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I had been wanting a better antenna for the SSB portion of 6m for the openings over summer. Having seen how a local amateur built a 2m 11 element YU7EF yagi, I decided to build one for 6m.
The design I chose was a YU7EF EF0606. This seemed to be a popular build with good gain and F/B. The antenna can easily be constructed using a standard 6.5m length of tube which was obtained locally with a short "extension" on the end for the last element. There is a EF0606W1 design Pop has optimised for fitting on a 6m boom (1WL), however I didn't want to waste any material.
Modelling the antenna in MMANA looked as per Pop's design.
I started out by cutting all elements to their specified lengths (within a mm or so). I used 10mm tube, which I chose for extra strength due to the size of the elements. All of Pop's designs are free space calculations, therefore I mounted my elements above the boom with clamps from Pirtek. These are simply attached to the boom with two 1/4 inch roof screws. The reflector clamp sits flush with the back of the boom, therefore when measuring each element I measured and marked the point where the back of the clamp would sit as detailed below.
One half of the Pirtek clamp used for marking out hole positions.
I rather crudely tapped the holes beforehand so that it would make it easier when tightening them down with the element in place. I did this with one of the screws and a socket wrench. I never used a drill to do up the roofing screws.
My method attaching the driven element to the boom. Two pieces of aluminium angle were cut and attached to the boom as pictured. Then two of the Pirtek clamps were screwed on either side to hold the end of each side of the driven element. Above there is an error in the photo, I drilled holes on the main boom at the driven element position without remembering that the dipole is split.
The driven element was cut in half and a plastic spacer inserted. I used silicon sealant to hold the plastic spacer in place until it was mounted to the boom.
Next step was to figure out an effective way to feed the antenna. As the dipole has a balanced feed and we feed it with unbalanced 50 ohm coax, I needed to investigate constructing an appropriate 1:1 balun. This will allow a symmetrical distribution of current through the dipole and avoid the coax from becoming part of the antenna and radiating.
I first looked and constructed a Pawsey Stub. This is basically just a 1/4 wave conductor (can be a piece of wire) calculated in free space shorted to the shield a 1/4 wave down the feedline. This seemed to work, but I had inconsistent results.
I then made a coaxial balun. There is two slight variations of construction on this. I used two pieces of coax, one 1/4 long and the other 3/4 in length (velocity factor IS applicable). This particular design produced reasonable results. Using the MFJ, R measured 58 ohms, and X was 0, or an SWR or 1.16 or a return loss of around 23dB. Considering my test load was two 100 ohm resistors in parallel I thought this was quite reasonable.
The balun attached to the boom. The 3/4 section was folded over on itself to reduce it's physical length to that of the 1/4 section.
Driven element balun attachment.
Return loss looks pretty good. R=54, X=0. Best is around 50.350MHz.
On the tower. Also is a EF0211 2m yagi above the 6m beam.
**Edit** - I did forget to document the process of mounting the boom to a clamp for suitable mounting. In the photo below you can just see how the mounting is done. Get a piece of angle iron (I think the 6m beam length was about 500mm long) and weld (or bolt) a square plate to the middle. I cannot remember the dimensions, but it must be large enough to allow a couple of U bolts. Then hose clamp the angle to the boom as pictured. This means you do not need to drill into the boom and you can use two clamps as opposed to just one. I also use galvanised "Hills" clamps and the brackets with teeth to stop the antenna spinning on the mast.