4-Element Yagi Antenna for 70 MHz
Get back to the self made overview...
In April 2017 i got the information, that it's planned to set the 4m Band free again for some months this year. In the last years
i did not really use this frequency range, because i did not have a transverter for it. But as i got this news, i remembered
that my new IC-7300 is able to receive and transmit on 70MHz. I had everything except an adequate antenna.
So i began to study the internet, read Rothammel's antenna book, search the annual lists of CQ/DL and Funkamateur for reasonable
construction proposals. At the end i found an article in Funkamateur magazine, where Martin Steyer / DK7ZB among other antennas
reported about a 4 element yagi for 70MHz in 50 Ohm style. Some informtion about this antenna can be found directly here on Martin's web page.
I was investigating to solve the construction -particularly the mounting of the elements- with POM, that i usually use for such
function. I did not want to pay and wait for delivery of such element holders, when i could also produce them by myself...
The driven element was mounted using a small junction box as protection against weather and humidity. These boxes are not very solid, so
i had to find a way to give the elements more mechanical stability. This problem has been solved by using a POM block, that i prepared
with the milling machine.
After face milling and drilling the necessary holes the POM block is ready for mounting. The junction box is not screwed to the block. In this case the block has just to press the box down on the boom. For the position of the driven element i chose a point in the middle of the junction box, where the
prefabricated feedthrogh points are.
The most important step at this point is the attachment of the driven elements. They must be isolated
with a very distance in the middle. I solved this by drilling an 8mm bore from one side to the other and then use a milling cutter to exactly drill
the desired depth on both sides. I first used the drill to center the holes on the milling machine. That's not very exact, but accurate enough for
our work. Because the milling cutter and the tube have the same diameter, the tube has to be hammered in very carefully until the nose piece at the
end stops it.
Next step is to drill the holes for the contact screws. They have to establish the electrical contact to the RF jack but also to fix the driven
element mechanically. On the picture above you can see this cross slot screw fixing the whole construction on the boom.
For the other side of the boom i u chose a wing nut. The RF Socket is easily fixed with some screws. Two pieces of enameled copper wire are responsible for the electrical contact. To avoid rain coming in at that point, i decided to use some silicone as aqua stop.
Just by the way: The picture above shows how i solved the tuning of the driven elements. I was using the saw to slit in for about 15mm at the center
on both sides. The flashes must be removed with sand paper. This work step also improves the electrical contact to the following tube.
The other elements are mounted using some brackets i also built with the mill. The dimensions for the boom notch shoud be absolutely exact, so that
it clicks in, when being attached. The more it fits, the more stable the elements will stay in their angle. The elements are fixed by a counter sunk screw and a wing nut at the lower end. When drilling the center elements, it's an advantage to also drill only about 2mm with a spot facer at one end. This will give the screw more effective area and less overlapping on the top.
A simple sheet metal screw fixes the lenght of reflector's and director's tubes. Just take care for the right lenght, so that the center really will be central. Concerning the installation on the mast i had several ideas, but at the end i decided to realize the one with less weight. I was using two pieces from the same materia, which i made the boom of. The single steps also have been milled, but with a small molding cutter now. The clamps can be tightened with nuts on two threaded rods. Last but not least: I resommend to use washers on each nut.
Currently this article is not really ready made. I will add some more details - particularly mechanical dimensions and a maerial list - later on.