Sunday, October 3, 2010

Top Loaded 40 Meter Antenna Project by KD8JHJ

This is a project I started shortly after receiving my amateur radio license.  After doing some research on shortened vertical antennas for high frequency I learned that placing a loading coil at the top of a vertical radiator along with a capacitance hat is the way to get the most performance out of a shortened vertical.

Why a shortened vertical?  The reasons I have settled on this design are portability and ruggedness.  This antenna's modular construction allows for the mast and capacitance hat to be broken down for easy transport.  The components are robust and sturdy allowing for many assemblies and tear downs.  The design, by Dick Stroud, W9SR was published in the 2008 ARRL Handbook.  Since I can't show the diagram from the text as it is copyrighted material I will simply post along photographically in the blog as I complete the stages of the project.

Overall height of the radiator will be approximately 15 feet.  The Antenna lower portion is two six foot sections of aluminum tubing.  A two or three foot piece of aluminum tubing of a slightly narrower diameter will sleeve into the two main sections and I will fasten them with stainless steel bolts and wing nuts so I can easily pull the bolts without tools.  At the upper end a similarly sleeved section of fiberglass tubing will isolate the top mast and main mast and also provide a form for the winding of the loading coil.  Above the loading coil is the top mast, a 2' 6" piece of aluminum tubing that the machined hub of the capacitance hat attaches to.  This hub is the center piece of the design (Pun intended!) and I am very proud of my handiwork.  I turned the two pieces of 1/2" aluminum from blanks I ordered from McMaster-Carr using my lathe.  All the hole drilling was accomplished using my bench top drill press.  To cut the large center holes through the two plates I used  a hardened steel hole cutter chucked up in the drill press.  Six 4' 6" solid 3/16" aluminum rods fit into the sockets around the outside of the hub and are locked in place by two set screws.  The hub is located about six inches above the loading coil and held fast by three machine screws.  The hub can be moved up and down to adjust the antenna's resonance slightly.

The aluminum and fiberglass tubing I purchased from Texas Towers.  Because they supply the commercial antenna manufacturers I knew the quality of the materials would be good.  As advertised all of the narrower tubing slides precisely into the larger sections.  Very nice and no slop whatsoever.  The stainless steel hardware for the hub assembly I obtained at the local hardware store.  Expensive yes but if I am going to spend my time making something and putting my call sign on it I want the best materials I can get.         

Capacitance Hat Hub Assembly

This project has been a work in progress for too long and maybe If I commit by blogging about it I will get busy and see it through.  If I get back to work now I should have no problem having it tuned up and ready for Field Day next summer.  In following posts I will detail my plans for an elevated radial system and my ideas for antenna deployment.


  1. Nicely done! Great job on the hub piece. I'm anxious to see the finished product! I think you should neatly stamp KD8JHJ into that hub :]

  2. Good Night esteemed colleague.
    I wonder if it would be possible to supply this project via email, because here in Brazil find no copy of this publication to buy ARRL HandBook 2008 in which your lordship informs that pulled such a project this vertical shortened to 40 meters.
    Since you already appreciate the attention and time spent reading this message.
    I wish lher a strong 73s extended to their families.
    By PY5KYW - Marcelo