Sunday, May 31, 2015

Short Dipole & Horizontal Loop for 3.5 - 30 MHz

A recurring discussion topic during our morning QSOs involves two of the multi-band HF antennas presented in the article Choosing the Correct Balun, by W8JI. The antennas are:
  • Multi-band Dipole / Doublet - described on pages 7-10
  • Horizontal Loop - described on pages 16-19
The focus of our discussions has been on versions of these antennas that are sized for 3.5 - 30 MHz. To gain some additional insights about how these antennas compare, I modeled each with antenna analysis software. I used a height of 70 ft above average ground conditions, to be consistent with some of the data presented by W8JI. The following PDF documents present the analysis results:

The radiation pattern graphics show only the total gain views. The software can also present the gain views for horizontal polarization or vertical polarization separately. I limited the radiation pattern analysis to the 80, 40, 20, 15 and 10 m bands, but any frequency can be entered to generate any desired patterns. Would anyone like to see what the patterns look like for an 80 m loop on 1296 MHz?

The SWR and impedance results cover the full range from 3.5 to 30 MHz. The last page shows the impedance for selected frequencies within the ham bands. Only the points are plotted on that graph.

Further analysis will compare the SWR and impedance results for 300 ohm and 450 ohm feedlines. I believe that the software can also provide an overall efficiency figure for the combination of feedline and antenna.

Update 7/4/15: Below is a graph of impedance (for the shortened dipole) calculated at selected frequencies within the HF ham bands (3-30 MHz). These points are for the transmitter end of a 300-ohm feedline for 3 different lengths: 1/8 wavelength, 1/4 wavelength and 3/8 wavelength (at 3.5 MHz).  The 1/8 and 3/8 wavelength cases correspond to the recommendations of the W8JI article referenced above.  I was a bit surprised at the high impedance values for the 3/8 wavelength case in the 75/80 m band, and turned to the venerable Smith Chart to double check the impedance transformation of the 3 feedline lengths. Its results agreed with the results of the NEC software. I can provide a table of the graph data to anyone who is interested.

It was interesting to see that the 1/4 wavelength feeder gave reasonable impedance levels on some of the bands, and that the 1/8 wavelength case was sometimes better than the 3/8 wavelength case. Impedance levels for a 5/8 wavelength feedline should be similar to those for the 1/8 wavelength case.
I plan to analyze 3 lengths of 450-ohm feedline in the same manner to include in this article. Also planned are studies of 300-ohm and 450-ohm feedline impedance for the loop antenna.

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