Building a Delta Loop Antenna (part 2)

I discussed an educational club activity in my previews blog post. The goal was for the club's junior members to build a Delta Loop antenna. I gave them the minimum number of instructions, and they had to figure out all the antenna dimensions and the mechanical structure to support the loop.

Building the Antenna

The delta loop is a full-wavelength antenna. The formula is the speed of light divided by the lowest frequency plus 2%. We wanted to build an antenna for the 40m band. This antenna will be able to work on 40 and the harmonic frequencies corresponding to the 20, 15, and 10-meter bands.

$$ \begin{aligned} L_{meter} &= \frac{300}{F} \times 1.02 \\ \\ &= \frac{300}{7.150} \times 1.02 \\ \\ &= 42.78_{meter} \end{aligned} $$

The length of each branch of the antenna is:

$$ \frac{42.78}{3} = 14.26_{meter} $$

This is the formula we used to determine at what height minimum the top of the antenna needed to be installed.

$$ \begin{aligned} &= \sqrt{a^2 - b^2} \\ &= \sqrt{14.26^2 - \left( \frac{14.26}{2} \right)^2} \\ &= \sqrt{152.5107} \\ &= 12.34952 \\ \end{aligned} $$

We built the antenna using an 18 AWG stranded and insulated wire. As shown in the schematic on the right, the antenna's top must be 13m off the ground.

The pneumatic antenna launcher allowed us to throw the ball at 13 to 12 meters. Then, we strung a 100m polyethylene line between 2 trees to support the antenna. Once erected, the feed point was 1 to 1.5m off the ground.

We intentionally installed the antenna with the top 4 meters longer than initially calculated. The literature on delta loops suggested that a narrower top would lower the antenna's characteristic impedance while a longer top would increase it. We aimed to maintain an impedance of around 200 ohms at the feed point across all the bands. To lower the impedance to 50 ohms, we planned to use a 4:1 transformer.

Measurements

Once the antenna had been installed, we used the RigExpert antenna analyzer to determine the resonant frequency. We cut the wire longer than the calculated length to ensure the antenna resonated below the target frequency. It is always easier to shorten than lengthen a wire while we tune the antenna.

At first, our antenna was resonant at 6.7 MHz. We used the following formula to calculate the length of wire to cut to make it resonant at 7.150 MHz.

$$ \begin{aligned} &= L_{wire} - \left( L_{wire} \times \frac{F}{F_{target}} \right) \\ &= 43.8 - \left( 43.8 \times \frac{6.7}{7.15} \right) \\ &= 2.75_{meter} \\ \end{aligned} $$

A final 40 to 10m scan revealed the VSWR and impedance between 100 and 200 ohms, as shown in the following graphs.

Delta look with and without the 4:1 votage transformer

We used a 4:1 voltage transformer to decrease the impedance from around 200 to 50 ohms. As a result, the VSWR on the ham bands 40, 20, 15, and 10 became less than 2:1, rendering the antenna functional without a tuner.

Conclusion

The project of building a Delta Loop antenna proved to be an enriching and educational experience for our club members. Through hands-on experimentation and problem-solving, they grasped the technical aspects of antenna construction and gained insights into the intricacies of radio frequency engineering. The collaborative effort in determining the antenna dimensions and mechanical structure fostered a spirit of teamwork and innovation among the participants.

This project exemplifies the power of experiential learning and hands-on exploration in cultivating technical proficiency and fostering a passion for amateur radio among enthusiasts.