a 40m or 20m compact loop, and that's where it gets a bit stickier. It's the loop or short whip on the balcony railing or picnic table, with a high duty cycle mode (like psk31, rtty, or SSTV) and turning up the power knob beyond a few watts that raises the concern. The antenna doesn't radiate well, and the QSO is a bit marginal, so the OP turns up the gas a bit. |
RF Exposure from Small Transmitting Loops
Jim Lux wrote:
[...] I know of only one fellow who worked on radar for the military and used to warm his hands by a slit on the waveguides, who seriously thought he suffered any disease from rf; his complaint was that it increased the severity of his arthritis in his hands. Now, I can see that "cooking" your hands with microwaves is not good ... how he determined that this caused more pain from arthritis was probably less than scientific--however, it would be quite easy for me to believe. I also note that he did have to have cataract surgery, however, so did many in my family who were not exposed to any rf that they were aware of. My statement stands, other than mentioned here I am aware of no hams with legitimate injuries from rf from hf antennas (and, I have no way to determine the validity of such anyway.) Now, I have heard many, many stories of friends of friends, or someone who knows someone--somewhere who has suffered noticeable damage from rf--however, I have also met individuals who have claimed to have been abducted by aliens ... Not long ago I had dental surgery and a "rf scalpel" was used. I suppose I could go find some loony attorney to help me sue the dentist .... I think I'll pass ... other than the "normal" longer-than-usual time to heal which is common to rf burns, I noted no abnormal complications. But, wives tales can make for entertaining reading, feel free to entertain me ... Regards, JS |
RF Exposure from Small Transmitting Loops
Michael Coslo wrote:
Jim Lux wrote: The big problem is this: a small loop stores a lot of energy in the fields around the loop (if the loop has a Q of, say, 100), and you're radiating 100 Watts, that implies that there is 10kW circulating in the loop between the loop itself and the tuning capacitor. The energy moves between the magnetic field of the loop and the E field of the capacitor every 1/4 cycle. Jim, are you really saying that there is 10KW in the loop? Who needs zero point energy if that is so? Or did you mean 10KV? technically 10kVA.. it's reactive power circulating between the L of the loop and the C that tunes it. The very definition of Q is the ratio of stored energy to that lost per cycle. In the case of the antenna, assuming it's lossless, the lost energy is that radiated away, and presumably replaced by the transmitter (assuming a steady state sort of system). If you have X Joules radiating away each cycle, there has to be Q*X Joules stored in the system, and X Joules added to the system. In a lossy antenna (which these loops will inevitably be, barring superconductors, etc.), some of the energy is lost to heat, but, again, if you measure the Q, that's rolled in. (The Q of a lossless resonant loop 1 meter in diameter at 7MHz would be spectacularly high, since the radiation resistance is tiny compared to the reactance of the loop) This is why small loops need HV capacitors and low resistance loops. For what it's worth, the same sort of problems with near fields crop up in superdirective arrays, because there's a lot of reactive power stored in the near field that circulates among the elements. Fortunately from the RF exposure standpoint, most amateur superdirective arrays (i.e. Yagis) are mounted several array sizes away from people, and in these arrays, the high energy density is almost entirely within the volume of the array. Take a look at the cover of one of the Antenna Compendiums (#3?) for a picture of this. jim, W6RMK |
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