Off-Focus: Offset Dish Focus & Feed Arm Calculator
Find the focus of your offset dish and design a feed arm for your helix, from a few measurements or a single side photo. In your browser, with a printable A4 drawing.
Off-Focus started with a very practical problem. We had a large offset dish lying around, roughly 120 × 130 cm, but no feed arm for it. At the same time, we had built our own helix antenna and wanted to use the dish for receiving L-band weather satellites.
The tricky part was figuring out where exactly the helix should go. With an offset dish, the focus is not where you instinctively expect it to be. It sits off to the side, somewhere in front of the lower part of the reflector, and being just a few centimetres off can already cost you a noticeable amount of signal.
So we started measuring.
We measured the dish, worked out the geometry, tried to understand where the focal point should be and at what angle the helix needed to face the reflector. Then came the next question: how do you actually build a feed arm that holds the antenna in the right place without getting in the way of the signal?
That is how Off-Focus came about.
Try different dish sizes, feed arm shapes and helix positions, or load a side photo of the setup you already have and see how close it is. You cannot really break anything, and moving things around is probably the easiest way to understand what is going on.
The tool lets you enter a few basic dish measurements and shows you where the focus should be, where the helix should sit and how it should be aimed. You can also work from a side photo of your own dish, mark the important points, and see the calculated focus, incoming rays and suggested antenna position directly on the image.
We still have not built the final feed arm, but we did test the important part. We placed the helix where Off-Focus said the focus should be, and the signal was clearly better than before. So at least we know we are looking in the right place.
Hopefully it can save someone else a bit of measuring, guessing and moving antennas around.
What it does
Off-Focus can calculate the focus of an offset dish from just three measurements: width, height and depth. It shows where the helix should sit and how far that point is from the dish edges.
You can also use a side photo of your own setup. Mark the reflector edges and the tool draws the focal point, incoming rays and suggested helix position directly onto the image. The points remain draggable, so you can fine-tune everything by hand.
If you are building a feed arm from scratch, Off-Focus can suggest a straight arm with a bracket or a bent arm, together with the required lengths and angles. You can also ignore the suggestions and drag the arm and antenna around yourself to try different ideas.
Already have something built? Mark the existing feed arm and helix in the photo and the tool will show how far you are from the calculated focus, including how much the antenna should move or tilt.
It also looks for practical problems such as the arm or reflector blocking part of the dish, components colliding, or the helix illuminating the reflector poorly.
For satellite pointing, you can enter your location and the satellite position to calculate azimuth, elevation and dish tilt.
And when you are done, the whole setup can be printed as a single A4 workshop sheet, with the drawing, labels and calculated values, or saved as a PDF.
Off-Focus is available in English and Slovak. Everything runs locally in the browser, so photos you load into the tool stay on your own device.