RF Hazards

TO: High Power Microwavers
FROM: Dick, K2RIW. 
RE: Non-ionizing high power microwave energy.

A Discussion of RF biohazards as related to microwave radio energy
by Dick Knadle, K2RIW, 19 Feb 2003.

INTRODUCTION -- The high power Radars of the Marshal Island experience of Dave, K4TO, and the Trinidad experience of Jim, W4RX are good for illuminating many of the principles of RF Bio-hazard Assessment. Dave's situation is calculated here, and I suspect the conclusions could be equally applied to Jim's situation (repeated below - Attach. 1). The aircraft that flew in front of Jim's radar were probably also at a safe distance.

High Power Microwave (HPM) energy is something to treat with a healthy respect. When in doubt, please err on the side of being conservative.

However, the subject is often misunderstood, and many people respond in a way that is more emotional than factual. As you will see, the most dangerous conditions that are possible are quite different from those you would consider by using "good common sense".

CALCULATION METHOD -- You will notice that calculations are performed in a step-by-step signal flow manner, so that the RF principles are illuminated. To many readers this methodology improves the memorization, and it increases the understanding, compared to simply plugging in numbers to a large equation with many variables.

K4TO's SITUATION -- Dave, K4TO gave us a "war story" (repeated below - attach. 2) concerning his experience in the Marshal Islands with one of the world's highest powered Radar systems. The 10 and 20 Megawatt Radars that fed a 150 foot dish sound like very dangerous pieces of hardware. As you will see by the following calculation, they probably were quite safe, particularly considering the way in which they were being used.

NON-IONIZING RADIATION -- The wavelengths that are far longer (lower in frequency) than the optical wavelengths are often called "non-ionizing". This means that the smallest unit of transmission (a photon) at this wavelength doesn't have enough energy to disrupt the molecules of your DNA. Thus, their only documented effect seems to be RF heating.

REFERENCE STANDARDS -- Arguably, one of the most authoritative documents on this subject is the ANSI IEEE C95.1-1991 Standard, published by the US American National Standards Institute. It has the title "IEEE Standard for Safety Levels with Respect to Human Exposure to RF Electromagnetic Fields, 3 kHz to 300 GHz". This document contains 450 references from many of the world's best research institutes, and it describes the compendium of opinions on this subject that have been gathered over the last 50 years. The RSGB uses a similar document that is recommended for the UK by the National Radiological Protection Board (NRPB).

MPEs -- The latest version of these documents describes the Maximum Permissible Exposure (MPE) levels that are allowed at various frequencies. It is interesting to note that the most stringent MPE is for the VHF frequency region of about 30 to 300 MHz. In this frequency region human tissue absorbs a majority of the RF energy that impinges on it, and converts it into heat. The most vulnerable organs are the eyes (possible cataract genesis) and the reproductive organs (possible temporary or permanent sterility), due to the fact that these organs have a low heat tolerance, and a restricted ability to dissipate excessive heat.

OTHER FREQUENCIES -- At lower RF frequencies a majority of the RF energy passes through human tissue and is not absorbed. At higher RF frequencies a majority of the RF energy is reflected at the skin surface. The so-called Microwave region above 1,500 MHz has an MPE that is 5 times higher than the 30 to 300 MHz region. To repeat, Microwaves are pound-for-pound 5 times safer than VHF frequencies.

SAFER MICROWAVES -- Although this is well documented, try to convince your local Zoning Board of this fact, and they are likely to think you are crazy. In the last 10 years I have given testimony before 12 Zoning Boards while defending amateur radio towers on Long Island, New York. The task is very challenging.

THE ALTAIR RADAR -- Dave's (K4TO) Marshal Island "Altair" Radar had a peak pulse power of 10 MW (at 155 MHz) and 20 MW (at 422 MHZ). Believe it or not, the most dangerous condition would occur if that RF power was radiated directly by the transmission line to that Radar dish. If the 422 MHz wave guide flange was simply "open circuited" to the air, that would constitute an "antenna" that has about 6 dB of gain and a VSWR of about 1.4:1. A person standing near that open circuited WG flange could absorb the average power of the RF transmitter (that hits him) and is converted into heat. If the vulnerable organs experienced a significant rise in temperature, then possible damage could occur.

THE DUTY FACTOR -- I'm assuming that the Altair Radar was a Long Range Search Radar, or an Over-the Horizon Radar. It is likely that the Radar had a pulse duty factor of 0.25% (0.0025) or less. Thus, the average transmitted power was probably about 50 kW. The ANSI C-95 specification says that the most stringent MPE for a "controlled environment" in the VHF frequency region is 1.0 milliwatt per square centimeter, averaged over a 6 minute period (360 seconds). The standard assumes that the vulnerable organs have a thermal time constant of 6 minutes.

SAFE DISTANCE -- Let's calculate how close you would have to stand to that open-ended WG flange to experience that MPE. This is done by assuming that the signal source radiates isotopically on the inside of a uniform sphere, and we must calculate how big a sphere is required for the Power Flux Density (PFD) to go down to 1.0 mW/(square cm). Remember that 1.0 mW/(sq. cm) equals 10 watts/(square meter).

The transmitter's 50 kW average with a WG flange gain of 6 dB equals an Effective Isotropic Radiated power (EIRP) of 200 kW average. A sphere that has an area of 200,000/10 = 20,000 square meters would be just right. The sphere's area is equal to 4*Pi*R^2 = 20,000 m^2. Solving for R equals 39.9 meters (131 feet). Therefore, a safe distance to the opened WG flange is 131 feet.

THE COMPLETE RADAR -- When the Altair Radar is operating, the feed horn tries to evenly distribute the 50 kW of average power over the surface of the 150 foot dish. That would give the dish the maximum possible gain of 46.1 dBi at 422 MHz. The best the horn can do is to create a 10 dB amplitude taper, which lowers the dish efficiency to about 60%, and the gain will be about 43.9 dBi (1.2 degrees of beamwidth).

THE PFD AT THE REFLECTOR -- The 150 foot dish has a projected surface area of 17,671 square feet (1,642 square meters), if it is a round dish. If the horn could evenly distribute the 50 kW of RF energy on the dish surface, that would equal a PFD of 30.5 watts/(square meter). Because of the amplitude taper and edge spill-over the PFD at the center of the dish is probably about 4 times greater, or about 122 watts/(sq. meter). This PFD is 12.2 times the current MPE. Because the ANSI Standard has a 10:1 safety factor built into it, that might actually be almost a safe exposure level. In other words, you could lie at the center of the dish while the Radar is operating for 360/12.2 = 29.5 seconds, and be safe by the most stringent portion of the current ANSI standard.

COLLIMATION -- The parabolic surface of the dish takes the spherical wavefront from the feed horn and collimates it into a nearly-parallel beam. As you stand at some distance in front of the dish, the spread of the energy will cause the PFD to drop below the 122 watts/(sq meter) level.

NEAR FIELD RANGE -- Let's see what happens if you stand at the best distance in front of the dish. A 150 foot dish at 422 MHz has a Near Field Range of 2^D^2/Lambda = 19,294 feet (3.65 miles or 5.9 km). At that distance (or greater) the dish achieves about the best performance of Gain and Pattern. 50 kW (average) into a dish with a gain of 43.9 dBi yields an EIRP of 1.23 GW, or 121 dBm (average power). At 3.65 miles at 422 MHz the Free Space Loss is equal to -100.3 dB. An Isotropic antenna in the boresight of that Radar at a distance of 3.65 miles would receive an average signal of 121 -100.3 = +20.7 dBm. An Isotropic antenna has an Effective Area of Lambda^2/(4*Pi) = 0.04 meter^2. Therefore an Isotropic signal of +20.7 dBm (117 mW) equals a PFD of 0.117/0.04 = 2.9 watts/(square meter). This is an MPE that is 3.4:1 below the current ANSI Standard of 10 watts/meter^2, and it is actually a safe PFD for continuous exposure.

DECREASING DISTANCE -- If you were at 3.65 miles in the boresight of the Altair Radar and you began walking toward the dish, the PFD would increase by less than 15 dB and peak at a distance of 1/5 the NFR, than it would decrease by 3 dB and continue to oscillate between two levels that are less than 3 dB apart. As you get closer two things happen: (1) the Free Space Loss decreases; (2) the apparent dish gain begins to decrease because portions of the reflecting surface becomes out-of-phase at your range. Experience with most dishes has shown that rarely does the PFD become more than 15 dB stronger than it is at the Near Field Range, and it almost never exceeds the PFD that exists at the apex of the dish reflector, unless the dish was intentionally made "nearsighted" by translating the horn beyond the normal focal distance.

The maximum PFD would occur at 1/5 of the NFR, or 0.73 miles. At that distance the 15 dB increase in PFD (from 2.9 Watts/meter^2) would be 92.4 watts/meter^2, which is below the estimated PFD at the apex of the dish reflector -- that was about 122 Watts/(sq meter). These worse case PFD's are very close to the old MPE, which didn't have the recently-added 10:1 safety factor.

At a 1.0 mile range (0.27 of the NFR of 3.65 miles), where Jim's living quarters were, the calculated PFD will increase by 10.9 dB, and increase from 2.9 W/m^2 to 35.7 W/m^2. That's 3.6 times the current ANSI Standard, but considerably below the standard that existed at that time (100 W/m^2). Thus, it was prudent that his living quarters were RF shielded structures.

All of these calculations assume that the dish is continuously staring in a particular direction -- that's the worse case condition. If the dish was continuously rotating, then the Bio-hazard assessments become many times safer, as long as it doesn't dwell for 6 minutes in a particular direction that contains biological material.

CONCLUSIONS -- The surprise of a real-world Radar situation is that the RF Biohazard actually decreases when you use a higher gain dish antenna. This is because it distributes the transmitted energy over a large surface area, and the PFD always decreases from that point as the range is increased. A bird that lands on the feed horn while the Radar is operating will probably not survive for more than a couple of minutes. Birds like to do this when the dish is not rotating. Under these conditions Radar operators will sometimes find dead birds lying below a Radar dish.

A naive technologist will sometimes use the peak power of a Radar instead of the average power. Then they will simply add the antenna gain (in dB's) to the peak transmitter power (in dBm) to calculate the EIRP. This methodology can give results that are doubly pessimistic in assessing the near-by RF Safety. They failed to realize that RF primarily creates a heating effect, and the heating is proportional to the average power, not the peak power. Also, the full antenna gain is only realized in the Far Field. When you're up close, portions of the antenna are out of phase, and do not fully contribute to the resultant PFD.

I hope this material is helpful in increasing the understanding of RF Bio-hazards. Feel free to correct the math errors.

      73 es Good VHF/UHF/SHF DX,
      Dick K2RIW.
      Grid FN30HT84DC27


ADDENDUM:  20 Feb 2003

Dear High Power Microwavers,

Eddie Currie, KB2VPO, has offered opinions concerning the safety of RF exposure (included below - Attach. 3) that deserve further comment.

I'll admit I have been too free with my statements concerning what amount RF exposure is "safe". It would have been more correct to say, "this exposure level has not been proven to be deleterious, according to the ANSI IEEE C95.1-1991 Standard".

Eddie's statements imply that he feels the non-thermal effects of RF exposure have not been given enough consideration. I feel there are a rather large number of experiments that are described within the 450 references of the ANSI Standard where well-equipped and prestigious laboratories have conducted long-duration experiments that were looking for the non-thermal effects. In most cases they didn't find any; and even in the cases were a small effect seemed to be present, the effect usually disappeared when the experiment was duplicated in another laboratory.

Yes, "absence of evidence" isn't "evidence of absence". However, if you look for an effect while conducting a large number of experiments over a long period of time, you at least build up a data base of the working conditions that appear to be non-deleterious.

I believe that part of the general public's misunderstanding about RF exposure concerns the emotional word "radiation". That word immediately evokes the concept of "nuclear radiation", and cancer. That mix up is putting us in a bad light. For at least two reasons we would be more correct to call it "RF Transmission", or "RF Emission" not "RF Radiation".

Nuclear radiation can be considered much more deleterious because it is an ionizing radiation, and the effects are cumulative. With nuclear: if you were exposed to one milli REM today, and one milli REM tomorrow, your total dose is close to two milli REM (the body does some self-repair during the day). However, with RF: if you are exposed to one mW/cm^2 today and one mW/cm^2 tomorrow, your total dose is almost exactly one mW/cm^2 -- it doesn't accumulate. The many experiments that have been conducted have not demonstrated a cumulative effect of RF exposures that are at or below the guideline levels.

There is a prevailing opinion among some very conservative people that can be stated as, "every exposure is an over exposure," or "every dose is an overdose".

I feel that this kind of thinking can become unreasonably extreme when addressing RF exposure. In many cases this kind of thinking represents a misunderstanding of the term "risk". There is a level of risk in everything we do. A prudent person considers the risk-to-benefit ratio, he minimizes the risk, and then he proceeds with the action; you can't live a life without risk. The American Association for the Advancement of Science (AAAS) has published a number of articles in its journal "Science" on the subject of the general public's misunderstanding of modern day risk assessment. The articles include examples of well-meaning parents who will not allow a nuclear power station to be constructed in their county (they feel it's too risky), but they will drive their child to school without connecting the child's seat belt. They fail to realize that the seat belt risk is a million times greater.

There is risk in surgery. The litigation of doctors seems to be based on the misguided opinion that if we punish them enough hey will not make any mistakes. The net result of this action is physicians leaving certain high risk professions (like Neurosurgery and OB-GYN) because the liability insurance premiums are over 1/2 million dollars per year -- guess who really pays those premiums (we do). One last example: 40,000 people per year die in automobile accidents in the US -- should we give up driving cars; of course not, it is more prudent to design safer cars and highways.

To sum up, there is an almost unmeasurable level of risk in the prudent use of RF Transmission devices. Wireless products are approaching one trillion dollars per year in sales, service, and the establishment of infrastructure. The benefit-to-risk ratio of the use of these products is enormous. Think about a driver stopping at an accident scene in a rural (or urban) area and using a cell phone to summon emergency services. Those precious minutes saved can easily mean the difference between life and death. There are those who believe that the risk of RF exposure from the use of a cell phone is unwarranted. I'm glad that only a small portion of the population holds that belief.

I have had QSO's with Eddie when he was using his 2 meter handy talkie. The RSGB Microwave Handbook, Volume 2, Chapter 11 on Safety, page 11.15, recommends that a handheld radio should be held horizontally while transmitting so that the RF Emissions from the rubber duck antenna are spaced further from your head. Next time I talk to Eddie I'll ask him how he is holding his radio -- Hi.

73 es Good Microwave DX,
Dick, K2RIW.



1.&nbps; At 07:45 AM 2/17/03 -0500, Dave Sublette K4TO wrote:

Good morning,

As long as we are swapping "war stories" I might as well put my hat in the ring. For five and a half years I worked In the Marshall Islands. Some of you may even have worked me as KX6DS. I was employed by the folks who ran the ALTAIR radar site.

ALTAIR is a dual band VHF/UHF radar using a 150 foot dish. The transmitted power on VHF is 10 Megawatts and on UHF 20 Megawatts, peak. This is BEFORE it gets to the 150 foot dish. You make the calculations on ERP.

We had safety limit switches that would disable the transmitters below a certain elevation and within certain azimuth limits that pointed at the living quarters, which were a mile away.

Also on the island was another radar called the millimeter wavelength radar. I don't know the power or frequency of that one, but before it was fired up, a truck with a red light and siren was sent all around the island (The island was about 1 mile in diameter.). Nobody was allowed outside while this radar was on. All of the building on the island were RF shielded.

BTW, the ALTAIR frequencies were 155 and 422 mhz. I was the receiver engineer and my "receiver" consisted of 60 six foot racks of electronics. There were eight feeds coming from the dish, LC, RC, Az, and El for each radar. All signal paths had to be phase matched to within less than a degree.

I am trying to remember what sort of test equipment we used, but I believe my present bench setup with a 22 ghz spectrum analyzer and 18 ghz counter exceeds the capability that I had on the island :-)

Thanks for allowing me to reminisce a bit.


Dave, K4TO

2.  From: "w4rx" on Mon, 17 Feb 2003 11:46:17 -0500

Along the same line, I remember when I was in Trinidad doing some satellite studies piggy backed on the prototype BMEWS tracker there. The prototype scanner ran even higher power into a fixed dish with pipe-organ scanning feed, beaming along the Atlantic missile test range. The width of the scanned beam was marked on all the aviation charts as a restricted zone but was largely ignored by the BWI Airline pilots. That is, until this letter was circulated:


Thereafter, the restricted zone was strictly observed.


Jim W4RX

3.  From: "w4rx" on Mon, 17 Feb 2003 11:46:17 -0500

At 08:49 PM 2/19/03 -0500, KB2VPO wrote:

Hopefully it is not being suggested that the IEEE, or any other legitimate professional organization, is claiming that exposure to radiation below the limits set by the cited standard are without potential hazard. Specifically the citation states only that the IEEE is unaware of any "verified reports": to this effect. Radiation standards such as that forth in the IEEE standard speak to radiation levels known to be harmful (to the IEEE) and as such should be used as guidelines without implication that all other levels are ipso facto "safe". Furthermore, "ionizing radiation" is precisely that and has no specificity to DNA. It is not all clear what damage might be done to DNA or DNA replication as a result of microwave radiation. Whether or not radiation has an ionizing effect is also a function of the material being irradiated. The IEEE standard focuses primarily on the 'thermal effects" of radiation on tissue, there are believed to other effects as well. All exposure to microwave radiation should be minimized to the extent possible and practical if one wants to experience absolutely minimum risk. The only radiation that can be definitely stated as "safe" is radiation of intensity zero. Those citing "war stories" were wise to err on the side of conservatism, if they erred at all A standard in such cases is merely intended to serve as a guideline and as history has shown time and time again subject to change and modification. Who among us is prepared to risk injury to another simply because the IEEE is alleged to have said such exposure was "safe". And that is precisely why the IEEE does not say what is safe only what is believed to be unsafe. Heaven protect us all if the IEEE is to become the final arbiter of what is safe and what isn't . .. "Greatest safety lies in caution ..." Eddie Currie