A Cutting Jig for VHF/UHF Yagi Elements

If you want a top-performance VHF/UHF Long Yagi, you will probably have to build it yourself.

It's as easy to build a good design as a poor one - so take the trouble to find a good design!

Here's how to cut the elements accurately to length, quickly and easily.

CONTENTS

1. Introduction
2. Drawing
3. Construction
4. Making Elements

Introduction

The DL6WU-DJ9BV series featured in the main Long Yagi Workshop page are highly recommended for amateur construction. They have been computer-optimized and extensively tested, but they do have the problem that almost all the element lengths are different. For 23cm the lengths generally go in 1mm or 2mm steps, and for 70cm the step is usually 2, 3 or 4mm.

Commercial yagis cost more to manufacture if they have varying element lengths, but for home construction it is very easy to cut elements with an accuracy of ±0.25mm or better, using the simple jig described here.

The jig takes advantage of the fact that an M10 metric screw has a 1.5mm thread pitch, so turning the screw by one 'flat' of the hexagon head advances the screw by exactly 0.25mm. When you have set the jig accurately for one element length, you can simply 'dial' the next length by turning the screw. This avoids need for re-measuring for every element - I was able to cut 168 elements for a 12-yagi array in a single afternoon!

Holes 1 - 6 are all the same diameter, slightly larger than the element material. Cut holes 1 and 3 to make open notches. Hole 7 is drilled 8.5mm and threaded M10x1.5mm.
CONTENTS Top 1. Introduction 2. Drawing 3. Construction 4. Making Elements End, Back	Construction Parts A, B and C are three pieces of heavy steel angle, and the outside face of part C is your cutting guide. There are two separate sets of holes and notches. You will use 1, 3 and 5 first, to make a flat, square end on a long piece of element material. Then use holes 2, 4 and 6 to cut the element to the exact length. Begin construction by marking six drilling centres in parts A, B and C, in the same relative locations on each part (if you are left-handed, mark holes 1, 3 and 5 on the opposite corner). Mark the centre of the end of the M10 screw, which should have 100mm of thread. Drill holes 1-6 all the same size, a little larger than the diameter of your element material; the DL6WU/DJ9BV designs require 4mm rod. Cut out the corners of holes 1 and 3 to make open notches. Drill out hole 7 in part A to 8.5mm and thread it to M10; alternatively, drill the hole to 10mm clear and use two M10 nuts. Mount parts A, B and C in line on a rigid plywood or similar baseboard. Make the board narrow enough to grip it firmly in the bench vice between parts B and C. Use a piece of element material to make sure the parts line up properly. Parts B and C need to be quite close together, to support the end of the element when you are cutting and filing. You may have to provide more than one mounting position for part A, to cover the full length range for 70cm reflectors and directors, and also for 23cm elements. If you make a long version for 2m elements, it may need an additional support in the centre, to keep the elements straight.
CONTENTS Top 1. Introduction 2. Drawing 3. Construction 4. Making Elements End, Back	Making Elements Lay the element in notches 1 and 3, and push the end through hole 5. Hold the element firm by pressing with your thumb between parts B and C, and make the end square and flat by filing against the face of part C. Remove the element. Measure carefully from the flat end, and mark where the other end is to be cut. Double-check this measurement because all the other lengths will depend on it! Insert the flat end of the element through holes 6 and 4, and into hole 2. Adjust the screw until the mark on the element is exactly level with hole 6 when the locknut is tightened. Holding the element steady by pressing between parts B and C, cut and file the element to the correct length. Remove the element and re-check the length with a ruler to make sure it's exactly right. Now you can repeat steps 1 and 3 to cut as many elements as you need of this length. Can you beat 40 seconds per element? Turn the screw to set a new element length. One turn = 1.5mm; one 'flat' = 0.25mm. Continue until all elements have been cut. Optionally, cut an approximately 45° 0.5-1.0mm bevel on the ends of the elements to reduce sensitivity to rain. You can do this very quickly using a pencil-sharpener held at the correct angle. Any half-way experienced engineer could make a much better jig than I did, but the rough-looking prototype is plenty good enough!
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