Syllabus Sections:-

9d RF

9d.1 Recall that the Health Protection Agency (HPA) has published Investigation levels for exposure to RF radiation for UK amateur radio.

Recall that compliance with HPA investigation levels will ensure that exposures are below the recommended limits and that the lowest investigation level for electric field strength is 28V/m (at 10-146MHz).

Understand that if the investigation level is exceeded the cause must be investigated and steps taken to reduce the exposure to below the investigation levels.

You have to be able to recall from memory that :-

• Health Protection Agency (HPA) have published Investigation levels for the maximum recommended exposure levels for electromagnetic radiation generated by UK amateur transmitters.

• that compliance with HPA investigation levels will ensure that exposures are below the recommended limits and that the lowest investigation level for electric field strength is 28V/m (at 10-146MHz)

The lowest investigation level

Note particularly the words the lowest investigation level as it comes into the next section. The lowest investigation level means the level above which investigations must take place - or if you are reducing power then when it drops to the maximum allowable field strength of

28V/m (at 10-146MHz)

a figure you must learn!! Note that the reading is frequency dependent.

If you find that the investigation level is exceeded then the cause must be investigated and steps taken to reduce the exposure to below the investigation levels.

What that means is that you have to be able to firstly calculate what levels of field strength you anticipate taking into account the power of the transmitter and the distance from the radiating elements or stray radiation.

Having done a calculation have some way of checking what levels you are experiencing.

A homebrew field strength meter is usually only a relative strength device as calibration is a little difficult if you do not want to have ground effects altering readings

Few of us have many big antennas, also in amateur radio called an antenna farm, to carry out tests.

It is however your requirement to understand the problems and dangers.

Also consider

Electromagnetic Radiation - Power Flux Density -

 ‘Converting this figure, a Power Flux Density (PFD) to Field Strength (FS) is very difficult when in the near-field of an antenna where capacitive and inductive coupling effects occur alongside the ‘normal’ RF field. The human body reacts to all three modes of coupling energy into the body.

Health England has replaced the Health Protection Agency and provides RF advice for all the UK. Its advice is to follow the International Commission on Non-Ionising Radiation Protection (ICNIRP) guidelines. The ICNIRP are currently re-evaluating their recommendations on safe exposure levels (as at December 2014). The latest advice is on their website http://www.icnirp.org.’

 ‘Similarly a field-day or demonstration station might have 400W to a 6dB gain antenna which would give a field strength of 28 V/m at 10m. Although no longer a formal limit, 28V/m is often used as a guide above which you need to conduct a more careful analysis and take steps to reduce it.

Conveniently that distance, which should be cordoned off to prevent public access, will also provide a safe ‘drop-zone’, should a mast collapse. A taller mast will obviously need a larger zone.’

 ‘It will be noticed that as the distance from the source doubles the illuminated area increases by a factor of four so the Power Flux Density will be a quarter of the previous value.  That effect is termed an Inverse Square Law; the PFD reduces as the square of the distance.’

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