Monday, June 20, 2011

Field Day Vertical Antenna

Field Day Vertical Antenna

This Field Day Vertical Antenna project is the result of many years of attending various field day sites and realizing that what was needed is a simple, easy to assemble vertical antenna. The design of this Field Day Antenna is not very novel and leverages ideas from Butternut verticals and ARRL publications. The one essential requirement was that the antenna can be raised by just one person.

The design of this antenna is an above ground mounted ground plane vertical. The bottom section is a guyed mast, which is guyed with four 1/4 wave tuned radials. The decision to use four radials is to simplify the raising of the antenna rather than for performance. The 1/4 wave vertical element is mounted above the bottom mast separated by a fiberglass rod insulator. The mast and element material is telescoped six foot aluminum tubing sections.

The materials for a 20 Meter antenna mast, insulator, and radiating element, from bottom to top, are:

  • 1.250" x 6' aluminum tubing
  • 1.125" x 6' aluminum tubing
  • 1.000" x 10" fiberglass rod
  • 1.125" x 6' aluminum tubing
  • 1.000" x 6' aluminum tubing
  • 0.875" x 6' aluminum tubing

The aluminum can be purchased from various vendors, one of which is Texas Towers. Look at the Texas Towers Aluminum Tubing Page for details.

I have found only one source for the fiberglass rod material. The fiberglass rod was purchased from MSC. Search on the part number 31983919. The fiberglass rod is cut to a length of ten inches and drilled to accept #8 hardware. Refer to the photograph below for more details.

The radials are made with #14 AWG stranded copper insulated wire from Lowe's HW. The wire comes in 500' rolls at $20 a roll. Not exotic wire, but great inexpensive wire for portable temporary antennas. Insulators are not used, but the ends are terminated in a loop such that the insulation material remains on the loop. Refer to the photograph below for more details. Shrink tubing can be added to the soldered joints to remove any shock hazard. Short lengths of rope are used to attach the radial guys to the ground stakes. Simple tent stakes from WalMart are used.

The fiberglass rod, aluminum tubing, feedline coax, and radials are all attached using #8 stainless hardware. Refer to the photograph below for more details.

Dimensions that I use for a 20M antenna are:

  • Radials - 240 / MHz or 240/14.1 = 17'
  • Vertical element - 234 / MHz or 234/14.1 = 16' 7"

20 Meters was chosen for a test case because the size is easy to work with and it is a good band for testing and making contacts on most any day at any time. Also, 20 Meters is about the lowest frequency antenna with a low VSWR bandwidth that covers the full band. Use the formula above or Vertical Antenna Calculator for other frequencies or bands. I used the very informative "Ground/Radial System" Tech Note supplied by Butternut for information on radials.

The base of the vertical element is not large enough to get all the sections needed, in six foot lengths, for a full size 80 Meter antenna. Twelve foot tubing sections could be tried, but I'm not sure that the base will have the physical strength needed. You're on your own for 80 Meters. Try a bottom loading coil. Let me know what works.

Using four radial guys makes it easy to find the four guy stake locations. Lay the bottom mast section on the ground. Stretch two guys out to make a triangle on the ground with a straight line intersecting the two stake locations and the bottom of the mast. Pace the distance between one stake and bottom of mast. Use that distance for a third guy stake. Than raise the mast, leaning it towards the one missing guy. Install the fourth and final guy stake. Lower the mast, install the vertical antenna sections, and then raise the full antenna.

Band options can be selected by having an antenna for each band that is wanted. Or several sets of radials can be mounted on one mast and the vertical antenna section can be changed or adjusted to change bands. Or maybe this is just another idea for mounting that old hy-gain 14AVQ multiband vertical antenna for field day.

Field Day Antenna Pictures

Click thumb for full size picture

All the partsInsulator installedInsulator detailsRadial detailCompleted antenna
All the partsInsulator installedInsulator detailsRadial detailCompleted antenna
Bottom plateBottom plate installedImproved guy collar
RadialsButternut Vertical
Bottom plateBottom plate installedImproved guy collarButternut Vertical at the 2003 PARK FD

Notice that there are no discussions about the performance, impedance, or any other vertical antenna technical details. Except for two, the length formulas that I used and as the radials of a vertical antenna droop, the antenna impedance rises. The above 20 Meter antenna, using RG-8U feedline, had a very low VSWR. There are limitless sources of technical publications about vertical antennas!

As I said in the beginning of this page, this is not novel or profound, just a lifetime of observations and ideas integrated into a simple antenna that can be used for a Field Day Antenna. Or, maybe these ideas will help you with your fixed antenna.

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