First, think a bit: 1. What frequency should it be used for? 2. What gain should it have, how long is the boom?3. What kind of material do I have (can I get)? 4. Indoor, portable or outdor use, mechanical properties waterproof, windload.
Preparation
If you know what you want, download this excel sheet (by VK3AUU and VK3AKI), start it and put the values in. As an example I built a short 13 element Yagi using one meter long 15 x 15 mm square Aluminum boom, elements are 4 mm hard aluminum wire.
Elements
Cut elements according to the software results. Accuracy should be within 0.1 mm. Phase the edge at one side of the elements. Drill holes in the boom, first 3.8 mm then 4 mm. Hammer elements thru the boom, phased side ahead. Elements must fit tight and must have and keep good electrical contact to the boom!Reflector
Reflector can be made from the same wire as the elements. To improve f/b I use perforated sheet (see picture below). The reflector is mounted to the boom using small brackets cut from 90° aluminum profile.
Radiator
Radiator is made from 3 mm copper wire. Wire diameter is not critical, large diameter = more broadband, small diameter = easier to bend.
Balun
To match the 200 Ohm (symmetric) radiator to 50 Ohm (unsymmetric) coax a 4:1 Balun is required. It could be made from coax cable cut to one half wavelenght (l) and bent to a loop.Do not squeeze the cable during bending, the diameter ratio of inner and outer conducter must be maintained.
Example: 1278 MHz, UT85 semi-rigid, v = 0.695
Assembly
Simulations with MMANA
I did some simulations with MMANA software. You may download the file here (shift-leftclick)
Simulation seemed to proof what was promised from the Excel spreadsheet. Measurement will be done later.
Measurements
yet to come.....
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