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.

Tuesday, May 12, 2015

W4BXI - Net control on RV Service Net

Click here for an audio recording of W4BXI on the RV Service Net on 7191 kHz, as heard on my transceiver in Baton Rouge. This was at 0645 CDT this morning. John's signal was reading S9.

Monday, May 11, 2015


Here are a few selected articles that were referenced during our recent on-air discussions of baluns, and voltage vs current baluns in particular:

Balanced - Unbalanced - Balanced, by VK5AJL
Lots of good definitions and illustrations.
See the comment posted below by N4NR. Other sources refute the author's contention that current baluns are only designed for 1:1 ratio.

Baluns: What They Do and How They Do It, by W7EL
An illustrated discussion by the author of EZNEC software.

Choosing the Correct Balun, by W8JI
An article that our group has consulted many times.

Broadband Baluns
Focus on air-core and ferrite-bead choke baluns.

Common Mode Chokes
A chart of impedance measurements

Special thanks to W4PRE who directed me to the Chattanooga Amateur Radio Club website that includes many good antenna-related topics on the Links page.