Sloper Antenna for 20 m thru 10 m

A sloper dipole antenna is planned for evaluation at W4UOA to supplement the two horizontal dipoles and one horizontal loop antenna. The hyperlink in the previous sentence has a very nice analysis of the sloper antenna. I offered to do some modeling of the new antenna at W4UOA, and some results are presented below. The antenna is sized (length = 29 ft)  according to W8JI guidelines for a multi-band dipole. The radiation plots are oriented for a westward direction of greatest gain. The heights of the antenna ends and the slope angle are arbitrary starting points. The antenna is to be fed with 450-ohm balanced line that transitions to 50-ohm coax cable thru a balun.

Click on any plot to magnify.

This plot shows the total gain (horizontal and vertical polarization) of the antenna at 14.25 MHz in the horizontal plane. The blue curve maps the gain (dBi), with the scale progressing outward from the center of the plot. The labels around the plot are azimuth, like compass bearings, going clockwise around the circle. 0º is North. The antenna is in the plane oriented east-west. The maximum gain is 2.25 dBi at azimuth 334º.

This plot presents the total gain in the vertical plane. The labels around the circle are elevation (above horizon) angles. This shows the maximum gain (2.25 dBi) occurring at 46º at azimuth 338º.

This plot shows the vertical gain in the horizontal plane. The maximum gain is 2.11 dBi at azimuth 270º and elevation angle 72º.

Here is a 3D view of total gain with colors to signify the gain according to the scale at the left side of the figure. The slanted white line represents the antenna structure. The web link at the top of this article has a very nice animation of 3D and 2D views showing how the gain varies with the angle of the antenna from vertical to horizontal. The author of the article uses the same modeling software as I have used for these plots.





The W8JI article with guidelines for multi-band dipoles recommends balanced feed line of 300-ohm characteristic impedance.  Since Carl has 450-ohm balanced line on hand, I ran comparisons of the design using both feed line types, for the frequency range of interest -- 14 to 30 MHz. The modeling software generated tables of data including: SWR, gain F/B ratio,impedance (R, X, |Z|, phase). I made graphs to compare the SWR vs frequency and impedance (magnitude) vs frequency for both feed line impedance values. The graphs below show that for most frequencies there is not a large difference between the results of the two impedance values.  The SWR data are based on a 50-ohm source impedance. These results do not include the coax cable that will extend the balanced line to the antenna tuner. That detail can be added to the model at any time.

I would prefer to start these graphs at 14 MHz, but I don't know how to prevent Excel from starting at zero on the x-axis.

It will be interesting to see how this new antenna performs at W4UOA in comparison with the other antennas.

73,
John WA5MLF