HF Work

In preparation for getting my first HF radio, I have been looking at what I can do about an antenna. There are many different designs with just as many people saying that their choice is the best. However, my brief is restricted to space. I would love to live somewhere with a bit of land to be able to experiment unhindered, but alas, this is not the reality of the situation. Realising that the length of antenna is the key to good HF performance, I need to be able to get that into a small space. After looking at lots of designs, I settled on building a BBTD (Broadband Terminated Dipole). The diagram above shows a loft installation. If it works it will allow me to use just one antenna over a broad range of frequency bands. With this design, some amount of 'zig-zagging' the wire runs will be acceptable, allowing me to fit it into a smaller space. I have already built the terminating resistor and the impedance matching transformer ready to be deployed. It consists of a high power, non-inductive 1000 Ohm resistor and a 16:1 ratio transformer(unun).

I reckon that to get any decent performance from this I need at least 66 feet of wire on each leg. The longer the wires, the better it will perform. These lengths rule out putting it in the loft so I may run the wires around the garden perimeter somehow. The theory behind the antenna is quite simple. The terminator resistor fixes the impedance at about 1000 ohms. So to match this, the transformer has to have a ratio of about 16:1 to match the system to the feed-point impedance of 50 ohms. (16 x 50 = 800). Considering that the impedance won't be exactly 1000 ohms at radio frequencies because of inductance and capacitance, I am lead to believe that the calculations are in the ballpark.