Fortec 1.2m offset dish focal length discrepancy

[radeohedca]

A local sat supply store has a Fortec 1.2m offset dish that I want to use for Ham Radio moonbounce experiments. When I measured the focal length of the display unit it comes up close to 90cm however the datasheet says it should be around 72 cm. My calculation based on the formula FL=(width ^3)/(16 X depth X height) confirms that it should be around 75cm.

Width = 120cm
Height = 130cm
depth = 11cm

1728000 (120^3) / 22880 (16x11x130) = 75.5cm

FWIW for focal length I measure from the middle of the dish (deepest) to the aperture of the LNB.

I'm sure the dish would still work for sat TV albeit not optimally, but regular consumers wouldn't notice. For my application I need every dB of gain that I can get so I want to make it right.

If the focal length really should be ~90cm then would anyone care to explain the discrepancy ?

Thanks.

 

[qwert1515]

Moved to the FTA section.

Could the discrepancy be because it is an offset dish (LNB is not at the center of the dish)?

EDIT: Well I guess not, it looks like you are using the correct equation for an offset dish:
Focal length = (width ^ 3) / (16 * depth * height)
(Legon's equation for the focal length of an offset dish antenna)

The datasheet for my 1.2 meter dish says the focal lenght should be: 60 cm, when I measured the focal distance of my 1.2 Meter dish it is about 76cm as your calculations suggest.

 

[mc6809e]

I've got a Fortec 1.2m dish aimed at SES2 (87W). The setup has been peaked using a SatHero 200 and my Alien2 has a quality of 85 for LPBHD. I went out to the dish and took some measurements. From the face of the LNB to the point it's aimed at on the dish surface is 86.6cm. I stretched a string across the dish and measured from the face of the LNB to the string. That measurement was 75.8cm. The measurement from the center of the LNB face to the dish surface (near the bottom of the dish) taken parallel to the LNB support arm is 72cm. I've always been under the impression that the focal distance was measured from the surface of the dish at the center to the LNB.

Ernie

 

[FaT Air]

Focal length for an offset dish is measured from the bottom lip to the feed.
Focal length for a prime focus is measured to the center of the dish.
Porabola Calculator 2.0 For offset dishes click the WIFI calculations tab.

 

[mc6809e]

Thanks FaT Air.

I didn't think it was just a coincidence that the measurement taken parallel to the LNB support arm was 720mm (the published focal distance) but I didn't know for sure.

Ernie

 

[FaT Air]

Your welcome.
Watch the LNBF shadow during sun outage and you'll see the top edge of the LNBF cast a shadow on the edge of the lower lip of the (upright*) offset dishes. *LNB support arm on bottom.
A prime focus dishes feed shadow is in the center of the dish during sun outage.
Parabola Calculator 2.0 also includes the measurement to the top lip - to get the feed in the proper focal point. (If you get a dish with no arm(s) so they can be DIY 'fashioned' correctly)

 

[radeohedca]

Thanks FaT Air.
That would explain the discrepancy. Still curious as to why it's measured this way but I will go back to the store and measure from the LNB to the lower lip, I suspect it will come in close to the published FL.