I had embarked on a project to build a 925MHz transverter out of junk bits with the aim of building it for as little as possible which meaned designing from the ground up, using easily recreatable construction techniques etc.

Next problem was an antenna, so I set about designing a small one that would fit easily in the boot of the car and be quite portable.

I settled on a 9 element yagi as the calculated gain was nearly 14dBi. Next size up was a 14 element that gave about 16dBi. Only 2 dB in it so decided on the 9 element one.

I often use my old friend Chuck, WA7RAI�s quick yagi program that has been a good steady performer with predictable results, as well as being easy to drive.

In my theme of building this whole project from junk, I searched through my aluminium stocks and found I had just the bits.

Boom was 12.7mm OD tube, all elements bar the DE were 4.7mm ali rod and the DE was some surplus 3mm copper tubing from an air conditioning supplier. I had this left over from building helix feeds for my AO-40 dish setups.

Construction
Start by cutting all the elements to length. The first director D1 has a profound effect on the frequency of the yagi so be accurate on all measurements.

Element	Length (mm)	Dist From Reflector
Ref	166	0
Driven El	149	58
D1	139	104
D2	140	175
D3	135	246
D4	134	342
D5	131	432
D6	132	528
D7	129	618
Folded Dipole Dimensions
Make so the length includes the outer edges of the bend. Bend aroud a 13mm drill shaft to get about a 13.5mm spacing from the inner edges of the tubing.
Used 3mm OD copper tubing for the DE from aircon supplier.
Use 4.3 - 4.7mm Ali rod for the other elements.
Boom Dia 12.7mm Tube.

If you have a drill press, you can put the elements in the chuck and get a nice chamfer on each end which is needed so you can hammer / press fit into the boom.

Be careful doing this, while the elements are short and quite rigid, too much sideways force while spinning at high speed could turn it onto a propeller with a possible chance of injury. You have been warned!!

Fold up the DE as per the dimensions. After testing with the network analyser, we had to shorten it slightly from the modelled figure so the table has the right dimension. Bend around a 13mm former such as a drill bit to get a smooth bend. This tube is annealed so is soft to bend.

The Feedpoint Coax is RG174 and RG58 Teflon.

The open ends for the folded dipole where the balun will be soldered to has a spacing of about 4mm.

Mark the physical centre of the cold side of the folded dipole for easy centring on the boom.

Mark the boom at the dimensions from the table. I start at 0 from the reflector and measure forward. I find this to be more accurate than measuring the individual spacings as there is less cumulative error. Just tape a tape measure to the boom and work from there, making sure you don�t put tape where you are going to mark the boom!

Drill carefully with pilot holes right through the boom, making sure all elements are in the same plane and not crooked in any way. Easy way to do this is with a drill press and a drill press vice holding the boom and have a large piece of supporting wood that rests on the drill press table. Then support the boom, drill the hole and pull through to repeat for all the other holes. Hopefully all holes will be square to the boom and all in the same plane. The vice will slide along the wood to keep it in one plane.

Now for the tricky bit. Drill slightly undersize holes for the rods to be a tight press fit in the holes. The idea is to hammer the elements into the boom for a snug fit. Chamfering the ends of the elements makes this easier.

If the exact drill can�t be found then go carefully with a tapered reamer.

Hammer the elements through so they are equal on both sides of the boom. Hammer them through while supporting the boom over vice jaws and the element will pass between the jaws.

Any elements that are loose can be tightened by using a pointed punch to "peen" the hole so that it grabs the element.

Peening the holes if they are loose. Try not to do it but if you have to, this is how it's done.

Balun construction.

I used a 4:1 balun here, made of RG-174 Teflon coax for the � wave section and RG58 Teflon for the feed cable. Teflon is good as it is heat resistant.

Using 0.66 as the velocity factor. The effective braid length should be 106mm long. The centre conductor is a little longer at each end to be able to connect to the DE.

All braids to be joined together and earthed to the top side of the boom.

I used a small piece of copper with a PK self tapper into the boom. Be sure to use some anti corrosion paste here as we want to reduce the dissimilar metal corrosion effect here. Lanocote is good.

DE mount to the boom.

I used a small strip of copper to solder to the centre of the DE with 2 holes to screw to the boom.

I filed a small channel in the boom to take the dipole element so that it gave rigidity, made room for connection of the balun above and to allow the copper strip, just soldered to the DE to lie flat against the boom.

The accompanying photos should make all this clearer.

Mounting the DE. See how I have recessed the DE into the boom to give me room for the connection above. Make sure you make the groove square to the boom. an't have crooked elements can we?? Two PK screws hold it to the boom.

The Finished Unit..		Return loss about 20dB - it's a bit wriggly due to metal objects around the workshop but at least I know where it at.

Happy constructing!

73�s de Simon.

ZL1SWW