FCC RF Exposure Rules 2021 and You

What’s New?

The FCC made a rule change effective for May 3, 2021. Nothing about any actual RF exposure rules have changed but the FCC has removed the automatic exemption for amateurs.

What has changed: Amateurs must now determine if they qualify for an exemption. That’s it, folks. That’s all that has changed. If the amateur stations cannot show reasons for an exemption, they must perform a routine environmental evaluation. It’s the same evaluation that has been around for a long, very long time.

Note that there is a 2-year grace period. Amateurs have until May 3, 2023, to perform these evaluations if not exempt.

For reference, the FCC’s full report on what it requires regarding RF exposure may be found here. In addition, the FCC has published a 65-page booklet, Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnet Fields.

The Elephant in the Room

But let’s consider the obvious elephant in the room. This is not an item for enforcement by the FCC. While the rules name an effective date and define a grace period, nobody is gonna’ be check’n noth’n about nobody, no how.

But here’s the thing as this author sees it: Amateurs have traditionally been known to be self-policing, courteous, and most of all…knowledgeable with reference to the physics of communications. People, including communications professionals, listen up when amateurs speak. Being an amateur radio operator is a credential that is universally recognized in our society.

So, what happens when your next-door neighbor questions your station’s safety with regard to harmful emissions? Your next-door neighbor likely understands nothing about electromagnetics. But what he or she does see is an ugly antenna high up in the sky on your tower supposedly pointed at their baby’s bedroom. He or she will not be satisfied by textbook discussions and hand-waving. Wouldn’t it be nice to have a credential to present? Wouldn’t it be nice to be able to say, “My station is in compliance with FCC rules on harmful emissions.”

We are not talking about satisfying a meaningless FCC requirement that will never be enforced. What we are talking about and interested in is understanding why we are safe and being able to present that to someone without hand-waving.

Here’s What’s Up in this Paper

Here’s what this paper will present: We will look at what the FCC is asking of amateurs and then I will walk through my own test, step-by-step, to see if I was exempt. Finding myself non-exempt, I then will move on to evaluate my station.

Here’s the good news: Most licensed amateurs will likely qualify for an exemption. Yours truly, however… did not. I would have qualified except that my Yaesu mobile rig is rated to output 65 Watts which put me over the line. But there is even good news for yours truly. Although I had to do an evaluation, my evaluation showed that I need not perform anything safety-wise beyond that.

An Overview of What I Did

The ARRL has for decades published a couple hundred page document covering all of the ins and outs of this process and more. This document may be found at the ARRL website.

So, what is a peak envelope power (PEP) figure? It is the average power of a single cycle of RF at the modulation peak when the transmitter is being operated normally.

Your task at this point (to be completed by May 3, 2021, but with grace until May 3, 2023) is to see if you are exempt. There are two ways to do this: 1. The easy way; and 2. the hard way.

Always choose the easy way. If you fail the easy way, the hard way may save you. The difference between the two ways is that with the easy way you just simply state the rated power of your transceiver. With the hard way, you start with the rated power but then subtract out losses to arrive at an antenna power output.

It should therefore be no surprise that I started with the easy way. Please refer to the illustration below where I followed the outlined procedure given in the ARRL RF Exposure and You booklet.

Filling out all that personalized info was a waste of time because nobody needs to see this thing.

This evaluation will be for my 2-meter mobile rig. The ham shack will be another matter.

Step 1: The Easy Way

My Yaseu mobile rig is rated for 65 Watts. I compared that with the limits of Table 1.1 where it is shown that the power limitation for all VHS bands is 50 Watts. Check out the illustration below. I do not qualify for being non-exempt. My assumed PEP is 65 Watts and the FCC assigned limit is still 50 Watts.

Step 2: The Hard Way

I, therefore, moved on the hard way. As a starting point, I listed my cable type and length.

The next step was to look up the specified attenuation for the coax I was using. My mobile rig uses a mag-mount with 12 feet of RG-8 cable. Looking at the manufacturer specifications they list attenuation at 50 MHz and 400 MHz leaving any frequencies in between up for interpolation.

Interpolation can be tricky or it can be easy. The easy way is to eyeball it. I just looked at the numerical values of 50 and 400 and with the mind’s eye pictured where 146 MHz would lie. In the illustration, I marked where I thought it would lie. Then, with my mind’s eye again I imagined what numerical value would fall in the spot for attention between 1.3 and 4.1. This leaves a lot of room for slop so I threw out a wild guess saying it was 2.1. I figured that it had to be way less than 2.5 and a little greater than 1.3.

But then I thought, “What the heck. Why not just solve for it.” As an engineer you know how to do those things so I solved for a 2nd degree polynomial equation representing the specified attenuation for this coax valid for frequencies between 50 MHz and 700 MHz. The precise value, using interpolation, turned out to be 2.040432 dB. I decided to just call it 2dB.

Thus, see in the lower illustration we now have the loss media fully defined at 12 feet of RG-8 coax exhibiting 2dB loss for every 100 feet of cable. But wait a minute…I only have 12 feet. We will get there.

At this point, the beauty of using Bel measurements pays off making life so incredibly easy. With 2dB per 100 ft. loss, we, therefore, have a net loss of 0.24dB. It’s just simply 12 divided by 100 times 2dB.

Minus (-) or Plus(+): Which to Use

Here is something supplementary to evaluation but likely of interest. When we talk about “loss” in dB, we need to show it as a plus (+) quantity if it is labeled as a loss. But when plugged into an equation the label is lost and it must become a negative quantity. Admittedly, this is confusing and thus this lengthy discussion.

Let’s discuss it in a slightly different light. We have 0.24dB loss which we humans understand but computers and equations are incredibly stupid. You have to tell them everything. What would happen if you plugged “0.24dB loss” into an equation? A computer will not understand that unless a special provision has been made. But then that will become the ONLY way that the value can be put in. We like to keep things simple so we will keep the equation simple and tell it that our 0.24dB loss is a loss by entering “-0.24.” Often we think of computers as smart but they are not.

But let’s bring up another peripheral subject. Many of us have seen these efficiency log functions as using a coefficient of 20 instead of the 10 that is used here. 20 is used when our in/out radio is a voltage. Voltage is a linear function while power is exponential. Here’s the thing: dB is a logarithmic function. If describing power (as in this case) its coefficient is 10. If voltage then its coefficient is 20 (owing to a squared function).

For a loss function, p_out will always be less than p_in. Therefore, the numerical value given to the log function will always be less than unity. The value of such a number coming out of a log function will always be negative which is indicative of a “loss.” The resulting dB from the equation therefore will ALWAYS be negative. This one is easy to keep track of because the power figures are simply numerical without exponentiation or logarithmic.

I promise you, you will find this informative so bear with me. I’m getting there.

What happens when we algebraically reformulate this equation around to solve for a power out, p_out. If we were talking about a gain then p_out would be greater than p_in. But our immediate reference is with losses owing to coax feed lines. Therefore, p_out will always be less than p_in by simple definition. Because the power out is less than the power in we are talking about losses and our dB figure must be negative. It’s easy to get mixed up on this so don’t feel bad. The professionals get it mixed up as well but at least they can easily see when they have made a mistake. But if you get it wrong at least you will see it right away since your numbers won’t make sense.

So now we move to where the rubber meets the road. 0.24dB equates to a 5.4% loss when dB is translated into linear terms. I’ve embedded the equation within the illustration for solving for percentage given a dB loss but you can also consult tables such as that shown in the illustration. The table lists losses for 0.2dB (4.50%) and 0.3dB (6.67%). You can pretty much estimate what value lies in between for 0.24dB.

When we subtract 5.4% from our transmitter’s specified 65 Watts out we arrive at 3.51 Watts consumed by the coax. That leaves 61.49 Watts for a final value to compare against the FCC table listing 50 Watts as the maximum.

I Failed to Qualify for an Exemption: What Next?

What does this mean? It means that the FCC is requiring me to evaluate my mobile rig station. I have just proven to myself that my mobile rig station is not exempt. I must now evaluate the station for the environment it “lives” within.

Let’s move on to the evaluation which can be illustrated with just one slide. Does that make you happy and give you high hopes. It should because to this point I have been making a big deal out of something that is pretty simple.

A 1-Page Station Evaluation

In the illustration above note that I added much of the same mumbo-jumbo that I had in the earlier form. Doing this is pretty much non-sensical because the FCC does not require you to keep these papers. But it looks good so I’m showing mine here. Also keep in mind that a form like this is good to show your neighbor who wants to know more about your compliance with FCC regulations.

For item D, my mode for my mobile rig is FM. Evaluations for FM stations are a little simplified because they are assumed to be running at 100% whenever the PTT key is down. For item D I put down FM and a pre-defined 100% duty cycle.

I then had to estimate my typical transmission periods for “controlled” and “uncontrolled.” The mag-mount antenna is controlled in that I am always around when it is transmitting and am, therefore “in control” of it. If a neighbor’s toddler decides to crawl up on the roof of my car I will at least know it and not allow my mag-mount antenna to emit RF energy while the baby is licking the antenna.

I estimated that my mag-mount 2-meter antenna will transmit a maximum of 3 minutes given a 6 minute period for a 50% figure in line item H. Likewise, for line item I I also estimated 50%. In line items F through J and K I then took 50% off from my transmitter power to arrive at 30.75 Watts antenna output power. This is the figure that I then compared with the Table 1 limit of 50 Watts. This satisfies my evaluation and there is nothing more that I need do.

I Didn’t Qualify for an Exemption AND I Failed My Evaluation

But, suppose that I DID NOT satisfy my evaluation. I would then be required to make sure that the antenna is arranged such that people and animals won’t injure themselves from my station. That is a subject for another blog post but you can always google some search terms for loads and loads of information on this.