This is a 8 dBi to 9 dBi gain bi-directional "door frame" loop antenna for 50 MHz. It
competes in the gain with a 2-element yagi when installed at suitable
height. The rectangle ratio of abt 1:2 provides straightaway 50 ohm impedance, so no matching
is needed for good SWR on whole the 50 MHz band. You need only a 5 to
14 meter long mast of any material and few other parts to construct this
efficient antenna for 50 MHz. Additionally this antenna can be scaled to
70 MHz by multiplying the dimensions with 0.716.
The horizontal wires are 1.0 meter long inside the plastic tubes.
The vertical wires lengths (abt 2 m) are depending on the wire type. Here are the guidelines for the wire lengths for
50.1 MHz.
Note2. A common mode choke balun is recommended in the feeding point. Eg 3 - 4 pcs Fair-Rite 0431164181 snap-on ferrites on the coax near the feeding point.
Note3. If you install this antenna higher the takeoff angle will be lower and max gain will be a bit better but the elevation pattern will break down to multiple slopes. See different heigths here
Note4. You can install a second similar loop in the same mast at 90 degrees angle to cover all directions by using one of those antennas at a time.
Here are the simulated performance of the 50 MHz door frame antenna at a height of 2 meter of the lower wire.
- Plastic covered wire (D = 1mm + 1mm PE insulator eg "Killu") vertical wire lengths 1.98 m, total wire length 5.96 m
- Enamel copper wire (D = 1mm) vertical wire lengths 2.10 mm, total wire length 6.20 m
Note2. A common mode choke balun is recommended in the feeding point. Eg 3 - 4 pcs Fair-Rite 0431164181 snap-on ferrites on the coax near the feeding point.
Note3. If you install this antenna higher the takeoff angle will be lower and max gain will be a bit better but the elevation pattern will break down to multiple slopes. See different heigths here
Note4. You can install a second similar loop in the same mast at 90 degrees angle to cover all directions by using one of those antennas at a time.
Here are the simulated performance of the 50 MHz door frame antenna at a height of 2 meter of the lower wire.
Elevation plot. Best takeoff angle gain at 25 degrees 8.1 dBi. (7.8dBi @ 20 degrees, 6.6dBi @ 15 degrees, 4 dBi @ 10 degrees) |
Azimuth plot directivity of 14.2 dB at 25 deg takeoff angle. |
3D plot shows the bi-directional directivity. |
Good SWR on whole the 6 meter band. |