We had a discussion about this on our club net on Tuesday morning ji tried to get onto the web to fill out a form for my station of course I couldn't get it to work so another local ham is going to do it for me. I think its all ridiculous think about it people live and work near high power  transmitters all over the place and there's no ill effect come on ofcom get your act togethet

Are you two a health risk?

In reply to the original question "No". 


In reply to the last one 'I am definitely a health risk" :)


G6IRW

To an extent Ofcom's hand is being forced by exposure legislation drafted by others, that has affected  those of use who work with high power RF and high voltages for a living for several years now. Attempts to kick this into the long grass are running out of steam.

(the basic restrictions identified in Tables 4 and 5 of the 1998 ICNIRP Guidelines. At work we have had the Control of EM fields legislation since 2016)


In terms of the average Ham station, all that is required is a half side of A4 sums that will show that for a full legal limit amateur station (well unless you have the extra power  licence variation for moonbounce experiments or huge dishes or something..  and to be honest then you really should know where you are firing your kilowatts) that the public exposure limits are safely met unless your victims come within a few metres of the antenna metalwork. 


If your antennas are in a private garden, and not running so close to the boundary as to expose the neighbours, that is about  it, and in terms of lower power, if you have a transmitter of 10 watts or less (most hand held) then there is no issue even in a crowd, you'd have to stick the antenna into someone to hurt them more seriously  than a spot burn, which even so is only just about possible with exposed metal on the Tx output.

Relating the primary limits in watts per square metre, or the derived ones in volts per metre or amps per metre may be unfamiliar, but once done a few times, should not be to hard for someone who can install their own antennas and knows how they work.


There is a rather simplified  spreadsheet (by ofcom) that over estimates the safe distances in most cases see here   It does not work as is under Linux but can be made to.


Note that we can still carry on as before putting ourselves and other enthusiasts in the line of fire as we see fit so long as at field days etc we put up some barrier rope, or put the antennas a few metres up and out of casual reach of gen public .

Licensees, installers and users are not required to comply with the ICNIRP general public limits if only the licensee owner, installer or user of radio equipment may be exposed <snip> For example, an amateur radio licensee does not need to comply <with the licence> condition in respect of their own exposure to EMC

(here)


In rather slow time (reply may take a few days if hard  sums are needed at legally binding accuracy.. )  I am quite happy to advise any fellow ham in distress  about this, but I suspect most will have very little to worry about,  other than having to think about where the field maxima actually are  is a bit unfamiliar.


Mike


73 de G7VZY

Mike



I must admit that for 20 years as a MN Radio Officer, I often wondered about whether I was working in a short-wave oven. Most radio rooms were fairly well RF screened - one even had copper wiping strips on the doors, other than the glass windows.  Every ship had the aerial fed from the top of the transmitter via copper tube to an aerial switch, then copper tube to the lead out insulator. Transmitters varied, but for 12 years on the City of Durban/GXIC there was a nice Marconi 1.5 kW transmitter: 410 - 512 kHz, 2, 3, 4, 6, 8, 12, 16, 22 & 25 MHz.  Aerial current could be anything up to 12 amps depending on frequency. The main aerial on that ship being a proprietary unloaded vertical, a Dieckmann & Klapper MAS aerial.     https://www.radiomuseum.org/r/dieck_klap_mastsendeantenne_mas140ma.html

"The description is a vertical tapering fibreglass mast, about 400mm in diameter at the base tapering to the top. At the top of this mast, besides a fibreglass whip aerial was a tubular structure with hexagonal form. From this structure were six aerial wires connected to a ring which was about a metre in diameter. This ring then being tensioned with plastic insulators and bottle-screws to ring-bolts into the base of the fibreglass mast. The overall height being 14 metres." "the rated frequency range was 0.2 - 25 MHz"

Other ships used wire aerials either end or centre/off-centre fed and simple unloaded verticals.


I did also have a 750 watt transceiver primarily for fsk radio-telex - fully synthesised, which I also used for RT calls as it was so easy to use. Key in the channel number, say 1602 (an ITU duplex frequency pair on 16 MHz) and press the talk switch and the ATU on the monkey island would tune the 10 metre whip in less than a second.


So what soft of field those open feeders in the radio room produced I hate to think!


73's

Clive

Interesting putting in figs for mobile radio. My 50W on 2m requires 2.26m separation but I somehow don't see this being an issue at warp speed on the M5!! I suspect this will be another thing which everybody will pay lip service to and absolutely nothing will come of it.

Yes there is no real risk in most cases, all that is needed is folk to think a bit about the more extreme cases.

The other issue is that if nothing is calculated ever, you leave the gates open to folk claiming injury from fields whose strength you cannot even provide an estimate for - even if the answer was they are at a level of no concern at all. It is good to have the numbers to close that sort of argument down before it starts.


Note that it is not made clear but for HF and VHF there is an averaging time to consider - the body can dispose of quite a lot of heat - several hundred watts actually if it is spread over the whole body, by raising heart rate, dilating blood vessels  and sweating, but it is not good to do this for a long time.

In some designs of  MRI scan the whole body RF  dose is about a kW, but only for  a matter of minutes, and there is aircon, so the body starts cool and does not have time to get too hot by the end. The same kW concentrated into say the head only however would be very serious indeed, and  even ten watts, if just into one eye or ear, would also be bad news.


For most Ham radio frequencies and distances of more than a body length, this slow heating  is the dominant problem, so  a short flash of 10 times the limit power for a few seconds followed by 90% of the time off, is also not an issue - the example of the mobile transmitter comes into that category for any other car or pedestrian, unless you are stuck in a traffic jam.

With microwaves the penetration is less, so the energy is all dissipated near the skin - which is bad in that it is concentrated in a small volume, but not so bad in that the skin is much better cooled than say internal organs.


To get a feeling of RF cooking powers, I once had dealings with parts designed for a 27MHz system  where with 40kW -50kW of RF a box of a meter cube or so  was specified as capable of thawing and heating approximately 1 tonne of frozen chickens per hour on a conveyor belt...


Mike.

Funnily my father started as sparks on ships. way back in the middle of the last century, and has tales of measuring the 'meter -amp' product of an antenna to confirm power going out for some inspection or other.

On MF  and the longer HF bands everything is electrically short so really it needs a lot of volts to get a decent current into any open circuit structure,  almost regardless of shape so yes if you were under an open line, you may well have exceeded the modern  H and S limits - did you ever feel particularly hot? Ankles in particular are a 'pinch point' for whole body currents, so aches there tend to be an early warning of high exposure.


There is a test cell for calibrating E field meters that is essentially an open line transmission line of known dimensions in a metal box (here is an example) I suspect your radio room would have accidentally made  a similar structure. Dividing line volts by the floor to ceiling distance in metres  would be a good order of magnitude estimate for the field  experienced sitting under the line.

M.

Yes, the metre-amps calculation.  It was carried at each annual radio survey by an appointed Radio Surveyor, the same bloke who in those days could carry out the 12-wpm Morse Test for an amateur licence. In fact I took my 12-wpm test  at Riversdale College of Technology when the Liverpool Radio Surveyor attended to examine those who were obtaining either a Radio Telephone Certificate of Competency or were revalidating their Morse at either 20 or 25 wpm. 


I digress. I will have to dig our a copy of a Radio Survey form that I have. I copied it because the Radio Surveyor's comments on it were full of praise for me!  The metre-amps related to 500 kHz and were the product of the aerial current at the transmitter output and the height of the aerial in Metres above a "certain point". This "certain point" escapes me, it was related to the load line of the ship, but I cannot remember which one. There was also a correction applied due to the fact that this particular ship - I was on her for 12 years - had a Dieckmann & Klapper MAS aerial. So could not be treated like an end fed or T aerial.


Often felt hot in the Radio Room, but that due to the location of the ship, the effectiveness of the air conditioning and whether the Captain or Chief Engineer felt hot....  On one ship which had a Reddifon Main Transmitter, I was warned on joining to expect shocks when transmitting on 22 MHz  Not nice if you were wearing headphones... It was caused by a break in the earthing copper sheet bonding between the transmitter rack and the steel structure. The transmitter being mounted on shock absorbers both below and at the rear, Once repaired was OK.


My real worry about radiation, was when I was using my mobile phone from inside my cabin, another virtually RF tight box. I had to sit on my bed and get as close to the rectangular window as I could and when passing Cape Town could always get a good signal. However, during the call I could always feel a buzzing adjacent to my ear which I put down to my phone going to full power due the constraints of the shielding.  Solved that with a magnetic base case antenna plugged in to the external antenna socket.


You can see a D&K Antenna on the starboard side of City of Durban/GXIC here https://photos.marinetraffic.com/ais/showphoto.aspx?shipid=924471


Plus a INMARSAT Std-A on the port side. Pity that the funnel was not made out of fibreglass.......


Clive