Shortwave Antennas Page 10

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

Understanding Shortwave Antennas: Page 10

Shop for Shortwave Radios here.

Outdoor Multi-Wire Shortwave Antennas

The easy way to overcome undesirable directivity shift is to design a multi-band antenna. The simplest multi-band antenna for shortwave listening is the multi-wire. This may be a Zepp (end-fed) or a dipole (center-fed).

The theory is simple: if a ½ wave antenna is best, then we need several of them, at different lengths, tuned to the various shortwave bands. Since the ½ wave element is most efficient, the one whose length is nearest our current frequency will predominate, while the others are relatively inactive. The top wire is the longest, and we use it to suspend the lower, shorter ones.

img

Note that ½ wave antennas are also resonant at 1½ waves. This is called “third-harmonic” operation. Knowing this, international radio treaty makers long ago placed the shortwave broadcast bands at convenient locations. The result is that we can use 4 wires to pick up 8 shortwave bands with excellent efficiency. Below are the calculations for this antenna (frequencies shown are approximate band centers for the meter bands shown):

Wire Fundamental 3rd Harmonic Length
1 3.25 MHz (90 meter) 09.75 MHz (31 meter) 468 ÷ 3.25 = 144′ 0"
2 3.95 MHz (75 meter) 11.85 MHz (25 meter) 468 ÷ 3.95 = 118′ 6"
3 5.10 MHz (60 meter) 15.30 MHz (19 meter) 468 ÷ 5.10 = 91′ 9"
4 5.90 MHz (49 meter) 17.70 MHz (16 meter) 468 ÷ 5.90 = 79′ 3"

 

The wires are spread 3-4 inches, held in place with simple Plexiglass spacers. Just cut a few pairs of the acrylic about 2 by 12 inches and run a few small bolts through them, pinching the wires between. Obviously, you stretch the whole mess out on the ground, assemble it, then pull it up with your rope and pulley.

The wires all join at the peak of the house and connect to the center wire of 50-ohm coax (RG-58). The shield of the coax connects to a wire which runs down to your ground rod. Solder and tape all connections to keep water out. Don’t forget the lightning arrestor.

If you have a big tree about 170 feet away, this antenna will give fabulous results.

The next design is a center-fed multiwire dipole (below). The big advantage is, since the array is supported at the center, you can use lighter (cheaper) materials, since each span is only 72 feet long. Also, using two tall trees puts more of the antenna higher off of the ground.

The only way you that may further improve on this design is to raise it higher. If you have thousands of dollars laying around, you can string it across three 100-foot towers - and probably get a write-up in a national magazine.

Ok - enough dreaming. Let’s get realistic here. We have no trees, and dozens of property association rules. We need a good shortwave antenna in the attic.

img

Continued…

14 thoughts on “Shortwave Antennas Page 10
  1. Thank you, Sir. I’ll send the RG 58 back when I get it. I’m also in the same situation that John Murphy is in, I don’t have the real estate for the 144′ long top wire, and will make the 118′ the top strand. The bottom wire will be 48′.
    The thing I’m not sure about, is after I run the four wire antenna in a straight line, it will have to go to a single wire, and run through a pulley, and then run about another 47 feet to get to the house, at a different angle. I didn’t know if this would be a problem, that’s why I elected to use the RG 58 for this purpose.

    Thank you, and John Murphy for providing such great information.

    Wayne Combs

    • Well if you’ve already got the RG-58 bought it won’t hurt to use it (the center conductor) as a feed-line. Connect the center to the 4 antenna wires, and to the 500 ohm terminal. Ground the shield to the radio ground, along with the ground wire. Keep your ground wire short and run it to a ground rod if you can.
      Make sure to waterproof the antenna end of the coax with silicone caulk to keep water out of the shield, trimmed off, as it will not be connected to anything at the antenna end of an end-fed half-wave fan antenna.

  2. I really appreciate these plans. I’m going to put up an end-fed dipole but I don’t quite have the space for the 144′ first wire. Initially I thought I would just start with the second length at 118′. My concern is that I won’t have any coverage for 31 meter. Should I put up an additional wire 48′ long as the bottom wire? Will that interfere with the other bands? Or just not worry about 31 meters? I’m in Connecticut. Thanks for any info you can provide.

    • That’s a great idea! Start with the 118′ wire on top, and a 48′ wire on the bottom for 31 meters. That’s an important one to have as there’s a good bit of SW around 9-10 MHz.
      Not having the 144′ wire won’t matter, as the only thing around 3.25MHz is Bro. Stair 🙂
      73, –kv5r

        • I’ve built and raised this antenna, as described above with the 31 meter 1/2 wave wire on the bottom. I am getting fantastic results. It took some time to build, mostly sorting out the separators for the wires, and coming up with a mounting plan for the mast on my house and the tree at the edge of the property. The most difficult part was keeping it all from tangling as I moved it from the workshop to outside. I can post construction photos somewhere if anyone is interested. Thanks again for the information provided on this website.

  3. With long wire anteners and diapole. Is the total length the critical thing?
    As in. Does not matter if centre or end feed, I have a maximum length of 40ft. And trying to mount above roof on a motorhome, about 8″ above roof And to use a end feed would be easier, but if centre feed is better I can manage this, just wanting advise on best set up, any other things I should know about, and your site is very helpful

    • The length is important only is you are transmitting. I’ve heard of some people putting a SWL antenna on motorhome by running a loop around the perimeter, ~12″ above the roof, using about 6-8 pvc standoffs on brackets screwed (and sealed) to the perimeter of the roof. The efficiency isn’t high but it’s probably the best you can get under the circumstances.
      If not a loop, I’d prefer the center-dipole over the end-fed.
      73, –kv5r

      • Thanks, I will try w centre diapole. We are still building the motorhome so will know how it works I about a year when finished

        • One other question. Would it be better to have a single wire diapole. At 40ft Or make one using ribbon cable ,cut at the various lengths I can with 40 ft max length or it all about the same for receiving signal with signals or ribbon wire. Thanks, I will try w centre diapole. We are still building the motorhome so will know how it works I about a year when finished

          • Well, a 40′ dipole will do well down to 11.7 MHz. To get lower, at night when parked, you could add clip-on tails thrown over something, to bring length to ~120 feet, that’d do well down to ~4 MHz and cover all your nighttime bands.
            73, –kv5r

  4. Love your site, very informative/helpful!
    I wish to add one more wire to my multiband, to address the 13 Meter band on my Zenith Trans-Oceanic Royal 7000. Middle of band is 21.5 MHz (full range 20.6-22.4). Would I be best served with a Full wavelength, Half, or 3rd harmonic length?

    Similarly, for SWL (and not transmitting) is the 31 Meter band best served with 144′ or 48′ if one ignores multi-band, cost of wire and length of antenna being moot? i.e. is there an advantage to the longer 3rd harmonic over a half wavelength?

Leave a Reply

Your email address will not be published. Required fields are marked *