Shortwave Antennas Page 11

Copyright © 1999-2011 by Harold Melton KV5R. All Rights Reserved.

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Understanding Shortwave Antennas: Page 11

Indoor Multiwire Shortwave Antennas

Indoor, or attic, antennas have several advantages: (1) no rain, no corrosion, no wind damage, (2) little possibility of lightning strike, (3) no problems with property association communists, and (4) no need for trees or other external supports.

It’s possible to make a multiwire dipole out of 4 or 5 conductor TV antanna rotator cable. This type is a flat ribbon wire. Since it isn’t very strong, it must be supported at frequent intervals. This means we can string it through our attic, diagonally, hanging it on the undersides of the rafters with bent nails.

Here comes the compromise: We will have to forego ½ wave operation on the lower bands. Below are the calculations for this antenna, which uses 5-conductor rotator ribbon wire (frequencies shown are approximate band centers for the meter bands shown):

Wire Fundamental 3rd Harmonic Overall Length Per Leg
1 (top) 05.90 MHz (49 meter) 17.70 MHz (16 meter) 468 ÷ 05.90 = 79′ 3" 39′ 6"
2 07.20 MHz (41 meter) 21.60 MHz (13 meter) 468 ÷ 07.20 = 65′ 0" 32′ 6"
3 09.75 MHz (31 meter) 29.25 MHz (10 meter) 468 ÷ 09.75 = 48′ 0" 24′ 0"
4 11.85 MHz (25 meter) 468 ÷ 11.85 = 39′ 6" 19′ 9"
5 15.30 MHz (19 meter) 468 ÷ 15.30 = 30′ 6" 15′ 3"

 

Since this is a dipole (center-fed), we need to buy ((79.25 + 30.5) ÷ 2) + 2 = about 55′ 2" of ribbon (leaves an inch on each end for connection). We then measure, mark, cut, and peel it all apart to make the two halves of the dipole (notice that the center wire is cut twice - 7 feet is discarded) (cut each conductor by pushing a small screwdriver through it):

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If you can’t get in the attic, you can lay it on top of the roof. Bring the coax out through a ridge vent, if possible. Waterproof the connections. NOTE: If you don’t want to build it, Radio Shack sells this type antenna for about $35 (+ coax).

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Continued…

13 thoughts on “Shortwave Antennas Page 11
  1. Will this indoor/rooftop multiband dipole work well without some kind of balun? I have read just enough about such things to be dangerous. I would be connecting the coax to a restored 1938 Zenith Long Distance. Thanks! Great article!

  2. Can’t make out how the dipole is attached to the coax? I’m new to this. It looks like one side is attached to the center and the other to the shield?

  3. I really like your site! Good info for the novice. Definitely appreciate the antenna information, and plans for setting one up.

  4. My radio has an antenna jack that looks like a regular headphone jack, which wire do I connect to the ending bevel and which to the larger portion. I have a Tecsun PL-660, if that matters. Does it matter at all?

    Here’s an image: http://snag.gy/R99iT.jpg

    • The antenna wire goes to the beveled tip connection of the jack. That will be the center conductor of the cable in your picture.
      The shield should be grounded, or if you’re putting up a dipole, to the other half of the dipole.
      –kv5r

  5. Probably a dumb question,… But the attic ribbon SW dipole antenna you describe, can it be used for transmitting,… Also, would the 3rd harmonic from the 19 meter band work for transmitting and receiving on 6 meters?

    • Yes, with a tuner, but not very well. I don’t think you’d be happy with the results. Multi-band dipoles made of ribbon don’t radiate very well, as the wires are too close and tend to cancel each other. Better to use individual wires and fan them out, or perhaps just add a 6M dipole to the ribbon’s feedpoint and run it perpendicular to it.
      Also, transmitting in an attic is pretty lossy, with metal ductwork and house wiring eating up considerable RF — and getting interference into various electronics.
      73, –kv5r

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