This 2m yagi uses design data from DK72B’s excellent yagi website. The element to boom mounting used here may be of use to some as it does not use any polamid mounting / insulating clamps at a cost of 2 euro each ( this is a 7 el crossed Yagi so twelve clamps would be needed ).
The photographs show how the 12mm dia tubular elements are mounted to but insulated from the boom ( apart from the electrical center of the element where the bolt passes through ).
A 25mm ( the width of the boom ) length of plastic 15mm dia central heating pipe is cut along its axis. A suitable size hole is drilled through the center of the pipe to allow the mounting bolt to pass. The plastic pipe is then slid over the element and the mounting bolt passed through the element / plastic pipe and secured to the boom. End cheeks made from perspex sheet are then slid over the element and screwed to the boom using stainless steeel self tappers.
All of the aluminium used for this antenna came from a disused 6m Yagi and a scrap 2m ZL special. The dipole centers from the 2m / 6m antennas are also utilised here.
The driven element feed point has a impedance of 28 Ohms, so a quater wave transmission line transformer with a characteristic impedance of 37.5 Ohms ( two 75 ohm coax cables in parallel ) is used to transform the driven element feed point to 50 Ohms.
The quater wave matching section is made from Webro WF100 75 Ohm low loss satelite cable. A MFJ-259B antenna analyser was used to determine the physical length of the matching section. Using the above cable the length is 38cm from one end of the outer sheath to the other ( i.e. the coaxial intact length, not including the flying lead ends ).
The matching section connections to the N connector are thoroughly insulated / sealed using self amalgamating rubber tape. Small drain holes are drilled in the dipole center’s lowest point to allow escape of water rather than it going down the coax, over the years I have given up trying to seal such connector housings, numerous methods have been tried and all have eventually failed, a couple of 4mm drain holes saves a lot of grief.
The driven elements were adjusted slightly ( 3mm ) to bring the antennas exactly resonant in the two parts of the 2m meter band, I set mine for 144.3 MHz ( SSB ) and 145.3 ( FM ), my thanks to my ever patient Wife for holding the antenna / mast whilst these adjustments were carried out.
Article by M0DGQ originally available at http://www.m0dgq.co.uk/2m crossed Yagi.html