Dishes and wind
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John Lemay
I have two mesh dishes on my mast. It's not obvious (to me) whether to point
them facing the prevailing wind, away from it, or somewhere else. What's the usual practice ? I'm not keen on luffing the mast because not everything is totally weatherproof when it isn't upright. Thanks John G4ZTR
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John Quarmby
A good question! Pointing the dishes into the wind balances the torque on the rotator, but results in maximum wind load on the stub mast. Setting the dishes so the wind blows across the face of the dish reduces wind loading overall but increases the stress on the rotator. So either might be correct depending on the relative strength of the mast and braking torque of the rotator. Like you I haven't luffed over as that compromises the weather proofing.
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I rather suspect I won't be coming on for tomorrow night's 70cm UKAC, with winds up to 60mph forecast here. 73 John G3XDY
On 13/01/2020 19:46, John Lemay wrote:
I have two mesh dishes on my mast. It's not obvious (to me) whether to point
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Paul G8KFW
Hi john
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As the wind is not constantly in the same direction and you are not there to monitor it I personally would have the wind hitting the back of the dish With the back to the wind the wind can skim over the outside causing less force to the mounting metalwork The problem hear is the method of mounting as the mounting bolts can putt through the dish skin as your dish is a mesh type I would think this is your safest option If you had automatic control you could go for nearly side on but the tenderncy is the dish can act as a wing of an aircraft and push back on the mountings
-----Original Message-----
From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of John Lemay Sent: 13 January 2020 19:46 To: UKMicrowaves@groups.io Subject: [UKMicrowaves] Dishes and wind I have two mesh dishes on my mast. It's not obvious (to me) whether to point them facing the prevailing wind, away from it, or somewhere else. What's the usual practice ? I'm not keen on luffing the mast because not everything is totally weatherproof when it isn't upright. Thanks John G4ZTR ----- No virus found in this message. Checked by AVG - www.avg.com Version: 2016.0.8048 / Virus Database: 4793/15886 - Release Date: 08/14/18 Internal Virus Database is out of date. -- Paul Bicknell G8KFW South Coast UK
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Ralph
Hello John,
I point my dish so it looks down wind . As we get many strorms and strong gales In the SW It has proved fine for me, but Some people may suggest differently hi.
73
Ralph
Sent from Mail for Windows 10
From: John Lemay
Sent: 13 January 2020 19:47 To: UKMicrowaves@groups.io Subject: [UKMicrowaves] Dishes and wind
I have two mesh dishes on my mast. It's not obvious (to me) whether to point them facing the prevailing wind, away from it, or somewhere else.
What's the usual practice ?
I'm not keen on luffing the mast because not everything is totally weatherproof when it isn't upright.
Thanks
John G4ZTR
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Chris Bartram G4DGU <chris@...>
I've recently been working on the design of a new mast system and I want it to support a reasonable dish for terrestrial operation. A lattice tower wouldn't be appropriate here, and I need something a bit better than a beefed-up domestic installation. I also need easy access to the antennas. As I've never been particularly interested in 2.3, 3.4, and 5.6GHz, I plan to use a domestic 1 or 1.2m offset dish with an efficient dual-band 1.3/10GHz feed. A snake yagi would be unlikely to survive for very long here.
Living in Cornwall where it's intrinsically windy, at a location which is also subject to katabatic winds falling off the range of hills on which the GB3MCB beacons sit, I've been looking at the problems from first principles. This seems to be an everyday problem on a par with circuit analysis for structural engineers ... My initial reading of a number of sources does seem to suggest that a worst case calculation of the force developed by the wind on a dish antenna is something which isn't that difficult to perform: the principles behind the building regulations take into account the aerodynamics of the dish in arriving at their figures. One very relevant point - which I have seen made in amateur radio publications, but which doesn't seem to be generally appreciated here - is that mesh dishes aren't that much better in terms of wind loading than than solid dishes of the same diameter at higher windspeeds. The difference seems to equate to a small penalty in dish diameter and the increased efficiency of the solid dish probably counteracts that. I hope these comments are useful: I'll probably understand this a lot better with further reading, so please treat them as provisional! 73 Chris G4DGU
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John Lemay
Firstly, many thanks for the helpful comments yesterday. I went with my gut feeling, and pointed the dishes downwind, and all was well. There's more to come later today, probably worse than yesterday here if the forecast is correct.
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My larger dish (2m diameter) is home designed and home built, so I have plenty of interest in its survival ! Chris - I've read the same web pages as you have by the sound of it, regarding wind loading of mesh dishes v solid dishes. And I'm not convinced. But it occurs to me that we should be considering not two types of construction, but three types. These are solid, perforated material, and mesh. If we compare solid and perforated material I can understand that the difference in wind load is not great; I'm looking at a sample sheet of perforated aluminium and the holes probably represent 30% of the area. But with mesh (like RF HamDesign dishes) the gaps represent 70 to 90% of the area, depending on the mesh option chosen. Nothing is simple, is it ? Regards John G4ZTR
-----Original Message-----
From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of Chris Bartram G4DGU Sent: 14 January 2020 10:50 To: UKMicrowaves@groups.io Subject: Re: [UKMicrowaves] Dishes and wind I've recently been working on the design of a new mast system and I want it to support a reasonable dish for terrestrial operation. A lattice tower wouldn't be appropriate here, and I need something a bit better than a beefed-up domestic installation. I also need easy access to the antennas. As I've never been particularly interested in 2.3, 3.4, and 5.6GHz, I plan to use a domestic 1 or 1.2m offset dish with an efficient dual-band 1.3/10GHz feed. A snake yagi would be unlikely to survive for very long here. Living in Cornwall where it's intrinsically windy, at a location which is also subject to katabatic winds falling off the range of hills on which the GB3MCB beacons sit, I've been looking at the problems from first principles. This seems to be an everyday problem on a par with circuit analysis for structural engineers ... My initial reading of a number of sources does seem to suggest that a worst case calculation of the force developed by the wind on a dish antenna is something which isn't that difficult to perform: the principles behind the building regulations take into account the aerodynamics of the dish in arriving at their figures. One very relevant point - which I have seen made in amateur radio publications, but which doesn't seem to be generally appreciated here - is that mesh dishes aren't that much better in terms of wind loading than than solid dishes of the same diameter at higher windspeeds. The difference seems to equate to a small penalty in dish diameter and the increased efficiency of the solid dish probably counteracts that. I hope these comments are useful: I'll probably understand this a lot better with further reading, so please treat them as provisional! 73 Chris G4DGU
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On mesh dishes, isn't it all about turbulence around the (thin) mesh conductive elements causing a much greater effective thickness? But I don't even try to understand mechanical things, and just quoting summat I read yonks ago mixed with a gut feeling. Andy
On Tue, 14 Jan 2020 at 11:20, John Lemay <john@...> wrote: Firstly, many thanks for the helpful comments yesterday. I went with my gut feeling, and pointed the dishes downwind, and all was well. There's more to come later today, probably worse than yesterday here if the forecast is correct.
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Richard GD8EXI
One of the problems with dishes in windy locations is flutter (vibration in the wind). Standard sky dishes only last about two years here because of flutter induced metal fatigue. As G8DOH has already pointed out a solid dish has a high mechanical Q so is prone to flutter. I suspect mesh dishes have lower mechanical Qs so should do better. This is born out by GD4GNH’s experience with a 1.9m mesh dish at a very windy location.
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To get back to the original question I suggest you park it in the direction, which leads to the least flutter as found experimentally. We had a mean wind speed of 55Knots and gusting over 70Knots here yesterday so my largest Yagis were locked into wind. Richard GD8EXI
On 14/01/2020, 10:50, "Chris Bartram G4DGU" <chris@...> wrote:
> I've recently been working on the design of a new mast system and I want > it to support a reasonable dish for terrestrial operation. A lattice > tower wouldn't be appropriate here, and I need something a bit better > than a beefed-up domestic installation. I also need easy access to the > antennas. As I've never been particularly interested in 2.3, 3.4, and > 5.6GHz, I plan to use a domestic 1 or 1.2m offset dish with an efficient > dual-band 1.3/10GHz feed. A snake yagi would be unlikely to survive for > very long here. > > Living in Cornwall where it's intrinsically windy, at a location which > is also subject to katabatic winds falling off the range of hills on > which the GB3MCB beacons sit, I've been looking at the problems from > first principles. This seems to be an everyday problem on a par with > circuit analysis for structural engineers ... My initial reading of a > number of sources does seem to suggest that a worst case calculation of > the force developed by the wind on a dish antenna is something which > isn't that difficult to perform: the principles behind the building > regulations take into account the aerodynamics of the dish in arriving > at their figures. > > One very relevant point - which I have seen made in amateur radio > publications, but which doesn't seem to be generally appreciated here - > is that mesh dishes aren't that much better in terms of wind loading > than than solid dishes of the same diameter at higher windspeeds. The > difference seems to equate to a small penalty in dish diameter and the > increased efficiency of the solid dish probably counteracts that. > > I hope these comments are useful: I'll probably understand this a lot > better with further reading, so please treat them as provisional! > > 73 > > Chris > > G4DGU > > > >
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Chris Bartram G4DGU <chris@...>
John and Andy:
I understand that something like that is the case. I have a
feeling that turbulence effectively applies an exponential term to
the area term of the equation relating wind speed to wind
pressure. Hence streamlining!
I'd normally run a mile from such mech. things! Bad memories of a wrong turn I once took in my 'education' ...
Richard: I'd not seen any maths or comments on vibration fatigue of dishes, but a quick search does bring-up a couple of papers. I'll see if I can make sense of them later. The Sky dishes here do seem to suffer a bit more from corrosion, but given my location is only a few km the coast, that's unsurprising. My initial response to your comment would be to de-Q the dish. That could possibly be accomplished by the choice of a composite reflector eg. GRP or by use of some form of damping material on the rear of a thin metal dish. The design of the dish bracketry would probably have a significant effect on vibration modes. I'll do some digging ...
73 All. Chris G4DGU
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Conrad, PA5Y
A long time ago when I did my dissertation at Uni I read a JPL paper that described mesh versus solid dish windload results from experiments in a wind tunnel. It was for scaled down model of BIG dishes. The conclusion was that the turbulence above 50 mph around
mesh wires makes the wind load of a mesh dish close to that of a solid dish at wind speeds approaching or above this. I don't remember the exact wind speed at which they became the same but it was certainly in the range we would typically see on an average
British hilltop. I could not find the original paper, there was a very telling accompanying graph, I will find it if I can find my dissertation. Most likely it is in the UK.
This is quite interesting:
73
Conrad PA5Y
From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> on behalf of John Lemay via Groups.Io <john@...>
Sent: 14 January 2020 12:19 To: UKMicrowaves@groups.io <UKMicrowaves@groups.io> Subject: Re: [UKMicrowaves] Dishes and wind Firstly, many thanks for the helpful comments yesterday. I went with my gut feeling, and pointed the dishes downwind, and all was well. There's more to come later today, probably worse than yesterday here if the forecast is correct.
My larger dish (2m diameter) is home designed and home built, so I have plenty of interest in its survival ! Chris - I've read the same web pages as you have by the sound of it, regarding wind loading of mesh dishes v solid dishes. And I'm not convinced. But it occurs to me that we should be considering not two types of construction, but three types. These are solid, perforated material, and mesh. If we compare solid and perforated material I can understand that the difference in wind load is not great; I'm looking at a sample sheet of perforated aluminium and the holes probably represent 30% of the area. But with mesh (like RF HamDesign dishes) the gaps represent 70 to 90% of the area, depending on the mesh option chosen. Nothing is simple, is it ? Regards John G4ZTR -----Original Message----- From: UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of Chris Bartram G4DGU Sent: 14 January 2020 10:50 To: UKMicrowaves@groups.io Subject: Re: [UKMicrowaves] Dishes and wind I've recently been working on the design of a new mast system and I want it to support a reasonable dish for terrestrial operation. A lattice tower wouldn't be appropriate here, and I need something a bit better than a beefed-up domestic installation. I also need easy access to the antennas. As I've never been particularly interested in 2.3, 3.4, and 5.6GHz, I plan to use a domestic 1 or 1.2m offset dish with an efficient dual-band 1.3/10GHz feed. A snake yagi would be unlikely to survive for very long here. Living in Cornwall where it's intrinsically windy, at a location which is also subject to katabatic winds falling off the range of hills on which the GB3MCB beacons sit, I've been looking at the problems from first principles. This seems to be an everyday problem on a par with circuit analysis for structural engineers ... My initial reading of a number of sources does seem to suggest that a worst case calculation of the force developed by the wind on a dish antenna is something which isn't that difficult to perform: the principles behind the building regulations take into account the aerodynamics of the dish in arriving at their figures. One very relevant point - which I have seen made in amateur radio publications, but which doesn't seem to be generally appreciated here - is that mesh dishes aren't that much better in terms of wind loading than than solid dishes of the same diameter at higher windspeeds. The difference seems to equate to a small penalty in dish diameter and the increased efficiency of the solid dish probably counteracts that. I hope these comments are useful: I'll probably understand this a lot better with further reading, so please treat them as provisional! 73 Chris G4DGU
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Living in the second windiest city on the planet after Chicago I park my dishes in the inverted umbrella position, feed pointing directly upwards.
Several years later and still no issues Cheers Keith VK6KB / VK6EME Perth Western Australia.
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Paul G8KFW
Hi Keith
I think that is how Parkes park theirs during high winds
Regards Paul From:
UKMicrowaves@groups.io [mailto:UKMicrowaves@groups.io] On Behalf Of Keith
Living in the second windiest city on the planet after Chicago I park my dishes in the inverted
umbrella position, feed pointing directly upwards. No virus found in this message. -- Paul Bicknell G8KFW South Coast UK
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g4cch_1
My 5.4m EME dish is parked at almost 90deg elevation. The counterweight arms on the back of the dish are strapped to the tower to reduce the twisting motion on the azimuth shaft.
If wind is forecast for any length of time over 40mph, then I luff the dish over like this, and support/brace with scaffold tubes. The feed has to come off unfortunately to avoid rain getting in. 
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Hi Paul, All the VK dishes I know of park like that. But then we don't
have issues with snow either. You can see some of the DSN dishes
in Tidbinbilla parked here... https://www.cdscc.nasa.gov/Images/Gallery/aerial4_fs.jpg And for the Parkes dish in particular, a quote from.... https://www.csiro.au/en/Research/Facilities/ATNF/Parkes-radio-telescope/About-Parkes "Because the large surface of the dish catches the wind like a sail, the telescope must be 'stowed' (pointed directly up) when the wind speed exceeds 35 kilometres an hour." regards Tim On 15/01/2020 2:02 am, Paul G8KFW
wrote:
-- VK2XAX : QF56if : ITU59 : CQ30 : BMARC : WIA
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Paul G8KFW
Hi Tim
The first picture you say from the gallery of aerial 4 but can you come back with the actual site
OK about the snow that is how you achieved a first at 122 Ghz with reduced water vapour at the moment in parts of the UK it is 100 % water
Regarding Parkes telescope and the film 'The Dish' I think there was a bit of licence apparently the pictures coming from Parkes during Apollo 11 where better than the other station hence it was the major receiving station for Apollo 11 and gained extra work due to its better reception from space and is a credit to Australia the designers and those that built it
I think you will find the Parkes telescope has actually not always been stowed at winds above 35 kilometres an hour as during the Apollo 11 mission
any way best of luck with the samples for the 122 GHz project certainly cracked you first wish to go for 50 sets of equipment nearly 10 times that now
Best Regards Paul
From: UKMicrowaves@groups.io
[mailto:UKMicrowaves@groups.io] On Behalf Of Tim
Sent: 14 January 2020 23:56 To: UKMicrowaves@groups.io Subject: Re: [UKMicrowaves] Dishes and wind
Hi Paul, All the VK dishes I know of park like that. But then we don't have issues with snow either. You can see some of the DSN dishes in Tidbinbilla parked here... https://www.cdscc.nasa.gov/Images/Gallery/aerial4_fs.jpg And for the Parkes dish in particular, a quote from.... https://www.csiro.au/en/Research/Facilities/ATNF/Parkes-radio-telescope/About-Parkes "Because the large surface of the dish catches the wind like a sail, the telescope must be 'stowed' (pointed directly up) when the wind speed exceeds 35 kilometres an hour." regards Tim On 15/01/2020 2:02 am, Paul G8KFW wrote:
-- VK2XAX : QF56if : ITU59 : CQ30 : BMARC : WIA
No virus found in this message. -- Paul Bicknell G8KFW South Coast UK
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Paul Randall G3NJV
The original 1960’s Telstar 85ft antenna (Goonhilly 1) stowed on the horizon with “back to wind”. On the other hand, the 32 meter KU band Mitsubishi Goonhilly 5 stowed at zenith - which turned out to be the most common position for all the big antenna. Some small cheap <15m antenna did not have full motion so could not be stowed. Although 100 mph wind gusts were common on the extremely exposed Goonhilly Downs site on the Lizard peninsular in Cornwall, no dish was ever taken from service and stowed. However, it was interesting to watch the active tracking equipment trying to keep pointing errors to a minimum whilst the control building shook in the wind. A lot of the “tracking error” that was being corrected was of course caused by flexing of the steel structures.
I recall one major “off track” event on Goonhilly 2 (now scrapped) that was later attributed to one of the railway-track type “bogies” lifting clear from its track under wind pressure.
My brother managed to slightly bend a steel girder on Goonhilly 1 by trying to move it with the stow lock still engaged. This original dish was based on a naval gun turret, designed by Husband & Co to track the first LEO comsat; it had to spin really fast in azimuth on near overhead passes! It had Ward-Leonard steering motors, something like 750HP. To put this in perspective, the later and larger 1970s Marconi Goonhilly 3 antenna had just 2HP steering motors with 20HP slew motors for fast repositioning.
Happy Daiz Paul G3NJV
Sent from Mail for Windows 10
From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> on behalf of Tim <VK2XAX@...>
Sent: Tuesday, January 14, 2020 11:56:04 PM To: UKMicrowaves@groups.io <UKMicrowaves@groups.io> Subject: Re: [UKMicrowaves] Dishes and wind Hi Paul, All the VK dishes I know of park like that. But then we don't have issues with snow either. You can see some of the DSN dishes in Tidbinbilla parked here... https://www.cdscc.nasa.gov/Images/Gallery/aerial4_fs.jpg And for the Parkes dish in particular, a quote from.... https://www.csiro.au/en/Research/Facilities/ATNF/Parkes-radio-telescope/About-Parkes "Because the large surface of the dish catches the wind like a sail, the telescope must be 'stowed' (pointed directly up) when the wind speed exceeds 35 kilometres an hour." regards Tim On 15/01/2020 2:02 am, Paul G8KFW wrote:
-- VK2XAX : QF56if : ITU59 : CQ30 : BMARC : WIA
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alwyn.seeds1
The same Husband & Co. that did the structural design for Jodrell Bank.
Interesting description of the issues faced in Lovell’s book “The Story of Jodrell Bank”. Regards, Alwyn _____________________________________________________ Alwyn Seeds, Director SynOptika Ltd., 114 Beaufort Street, London, SW3 6BU, England. SynOptika Ltd., Registered in England and Wales: No. 04606737 Registered Office: 114 Beaufort Street, London, SW3 6BU, United Kingdom. _____________________________________________________
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Alan Beard
Taken on Ponape, Federated States of
Melanesia
What happens if the wind blows the wrong way???
Taken in 2010
Alan VK2ZIW
On Wed, 15 Jan 2020 10:24:03 +0000, alwyn.seeds1
wrote
> The same Husband & Co. that did the structural design for Jodrell Bank. > > Interesting description of the issues faced in [UTF-8?]Lovell’s book [UTF-8?]“The Story of Jodrell [UTF-8?]Bank�. > > Regards, > > Alwyn > > _____________________________________________________ > > Alwyn Seeds, Director SynOptika Ltd., 114 Beaufort Street, London, SW3 6BU, England. SynOptika Ltd., Registered in England and Wales: No. 04606737 Registered Office: 114 Beaufort Street, London, SW3 6BU, United Kingdom. > _____________________________________________________ --------------------------------------------------- Alan Beard OpenWebMail 2.53
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Pete - GM4BYF
If the wind blows the wrong way, this is what happens !! Fortunately no damage to anybody or anything - except my pride! See attached 73 Pete GM4BYF On 16/01/20 10:00, Alan Beard wrote:
Taken on Ponape, Federated States of Melanesia --
vry 73 Pete GM4BYF
.JPG)
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Paul Randall G3NJV
I think that picture shows the elevation actuator has been removed.
Paul
From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> on behalf of Alan Beard <beardal@...>
Sent: 16 January 2020 10:00 To: UKMicrowaves@groups.io <UKMicrowaves@groups.io> Subject: Re: [UKMicrowaves] Dishes and wind Taken on Ponape, Federated States of Melanesia
What happens if the wind blows the wrong way???
Taken in 2010
Alan VK2ZIW
On Wed, 15 Jan 2020 10:24:03 +0000, alwyn.seeds1 wrote
> The same Husband & Co. that did the structural design for Jodrell Bank. > > Interesting description of the issues faced in [UTF-8?]Lovell’s book [UTF-8?]“The Story of Jodrell [UTF-8?]Bank�. > > Regards, > > Alwyn > > _____________________________________________________ > > Alwyn Seeds, Director SynOptika Ltd., 114 Beaufort Street, London, SW3 6BU, England. SynOptika Ltd., Registered in England and Wales: No. 04606737 Registered Office: 114 Beaufort Street, London, SW3 6BU, United Kingdom. > _____________________________________________________ --------------------------------------------------- Alan Beard OpenWebMail 2.53
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