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I am new to HF. I'd like to put up an 80m delta loop. Due to aesthetic considerations I need to use coax to the shack. If tuned for 3.55MHz, I anticipate having a good SWR on 80m, 40m, and 20m. The transceiver is an Icom 7300 which tunes SWR 3:1 or less. Trying to avoid buying a tuner.

My question, essentially, is whether the feedline and balun choice can be optimized for not just one, but two bands — 80m and 40m.

Do I...

— Bring down a length of 450 ohm line, then a balun, then coax? (What length, what balun?)

— Balun in the air at feedpoint (4:1?), then coax all the way?

— Something else?

I have read much about impedance, SWR, quarter wavelengths, and I simply do not have the background to make sense of it. Maybe one of you will make it click. Thanks.

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  • $\begingroup$ What is the height? Do you already have specific dimensions in mind? $\endgroup$ – Glenn W9IQ Jan 31 '18 at 19:00
  • $\begingroup$ The height will be about 70 ft at two corners and 40 ft at the low corner. It will be about equilateral. By "tuned for 3.55MHz" I meant trimmed. $\endgroup$ – enzo Jan 31 '18 at 19:41
  • $\begingroup$ You should watch a Smith chart tutorial. $\endgroup$ – Phil Frost - W8II Jan 31 '18 at 21:14
  • $\begingroup$ What parts of the bands will you use? CW, SSB, FT8, etc.? $\endgroup$ – Glenn W9IQ Feb 1 '18 at 11:14
  • $\begingroup$ Yes I am mostly interested in data and CW. $\endgroup$ – enzo Feb 1 '18 at 12:16
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Edited to reflect a horizontal loop:

The delta loop should be at least 1$\lambda$ in circumference for the lowest band. The following analysis is for a horizontal, equilateral triangle loop with all three corners at 60 feet. Each leg is 94.37 feet.

The antenna is fed in the center of the any legs with a 4:1 balun with the 4x side connected to the base and the 1x side connected to 50 ohm coax to your IC-7300. The center feed optimizes the results compared to a corner feed.

The performance by CW/digital band segment of this antenna is:

  • 80 meters, SWR 1.7:1, 6.7 dBi gain at 87°, 0 dBi gain at 22°
  • 40 meters, SWR 1.7:1, 7.1 dBi gain at 32°, 0 dBi gain at 9°
  • 20 meters, SWR 1.6:1, 9.6 dBi gain at 16°, 0 dBi gain at 4°

These results were obtained over real, average ground.

The 4:1 balun was chosen because the impedance of the feedpoint of the antenna for each band is:

  • 80 meters 133-j50
  • 40 meters 272-j105
  • 20 meters 279-j80

Since you specified that you need to run coax into your shack, it is best to transform the impedance at the antenna in this case and run coax all the way into the shack. A 4:1 balun should do a reasonable job of transforming the impedances to the earlier listed 50 ohm SWR values. Other combinations of ladder lines and baluns are possible but with no practical advantage.

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  • $\begingroup$ Thanks very much Glenn. That's very interesting that it continues to be a "cloud warmer" at 40m and 20m, contrary to the conventional knowledge of delta loop behavior. Maybe it has to do with the unequal sides and tilt? Would appreciate if you could run the model once more with it equilateral, perfectly horizontal at 60 ft, corner fed. $\endgroup$ – enzo Feb 2 '18 at 3:47
  • $\begingroup$ Could you or anyone add some information on the choice of balun? (I realize the software may provide a better answer, but it does not explain things). For example why not feed with 450ohm line, then a 9:1 balun into coax? $\endgroup$ – enzo Feb 2 '18 at 3:52
  • $\begingroup$ @enzo I see now that I misinterpreted your layout for the antenna. I will redo the analysis and update the answer this weekend. $\endgroup$ – Glenn W9IQ Feb 2 '18 at 12:10
  • $\begingroup$ @enzo I have updated the answer. $\endgroup$ – Glenn W9IQ Feb 2 '18 at 13:11
  • $\begingroup$ Thanks again Glenn, that all makes sense. Will try to get it in the air! $\endgroup$ – enzo Feb 2 '18 at 15:43

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