Focal Point of a 90cm dish for LNB Positioning

**checkitout** · 24-01-09, 10:42 PM

Picked Up a 90cm dish yesterday, only thing is that the arm that holds the LNB was bent (really bad, like e.g. someone bent it to squash it into a removalist truck)

I had a go at bending it back to where it looked like it should have been and then tried hooking it up to receive signal e.g. C1 but could not get anything. If I stick my 65cm dish back, I can tune it in 20secs.

Having thought about it again tonight I am suspicious that my "guess" of putting the LNB arm back is likely the problem.

Does someone know how to calculate the location of where the LNB should sit in front the dish - I assume this is the focal point.

**beer4life** · 24-01-09, 11:45 PM

Originally Posted by checkitout

Picked Up a 90cm dish yesterday, only thing is that the arm that holds the LNB was bent (really bad, like e.g. someone bent it to squash it into a removalist truck)

I had a go at bending it back to where it looked like it should have been and then tried hooking it up to receive signal e.g. C1 but could not get anything. If I stick my 65cm dish back, I can tune it in 20secs.

Having thought about it again tonight I am suspicious that my "guess" of putting the LNB arm back is likely the problem.

Does someone know how to calculate the location of where the LNB should sit in front the dish - I assume this is the focal point.

G'Day Cobber,
Is it really worth the bother?
So much depends on the parabolic shape and depth of the dish.
OK, so it's a challenge.
If we could positively identify it, would help. Someone sure to have one exactly the same.
There aren't all that many manufacturers, although there are various models. Even the exact dimensions, round or oval? (High/wide). Depth of dish? Straightedge across rim to centre depth.
Type of LNB holder. Round, square or oblong fitting into arm?
A photo if possible. What does mast mounting bracket look like?
If you can supply this and any other information that you consider relevant, we should have a good chance to help you.

Kindest Regards, " The Druid ".....

**Godzilla** · 25-01-09, 01:01 AM

Post some pics of the front and back, i have a 90 cm here, if its the same one, one of us could take some measurements from reference points and post them for you.

**weirdo** · 25-01-09, 02:10 AM

Bit hard to calculate an offset dish (which I assume it is?)
If the old arm is still in one piece albeit bent, you should be able to measure the length (from bolt hole to bolt hole) and make one up the same.
Doesn't it have two arms so you can copy the good one or are they both bent?

EDIT, just read your post again (sober this time) and see you mention 'arm' (as in singular)
doh

**checkitout** · 25-01-09, 11:35 AM

Pretty sure it is an offset. Also looking into the dish it does have a big "S" logo towards the top of it.

It also has an oval shape, the longer side is the height - The 90cm is the horizontal width.

Yes this is a challenge.
As a 90cm dish it would be great to bring it back to life, only need to find that focal point

Thanks for the replies - I am hoping to have a chance to take a pic and post it today.

**beer4life** · 25-01-09, 12:32 PM

Originally Posted by checkitout

Pretty sure it is an offset. Also looking into the dish it does have a big "S" logo towards the top of it.
It also has an oval shape, the longer side is the height - The 90cm is the horizontal width.
Yes this is a challenge.
As a 90cm dish it would be great to bring it back to life, only need to find that focal point
Thanks for the replies - I am hoping to have a chance to take a pic and post it today.

G'Day again,
Methinks this is the little beauty.....

SRT D90B

• Type: Offset
• Vertical Size: 99 cm
• Horizontal “Diameter” Size: 90 cm
• Reflector Thickness: 0.8 mm
• Dish Material: Steel
• Surface Treatment: Polyester Powder Coating
• Focal Length: 540 mm
• F/D Ratio: 0.6
• Aperture Efficiency: >75 %
• Gain at 12.50GHz: 39.82 dBi
• Mount Type: Az/El, Fixed
• Elevation: 0 - 85 degrees
• Azimuth: 360 degrees
• Operating Temp: -40° C ~ +60° C
• Operating Wind: 90 Kph
• Wind Load - Survival: 180 Kph
• Net Weight: 7.75 Kg.
MADE IN CHINA

Does any one recognise it?
Kindest Regards, " The Druid ".....

.....

**Godzilla** · 25-01-09, 12:48 PM

Yep, with then "S" its a strong as sold by Satech.

Now i have a question...where do you measure to, to find the focal point on a offset dish, the middle or the center of the mounting point which is lower than that?

**Farmsky** · 25-01-09, 01:24 PM

**beer4life** · 25-01-09, 03:09 PM

Many Thanks " Farmsky ",
Now all we need is to establish the relative position of the LNB.
May I suggest the following:
From the throat of the LNB to centre, top rim.
From the throat of the LNB to centre of dish.
From the throat of the LNB to centre, bottom rim.
It is assumed that the LNB would be central to the side rims.

I think that should cover it.
I'll send you the bill on the 29th Proximo.

Kindest Regards, " The Druid ".....

**Godzilla** · 25-01-09, 04:41 PM

I have what looks like the same dish, all be it without the "S", looks like from the same manufacturer, same LNB holder, same square tube for the arm.

From the top inner lip to the center for the lnb holder.

From the inside of the bottom lip to the center of the LNB.

From the center of the area where the 4 bolts are holding the dish to the center of the LNB holder.

**weirdo** · 25-01-09, 04:58 PM

Geez God, you could have cleaned the shit of first.
It's all in the presentation you know

**beer4life** · 25-01-09, 05:24 PM

Many thanks " Godzilla ", I'm sure he will be pleased with the Red Carpet treatment.

Originally Posted by weirdo

Geez God, you could have cleaned the shit of first.
It's all in the presentation you know

Bleedin' nitpicker.

Kindest Regards, " The Druid ".....

**checkitout** · 25-01-09, 07:51 PM

Hey Thanks Guys - especially Godzilla

Please ... I do not care about the bird droppings (almost should say "I do not give a shit".. LOL..... but I do not think this is appropriate especially as members here have been kind enough to go out of their way to help)
The important point here is to see if I can resurrect this thing.

Here are some pics:

Note - The dish is slightly different from others posted here. Notably the LNB arm is aluminium and the dish has lips rolled back rather than forward.

On closer measurement I found that it is actually 85cm wide and 96cm height.

Measurement from the top outer edge to LNB center

Measurement from bottom outer edge to LNB center

Measurement form dish center to LNB

I reckon the LNB arm needs to go down about 3cm towards the ground.
From the comparison with Godzilla's 90cm dish that should be pretty close.

Most definitely there was no chance of reading any signal as it is at the moment. 3cm may not sound like much... however I am sure this should be exact.

Does anyone have this same 85cm dish?

**vk6xlr** · 25-01-09, 08:00 PM

Ah. Looking at your dish, it's one of those foldable arm types made for the caravaners.

**checkitout** · 25-01-09, 08:04 PM

Originally Posted by vk6xlr

Ah. Looking at your dish, it's one of those foldable arm types made for the caravaners.

Yes sounds right - For good measure I have also whacked a few screws in on the arm to the dish to stop it moving around

**mtv** · 25-01-09, 10:14 PM

This doesn't have measurements, but the pic may give you a visual idea of of where the arm should sit.

Cheers

Col

**Watusi** · 14-07-09, 12:06 PM

Hi guys am new member here but being testing fta for awhile now, anyways i came across this thread i found it interesting.I hope you guys can help me out.I pick two use dishes this morning, one's got Sat Radar logo it's 90cm the arm seems to abit short. compared to other one the i pick up, I've never seen one this before am wondering if it will work with say amc4 or 125 or 123w its a 90cm but the arm seems short, could guys advise, thanks in advanced, and sorry if am hijacking this thread i just thought this would be the ideal place to post this, there's a 4 inch difference between the Sat Radar dish and Ariza dish, along with my other eagle star dishes, here some pictures.

**Watusi** · 14-07-09, 12:09 PM

by the way i did do several query's on the net but came up empty handed
your help is greatly appreciated. I was thinking off making the arm i little longer, to measure the same length as the other one.

**LeroyPatrol** · 14-07-09, 12:12 PM

The Sat Radar looks good to me. It looks the same as my 85cm offset. They are two different dishes so things like focal point (arm length etc) will vary between brands. Don't touch or modify a thing just put them up.

Leroy

**Watusi** · 14-07-09, 12:23 PM

Thank you very much for your advise, like i said the arm seems 4 inches shorter compared to other dish. I will give the sat radar a try tommorow to see if can get it going, in the mean time i would like to hear from other members as well.

Originally Posted by LeroyPatrol

The Sat Radar looks good to me. It looks the same as my 85cm offset. They are two different dishes so things like focal point (arm length etc) will vary between brands. Don't touch or modify a thing just put them up.

Leroy