Bob Glorioso, W1IS, Bob Rose, KC1DSQ
Impending Field Day many years ago was the genesis of W1IS’s efforts to build a portable 2-element 40M wire beam. After several years and iterations with more or less success and recent collaboration with KC1DSQ, this latest design met our requirements for a beam that can be hung between trees, easily be moved in a mini-van, have good forward gain, and low SWR. This meant that, because the element spacing depended on fitting into a mini-van, the spacing had to be ten feet, the front to back ratio would be lower than with wider spacing and the configuration would be Driver and Director rather than the usual Driver and Reflector.
The constraints above were used as the basis for an EZNEC simulation. Given the close spacing, we expected the impedance of the antenna to be low, between 15 and 25 ohms, but we found that it was easy to configure it for 50 ohms eliminating the need for impedance matching. The SWR curve found on EZNEC in Figure 1 shows less than 3:1 SWR across the 40M band. Similarly, Figure 2 is the SWR from our FD antenna with 60 ft of RG-8X coax.
Figure 1. SWR from EZNEC Simulation
What about the gain and Front-to-Back, F/B, ratio? The gain is from one-half to one S-unit above that of a dipole at 40 feet, 3-6dB, for a 2-4X power gain across the band. The SWR, gain and F/B ratio are frequency dependent. Gain is 7.2dBi and F/B is 5dB at 7.05 MHz and gain is 8.4dBi and F/B is 8.5dB at 7.2 MHz with a take-off angle of about 40 degrees. As it is, this configuration provides reasonable gain across the band but you can squeeze a bit more gain and F/B at the bottom half of the band using the dimensions later in this article.
Figure 3. 40M Wire Beam Layout
The layout of the beam is shown in Figure 3. The spreaders are 10 feet by 1.25 dowels though other, equally rigid, lighter non-conductive materials may also be used. The wire is either #14 THHN from your local hardware store or #14 FlexWeave™ from Davis RF. The Balun we used on our antennas is 5 turns of RG-8X coax wound in a 5” diameter coil and wrapped in tape. It is light and provides about 10 dB common mode isolation though other 1:1 choke Baluns will provide more isolation but be heavier. Weight is an issue when hauling this antenna up with ropes slung over a tree limb.
First build the Driven element, a simple dipole. Put it up at least 30 feet with the Balun and 50-ohm coax. Use an antenna analyzer or a rig with an SWR meter and adjust the length of the dipole for minimum SWR at 6.8 MHz. Then add the spreaders and director. Note that the antenna may sag on the driven side from the weight of the Balun and coax. As in Figure 3, shorten the length of the harness between the attachment to the spreader harness and the support rope until the elements are level. The antenna can easily be “rotated” 180 degrees by pulling the coax and flipping the antenna, a good feature for FD stations not on the coasts.
Adjust the length of the Director until the SWR is minimum at 7.16 MHz. Your SWR curve should look similar to the curve in Figures 1 and 2. You now have a portable 40M beam with good gain and F/B on the upper half of the band and low SWR with about 1.5 dB less gain and 7 dB less F/B on the lower half of the band. Note that the gain and F/B improve as the SWR goes up on frequencies above the minimum SWR.
CW and Digital operators can get another 0.5-1 dB gain and up to 7 dB more F/B by tuning the stand-alone driven element, the dipole, for minimum SWR at 6.7 MHz. Add the spreaders and Director and adjust the length of the Director for minimum SWR at 7.02 MHz. The final lengths should be close to that in Figure 4. Tune your antenna until it is close to the SWR curve in Figure 5. In the interest of space, we don’t show the actual SWR curves we obtained because they are so close to the simulated SWR curves as in Figures 1 and 2 above.
Figure 4. Layout of the 40M Beam cut for 7.0-7.15 MHz
How does it work?
Though different versions of this design have been used successfully on several Field Days by our club, PART of Westford, MA, we always test the final versions of our antenna projects by putting them on-the-air. We put the full band version, Figure 3, up in the back yard of W1IS. about 25 miles west of Boston, and got into our antenna testing position, Figure 6. With the beam headed SW at 11 AM using a KX3 running 5 watts SSB we worked stations in VA, NC, OH and were heard in FL.
Figure 6. W1IS and KC1DSQ, masked and socially distanced, using a KX3 to test the 40M Beam. (Dee, W1MGA, photo)
A combination of simulation and testing led us to the design of an effective, portable 40M beams for the upper and lower halves of the 40M band that can be transported in a mini-van, and provide useful gain and respectable F/B. We are hoping that we can use these antennas for FD 2021!