D3+ High Performance Antennas for Field Day
For over forty years, I have been an avid Field Day (FD) participant with clubs both small and large, from coast to coast. During these experiences I observed that top performing FD stations have one thing in common – superior antennas – typically large wire antennas and/or beams. Additionally, Iâ€™ve observed that aerials, which required use of antenna tuners for frequency excursions, often confound the inexperienced operator resulting in sub-optimal performance, or even damage to equipment.
This article describes versatile broadband wire antennas. These antennas will double your effective radiated power over a dipole, will be easy and inexpensive to build and install, and will be simple to match. They can be operated as traditional dipoles or V-beams. Through careful choice of leg length and use of a matching unit, they can also be used on multiple bands. Because they are broad banded, newcomers to the hobby normally need not worry about antenna tuner adjustments. Perhaps most importantly, theyâ€™re fun to build and provide insight into antenna theory that you wonâ€™t get from working with dipoles alone.
Many hams use half-wave dipoles because of their simplicity and outstanding performance. Increasing the length of each leg has long been advocated as one means of increasing gain. The most popular of the extended length antennas is Louis Varneyâ€™s, G5RV configured for 20 meters. The G5RV antenna on 20 meters is over three times as long-tree half wave lengths as a ordinary dipole with a feed point impedance of about 100 ohms. Although Varney initially set out to build a 20-meter antenna, he found that, with some minor changes, the antenna could be used on several other bands. A longer antenna offers significant advantages over a dipole for both portable and permanent installations.
These advantages are:
- Gain of 3.4 db over a dipole
- Low feed point impedance; easy to match
- Relatively broad-banded
- Six-lobe (omni-directional) radiation pattern
- Performs well at any height
- Easy to adjust
- Can be configured as a V-beam
The remainder of this article will show how to design and build three-half wave antennas for any band 160 to 6 meters. Figure 1 below shows the basic design of a three-half wave antenna with matching transformer.
The D3+ antennas should be installed as high as possible. However, for field day use, I recommend you construct a system which allows the antenna to be lowered or raised. Antennas low to the ground will produce a high angle of radiation, which will improve the receive and transmit signal within 500 miles. For distant communications raise the antenna just before sunset to decrease the angle of radiation. Return the antenna to its lower height just after sunrise to improve close-in performance. The antenna may also be oriented as a horizontal V-beam, if directivity is desired. Furthermore, the V-beam can also be installed with its ends sloped to the ground. The later arrangement, allows the D3+ to be easily moved to change signal direction. See Figure 4. These installation suggestions will maximize the antennaâ€™s performance resulting in more contacts. And, youâ€™ll rack up the points. If you can only place the antenna one way, install it as an Inverted-V as high as possible.
Tuning the D3+ is simple. Leave a small length of wire at each end-see Figure 3; bend the wire 90 degrees (this is why I recommend solid wire). Use an SWR meter or similar device (an MFJ Antenna Analyzer works great for this step) to determine whether the antenna length needs to be decreased or increased. If the antenna needs to be electrically lengthened, just un-wrap the wire on each leg and move each end insulator, the same distance, away from the center. If it needs to be shortened, cut an equal length (a couple of inches at a time) from each leg. I have found that on 40 meters and higher the SWR is relatively flat across the entire band. On 80/75 and 160 meters the antenna has a much broader bandwidth than a standard dipole.
Although this article focused on antennas for Field Day use, the D3+ Antennas make great base antennas as well. Have fun and good luck.
[i] Giant Wire Antennas by Alan M Christman, WD8CBJ, 73 Magazine, Year unknown.
[ii] For a discussion of how quarter-wave transformers work and are constructed see the â€œMatching Devices At The Antennaâ€ section in the ARRL Antenna Book.
[iii] See Section on Coaxial Cables in The ARRL Antenna Book, Table. Table. Characteristics of Commonly Used Transmission Lines
[iv] The Wireman carries both the 300 and 450-ohm lines.
Copyright for this article is W5GI
Originally available at http://www.w5gi.com/D3antenna.htm