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Feeding an off-center fed dipole

Jul 8th 2015, 23:47


Joined: May 20th 2014, 01:06
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Total Posts: 0
I am finishing an off-center fed dipole for 20m and 10m (with a coupled resonator for 15m), and am nearly ready to raise it. Before I do, I am curious about one detail regarding the feed.

The feedpoint impedance is close to 110 Ohms on all resonant bands, and I wish to use a 2.25:1 Ruthroff-type transformer for impedance matching. (I realize the Guanella type is wider in bandwidth, but I won't be using the bandwidth and can't yet wrap my head around Guanella baluns with rational impedance ratios.)

The impedance transformer has a trifilar winding (five turns on Fair-Rite 5961002701 (A_L = 140 nH). The input will be across the first two files and the output across all three files.

To choke off common mode, I will also use an ugly balun made from five turns of RG-58. This will be just before the Ruthroff transformer. Clearly, it is easier to place the common mode choke here than on the output side of the transformer, where a 110 Ohm transmission line would be optimal.

Because I will be addressing impedance matching and common-mode rejection separately (inspired by the thoughtful article by Zack W1VT in an old issue of _QEX_), I can connect the end of the second file/beginning of the third file to the coax braid, making this, for lack of better terminology (any constructive suggestions?) an unbalanced balun. The coax center conductor will be connected to the beginning of the first file (as will one side of the antenna). Refer to the upper diagram on page 44 of Sevick's 1997 handbook for Amidon; I will be swapping the coax shield and center conductor.

My question: Which of these two alternatives would be preferable: to connect the common terminal of the transformer to the shorter dipole leg, or to the longer? I note that Zack W1VT connected (either directly, or through a balun) the coax core to the longer leg, and am inclined to do likewise.


TU es 73 de John/NV2K

Jul 9th 2015, 13:41


Super Moderator

Joined: Apr 4th 1998, 00:00
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Total Posts: 0
There are modeling tools that can handle that level of detail, but they are prohibitively costly--3 dimensional finite element analysis programs. Most hams would choose the alternative that provides the best SWR curves--though there are now a variety of ways to compare signal strengths using remote receivers--reverse beacon network,, and WSPR.

Zack W1VT

Jul 9th 2015, 19:31


Joined: May 20th 2014, 01:06
Total Topics: 0
Total Posts: 0
Thank you, Zack! The take home message that I am receiving here is "Time to raise the antenna and try it."

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