Parabolic dish Accuracy testing with lasers

de N6CA 09.2000

Presented at Microwave Update 2001

Using careful construction techniques, a small HeNe or solid state laser can be used for checking the real shape accuracy of any "dish" reflector. Some dishes aren't parabolic and they can't even be used as bird baths with the hole that's usually in the bottom..... The next time you find a dish and wonder how good it really is, you can now check it out....without guessing. Once you have proven your dish is good enough for a particular frequency, all that remains is selecting the right feed to correctly illuminate it. Remember it's very easy to think your dish is working but it can be several db down in gain and still appear as if it's a good antenna.

By orthogonal (perpendicular) mounting the laser to point directly into the dish and translating the laser along a rigid support, one can simulate parallel rays entering the dish. These can be reflected off the back of the dish and the focus movement observed. By translating the laser from the rim of the dish towards the center (in at least two axes) the real upper limit of useable frequency for a particular dish can be measured. Using the laser gives a visible indication of the focus movement as the laser is translated. Some reflectors (dishes) aren't parabolic and therefore the focus will move as the laser is translated. A true parabolic shape will have no movement of the focus. Most available dishes aren't perfect but they can accurate enough for amateur microwave applications.

Many dishes I have tested have serious focus errors near the center but, that typically accounts for less than one half db loss in gain. The remainder of the dish is usually ok for a particular frequency.

Care must be taken to mount and align the laser and its support to the dish. We assume for these tests that the rim of the dish is a uniform reference for mounting the laser support. Good mechanical techniques and construction should be used. I prefer a grocery store check-out laser that's larger in diameter and stays on continuously. Pushing a button on a laser "pointer" makes for unnecessary movement and tighter mechanical tolerances in construction of the fixture that holds the laser.

Lasers won't reflect at all from most dishes, painted, polished or otherwise. I have tried just about every stick-on or flexible reflector known to man and the only readily available one is the ruler(s) made by C-Thru Ruler Company which is available in every stationary store. They are very flexible and inexpensive (<$1) Metal and plastic tape(s) give too much beam divergence which makes the reflected spot too large effectively masking the focus movement. Thickness variations of these rulers are less than .0001 inches so they contribute no measurable errors. They can also be used for the focus point indicating target. They are clear and will show the laser beam when viewed from almost any angle. The tick marks provide a reference point when observing focus movement. Taping both sides of the reflective ruler(s) down to the dish with masking tape will give good reflections for any size of dish.

An eighth wavelength of focus travel is a good number to shoot for. Since this is about 0.061" (1.5mm) at 24 GHz, care must be taken when building and checking any dish measurement components. Many forms of Aluminum extrusions will give straight support structures. Right angles can be easily provided using a "square". Angle, plate and channel Aluminum can be used for the laser holding fixture and rigid support. Lubricating the support with light oil will assist sliding the laser mount.

There are many other considerations when selecting a dish but, if the dish in question doesn't pass this test there is no point in using the dish. If it fails at 24 GHz it may however be perfect for 5.7 GHz use.

Excercise care when using any laser to avoid eye injury.

Dish data plots:

24 inch Antenna Center spun Aluminum

27 inch MaCom Aluminum

36 inch Antenna Center spun Aluminum

36 inch PanaSat steel

48 inch Andrews Aluminum

8 foot Andrews (light duty) Aluminum

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