A project by IM0JZJ
The antenna described here is a dual-band antenna for the 80-meter and 160-meter bands. Its total length is only 37 meters and 60 centimeters (37.6 meters), with no traps.
The drawing shows the antenna with the optimum dimensions, determined through the construction and testing of several Morgain antennas built by the author.
The wire used is stranded copper wire with a cross-sectional area of 1.5 mm², covered with vinyl insulation. In other words, ordinary electrical wire intended for domestic lighting installations.
Tuning the Antenna
Tuning is straightforward: the lateral jumper wires are moved to adjust the antenna for the 80-meter band. For the 160-meter band, the relevant dimension is a straight wire length of 18.80 meters per arm.
To tune the antenna, prepare two short wire jumpers, 12–15 cm long, with pins soldered to their ends. Insert the pins into the stranded wire, together with the corresponding jumpers, at the positions indicated by the dimensions.
Raise the antenna to its operating position, preferably more than 10 meters above ground and away from surrounding objects, particularly near the driven (hot) side. Then measure and plot the SWR curve, as illustrated in the drawing.
For tuning the 80-meter band, the jumpers should initially be positioned 30 cm from the ends of the respective arms. The antenna resonates between 3635 and 3715 kHz, with a maximum SWR of 1.5:1. On the 160-meter band, the resonance range is 1840–1850 kHz, with a maximum SWR of 1.1:1.
Refer to the graphical example in the original drawing.
The jumpers can be moved a few centimeters sideways or toward the center to adjust the SWR curve. However, avoid excessive movement, as this would also affect the antenna’s performance on the 160-meter band.
Once the tuning results have been verified, replace the temporary jumpers with permanent ones of the same length. Solder them in place and insulate the joints with a little varnish.
Note that tuning the 80-meter band interacts significantly with tuning the 160-meter band. Therefore, every adjustment must be followed by a new check of both bands.
When carefully tuned, this antenna should provide very good SWR readings.
Feed Line and Impedance
A conventional folded dipole has a feed-point impedance of approximately 300 ohms, whereas the Morgain antenna has an impedance of only 50 ohms.
This is an advantage because a 50-ohm coaxial cable can be connected directly, without adding a balun. The author recommends RG-8X coaxial cable, or an equivalent type, because it is lightweight, has a characteristic impedance of 50 ohms, and can handle approximately 500 watts of power.
For details on the mechanical ties used to secure the antenna, refer to the article Morgain 40/80 Meters.
Spacers and Center Plate Construction
The spacers and central plate are made from green fiberglass laminate (vetronite), approximately 1 mm thick. Install one spacer approximately every 1.90 meters.
The spacers measure 11 cm. Drill holes sized to accommodate the outer diameter of the wire, positioned 9 cm apart, plus a central hole, as shown in the original drawing.
The central plate measures 11 × 11 cm. Drill holes 9 cm from the lateral edges. Near the lower center, make an additional hole to secure the coaxial cable mechanically.
At each end, use two terminal spacers made of bakelite, 3–6 mm thick. Tie the assembly together using strong multi-strand twine, available from electrical supply stores, and tension the ends with nylon ropes.
Good building and enjoy the project!
G. Remo Campedelli – IM0JZJ