The Sloping Antenna
The two previous antenna designs, the inverted L and the T antenna, do not exhibit much directivity on the broadcast band frequencies. A directional antenna can help pull in some of the weaker stations while attenuating the stronger ones.
The antenna modeled here is 7’ high at the low end, and 35’ high at the far end. The sloping length is about 123’. The antenna is fed with a vertical piece of wire dropping straight down from the beginning of the sloping part of the wire at the 7’ level. It could be thought of as a sloping inverted L.
You could use two of these antennas, one sloping up from south to north and the other sloping up in the other direction from north to south. That would give you a low cost switchable directive array. You could use mercury switches at the top of the 7' section to switch between the two sloping wires. The switches could be operated with a rod that goes back into the house. A spring loaded arrangement with a twine lead to mechanically throw the mercury switches from one antenna to the other should also work fine.
The gain in the direction of the 7’ high end is about 4.5 dbi at a 35 degrees elevation angle at 1700 kHz. The gain moving in the direction of the high end is about -.5 dbi at 35 degrees. This gives a front to back of 5 db, or almost an S unit.
For 1000 kHz the front to back drops to 4 db for a 30 degree elevation angle. It drops to about 3 db at 530 kHz, measured at a 25 degree elevation angle.
Sloping Wire at 1700 kHz. The antenna slopes upward toward the right hand side of the plot. Combine this perspective with the top view (azimuth) shown below, and you can see the directivity and front to back of the antenna.
Here is a top view with a slice done at the 35 degree level. This plot has a different perspective, in that it has the antenna sloping up toward the top of the plot.
Sloping Wire at 1700 kHz, elevation plotwith data. The antenna slopes up toward the right side of plot. The point on the left is the maximum gain. (From this perspective, 180 deg - 145 deg = 35 deg elevation angle measured from the left side.)
Here is a view of the antenna.
It is worked
against a ground system, fed at the bottom (intersection of x, y, and z axes).
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