Electro Magnetic field Exposure, OFCOM & RSGB

[G3PIJpeter]

It's perhaps worth remembering the benefits from the Health and Safety at Work etc. Act (1974). In the year of the Act, 166 construction workers died through on-site accidents - see https://www.theb1m.com/video/health-...visual-history

In 2018-19, the figure was 30 fatal injuries (in a much larger industry)
see https://www.hse.gov.uk/statistics/in...nstruction.pdf Page 11

The problem, of course, is subsequent over-zealousness, backside covering and mis-reporting in the popular press. Where is RF radiation exposure in all this?

- Peter

[Radio Wrangler]

If someone dies on a building site due to a scaffolding collapse, a falling brick or something like that, the cause is clear, visible and understandable by the layman.

With radio stuff, the causal connection, if any, is invisible. It can also be very counter-intuitive to the layman. Rational people want to err on the safe side, but irrational people seem to either outnumber them, or just make more noise. Parents with children are extremely protective and if little Timmy went down with some mysterious lesion, then they can easily assume that the strange house one street over with the funny antenna on the roof is radiating some sort of bad ju-ju.

Look at the irrational panic over the next generation cellphone towers and the assumed link to covid 19?

On the other side of the fence there are vested interests, and that tends to reduce trust in their statements.

So the setting of standards in these areas are founded on a lack of real cases and quantifiable circumstances, an urge to do something, a desire to err on the safe side and the difficulty of holding back a crowd of people with torches and pitchforks clamouring for a witch hunt.

David

[Oldmadham]

Originally Posted by ms660

Must admit I never much saw the point of VHF ham radio. Across the world with 10 watts with a home brew rig then yes, high power squawk boxes, never saw the point in them, but hey ho.

Lawrence.

A couple of thousand km on 2m from tropo ducting in season, access to OSCAR satellites, possibility of EME, no nothing to see here!

That said, most VHF rigs used to be in the 10 watt class, and we did a lot of that stuff!

[Oldmadham]

Originally Posted by trh01uk

Well, lets take a few numbers here. Assume 400W is fed to a yagi antenna with a gain of 10dB. I am thinking here of operating on 2m, so our frequency is 145MHz.

At a distance of 10m from the antenna - and making the perhaps simplistic assumption that the antenna is a point source, the simple equation (30*P)^0.5 / r will give the field strength at distance r in V/m, where P is the radiated power. In this case P = 4000W (due to 10dB gain from the antenna) and lets take r as 10m, which many people would assume to be a pretty safe distance.

The field strength comes out as approx 35V/m. If you look at the attached field from the ICNIRP paper we had posted previously, this Fig.2 is the maximum field strengths allowed averaged over a 30 minute period. At 145MHz, its actually 28V/m approx. So nominally, in this notional situation we are over the limit.

However there are a couple of caveats here....

1. Who is going to transmit steadily for 30 minutes, and stand there in the full beam for that period? Most amateurs would get bored after 2 mins and head off for a cup of tea.....

2. Most yagi antenna as mounted well out of reach, often on a house roof or a mast of some kind. Actually standing in the beam would require some acrobatics.....or a very tall ladder in most practical cases.

So I conclude that while its possible to generate these high field strengths, in practice most amateurs are unlikely to breach the limits, unless they are doing something very odd indeed.

Richard

All good stuff, but as soon as you say "In this case P = 4000W (due to 10dB gain from the antenna) ", your argument falls apart.

Antenna gain doesn't increase the total available power, just redirects it.
400 W into a 10dBi gain antenna produces the same field strength as 4000 W into a isotropic radiator.

If the isotropic radiator is radiating 4000w, it is doing so in all directions, (really, it needs to be in free space to do this, but it is an ideal antenna, so we can assume it is), the field strength it can produce is distributed as a sphere around the radiator.

Unless you are ubiquitous, you cannot occupy all of those positions in space simultaneously, so you also cannot intercept all that energy.
In fact, you intercept the same amount you would from standing in front of our 10 dBi "gain"antenna in your example.(funny, that!)

If we could get real power gain from antennas, why would we need amplifiers?
We could just set up a chain of these wondrous "gain" antennas.

This is not ambitious enough!----Hell, we could solve the world's energy problems!!

As I said before. that 400 watts is all you've got to play with!

[Radio Wrangler]

Another curve ball: Antenna gain and also the 377 (volts per metre)/(ampere per metre) field relationship aren't defined in the near field and the far field is normally considered to be 20 lambda out....

David

[M0AFJ, Tim]

Originally Posted by ms660

Must admit I never much saw the point of VHF ham radio. Across the world with 10 watts with a home brew rig then yes, high power squawk boxes, never saw the point in them, but hey ho.

Lawrence.

Hope this comment was tongue in cheek?, I’m having regular contacts at 4000kms range down to Cape Verdi on both 2 and 70cms, MS contacts 1 to 2000 kms, SpE..
There’s a lot more going on than chatting on a handbag rig

[trh01uk]

Originally Posted by Oldmadham

All good stuff, but as soon as you say "In this case P = 4000W (due to 10dB gain from the antenna) ", your argument falls apart.

Antenna gain doesn't increase the total available power, just redirects it.
400 W into a 10dBi gain antenna produces the same field strength as 4000 W into a isotropic radiator.

If the isotropic radiator is radiating 4000w, it is doing so in all directions, (really, it needs to be in free space to do this, but it is an ideal antenna, so we can assume it is), the field strength it can produce is distributed as a sphere around the radiator.

Unless you are ubiquitous, you cannot occupy all of those positions in space simultaneously, so you also cannot intercept all that energy.
In fact, you intercept the same amount you would from standing in front of our 10 dBi "gain"antenna in your example.(funny, that!)

If we could get real power gain from antennas, why would we need amplifiers?
We could just set up a chain of these wondrous "gain" antennas.

This is not ambitious enough!----Hell, we could solve the world's energy problems!!

As I said before. that 400 watts is all you've got to play with!

Of course, you are quite right that we only ever have 400W here. Sorry - I was being very sloppy with the language!

So the basic equation for field strength, E in V/m, is 30*(P)^0.5/r, where P is the effective radiated power, and r is the measurement distance in metres. If we could make the theoretical "isotropic antenna" (which radiates equally in all directions) then the "effective radiated power" would be equal to the power fed to the antenna up the cable. We can't make such an antenna, so hidden in that "P" term is an allowance for gain of the real antenna. For a dipole that's roughly 1.64 times.

What we are interested here in this discussion is what a human being is actually receiving in terms of field strength. Clearly a human body cannot fill every point on the imaginary sphere around an antenna to absorb all the power being fed to the antenna (leaving aside the possibility that the antenna is embedded in someone's body!). So in the example given, of 400W being fed to an antenna with 10dB gain, if all the 400W fed to the antenna is concentrated into a beam with the area of a human body, its as though 4000W were being fed to an isotropic radiator. Even if there is "only 400W available" - would you really want that much power flowing through your body?

And the important thing here is "what E-field is the person actually getting in their body?" That's what the paper originally posted concentrates on, electric field strength and magnetic field strength - though I note it switches to power density above 2GHz. That's really just concentrating on what's easiest to measure in the real world. Electric field and magnetic field meters exist, though I recall they are devilishly expensive bits of kit, and unlikely to ever arrive in the hands of some radio amateur!


Richard

[trh01uk]

Originally Posted by Radio Wrangler

Another curve ball: Antenna gain and also the 377 (volts per metre)/(ampere per metre) field relationship aren't defined in the near field and the far field is normally considered to be 20 lambda out....

David

David,

I think that's why the original papers are quoting limits on electric field and magnetic field strengths separately - see the graph I posted in post#11. The near field is indeed where the danger is most likely to arise, where of course power density is at its highest.

The difficulty for any radio amateur is actually measuring E-fields or H-fields reliably. My vague recollection is that the E-field meter I bought back in the 1980s for EMC work, cost around £5000 back then. Of course, that wasn't material cost - it looked a fairly simple instrument - the cost is in getting it calibrated across a wide frequency range, which back then was something like 1MHz to 1GHz.


Richard

[Radio Wrangler]

In the near field, power density alone tends to become meaningless and you have to consider E fields and H fields separately.

We used to have an all-plastic robot arm that moved e and H field sensors around to sweep the working volume in our screened room to prove the calibration of applied fields for susceptibility tests. Calibration was a nightmare. There were loopholes too. It was assumed that the pneumatically positioned plastic arm would not affect magnetic fields, but the same assumption was carried through to electric fields. Ooops!


David