Simplifies Ground-Plane Antenna Construction
A custom plastic framework supports the elements of a homebrew VHF/UHF antenna.
John Portune, W6NBC angle between the vertical and Amateur Radio operators have a well- radial elements. deserved reputation for the ability to A full-size /2 dipole in free space has repurpose everyday materials for the a center impedance of roughly 72 . needs of our hobby. Now, with the A ground-plane antenna, with its wide availability of inexpensive 3D radial elements perpendicular to the printers, small plastic parts such as vertical element, has a feed-point dipole feed-point fixtures, antenna impedance of 36 . This is because trap forms, and special mounting with straight-out radials, the radial cur- brackets can be purpose-designed rents are equal in amplitude and and printed out at the amateur’s phase but run effectively in opposing desktop. Figure 1 shows such a fix- directions. Hence, radial currents ture — a framework that supports all oppose each other and do not con- the elements of a VHF/UHF ground- tribute to the radiation or the feed- plane antenna built around an point impedance. Only the half-size SO-239 coax connector that also con- monopole of top radiates with half the veniently mounts the assembly atop a 3 impedance. ⁄4-inch PVC mast. I have adapted this design from the one published by However, if the radials are bent down- Laagvlieger at www.thingiverse. ward away from the vertical element, com/thing:1938947. The design files the radial currents begin to participate are available for downloading from my in the radiation and feed-point imped- website at w6nbc.com/3d. ance. Think about it this way: If the radials were drooped all the way Ground-Plane Antennas down, a ground-plane antenna would The antenna is comprised of a half- effectively be a 72 dipole. So, if the wavelength (/2), center-fed vertical radials are bent about halfway down, dipole with the bottom half (/4 mono- the antenna will exhibit a feed-point pole) fanned out into two or more impedance of 50 , making it ideal for Figure 1 — Purpose-designed and radials. Gain and radiation patterns direct connection to 50 coax. 3D-printed plastic framework for an SO-239 VHF/UHF ground-plane antenna. The cylin- are essentially the same as a vertical drical cap at the right is sized to slip over the 3 dipole. A ground-plane antenna’s This antenna has excellent bandwidth end of a ⁄4-inch PVC pipe mast. Four ground- attractive features include its small (see Figure 2), but if you use larger plane radials bolt to the horizontal member at the left of the mast mounting cap. The size, easy mast-mounting, and direct diameter elements than described SO-239 coax connector bolts to the radial connectivity to coax. here, the bandwidth will be modestly elements through the bottom of the radial greater. Figure 3 compares the radia- mount with the antenna’s vertical element soldered to the connector’s center pin and In order to be directly connectable tion pattern and gain of this antenna projecting through the dome of the small cyl- to coax feed line, the antenna’s (red) to a vertical dipole or J-pole inder atop the radial mounting plate. Silicone is injected through the small hole in the side feed-point impedance is adjusted to (blue), with both mounted 10 feet of the radial mounting plate for waterproofi ng. that of the coax by varying the above average soil. With elements
www.arrl.org QST June 2019 1 QS1906-Portune02 plastic part is printed with a tiny hole 2.0 for the vertical monopole, which must be enlarged to fit your choice of whip.
1.5 Four-hole chassis-mount Type N con- nectors also fit the printed part, and
SWR have the added advantage of being
1.1 more weather resistant than SO-239 1.0 connectors. With either, complete the 144 145 146 147 148 weatherproofing of the coax with vinyl Frequency (MHz) electrical tape.
Figure 2 — SWR across the 2-meter band. I added a six-turn 1:1 coiled-coax cur- rent choke balun onto the PVC mast (see the lead figure). Secure the G% Figure 4 — Radials fashioned from balun coax with tie-wraps through 1 ⁄4-inch metal tubing and attached to 3 ⁄16-inch holes in the mast. Alternately, the fi xture with 6-32 hardware.
you may run the coax inside the mast and out through a hole in the side. In that case, small mix-61 ferrite beads radials in small increments, keeping can be used for the balun inside the all lengths equal. Note that each radi- )UHT 0+] 46 3RUWXQH mast. Also, as this is a VHF/UHF al’s 1-inch mounting screw is part of antenna, use a minimum of small- Figure 3 — Radiation pattern and gain of its length. As the last step, touch up the ground-plane antenna (red) vs a ver- diameter coax. the SWR by again adjusting the radial tical dipole or J-pole (blue). droop angle. Table 1 gives starting lengths by band for both the radials and whip. The made of either wire of tubing, the effi- lengths are greater than needed to Conclusion ciency is high. allow for tuning. Some antenna refer- The 3D-printed fixture eases con- ences show different lengths for struction while serving to align and Construction radials and whip. This helps with precisely mount all the antenna ele- ments and provide a convenient mast There are many materials suitable for impedance matching on ground-plane mount. As such, it makes a great the whip and radials. The most conve- antennas with non-drooping radials. teaching tool because its setup pro- nient is solid copper wire, with or Equal lengths work just as well with cedure is readily demonstrated to a without insulation. The lead figure drooped radials. Impedance is easily group. shows #10 AWG bare copper buss set by adjusting the radial droop. wire for the whip and #14 AWG insu- lated solid house wire for the radials. Matching and Tuning Attach the radials to the SO-239 con- Begin with the radials bent down from Photos by the author. nector with either crimp-on ring termi- the horizontal at roughly 45°. Use an John Portune, W6NBC, is an ARRL member nals or small loops bent at the ends. antenna analyzer to find the initial res- and frequent contributor to QST. He has onant dip, which will be below the been licensed for 53 years and has held an 1 Amateur Extra-class license since 1972. Soft ⁄4-inch copper or aluminum band due to the overlong elements. John has a BS in physics and also holds tubing also make excellent radial The first step when tuning and FCC Commercial General Radiotelephone 3 Operator and FCC Radiotelegraph licenses. materials. Just flatten ⁄8 inch of one matching almost any antenna is to He retired as a broadcast television engi- end and drill a hole for a 6-32 screw, achieve a low SWR (i.e., a good neer and technical instructor at KNBC in as shown in Figure 4. Burbank and then from Sony Electronics in match). Now, simply adjust the droop San Jose, California. You can reach John via The vertical whip gener- angle of the radials to email at [email protected] or through his obtain the minimum website at www.w6nbc.com. ally needs to be copper or Table 1 SWR. For updates to this article, brass so it can be sol- Starting Element dered to the connector’s see the QST Feedback page at Lengths The SWR will change center conductor. It is, www.arrl.org/feedback. only a little with fre- however, possible with Band Length (MHz) (inches) quency adjustment. Set acid flux and liberal the frequency (i.e., tune scraping to tin the end of 144 22 the antenna) by short- a stainless-steel whip for 220 14 440 7 ening the whip and soldering. Note that the
2 June 2019 QST www.arrl.org