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Best brand dish to retain shape in tropics

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Apr 9th, 2013 at 1:41am  
I'm not familiar with all the brands of Ku dish reflectors, but....

On checking the shape of 2 reflectors with the 2 strings trick,  I found 2 that are very much out of shape: one is quite similar to my own "Raven Type 122 1.2m Class 1". Incredibly, the tiny ribs on the back of the reflector skin are as tiny where they meet the heavily loaded feet of the 3 feed legs, as where they only have to support the reflector skin in the top area -absurd! The string crossing shows a gap of almost half an inch and the Eb/No is down well over 2 dBs, useless at most times. This dish has been in the tropics around a year n a half only! From brand new.

The other dish is an Andrew 3040942, also 4ft, and the string gap is an amazing 5/8, and none of the modems tested here will lock at all! The backside framing ribs look just as useless as above. In the Tropics for perhaps 4 years!

So my questions to the more experienced folks:

1-Are the similar looking Andrew, ChannelMaster, Raven, RavenSkyware all of the same "wet noodle stiffness" quality?

2-Any dish brand that is designed to DIRECTLY transfer the full load of the feed legs ENTIRELY to the steel mount? With properly engineered beam arms?

3-Are some batches of Raven-style reflectors made in China by shops that replace too much of the fiberglass load by fillers with no strength, in the Polyester mix?

4-Any aftermarket really stiff frame structure that could be retrofitted to the back of an existing "bent" reflector?

5-What do people recommend for tropical use? Buy a pallete-full of reflectors, and replace them yearly?

6-Does the "Class" number refer to a specific BUC etc weight? And written in stone lifetime in tropical sun???


I'd bet there is an endless number of people in low latitudes who have no idea why their Internet is often intermittent!

Thanks for any help.   Christian
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Reply #1 - Apr 9th, 2013 at 2:05pm  
Christian,

Im not a subject expert, but I am not aware of any brand of 1.2meter antenna that has the feed arm connected to the frame. (There may be one out there but Ive never seen one)  I prefer Skyware type 183 design but thats a 1.8meter. The feed arm is securely bolted to the frame (not the reflector) while it does have the support arms from the mounting block to the dish they are designed for ensuring the focal point and reinforcing the dish against wind instead of supporting the feed. I also think the 183 has the best (strongest) mast and EL/AZ adjustment design on the market. All just my opinion but Ive dealt with hundreds of 1.8 and 2.4's an have never had a problem with Skyware (formerly Raven) but Ive seen a few Prodelin's with sheared bolts, and other issues.
As for the Class question - Class III are heavyweight, "industrial use"designs and Class I seem to be "Home user" quality.

Unless someone absolutely needs a small form factor design of a 1.2m (like for building code restrictions) Id always advise to go to a 1.8m Type 183.
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« Last Edit: Apr 10th, 2013 at 1:20pm by N/A »  
 
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Eric Johnston
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Reply #2 - Apr 9th, 2013 at 4:27pm  
Some dish designs, particularly those that are very elliptical, may not be intended to have flat front rim and the crossed string test is not applicable.

Some 'plastic' dishes are suitable only for C band as the embedded metallic material is not of adequate quality for Ku band.

If a dish is distorted, then pull it straight if possible.

Best regards, Eric.
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Reply #3 - Apr 9th, 2013 at 9:10pm  
Thanks for info, Dave: quite useful! Sorry for late reply, but I am on HI time zone.

Did you mean you don't know of any 1.2 dish that does not have CONTINUOUS >metal< from the post pipe to the metal of the feed structure?  Lucky you!

Is there a STANDARD SPOT where I should search for "fine print" ID on dish reflectors? I haven't found much of anything so far.

My RavenSkyware 122 uses the VERY SAME SIZE ribs on the back, that hold a few ounces of skin at the top of the dish, as for holding the 3 foot cantilevered lower leg that is loaded with tons of cast aluminum BUC...   And also to take the pull of the side legs...

Just measured: 22' of plastic to go from the nearest mount bolt to the metal bottom leg foot. And 20 inches of plastic betweeen the mount metal and the feet of the metal side legs.  The metal mount is just tied to a center patch of the plastic reflector with 4 bolts on only a 7" square pattern. The ton of cast alum on this 4 footer dish is cantilevered over 3 horizontal feet away from the center of that 7" square!

The "Raven-like" dish with the 1/2" SE (String Error   Smiley ) is quite similar in insane engineering. 1.5 years from brand new un-packing.

The Andrew 3 040 942 with the 5/8 SE, is 5 years old, has the mount bolt pattern extending DOWN toward the bottom leg by a few more inches (like on pics of Eric's links, where possibly the ribs look taller as well as perhaps containing a higher concentration of glass fibers...), so a bit less stress on the plastic in this case, on that one leg, BUT, once again, the rib size is the same as at the top of the dish.

Eric: if your links show a much older Andrew, would you know if the bottom ribs going near the foot of the bottom leg, could be oversized? (I once had a DirectWay very elliptic dish done that way). Your pics show square-patterned ribs, and the dishes here are all "triangulated" pattern. So may be you have an earlier US-made original version?

I managed to have a peek through the fence, at a recently installed ChannelMaster 6 footer, and sure enough it is all the same, just scaled up, with just 4 bolts tru the plastic, that are quite a bit more spaced of course, but still same tiny ribs all around the dish and zero metal beam that I can see from a distance, from the metal cap mount to any of the 3 metal legs -just feet of plastic.

There is about 13 of these systems on the atoll, all from the same service provider, most 4 footers, most under 2 years old, and only 3 are 6ft.

Dave: where can I find www drawings of the back of your well engineered dishes? And get some ideas about designing a 3 pointed metal beam...   I spent a few hours yesterday but could not find any.


Eric: for various reasons my own Raven-122 is temporarily installed on a palette, in the "upside down" attitude, for minimum footprint, ie spun 180 deg aroung the "lower leg"; we end up with the reflector somewhat flat (around 11 deg from hz, for my 54 sat elev, and I had to drill a rain water drain hole), and the tripod is on the high side of the satellite beam, not below. I also have the whole thing resting, just with gravity help, on just 3 home made adjustable jack screws, located at the reflector PERIMETER.

a-this Raven is also 2 years old and the string test is rather perfect!

b-in a general manner, the 3-legged pyramidal feed support is now well upright, with the ton of cast alum rather pushing downward and evenly on the 3 legs. In my case the 3 jackscrews are near the 3 leg loads, for absolute minimal stresses.

c-the center of the skin, a the 4 mount bolt holes is fully unsupported, so perhaps in a few years the plastic of the dish center will sag down from the well supported rim, and the dish will take a deeper shape -wet noodle style!

d-as the huge cantilever (torque at the mount 4 bolts) is much eliminated, for people using the standard "pole and cap mount", why not remove the 4 center bolts, spin the whole dish 180deg, rebolt, and figure out what correction to apply to the factory elevation scale. If this was done at original install time, it might avoid all distortions whatsoever for a lifetime?

e- in general what are the cons with the upside down attitude? Horn looking more downward and getting a bit more noise from the ground? Or catching LESS noise if installed over a tin roof? Condensation more likely to drain to the horn window, and away from the electronics. In my case I'm unable to see a difference, over concrete floor...

f-another option to completely avoid distortion: built a gantry straight above the CG of the feed assy, rig a couple of pulleys, use a small rope and a carefully adjusted COUNTERWEIGHT!

g- I just can't believe how insane the whole mickey mouse thing is!


RE: if a dish is distorted, pull it straight if possible...

The problem is when we get to an extreme 5/8 in "SE", we really need to know what proportion needs to be "pushed back" into the sides, and how much pulled up from the bottom.

If not, applying the wrong proportions, in first approximation the dish becomes either shallower, or deeper, and the feed horn needs to be moved back or fwd, to the new focal length....

There is also the issue of plastic creep: if constant correcting forces are applied will the plastic keep moving mm's for weeks and months?


In my case I'm thinking of making a glued 4-arms reference jig (same geometry as the 2-strings) from my apparently perfect Raven; and as we are hunting for almost an inch, it will likely be appropriate for most brands of offset 4 footers that are wildly out of shape...


Thanks guys

Christian
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« Last Edit: Apr 9th, 2013 at 11:12pm by N/A »  
 
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Reply #4 - Apr 10th, 2013 at 2:48am  

Eric had said:

..... Some larger dishes have an optional extra part comprising a push rod attached towards the bottom of the main upright mount pole and pointed at the rear end of the feed support arm.  Once everything is set up and pointed, this is adjusted to push the rear end of the feed support arm forwards and reduce the stress on the dish caused by the heavy weight of the BUC/LNB feed assembly......

I had thought abt that also: perhaps a strut approx lined up with the feed lower leg, made of a length of "all thread", perhaps 3/8 or 1/2in; one end stuck inside the rim of the dish very bottom, and the other going through a simple hole in the mast pole, with a couple of nuts for fine adjustment.

The trouble with this is that any adjustment to the pointing necessitates a 3-way elevation adjustment, with the normal elev adjust, with the 2-strings, and with that added strut -back and forth.

A more user friendly set up would be to weld 2 long cheek plates on the sides of the mount (the moving half!), and that would end up together under the foot of the lower leg, with an adjustable little screw jack pushing at the bottom of the lower leg. Adjust per 2-strings only once.

This and the suggested "back beam" that would pull back the side legs would be perfect from an engineering point of view.

But would be better if the usual mount attachment via "4 bolts"  were plainly eliminated, and replaced at the factory with a 3 points structure that accepted DIRECTLY the existing 3 side bolts already holding the 3 legs.

Perhaps if wind load stiffness was a concern, then retain a single bolt toward the center -in place of the "4"... to regain the same wind stiffness as provided by just the "4 bolts" alone.

Would not increase the apparent sales price by much, while saving tons in the upcoming years.

Another 2 cts...

Christian

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Reply #5 - Apr 10th, 2013 at 1:52pm  
Sorry I misspoke (or mistyped!) I meant to say that Ive never seen a 1.2 that HAD the feed arm directly bolted to a metal frame!

Every 1.2 Ive ever dealt with had the feed arm mounted with a single bolt thru the fiberglass at the base of the parabolic.

I will look around and see if I can find a engineers drawing of those type 183 antennas.

Ive also pondered the idea of making a additional support arm using all thread rod to support the feed arm - especially when using larger BUC's.
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Reply #6 - Apr 11th, 2013 at 9:34am  
Quote:
Sorry I misspoke (or mistyped!) I meant to say that Ive never seen a 1.2 that HAD the feed arm directly bolted to a metal frame!

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What is the problem ? Smiley

We have done many installs in the tropics and other extreme (weather) locations and never a dish problem, using standard Channel Master or Prodelin dishes.
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« Last Edit: Apr 12th, 2013 at 12:46pm by N/A »  
 
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