BATC Forum

Following the excellent CAT 21 talk, Noel and I have been discussing the best way to present the QO-100 EMF information with Ian and Peter. While we have a fair understanding of the area in front of the dish towards the satellite, we need to provide data about the immediate vicinity of the dish. It would help if we had an understanding of the more unusual transmit dish installations.

Please could you post a photo here of your QO-100 dish installation if it is non-conventional? Examples would include dishes on a pole, on a wall of an inhabited building, or on a flat roof above a bedroom. Please state any major concerns you have about EMF on your own installation.

Initial indications are that not many people should have problems, but we want to be sure!

Here is a photo of my "conventional" installation. My only concern is the backspill through the fence to my neighbour. If yours is similar to this, there is no need for you to respond. Thanks

Dave, G8GKQ

My 1.8 meter QO-100 installation.

Martin
G8KOE

My dish in Harwell is well away from the boundary, but cases like Dave's some wire mesh on the fence would probably provide sufficient screening should a problem be discovered.

Bolton is another matter, my dish is 10ft up in the air so no issues in front of it, the place at risk is the shack below but two layers of engineering bricks and double glazed metallised windows are opaque to RF (ask how I know ) so that's OK.

Here is the dish in situ with it's Scandinavian class weather protection Here is the view from said dish towards QO100. So unless the victim is 10ft tall and wearing heals, we should be OK.

Mike

Picture of my 1.2mt dish below.

73
Noel - G8GTZ

My 1.2m dish.
I measured 2.8m from the POTY to the fence. Its 5.7m from the dish face to the other fence in the direction of QO100 and the dish centre is 1.3 above the ground. So its probably OK where it is.
Duncan G7VVF

Dave and Noel, maybe the following information is helpfull with the case study.
In Electron January 2015 PF1F published an excellent article and held a presentation during the VHF-and-above day in 2015 about 'safe' field levels and parabolic dishes.
In fact, used formulas are often fine for far field but not for nearby field.
ITU-R BS.1698 could be used for that (https://www.itu.int/rec/R-REC-BS.1698/en figure 1, page 15).


In the article by PF1F the mentioned example for 0.8 Watt on 24 GHz in a 50 cm dish. "The 'near field' here means: the distance within which the far-field calculations do not apply at all. For our 50cm dish, this zone 1 runs from 0 to 5 m from the parabola. The maximum S here is the aforementioned 9.8 W/m2, and we assume that it is present everywhere in zone 1.
Zone 2 is the transition area from the near to the far field. Here, the power density S decreases approximately linearly with distance. At the boundary of the far field, in the example at a distance of 12 m, S thus comes to 4.1 W/m2.
The ITU standard already starts the far field at 0.6 D2/λ, so here at a distance of 12 m. Using the far-field formula, we find here an S of 4.2 W/m2, which indeed corresponds very well with the result from the transition zone. (We would find an exact match with the factor π2/16 = 0.617, instead of 0.6.)
Once in the far field, i.e. beyond 12 m in our example, S decreases by the square of the distance. The far-field formula works well here.
The general formulas for the power density are:
Zone 1 (near field): S = 16 P / (π D2)
Zone 2 (transition zone): S = 4 P / (π λ R)
Zone 3 (far field): S = π D2 P / (4 λ2 R2)
Here P is the 'net transmit power, so P = ηPTX.
The gain of the dish is already included in the formulas.
Thus, at the beginning of the transition field, approximately S = 5.1 P/D2; at the beginning of the far field approx. S = 2.1 P/D2. If we know in which zone the limit value is exceeded (if that happens), we can use the corresponding formula to determine the safe distance. Some calculations are still necessary!"

Excel calculator that is made to calculate safe distance: The larger the dish, the safer it is

This article and calculator is not applicable for the following situation:
For field strength behind the edge of the dish it totally depends on the spill-over. In the other post -10dB is mentioned but in fact, that is what we would like to have. As can be seen in the recent article in CQ-TV273, the well used Poty had a lot of over-illumination with most used dishes. Just behind the edge of the dish, the Poty could likely be considered as an antenna with a gain of 0 dBi.

Chris PA3CRX

PA3CRX wrote: ↑

Mon Oct 25, 2021 12:08 pm

This article and calculator is not applicable for the following situation:
For field strength behind the edge of the dish it totally depends on the spill-over. In the other post -10dB is mentioned but in fact, that is what we would like to have. As can be seen in the recent article in CQ-TV273, the well used Poty had a lot of over-illumination with most used dishes. Just behind the edge of the dish, the Poty could likely be considered as an antenna with a gain of 0 dBi.

Chris PA3CRX

Chris - this is exactly the situation we don't already have an answer. Hence the study. Your assumption the POTY has 0 dBi gain at the edge of the dish is incorrect. The ideal illumination would by 10 dB down, the POTY has about 9.5 dBi of gain, therefore if the dish was optimised it would indeed be 0 dBi, however because the F/D of most offset dishes is a lot longer than 0.35, there is spill over. This doesn't really matter for efficiency for dishes up to about 0.6 like the Channel Master, it is about -6dB at the edge and you could consider a gain of about 3.5 dBi. For longer dishes (for which the feed was not intended but it works moderately well, the spill over will be higher.

The obvious first test is what if the dish was entirely absent? If a 9.5 dBi horn located at the feed point is still safe behind the dish then there is nothing more to do.

If it is not then the gain needs to be estimated at the edge of the dish. That involves geometry and looking at feed patterns. If you know the F/D you are half way there. If not this might help him https://www.satsig.net/pointing/finding ... -angle.htm - there is a calculator at the end of the page.

The other way is to simply measure the angles and read off the pattern. For example, say the feed arm is 70cm from the dish and the dish is 100cm across then the half angle is roughly 36 degrees. [It's an approximation, probably good enough]. The radiation pattern is approximately like this - someone really ought to measure it you know, it's been 3 years and nobody has -
So at 36 degrees it is only 4 dB down with a gain of ~6 dBi.

With the Channel Master dish the angle is ~40 degrees so 4-5 dBi.

The 10 dB down is at about 55 degrees which would be an F/D of 0.35. That's not the most efficient though*, as well as the energy that hits the dish vs that which misses it there is the illumination efficiency to consider, ideally the illumination is constant across the aperture, but it is not with a simple feed like this. Even though you are losing power past the edge, if the dish illumination is more even you may gain more than you lose. The net result, according to a study by Willi, HB9PZK using the full version of CST is 50-60% efficiency from F/D of 0.4 to 0.75 - which is not great but good enough for such a simple to build feed. Unfortunately Willi deleted his web articles so I can't share a link. A full simulation in CST would be the proper way to do these assessments. Perhaps the RSGB can negotiate with a university with access to do so. It would be a good student project, though probably postgrad rather than undergrad.

*W1GHZ has some excellent notes on this in his antenna book.

Mike

Mike,
I agree with you about the 3.5 dBi, my estimation about 0 dBi was too rough (I considered the gain of such wide feedpattern lower)..

As I understand, the level considered at ‘safe distance’ is for ‘public’ at this frequency is 61V/m (for us ‘worker’ 137V/m). The formula I found is:
d (m) = √ 30 EIRP(W)/ E (V/m)

When we fill out the data for a station with 25 Watts and your 3.5 dBi (= 2.3 times):
√30*25*2.3/61 = 0.68 mtr.
So if the distance of the feed opening to the accessible edge of the dish is less then 68 cm, it is likely the limit is passed?

All of this with a question mark because I am not sure if this all is correct. I am not sure about the (correct use of) the formula and the V/m level. Still a lot to read: https://www.icnirp.org/cms/upload/publi ... dl2020.pdf

If it is (more or less correct), larger dishes are safer because of the larger distance from the feed to the edge.

However, with this kind of power in front of a dish, safety distance should not be ignored. Especially with the dish on ground level the limit is easily exceeded with smaller dishes.

As my dish is already 4m AGL and well screened from passing people I think I'm OK, even when running enough power for 1Ms - only done to see if the amp could.
Any edge over-spill is going to hit the concrete roof tiles, sundry timbers and thermalite bricks. Not that anyone except me goes into the garage below.
About the only RF advantage of my location - a safe line the QO-100

Gareth

Hi Gareth,
that looks like a good solution for anyone with their dish up against their land boundary as the over-spill from over-illumination seems to be heading towards the ground. Mounting, particularly pole mounting, to get good height over a neighbour's fence is obviously more tricky.

Duncan G7VVF