Baluns

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.

FoxDelta Analyzer at Tuner Input

Here is an example of what the FoxDelta Antenna Analyzer sees when connected to the input of my LDG AT-100Proll. The graph has plots for three frequencies at which the tuner was tuned:

• 3740 kHz  - Blue curve
• 7191 kHz  - Red curve
• 14200 kHz - Green curve

Click on the image above for a magnified view. 
All plots were done with my 80 m horizontal loop antenna connected to the output of the tuner. For each frequency I performed the following steps:

• Tuned the tuner for minimum SWR with the transceiver as a source at the selected frequency
• Connected the analyzer in place of the transceiver
• Ran a scan from the analyzer software

Using the analyzer mode that enables 3 separate scans to be displayed on a single graph, I saved the composite graph and added frequency labels next to the color legend at the bottom of the graph.

You can see that a good dip in SWR is obtained for first two frequencies. At 14200 kHz, however, the short (84-ft) feed line length did not allow the lowest SWR to be obtained within the 20 m band.

Feeding a Multi-band Dipole Antenna

During recent on the air discussions we revisited the design options for a multi-band dipole antenna with balanced feeder. Design guidelines for a slightly-shortened dipole antenna are published in the document Choosing the Correct Balun by W8JI. The recommended lengths of balanced feedline are based on odd multiples of 1/8 wavelength on the lowest frequency of operation. This is covered on pages 7 & 8 of W8JI's document.

Some of our group wondered if the same feedline recommendations would apply to a standard half-wave dipole as they do to the slightly-shortened dipole. To explore this topic I used the 4nec2 antenna modeling software to analyze a half-wave dipole of total length 134 ft (12 ga copper) at 30 ft above rocky ground. This length is close to resonance at 3.5 MHz. The shortened dipole in the article, to cover 80 m and above, has length 110 ft.

I collected data for the feed point impedance at one frequency on each of six bands. I also expanded the model to use a 450-ohm feedline of length 30.48 ft (close to the 1/8 wave length of 30.9) and collected the impedance data at the end of the feedline for the same 6 frequencies. The results are shown in the graph and table below.

The graph clearly shows the wide variation of impedance (orange bars) at the feedpoint, while the impedance at the xmt end of the feedline (blue bars) shows that the impedance variation is considerably narrowed. This supports the comments on page 7 of the W8JI document.  At 7 MHz the dipole becomes a full wavelength, with very high impedance, but the feedline -- 1/8 wavelength at 3.5 MHz -- becomes 1/4 wavelength at 7 MHz, thus transforming to a much lower impedance.

The table below gives the data used in the graph. I did not include the resistance and reactance data, but they are readily available.

f (MHz)	Z (xmt)	Z (fp)
3.5	338	56
5.3	539	1156
7	105	5970
14	839	2433
21	453	1637
28	225	1180

This analysis covers only the 1/8-wavelength feeder case, but any other lengths (e.g 3/8, 5/8) can be analyzed easily.

John

Antenna Tuner Matching & Feedline Lengths

This is a recurring popular topic in our morning QSOs on 75 m.  Multiband operation of a single antenna system results in a wide range of impedance at the transmitter end of the feedline. The Hamuniverse web site provides this article with a discussion of some common scenarios that can result in a very high impedance being presented to an antenna tuner. A few good thumb rules are provided along with a table of feedline lengths to avoid and lengths that are acceptable.

The table focuses on the 160, 80, and 40 m bands. It is worth noting that the listed lengths to avoid are computed for one frequency on each band. For example, the 130 ft and 260 ft lengths to avoid on the 160 m band relate to 1.8 MHz (assuming a feedline velocity factor of 0.95). At 2.0 MHz you may want to avoid feedline lengths of 117 ft and 234 ft. Thus, considering the entire 160 m band you may want to avoid feedline length ranges of 117 - 130 ft and 234 - 260 ft. Similarly, on the 80 m band you may want to avoid length ranges of 58 - 67 ft, 117 - 135 ft, and 175 - 200 ft. On the 40 m band you may want to avoid 32 - 33.5 ft, 64 - 67 ft, 96 - 100 ft.

A table footnote says that "some trimming or adding of line may be necessary to accommodate higher bands". If one desires to use the antenna on ALL bands through 10 m, the set of feedline lengths needed to satisfy the guidelines on all bands may become more restrictive. Careful testing on all desired frequency neighborhoods should reveal any problem areas for matching. Then, hopefully, minor adjustments of the feedline length can accommodate the range of the antenna tuner.