BATC Forum

I am not wishing to start a debate over the proposed risk assessments required for Amateur use, so please can we keep that out of this thread.

I need to relocate my dish from it's present position, it's not the best thing to look out the patio doors and see my dish standing within 2 metres. So with the advent of better weather ahead, I am looking at where I can place it, so consideration of the new proposals might as well be made at the same time. One seems to require some better method then the proposed Ofcom spreadsheet, but I have used that for safe distances at 2410 MHz.

So I am hoping that within this thread I can get a better understanding of how things may be calculated and presented, which in turn may help others as well as myself.

I have a 1.05 metre dish (actual 1.05 wide 1.15 tall elliptical off set dish) From an on-line calculator the dish has a gain of 27 dB at 2410 MHz 0.7 efficiency, ( compared to an isotropic ).

From a bandwidth on-line calculator the 3 dB beam-width is 7.5 degrees (+/-3.75) and is 9 dB down at 13 degrees beam-width (+/- 6.5). It does not go less than this due to errors getting to be too much and also side-lobes can be a concern.
Using a checked Bird Thru line my r.m.s. power is 33 Watts.

So putting everything into dBm for ease at this point. 33 Watts is +45.2 dBm(i) The direct beam gain would be 27 dB so that would be a power level of 72.2 dBm
Now considering the 9 dB down beam-width as the best I can get from a calculator at +/- 6.5 degrees = 63.2 dBm

72.2 dBm would be equivalent to 16.6 KW Safe distance is 18.39 metres.
63.2 dBm would be equivalent to 2.09 KW Safe distance is 6.53 metres.


So I turned to a simple CAD drawing package to get ideas down into my head.
My centre of the dish is at 1.5 metres from the ground, the elevation to the sat is 23.5 degrees, so I have drawn lines for that and also the 9 dB down beam-width. I have also assumed a person at 1.85 metres tall.

At first I was way off on my ideas, but included here so others do not make the mistake I did. The first idea gave me a safe distance of over 23.5 metres which was more than standing in the direct beam, so I had a rethink: -

In the second half of the image, I am basically saying that if the top of the person is not within the -9 dB beam-width the safe distance is going to be 6.53 metres, and even at that distance I could actually have 1.57 metres clearance from the -9 dB beam to the head of a person, but can not back that up with any calculations.

How to account for 10 minute periods rather than 6 minutes of constant transmit I am not sure.

I would appreciate comments to see if anyone thinks I am on the correct or wrong track with this.

Adrian

references:
https://www.rapidtables.com/convert/pow ... to_dB.html
https://www.rapidtables.com/convert/pow ... _Watt.html
https://www.everythingrf.com/rf-calcula ... tenna-gain
https://www.satsig.net/pointing/antenna ... ulator.htm

Was having a similar conversation last night with Rob M0DTS and he pointed me to this calculator.

VK3UM Electro Magnetic Radiation Calculator (EMRCalc)
http://www.vk5dj.com/doug.html

Haven't played with it much yet but sofar it has put some of my concerns to bed.
There's a print option for you to save a copy to show you've looked at compliance.

Terry

Thanks for the posts Adrian and Terry, we realise a lot of BATC members will be considering this in the next few months.

BATC is working to issue some guidelines, which will be able to be adjusted to apply to most typical QO100 DATV uplinks, but they are not ready just yet.

More information to follow

Dave, G8GKQ

Hello Terry;

Had to go and find a windows PC, but had a quick play with it on both the dish and 14.2 MHz with a wire dipole, and on the dish part not sure about the mast height, can not get that to be below 2.5 Metres which would be above head height anyway, so anything less than this or floor mounted I am uncertain about.

I could not get it to match the Ofcom calc on 14.2 MHz either, so I am possibly driving things wrong on the program, will need to read more.

I think one would still need written words with explanations etc to go with any printed RA, if meant to be understood by a lay-person, (that includes me).

Adrian

Just seen Dave's post, so a quick comment.
I would have expected the BATC and others to be working on this, but as we all got emails from Ofcom recently it has put it into the minds of many, including me. I would guess for many the issue is ground mounted large dishes in gardens/yards etc. I do not have a large area to use, but can maintain 7 metres to a facing boundary, just have to stop the Mr's from walking in front of it

I can confirm the microwave committee has been working on this. Hold fire. We don't have to do this immediately - though to be fair, we should already be considering RF safety whenever we install antennas. Practical advice will appear soon.


Mike

I think moving the dish will be sooner than any deadlines.

Adrian

Hello All,

I put my objections in writing, and to my MP, but OFCOM have (20th May email) implemented compulsory ICNIRP EMC compliance with 6 months to 12 months for me to implement.

I now have run through two scenarios. 2.4 GHz Eshail2 and 70cms. Parabola and Co-linear antenna. Both at least 5 m AGL.

To cut a long story very short the most critical part in any calculation (as opposed to any field measurement) is where the side-lobes of the main transmit 'beam' are and how much reduced in power they are - as this is a huge power 'loss' compared to the main bore.

One dish measurement is -25 dB down [least loss] on the bore-sight for a Parabola. Diamond has published a radiation plot for my co-linear at -14 dB compared to side radiation of the main Torus [least loss, worst case].

This assumes that we/I are not purposefully aiming parabolas anywhere they should not which will be all satellite Tx cases I expect.
And there are no local distortions to a theoretical field pattern.

So what is the worst case side-lobe loss plot (least loss c/w bore) scenario with the now 'standard' G0MJW POTY design and a f/d 0.6 dish?
Maybe two popular sizes? 1 and 2.4 m. A far field measurement perhaps.
This is the only way I can think to simply tackle this problem.

For small half angle deviations this 'loss' is easy to find, but for larger angles, say greater than 6 degrees off bore...

Ofcom claim their measurement accuracy, for compliance assessment, is +/-4dB (95% confidence). But may be worse.

Anyhow you can see that off-beam-lobe power has a far greater effect on Amateur theoretical calculations than simple bore-sight ones based on the dish/antenna intended 'target' beam/s. Small dB losses like the feeder loss or modulation are not significant in this case.

I read the BATC is going to publish some guidance soon. That will be most helpful, I hope.

73, Mike

Hi Mike

One valid approach would be to apply a sidelobe mask - that is a maximum level the sidelobes will not be above. Start with the first null beamwidth and assume within this the gain is the same as boresight. The first null is about 70 * Wavelength/Diameter, measured in degrees from boresight - this is approximately plus or minus 4 degrees for a 2.4m dish and +-8 degrees for a 1.2m dish. Assume everywhere else it is 13 dB down. That is the level of the first sidelobe assuming even illumination. Any decent feed would normally be much better than this. This represents conservative assumption but if the test is passed at that level then nothing more to do.

If it is not then a more complex calculation is going to be needed, but really it should not be needed. We should not be operating so close to the wire that we need to worry about sidelobes containing power 20 times less than that in the mainlobe.

Wikipedia has a good article on this https://en.wikipedia.org/wiki/Side_lobe

I have to disagree with you about the need to comply with ICNIRP as we should already be complying. It has been a legal requirement for years. We can complain about the new dictat from Ofcom and its implementation and some of the really dumb thinking that surrounds the blanket choice of 10W EIRP threshold whatever the frequency, rather than a total power limit. When it comes to microwaves where dish gains over 30 dB are commonplace that might be milliwatts of actual power, but we should not challenge the need to comply with limits. If we persist in complaining we might well find we will lose those privileges. What matters is the implementation is proportionate and the trust that is put in amateurs to transmit safely is not undermined.

Mike

Hi Mike,

Thanks for your thoughts. I am aware of the Amateur EMC requirement (as anyone licenced should be). This stipulation is already present.

My complaint was in the manner of extra enforcement and unproven nature of any harm done. (I am not going to continue).

Yes, the idea of a side lobe mask is definitely the way to go. The problem I had was determining exactly what are the values to use.
Obviously we want something representative / 'accurate' (the "Ball Park" figure) but at the same time a figure that anyone and all can rely on not to cause the limit to be easily exceeded. I.e. biassed towards the least off-bore loss that anyone may encounter.

The -13 dB worst case works well for me (just ran through the field strength calculator check).

Thanks again,

Mike

OK I am going to post a few questions from an uneducated person, me.

Reading through the Wiki link page that Mike posted I have a couple of questions to try and clear up a few things in my mind: -

'For a circular aperture antenna, also having a uniform amplitude distribution, the first sidelobe level is −17.57 dB relative to the peak of the main beam.'

I guess the bit here is the uniform amplitude distribution, would something like -15 dB down on main beam be a reasonable guestimate for a circular antenna to use in the calcs

What efficiency does other put down for the dish, I appreciate it is really just a guess and without knowing more facts and figures that for me, is all it can be. Does anyone know, if you put into the dish feed a known power level at a known SR you should get an X-level on the WB spectrum with a 100% effective dish, so if you get 1.5dB down on this figure you have a 70% effective antenna, 3dB down 50% etc?

Just pondering.

Adrian