Shortwave Antennas Page 16

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

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Understanding Shortwave Antennas: Page 16

Support Planning And Erection Safety

It’s a pretty good idea to plan and graph any proposed erection which could fall into power lines — as opposed to killing yourself like several people per year do. This is easy to do using graph paper or a computer drawing program with a grid.

In the following example, a 40-foot aluminum tower will be anchored to a tilting base plate, bolted to a concrete pad, then attached to the house gable at 13 feet, then guyed near the top (39 feet) with one set of three 3/16″ braided steel cables with turnbuckles. Up to 20 feet of 1½″ heavy-wall steel mast pipe (commercial fence-post stock) will stand atop the tower, giving a height of 60 feet. The upper pipe mast will be guyed with three regular 7-strand galvanized guy wires.

Positioning the power line at the proper height and distance, then drawing an arc, will show where the tower must be placed, and the maximum height at that location, that will clear the lines.

To further eliminate the possibility of a line strike, the tower may be guyed with an additional safety cable extending away from the power lines. This will ensure that the top pole would topple before the tower, thus reducing the arc of fall to 40 feet. This failure mode assumes tornadic wind conditions. If it would take tornadic winds to topple the tower, the same would topple the power lines and poles! Thus, well-anchored steel cables, rated at over 2500 pounds working tensile, will totally prevent a power line strike.

Most electrocutions occur during tower erection. When pulling up an assembled tower or pole, at the very least a safety rope should be tied to the top, extending perpendicular, away from the power lines, tied securely to a tree trunk. If the tower falls during erection, the safety line will swing it down parallel to the power lines. Never try to man-handle a tower into position without safety lines !! One extra helper is ten times better!

No antenna, pole, mast, or tower should ever be raised alone, nor by unsupervised children.

Well, that’s all for now, folks! I hope this article has been helpful. If you wish to continue learning antenna theory and construction, obtain a few good books on the subject. I recommend you start with The ARRL Antenna Book.

73, de KV5R

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13 thoughts on “Shortwave Antennas Page 16
  1. Excellent article however I have a few questions. If I were to build a simple 1/2 wave dipole outside in order to cover the entire spectrum for SW listening(Tecsun PL-990x):
    a) what would be the total length of the antenna?
    b) is one side of the dipole antenna soldered to the center conductor of the coax cable? and the other side of the dipole soldered to the shielding of the coax?
    Thank you

    • Howdy Ray,
      a) The length of a dipole is 468/f(MHz)=feet, where f is the lowest frequency you want to receive. For receiving only, the length is not critical. For example, if you want to listen to the 80 meter band (3.8MHz), use ~125 feet. If you only want to listen down to the 49 meter SWL band (5.9MHz), use ~80 feet. etc.
      b. Yes. Connect the coax center and shield across a center insulator, and be sure to completely waterproof it with silicone caulk.
      Note however, that coax is very lossy unless it is terminated in a matched load, which it never is when used for all-band operation. Much better, feed the antenna with 300 ohm window line, as it’s not lossy when mis-matched.
      For SWL, an end-fed “longwire” (with no feed-line) is hard to beat, for performance and simplicity.
      73, –kv5r

  2. Hi Harold, thanks so much for all the information. I will soon be building a fan dipole for SWL with my SDRPlay RSP1A. Should I use 75 ohm coax, or 50?

    The receiver wants 50, but these antennas are closer to 75 ohms, I believe. Does it make much difference? Length of the coax will be about 100 feet.

    Thanks again!

    • Hi Tom,
      It’s true that dipoles are about 72 ohms but that’s in “free space.” Dipoles in close proximity to ground (like < 1/4 wave high) are more like 30-50 ohms at resonance. So use 50-ohm coax. And at 100 feet, use one of the large ones like RG-8, 9913, or LMR-400.
      73, –KV5R

      • Hi again Harold. I ended up building just a simple, inverted dipole. Propagation conditions at the moment seem terrible, but I am getting a lot of noise (household – I presume) across the entire SW spectrum. It is worst at 6 Mhz, but still bad on 5 and 7. Should I have a balun, or a choke? Using RG-8X, as per your recommendation. Thanks again for your help.

    • I got a Tram 1410 discone last year & really like it. They’re about $40 if you shop carefully. Mounts atop a 1-3/8″ pipe with coax connector inside. Chain-link fence top-rail works well. It’s feeding an RTL-SDR dongle, thru an FM braodcast band notch filter.

  3. Was looking for replacement tuning slugs for a 1946 Stromberg-Carlson when I just clicked on your website. Started reading page one of “Short Wave Antennas” and couldn’t stop. Very informative, very candid, good writing, too. Became a radiophile while growing up in the nineteen-sixties in Allentown, Pennsylvania. Along with my dog Buttons, radio was my best friend. (Lived a deeply sheltered though not uneventful life.)
    Thanks for taking time to be clear and concise about a highly theoretical and abstract subject.

  4. Wow! Impressive article and extremely thorough. Quick question: We have an Icom SSB on our boat, and for backup listening, a Tecsun PL-660 portable in case of loss of electronics (lightening strike, etc.,). Would it be best to attach a wire from the Tecsun to the lifelines (horizontal s/s wire around perimeter of boat), or attach to a stay (wire which holds up the mast)? Thanks, and look forward to your reply! Smooth sailing, Tom

  5. I’ve been trying to learn more about antenna theory and also find some practiical projects to try out for quite some time now. Having read your entire article I finally heard the penny drop deep in my grey matter.
    Superbly written and very intelligible to the lay person such as myself.
    Thank you for taking the time and rest assured, I’ll be buying some ribbon cable in the morning.

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