The Festive Antenna
Mark Foreman offers a fun Christmas project, with a set of challenges to readers. Why not give it a go?
Ican imagine many of you preparing for Christmas, a time to celebrate, rest and reflect. A time to be happy. Some of you may want to switch on the radio and enjoy the festive season with fellow radio amateurs. Last year I brought you the idea of Morse anagrams.
Something Different
I would like to present to you something different this year. It is a festive antenna designed to bring happiness and joy to all. There are several different ways in which you can enjoy this antenna. The prototype was made using a long narrow plank of wood and PVC conduit pipe, which was fixed to the plank with steel brackets and some epoxy glue. Next, the wire was attached to the conduit pipe to create the antenna. In the diagram, Fig. 1, the wood and plastic pipe is shown in bold while some of the wire can be seen as non-bold lines.
Table 1 suggests some dimensions to enable you to build the antenna. As the frequency of operation of an antenna is inversely proportional to the length of the parts of the antenna and as I am unable to know what frequencies you want to use, I hold the view that you are free to scale the antenna up or down to suit your needs.
The second diagram, Fig. 2, is a view of the antenna showing both the wire and the wood/pipes that support it. The wire is in narrow lines while the wood/plastic is in bold. Fig. 3 is a drawing of the antenna without the branches made of plastic pipe. The shape of the wire is easier to see.
What you do is to start at the top of the tree and fix the wire to point C, next bend the wire so it goes back along the pipe BC until it is at a point half-way between B and C. Now from this half-way point the wire goes to point E. We continue in this way to the bottom branch of the tree. From halfway along branch JK we bring the wire to the very bottom of our tree.
After building your antenna you have almost complete freedom as to how you orientate it and feed it. You can place it above a ground plane and then connect both wires at L together and use it as a Marconi type antenna. You can connect one of the wires at L to the ground plane and the other to the core of the coax and use it again as a Marconi type antenna or you can connect each of the wires at L to a balanced feeder. For this final suggestion I don’t think a ground plane will be needed, but if you very much feel the urge to, you can still have a ground plane below the antenna. I would suggest using a 1:4 balun at point L. You could connect a broadband transformer wound on a ferrite ring such that the ground connection goes to the ground plane and the braid of the coaxial cable. One of the two ends of the transformer goes to both the core of the coaxial cable and one of the wires at L and the final connection to the transformer goes to the other wire at L.
For those of you who like Christmas or are traditional I suggest you put the antenna in a vertical manner with point A at the highest point. Those of you who are indifferent to Christmas or want to use horizontal polarisation should have all parts of the antenna the same height above the ground. If the unrepentant version of Ebenezer Scrooge was to build the antenna, I imagine he would invert it putting point L at the top and A at the bottom as a protest against Christmas. I sincerely hope that he would see the error of his ways and then reinvert the antenna. Those of you who just want to be awkward can keep points A and L at the same height above the ground and turn the central plank such that pipe BC is vertical. If you can imagine other ways of orientating the antenna, then please feel free to do as you wish.
EnjoyingYour Antenna
Now we get to how you can enjoy this antenna. There are three different ways of doing so:
First, you can calculate or measure the impedance of the antenna over the
AB, BD, DF, FH and HJ ...................... 400 BC...................................................... 250 DE ...................................................... 500 FG ...................................................... 750 HI ..................................................... 1000 JK .................................................... 1250 JL .................................................... 1400