Sunday, 21 March 2021

OFCOM RF Field calculations and some modest openings.

First, a general update:

I have been too busy to be on the radio much. Work, sadly, still has to be done. But...

I have been around often enough to watch the tropo predictions. There was one opening which was predicted and just did not happen. then another which produced good results on 2m. 

The one that did happen produced these maps:- 

144MHz FT8 contacts at GM4FVM 27 February to 3 March 2021
2m:  52 QSOs, 8 DXCC, ODX DL1BAK JO43 775km. 

This tropo opening lasted for several days, as they often do, and just managed to extend into the RSGB 2m FT8 contest, which occurs at the same time as various European data contest. Surprising that, having good conditions for a contest.

432MHz FT8 contacts at GM4FVM 27 February to 3 March 2021
70cm 17 QSOs, 5 DXCC, ODX DL7APV in JO62 1027km

23cm Nothing heard during this event.

Plus I noticed that there was a ionospheric disturbance predicted which resulted in me being ready for a small Es opening early on 13 March on 6m.

50MHz FT8 contacts at GM4FVM 13 March 2021

Just imagine it, successful predictions of a 6m Sporadic E opening and no jet streams involved.

Now, on to the EMF calculations:

For a few months I have been watching developments with the consultations the UK regulator Ofcom, has been having regarding their proposal to apply RF field calculations to radio amateurs (and marine band operators amongst others). These standards already apply to most operators of transmission equipment in the UK, such as at sites where mobile phone towers transmit RF as part of the cellular network.

Phone mast less than 2km from GM4FVM, no doubt ICNIRP compliant (no signal at FVM though)
Strictly, Ofcom proposed to extend the International Commission on Non-Ionising Radiation Protection (ICNIRP) general limits on Electro-Magnetic Field (EMF) exposure to apply to all spectrum users. Which for me means that I have to calculate a horizontal separation distance which, if it falls within the property, will ensure that nobody outside the property will encounter higher levels of EMF than would be wise.

There seemed to be quite a lot of fuss about this proposal in the radio amateur community. The Radio Society of Great Britain responded to the consultation in a way (or so it seemed to me) that this is all fine and good for everybody else, but radio amateurs should not be affected by it. They described complying with the measures as "onerous".

I think we have to understand the power structure here - Ofcom is the boss and they make the rules. Radio amateurs follow the rules. Ofcom, of course, were using "consultation" as meaning "we intend to do this anyway so you can write to us, but you might as well save the wear on your keyboard".

With people making up stories about EMF issues Ofcom say that they have to ensure that everyone toes the line. Quite so.

And anyway, isn't amateur radio liable to be onerous? It is hard to get into this hobby. The exams are difficult and so they should be. Is it really all that bothersome to do a few sums just to check that you are not allowing someone, including yourself and your family, to encounter a high level of EMF energy? I know that the ICNIRP limits are set very much on the side of caution, and it would take many times that level of EMF to come any where near affecting anyone, but still we radio amateurs should comply. In my view.

So a few months ago I decided to have a try to check what my minimum separation distances actually are. I already had done some effective radiated power (ERP) calculations a couple of years ago which I updated. Then I tried to work out what the field strength might be at various places around the FVM Estate Grounds. The idea is that "the general public" should not be exposed to field strengths which might cause them bother ... which sounds like a good idea to me.

In the past I had taken my ERP as being more of a target I should aim for. This is at least in part due to my interest in moonbounce where every last decibel counts. The same argument applies but less so to meteor scatter, and to some extent to my efforts to work DX on 6m.

I started off by trying to update the earlier calculations to fit the model Ofcom were using. For the earlier calculations I had taken my maximum transmitter power, subtracted losses in the coax, added the antenna gain (in dBi) and came up with an ERP figure for each band and antenna.  

For this first attempt at calculating separation distances I took the ERP figures and the antenna heights and put them into a spreadsheet on the Ofcom site. At the time the information available from Ofcom on how to use the spreadsheet was a bit sketchy. It looked initially that I might have to re-organise my antennas a bit to comply. In other words, the horizontal separation distances in a couple of cases extended beyond the property. But as we shall see, my initial calculations were wrong due to lack of information.

To align with the Ofcom proposals I tried to sort out the "mode factor" and the "Tx percentage". This is the equivalent to working out the average power (100% for FM, lower for SSB) and then applying the average percentage of time you would transmit over 6 minutes. This flummoxed me a bit. There are a few windbags on FM who could easily waffle for 6 minutes and get this to 100%. Me, I guess being mostly a data modes operator I would never transmit for more than 50% of the time on any mode in any 6 minute period.

I had not considered that my ERP figure, which I was trying to work out using the ISO standard, was based on gain over an isotropic point radiator. Later I discovered that the RSGB were using dBd gain figures based on gain over a dipole (and hence my 10m dipole records zero gain by their reckoning). So all my calculations were on the wrong side by a factor of almost 2.5dB which, was as if my power was almost double what it really is. I decided to wait for more guidance from the RSGB and Ofcom.

When more guidance came, the RSGB website contained a spreadsheet based on the Ofcom one. This is more amateur-friendly, and I decided to follow it. For the most part it produced figures very similar to my own, taking into account the change from dBi to dBd. And it handily does a calculation relating antenna height to distance on the ground, saving me my "square on the hypotenuse" calculations in my home-brewed version.

My maximum output leaving the shack on any band is 200W or less, and a bit less again at the antenna. First I started thinking about all my lower powered activities, with 4m being about 150W and 23cms 50W. Lately I have decided to back things off a bit on 6m and after a successful spell on 200W with a 5 element beam I and going over to barefoot with a HB9CV. 

These three bands, 23cms, 4m and 6m, looked fairly easy to declare compliant. So too did my 2m and 70cms FM activities. I hardly ever transmit on FM anyway. Nevertheless I decided to do the calculations for that and even for my 4m FM vertical, which is presently in the garage. I know where I would put it up if I decide to listen to a closed squelch again this summer.

Fitting it all into the RSGB calculator showed that the separation I needed to do for all of those bands and antennas is NIL, save for the 10m vertical. In every other case the recommended minimum separation is short enough that the antenna height accounts for it.

I will put a screen grab of one of the calculations in here, but I doubt if you will be able to read it. You can click on the page to enlarge it but I doubt if that will show much detail ...

Separation distance for 6m at GM4FVM using the RSGB calculator.
Things are a bit different for the 2m and 70cms horizontal yagi antennas where the antenna gain is much greater. The ERPs are about 10dB higher than on 6m. With the mast down the antennas are about the same height as the 6m one, but the horizontal separation distances are 10.3 and 9.3 metres respectively.

Luckily, here I have enough space around the mast to provide this separation distance comfortably within our property and leave several metres to spare. It is fortunate I had the earlier calculations in mind when I moved the antennas around because it would have been tricky where they all were before.

I am aware that for some amateurs their calculations are going to produce be uncomfortable reading. For those using masts on gable ends of houses there may be very little separation available to the edge of their property. They may be helped by having more antenna height than me, especially if they live in two story houses.

The Ofcom advice suggests that if it is difficult to comply other measures should be considered including raising the height of the antenna. Grand idea, and it would work, but my neighbours would not be pleased.

For me, I am considering myself fortunate to be able to fit it all in within the boundaries here.





  1. When I looked at just the OFCOM calculator it did not take any account of the gain of the antenna, just the input power of the transmitter. It goes on to say that you should add together transmitters that will be transmitting at the same time.

    No account is taken of losses in coaxial cable, nor any gain for antennas, either in dBd or dBi. and it's only for frequencies abover 10MHz...

    All of my antennas are either on top of my house, and hence well outside the minimum access level, or in my back garden and, again. well outside where anyone can get to them.

    The regulations also relate to 'the public' so not you and I, or even our loved ones...

    It seems like a big fuss over not very much to me.

  2. The other thing that came to my mind. After you've put the power levels into the spreadsheet, calculated the minimum distances, and saved the results. What next? I don't think anyone is going to come round with a tape measure. Unless the people in your local area are aware of the regulations, and let's face it even many Radio Amateurs will not know, then what is going to happen?

    Unless you are unlucky enough to have someone who a) knows and b) can estimate that you are not compliant and c) complains enough so that OFCOM get fed up and come round, then it's all a big waste of hot air.

    Even if your 10MHz dipole is within a few metres of a fence line, it wouldn't take much to lift it up, or move it a bit. Maybe you could put in a planning application saying that you wanted to make sure that your antennas were high enough to ensure public safety?

  3. Hi Dave
    You are right there, I imagine that several local "expert amateurs" round here have never heard of this even though Ofcom has written to them all. I suspect they will not know what it is or how to do it anyway.

    I have done this because I wanted to know what the results were myself. As it happens I did use the earlier information when I moved the antennas around.

    It will soon become a licence condition to have done it. That has not happened yet so it was curiosity that drove me to do it. I then found it was useful afterwards for deciding the antenna positions.

    It looks like you found all the issues with the Ofcom calculator that I did. The current RSGB one is easier to understand than that was and surely even the most limited amateur could use it.

    EMF levels are becoming a public health issue. I think that it is not Ofcom who are pushing this as Ofcom have nothing to gain from it. I think that it is pressure from public health people who want to be able to refute crazy notions about the effects of electromagnetism. Really, baseless Twitter conspiracy theories are to blame for all this.

    Sure, this is probably pointless for most. So are many of the licence conditions. However, as I was capable of doing the calculations it seemed easier to get on with it.

    73 Jim GM4FVM