Log Periodic Antenna Design

This antenna was designed using a DOS based program that came with the ARRL Antenna Book. I tried several different designs and concluded this one provided the best gain versus boom length. Significantly longer antennas did not provide much gain increase. Antenna stacking seemed like a better option for more gain.

Design Input Parameters:

• Frequency Range: 174 to 230 Mhz
• Tau = .95
• Sigma Optimum = .18
• All Element Diameters = .25"

Output Parameters:

• Boom Length = 104.47" (A 9' boom was a nice round number to use.)
• Boom Spacing (Center-to-Center) - 0.81"
• Gain = 7.85 dBd
• Front-to-Back Ratio: 22 - 28 dB

Element Lengths and Spacing in Inches

Boom and Feed Design

The next step was to select the boom type (round or square), boom diameter, and desired impedance. I decided I wanted the impedance as close to 75 ohms as possible. The log periodic is a balanced antenna and I needed to convert it to unbalanced for coax feedline. I could have selected 300 ohms and used a commercial 300 to 75 ohm balun but that resulted in a large spacing of the two boom halves which I didn't want. Instead I went with a ferrite choke balun as described in the ARRL Antenna Book. More on the balun in the Construction page. The Tennadyne log periodics are used as the model for the mechanical design. It's a proven design and I knew I wouldn't have to worry about it not working. I selected 1/2" square tubing for the boom material. This helped keep the weight down since most of the antenna weight is contained in the two boom pieces. To get an impedance close to 75 ohms using 1/2" square boom, the center to center spacing of the boom halves was calculated to be 0.726". As it turned out in practice, having the boom halves this close together resulted in water bridging the halves when it rained. This caused the antenna not to function properly. If I selected an impedance of 85 ohms this opened up the spacing to 0.812" and reduced the water bridging problem. There was a small increase in VSWR that has no real impact on the antenna performance. Now it's time to build the antenna.