ICNIRPcalc

[Dave Dibley]

Hi All,

For those looking at the RSGB and Ofcom calculators and scratching their
heads the IARU has produced an extensive calculator called ICNIRPcalc
available here:


https://www.iaru-r1.org/about-us/committees-and-working-groups/emc-committee-c7/links-to-emc-resources/


73 Dave G4RGK

[jim-G0LHZ]

Hi Dave;

I have been playing with the ICNIRPcalc mentioned in your your message. It is very easy to use.

I have used it to estimate safety distances for a field day 160M inverted V wire dipole - typical CW FD antenna we use every year.

I have noticed that the 'Safety Distance Nearfield Conds' seems to be unchanged with respect to power or modulation type - it is the same for 1 milliwatt or 400 watts - it shows a distance of 26.36 metres.

There was a discussion on a recent RADARC zoom meeting in which it was said that the RSGB  calculator also shows that the nearfield distance is independent of power.

This is a bit counterintuitive as it seems to suggest that you need to keep the public at 26.36 metres away even if you are running 1 mW! 

Any thoughts?

Regards Jim - G0LHZ

[Bill Somerville]

Hi Jim,

you have misunderstood the implications of the extent of (reactive) near-field conditions. The calculator is not capable of determining safe separation distances where near-field conditions are effective. All it does is give the relevant extent of the near field and you should interpret that as:

If members of the general public can reasonably expected to be within the near-field extent of any part or your aerial system; then you must use a different modelling tool or make suitable direct measurements of E- and H-filed strengths to determine the safe separation values.

Note that neither the power level selected, nor the aerial gain in any direction, will have any bearing as it is exactly the effects of them that the tool is not capable of modelling. So only the frequency of operation affects the extent of the near-field.

73
Bill
G4WJS.

[Bill Somerville]

Hi Jim,

with respect to your specific example. You would seem to have the following choices:

1. Limit power to below 10W EIRP, to do that with a typical dipole or similar aerial I guess you would need to assume a gain of something like 2.15 dBi (gain of a dipole in free space with respect to a isotropic source) less any feeder losses, say 0.15 dB for example. I think you can also adjust for modulation and intermittency in a 6 minute period, say 50% for each (typical SSB operation) which allows you 6 dB higher power. So limiting transmitter power to about 25W (10W +4 dB) obviates the requirement to enforce safe distances with such an aerial system.
2. Find a modelling tool that allows E- and H-field strengths to be calculated in the near-field. That will require modelling of the aerial elements, the ground around the aerial including any artificial ground system, and any structures that might impact the E-M fields. This is not a simple exercise, and certainly not in the domain of basic tools like the one in question, or the Ofcom calculator. The RSGB calculator seems to be trying to cross this bridge but I doubt they will be successful. Note that the model will be complex and results will have to be recorded for every accessible location within the near-field extent.
   
3. Make E- and H-field strength measurements around the actual installation to directly determine the safe separation in each and every location within the near-field extent that can reasonably accessed by the general public. Note measurements will have to be of suitable accuracy for anyone who might want to view the records, that could mean that only properly calibrated and rather expensive measuring equipment is required. Although an extra safety factor that can be proven to encompass all measurement errors might be an option, but the results from that may be unpalatable and unworkable in fixed installations in tight spaces.
4. Exclude the general public from any location within the relevant near-field extent of any part of the aerial system. That is 26.36m for a top band aerial. Note for any reasonable 160 aerial system and legal power level the far-field separation distance will be much smaller (probably well under 3m) than the near-field extent and therefore of no relevance.
   

Note that I have not seen any discussion of how variable factors that might affect the near-field E-M field strengths should be taken into account, for example ground conductivity variations across seasons or after extended wet or dry weather.

73
Bill
G4WJS.

[Dave Dibley]

Hi Jim,

I agree some of the figures appear to be somewhat strange, although the
figure for 3cm / 70cm dish look reasonable to me. As I understand it
ICNIRPcalc is a simplified free version of the WATT32 program written by
DF3XZ and DL9KCE for the DARC and available only to members to enable
them to comply with their regulatory body requirements. So I guess the
near field calculation may not be fully functional, but that is just an
assumption, I will try and find out if that is true.

73 Dave G4RGK

On 08/03/2021 11:28, jim-G0LHZ wrote:

> Hi Dave;
>
> I have been playing with the ICNIRPcalc mentioned in your your message.
> It is very easy to use.
>
> I have used it to estimate safety distances for a field day 160M
> inverted V wire dipole - typical CW FD antenna we use every year.
>
> I have noticed that the 'Safety Distance Nearfield Conds' seems to be

> *unchanged* with respect to power or modulation type - it is the same

> for 1 milliwatt or 400 watts - it shows a distance of 26.36 metres.
>
> There was a discussion on a recent RADARC zoom meeting in which it was
> said that the RSGB calculator also shows that the nearfield distance is
> independent of power.
>
> This is a bit counterintuitive as it seems to suggest that you need to

> keep the public at 26.36 metres away even if you are running *1 mW!*
>
> Any thoughts?
>
> *Regards Jim - G0LHZ*

>
> On Sunday, 7 March 2021 at 10:26:41 UTC Dave_G4RGK wrote:
>
> Hi All,
>
> For those looking at the RSGB and Ofcom calculators and scratching
> their
> heads the IARU has produced an extensive calculator called ICNIRPcalc
> available here:
>
>
> https://www.iaru-r1.org/about-us/committees-and-working-groups/emc-committee-c7/links-to-emc-resources/
>
>
>
> 73 Dave G4RGK
>

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[Bill Somerville]

On 08/03/2021 12:34, Bill Somerville wrote:

Hi Jim,

with respect to your specific example. You would seem to have the following choices:

1. Limit power to below 10W EIRP, to do that with a typical dipole or similar aerial I guess you would need to assume a gain of something like 2.15 dBi (gain of a dipole in free space with respect to a isotropic source) less any feeder losses, say 0.15 dB for example. I think you can also adjust for modulation and intermittency in a 6 minute period, say 50% for each (typical SSB operation) which allows you 6 dB higher power. So limiting transmitter power to about 25W (10W +4 dB) obviates the requirement to enforce safe distances with such an aerial system.

Hi again Jim,

the above is probably BS. Because the regulations below 10 MHz use a different human exposure model, which is not based on RF heating, I don't think factors for modulation and intermittency can be applied, so the power limit to a dipole to avoid having to measure or model the system would be 6.1W at the transmitter, plus any compensation for feeder loss, regardless of the modulation mode used or ratio of Rx to Tx.

Thanks to Dave, G6AWF, for pointing out the different recommendations in ICNIRP for frequencies below 10 MHz.

73
Bill
G4WJS.

[Bill Somerville]

Hi Dave and Jim,

quote from the ICNIRPcalc documentation:

"Since the calculations are all done assuming far field conditions, in the pink field the distance to which the near field reaches is given. According to German law measuring or near field calculations have to be used beside the here calculated far field solution, if the near field conditions apply outside the ham operators property."

73
Bill
G4WJS.

[Bill Somerville]

On 08/03/2021 12:34, Bill Somerville wrote:

4. Exclude the general public from any location within the relevant near-field extent of any part of the aerial system. That is 26.36m for a top band aerial. Note for any reasonable 160 aerial system and legal power level the far-field separation distance will be much smaller (probably well under 3m) than the near-field extent and therefore of no relevance.
   

Hi Jim,

I am going to revise this option as well. After some research I am convinced that the most likely reference for the reactive near field extent is the centre of the radiating element. So this option should read:

"4. Exclude the general public from any location within the larger of the relevant near-field extent from the centre of the radiating element (middle of the dipole for a dipole aerial), or the relevant far field separation. That is 26.36m and 1.6m (for 400W digi-mode operation) respectively, for a top band dipole aerial."

This means that in this example, assuming the dipole is not greatly shortened by loading, the exclusion zone could be practically defined as outside of the near field extent (26.36m) from the centre of the radiating element and the physical extent of the aerial plus 1.6m.

I am still uncertain of what the reference point is for far field safe separation values, I have assumed that it is the nearest physical point of the aerial, I may be wrong and it too is the centre of the radiating element (or perhaps the nearest point of the radiating element). If anyone can clarify this it would help my understanding greatly.

As the ICNIRP recommendations are all in terms of E-field strength, H-field strength, and incident power density; there is no mention of distances within them. It is down to the modelling tool implementation as to what the reference is for any safe separation distance. I don't see anything in the ICNIRPcalc documentation that states what the reference is for safe separation distances, which is not helpful!

73
Bill
G4WJS.

[jim-G0LHZ]

Hi Bill and Dave;

Thanks for your comments - fuel for thought.....

My problem is with the near field calculation - if you put 1 mw or even zero as your power into the calculator you still get a near field number of  26.36 metres for a 160M Dipole. So, this suggests that even if you do not transmit (i.e. zero power) - the near field safety distance is still 26.36 metres - that cannot be correct :)

No big deal, I was just curious about the near field number.

Looks like at the RADARC Field Day site (assuming we do a field day eventually after Covid) we will need to mark out the near field distance and keep members of the public outside this distance. Not a problem at CW FD as we usually run 5 Watts (QRP section) so it is under 10 watts ERP but an issue at SSB FD where we run 100 watts.

I suppose we will need to put up a safety warning sign, although that may cause more trouble if it mentions RF Exposure - maybe we just mark out the distance from the mast and put a sign up suggest it is a safe distance should the mast fall over - hi hi.

73s de Jim - G0LHZ

[Bill Somerville]

On 09/03/2021 11:23, jim-G0LHZ wrote:
> My problem is with the near field calculation - if you put 1 mw or
> even zero as your power into the calculator you still get a near field
> number of  26.36 metres for a 160M Dipole. So, this suggests that even
> if you do not transmit (i.e. zero power) - the near field safety
> distance is still 26.36 metres - that cannot be correct :)
>
> No big deal, I was just curious about the near field number.

Hi Jim,

the near field number is not a safe separation distance, it defines an
area where the modelling tool does not give valid results. You can, and
may have to, use it as a safe separation distance in lieu of having a
better tool of measurement. Neither transmitter power nor aerial gain
have any large impact on the near field extent, it is simply something
like wavelength divided by two Pi. The reason for this being valid is
that the reactive near field radiation falls off extremely rapidly with
distance (inverse distance cubed), unlike far field radiation which
falls off only with inverse distance and thus extends to infinite
distance (as we know because that's how radio works!).

73
bill
G4WJS.

[jim-G0LHZ]

Hi Bill;

OK, thanks for the explanation - all understood. 

It does suggest that any of us with dipoles, doublets or long wires transmitting on 80 or 160 metres in a small or average sized garden will probably have a near field extending beyond our property limits. 

Not sure at the moment what OFFCOM will want with respect to the near field.  

Maximum of 10 watts on 80 metres?? - very depressing :(

73s de Jim

[Russell g4zrz]

Actually Jim I don't think it matters what kind of antenna you're using, the near field is similar. The difficulty is in calculating that near field and knowing what level is acceptable.

As for 10 Watts on 80m, it won't happen. There are far too many hip operations, medication changes etc that need to be talked about on there ;-)

Russell

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[Simon Farnsworth]

Hi Jim,

What Ofcom want is paperwork to show that you've assessed the field strength, and that the field strength will be below the limits shown in Table 5 of https://www.icnirp.org/cms/upload/publications/ICNIRPrfgdl2020.pdf (assuming that you transmit for less than 6 minutes continuously, which is fair for all but the most verbose rag-chewers).

For 160m (using a frequency of 1.8 MHz to determine the limits), this is an E field less than 444 V/m, and a H field less than 2.72 A/m.

The difficulty is that the far field is easiest to model - the magnetic and electric fields that form your EM wave are in phase in the far field. In the near field, the relationship between magnetic and electric fields is not simple, because it depends on all sorts of weird and wonderful physics. Incidentally, the near field relationship is what beam antennas of all forms take advantage of to obtain gain - if it were as simple as the far field, we wouldn't be able to build beams.

So, in the far field, I can determine field strengths based solely on EIRP and the impedance of free space; in the near field, I have to account for every object in the area with electrostatic, inductive, capacitive or conductive behaviours, which gets difficult fast, especially since power cables, people, plants, metal fencing, and many other objects fall into that category.

Because modelling is hard in the near field, the spreadsheet gives up. It tells you that the near field is of a certain size, and you'd then need more powerful tools to model the near field and determine field strengths in that area. Or, of course, measurements can be taken instead with calibrated equipment to show that field strength is below the required limits.

Simon

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[Graham Bedwell]

QRO = short overs  or    QRP = long  overs ?

 

Graham G3XYX

Virus-free. www.avg.com

[Vin Robinson]

Hi Simon

After all this hot air about ICNIRO, EMF etc. thanks for a really well written summary, particularly on the near field variables.

I've printed off the very useful table 5 in the pdf and will add it to my licence docs and then forget about the whole pantomime.

Regards

Vin, G4JTR

 

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[Simon Farnsworth]

And when I re-read, I notice a typo.

It's table 6, not table 5, for averaged over 6 minutes, and that's what I did my calculations on. Table 5 is for averaging over 30 minutes of transmission.

Simon

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[Bill Somerville]

Vin,

that table states some of the recommended limits (not necessarily the correct ones as it happens), it says nothing about your installation. You will not be compliant with your licence conditions, if the licence variations being proposed go ahead, by simply recording the recommended limits. You may call it hot air, but that would be a gross insult to those attempting to negotiate how to deal with these changes, the licence variation is coming in some form and taking an Ostrich approach of ignoring the new regulations seems unwise.

73
Bill
G4WJS.

[Simon Farnsworth]

Just knowing the limits is not enough.

You also need to demonstrate in your paperwork that you've either performed professionally acceptable measurements of the field strength that could affect people (both E and H fields), or a high quality model that shows that the measurements will be below the limit if taken.

Ofcom's spreadsheet has such a model embedded in it for the far field, and simply gives up on the near field. The RSGB's experts in the EMF group (volunteers, at that) have spend considerable effort with high quality antenna modelling software to put a front-end on the Ofcom sheet that gives you validation that takes in figures any licence holder should be able to give about their installation (power output, antenna gain) and outputs a guaranteed safe separation distance where the field strength measurement *will* be lower than the legal limits.

Unfortunately, because the near field is so hard to model, the safe guarantee from that spreadsheet becomes huge if you're working at lower frequencies - it simply says that within a certain distance of the aerial, the near field is important and must be considered.

To comply, you either need professional measurements repeated regularly with equipment that has a valid calibration and a way to trace that calibration back if it turns out to be giving wrong numbers, or a model that shows that your setup is within the limits at all times.

Simon

[Simon Farnsworth]

Also, note that this is very much a paperwork exercise (albeit one that's being introduced all over the world - hence the ARRL working with the RSGB on modelling).

Practically, I've only heard of two effects this has:

1. You can be ordered to show the paperwork on demand, just like you can be ordered to show your licence; this will happen at about the same frequency as it does today.

2. When you submit a planning application for a mast or similar, you can include this paperwork to show field strengths, and because it's part of your licence, the planning authority must accept that you won't exceed those limits.

So, it's not all bad for us - the requirement to present paperwork in our licences is rarely called upon (unless someone's complaining about your setup), but planning departments can no longer refuse a mast based on a risk to health from high field strengths.

Simon

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[jim-G0LHZ]

Hi All;

I think Simon and Bill have illuminated the issue exactly (thanks Bill and Simon), in summary:

1) Ofcom require you to demonstrate that your antenna is 'safe' as per the ICNIRP guidelines. 

2) The proposed spreadsheet and other calculations cannot model the near field.

3) Most amateurs will not be able to make a measurement of the near field strength due to equipment cost and calibration.

So, the only way you can demonstrate that your antenna is compliant is to show that members of the public (and your family?) CANNOT be in the near field of your antenna (i.e. the near field safety distance as specified by the calculator).

Most people with small and medium sized gardens will not be able to ensure this on 160M and 80M as the near field extends for 160M is 26.36 metres and for 80M it is13.30 metres. 

The near field on 40M extends to 6.82M which is still a challenge if you have a small garden.

(the above near field distances are taken from IcnirpCalc Version 1.5)

So, in my opinion, it will be 10 watts max power for most of us on 160 and 80, and (for a lot of people) 40M as well - very very sad :(

Sorry about the doom and gloom.

73s de Jim - G0LHZ

[Vin Robinson]

Hi Jim

Thanks for you contribution.

Despite my flippant remarks this morning on this group, I've looked at IcnirpCalc Version 1.5., accepting that for the near field, it depends only on frequency.

I've come up with the same answers  as you for 160, 80 & 40 etc.

I also tried it for  the 137KHz band, answer, several hundred metres, also low mobile phone frequencies 900MHz giving near field distance of 5cm.

Well,  my phone is closer than that to my ear! And on 2M do I have to shout at my mobile handy 1/3 of a metre away to comply?

SO, what I want to know is, what is the actual danger of being in the near field?  What does it do to you? What terrible fate are we guarding against?

Does anyone know this?

Regards, Vin, G4JTR

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[Bill Somerville]

Hi Jim,

yes that seems a fair summary of where things may be heading. You should qualify the "near field" as the "reactive near field", the near field extends further than the inner reactive zone but it seems far field modelling is acceptable there.

Note there is no such thing as "the near field safety distance" in tools like ICNIRPcalc, calling it a safety distance is wrong, it is merely the outer extent of the reactive near field (wavelength over two Pi distance from the centre of the radiating element). It is Ofcom that might enforce it as a safety limit but that is pragmatic since the real safety limit is certain to be less depending on transmitter power, aerial gain, and several other complex factors, except when using sufficient power and aerial gain such that the far field separation distance is greater than the extent of the reactive near field (highly unlikely below 10MHz with only 400W and typical Amateur aerials!).

You should be more specific about limiting to 10W, that should be 10W EIRP which with a dipole means something like 6.1W transmitter output since a dipole has 2.15dBi gain. A caveat here is that some parts of the proposed licence variation state that the licensed maximum power should be used for compliance documentation, not the actual power used. That might mean mere possession of 100W transmitter connected to an aerial would require compliance at that power or even at 400W!! Having said that, I don't think the lower power Amateur Radio licence classes require that transmitters cannot be used if they are capable of exceeding the licensed power limits, if that is correct then it would set a sensible precedent on equipment capabilities vs. licence limits.

73
Bill
G4WJS.

[Simon Farnsworth]

The dangers are the same for near and far field - EM waves cause heating when absorbed by the body. The only reason the near field is more of an issue than the far field is that it's harder to compute.

The figures in the ICNIRP document then tell you that if you exceed this field strength, the heating effect of those EM fields may be high enough to cause damage. This applies to both near and far field; if the field strength is too high, you may get unwanted localised heating, which can cause damage to your cells (by cooking you on the inside.

For any antenna, you can split the EM field into three vector components:

1. The far field, whose components drop in strength proportional to distance, and occurs when electric and magnetic field components are in-phase (i.e. E and H fields have a fixed relationship that we can predict).

2. The radiative near field, whose components drop in strength proportional to distance squared.

3. The reactive near field,  whose components drop in strength proportional to distance cubed.

Once you're far enough from the antenna, the only component with any appreciable contribution to field strength is the far field component - distance squared and distance cubed are so high that the resulting power / drop-off is as close to 0 as makes no difference. We call this the "far field" simply because the only vector that affects field strength measurable is the far field component.

When you get closer, the radiative and reactive near field components are no longer small enough to neglect. We call this the "near field" because you need to pay attention to those components of your E and H vectors, not just the far field components.

So, the danger is exactly the same in near and far fields, it's just that in the near field, we can't predict the EM power that will be applied to your brain as easily as we can in the far field. In the case of mobile phones, manufacturers use a dummy head and measure the field strengths to confirm that the absorbed power (SAR) is low enough to be safe - https://indexsar.com/wp-content/uploads/2014/06/SARA-C-datasheet150812.pdf is a data sheet for the sort of systems they use to check that a phone's SAR is within ICNIRP limits.

Also note that as amateurs, we are allowed to expose ourselves to stronger EM fields; it's only the general public (i.e. those who either have no licence, or who aren't aware that a transmission is ongoing) that we are supposed to protect from EM fields.

Simon

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[Sergei M0TLN]

OFCOM calculator only specifies power density for general public of 2w/m2 as level we must comply with. 

This is (referring to the ICNIRP document in this conversation) on Figure.2 (where chart start from 30MHz from 2020 and was from 10MHz before) and Table 5 for 100kHz-30 MHz says NA.

The same doc in 2nd paragraph after Table 5 explains why we use power density - seems no one can calculate E/H levels in near field.

And therefore all starts from 30MHz.

Do we still need to comply to all 3 values or just to one for HF as OFCOM calc says?

Sergei

[Simon Farnsworth]

Hi Sergei,

It's complicated; hopefully the following explanation will help decode the ICNIRP's dense documents.

The figure we care about is SAR (Specific Absorption Rate of an EM field by a human body); this is in W/kg, and represents the amount of energy a human body will absorb from a given EM field. The SAR limits are set conservatively, as the amount of RF energy you can absorb where the extra heat is completely harmless. Over time, we expect SAR limits to rise, as more studies show what's safe - but the goal is that (unlike asbestos) there will not be a use case for EM fields that is unsafe and yet too useful to stop doing immediately we realise it's unsafe.

Measuring SAR is possible, but expensive - it's done for mobile phones and similar mass market items via a machine like https://indexsar.com/wp-content/uploads/2014/06/SARA-C-datasheet150812.pdf which hams can't afford  (hundreds of thousands of pounds to buy the machinery, tens of thousands a year to keep it in working order, thousands of pounds of consumables per test). This is, however, the gold standard, and the reference we're all trying to meet - if you meet it, then the rest of the limits are irrelevant to you.

Recognising that a full SAR measurement for all uses of EM fields is impractical, we now need to find things that people can do without incomes in the millions per year ballpark (much as I would like to be able to afford to SAR test everything I do, money is a factor around here). The ICNIRP spent a lot of time (in conjunction with World Health Organization experts) looking at biological models of how EM fields and SAR are related. They came up with two much easier measurements you can make where you can't afford the full SAR measurements; if you are below these limits, then SAR *will* be lower than the safe level.

The three limits, *all* of which must be met are:

1. Power density (if above 30 MHz). This is the total power carried in the EM field we're thinking about, and represents an upper limit for heating.

2. Electric field strength (if below 2 GHz). We believe that electric fields above this strength below 2 GHz might be absorbed in ways other than just producing heat, and therefore should be avoided.

3. Magnetic field strength (if below 2 GHz). We believe that magnetic fields above this strength below 2 GHz might be absorbed in ways other than just producing heat, and therefore should be avoided.

So, for people who exclusively use 13 cm and shorter bands, only heating effect is believed relevant based on studies so far - therefore, all you need to care about is power density.

For those of us who use bands between 6 m and 13 cm, we also need to care about the strength of the E and H fields that make up the RF waves we send; this is because there are plausible ways in which the effect of the EM field is more than just heating, and thus we need to avoid high E and H field strengths as *well* as limiting power density.

And for people who consider 6 m too high to be fun radio, and stick to 10 m bands and longer, we reach a point where physics tells us that the heating effect cannot be too high if we comply with the E and H field strengths; thus, there is no longer a power density limit above 30 MHz, just E and H field strengths.

The Ofcom spreadsheet does the following calculations to find a safety distance:

1. Given an EIRP, determine the power density, electric field strength and magnetic field strength as functions of distance in the far field. This is a trivial calculation to perform, as the field strengths related to EIRP via the impedance of free space in the far field.

2. Given a frequency, determine the maximum E and H field strengths permitted, and the maximum power density permitted. If not applicable at this frequency, treat as infinite.

3. Find the distances from the antenna where the E field, H field and power density as computed by the functions in step 1 are below the limits computed in step 2.

4. Take the longest distance from step 3. This is your far field safety distance.

5. Determine the size of the near field. This is the area in which near field effects are large enough to affect the values computed in step 3.

6. Output the larger of step 5 and step 4 as the guaranteed safety distance.

You will note that step 5 does not take anything about your transmission into account - it simply gives up and says "field strengths in the near field are difficult to calculate", and expects you to either avoid the near field, *or* to do the safety checks another way (measurements, better modelling). It's worth noting here that near-field modelling is an active research area - there are people trying to find a good way to accurately model near field effects as seen in reality - but the current state of the art requires you to model not just the antenna itself, but all the objects in the near field. For now, the best way to deal with the near field is to take measurements and demonstrate that you're well under the safe limits.

As Vin has pointed out, though, unless you have access to the kit to take the measurements, and documentation showing that it's been used properly, this is, shall we say, "suboptimal" for HF and MF bands; you either need to keep people a long way from your antenna (28 metres for 160 m), do the measurements, or stop operating on those bands. The RSGB's EMF team (https://rsgb.org/emf) are looking at this, and are hoping that they'll be able to establish "safe by design" antenna setups, where the near field is well-understood and thus they can give you a more usable separation distance - e.g. being able to transmit on 160 m using an end-fed wire that's kept 5 metres above the ground and be inherently safe.

Simon

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[Bill Somerville]

Simon,

wherever the ICNIRP's recommendations reference the near field they qualify it as the reactive near field, I think we must do the same. I assume that they are OK with modelling the near field outside of the reactive near field to determine safe separation distances. This is important as the reactive near field is a relative small inner zone of the whole near field (0.159 x wavelength vs. ≥1 wavelength), small enough that even parts of the aerial may well be outside of it. This distinction seems critical for HF wire aerials in typical Amateur installations!

Whether the likes of the Ofcom calculator, the RSGB models, ICNIRPcalc, and similar tools model the parts of the near field outside of the reactive zone is unclear to me, but clearly they are claiming to give usable safe separation distances in those zones.

73
Bill
G4WJS.

[Gary Searle]

Do I really have to bother with this OFCOM dictate if I only use 10 watts or less on 40 or 80 in the middle of nowhere portable DE G4ELD

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[Simon Farnsworth]

I've just seen that, under pressure from the RSGB, Ofcom's latest draft guidance (attached) says that they do not expect us to do anything about operations below 10 MHz for now - further guidance will come out later once they've done their thinking.

Thank you, RSGB EMF team!

Simon

[Sergei M0TLN]

From 2020 this must be below 30MHZ which is why I do not see where this infinite value came from:

"2. Given a frequency, determine the maximum E and H field strengths permitted, and the maximum power density permitted. If not applicable at this frequency, treat as infinite."

So, if ICNIRP says n/a for below 30MHZ - then this is whole HF band with no exceptions if your EIRP is more then 10W.

Just because I see you used AND+AND which is a combination of all 3 values which I think is not really what the doc says.

[Simon Farnsworth]

Specifically, you can treat "NA" values in the ICNIRP document as infinite, because the ICNIRP says that these values do not need to be taken into account when determining compliance with the ICNIRP safety limits.

So, when ICNIRP says "NA" for below 30 MHz, that means that only the E and H field strengths need to be accounted for, not the power density.

The spreadsheet calculator handles this by using a value that's effectively +infinity for NA values, so that you comply with the ICNIRP limits as documented in https://www.icnirp.org/cms/upload/publications/ICNIRPrfgdl2020.pdf

Note that the Ofcom spreadsheet, based on my experiments, oversimplifies the ICNIRP guidelines, and is a little stricter; the ICNIRP permits more relaxed behaviour in the far field than the Ofcom sheet appears to model for frequencies between 30 MHz and 2 GHz, and has a set of rules for which limits you need to meet (or the basic limits) if you're in the reactive or radiative near field zones.

Simon

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