CQ May 2021
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A New Design of a 40-6-Meter Off-Center-Fed Dipole
BY BOB GLORIOSO,* W1IS AND BOB ROSE,# KC1DSQ
Figure 1. Center-loaded, off-center-fed dipole (CL-OCFD) with feed at 16.4%, total length = 67.35 inches.
A 40-meter off-center-fed dipole (OCFD) is a versatile antenna for portable operation or space-constrained fixed stations because it is a manageable length and can cover 40, 20, 15, 10, and 6 meters with low SWRs. To make an OCFD antenna work well on the higher bands, the resonant frequency must be placed at the bottom of the fundamental band, typically between 7.0 and 7.05 MHz. This article explains how to build a 40-meter OCFD with resonances within the bands and is especially useful on rigs without a built-in tuner (such as the Yaesu FT-818 or the ICOM IC-705) or with lengths of coax less than 100 feet. It will draw upon the techniques described in our 160 and 80-meter OCFD CQ article1 to deliver less than 3:1 SWR across all five bands using only 50 feet of RG-8X.
A 40-meter center-fed dipole resonates on odd multiples of the fundamental frequency. By moving the feedpoint away from the center, we get resonance on even harmonics as well. However, due to the lack of end-effect on the half wave/s in the middle of the antenna, the harmonics occur at slightly higher frequencies than integer multiples of the fundamental. For example, a center-fed dipole tuned to 7.15 MHz has a third harmonic resonance at 21.7 MHz instead of the expected 21.45 MHz. This is explained in some detail in our OCFD article in the June 2020 issue of CQ.1
The resonant frequencies of the fundamental and harmonics are determined by the length of the antenna and the number of half waves without end effect. Moving the feedpoint changes the SWR at those resonances, but does not change their resonant frequencies. The resonances for any 40-meter OFCD tuned to 7.05 MHz are shown in Table 1.
Table 1. Resonant Frequencies for an OCFD tuned to 7.05 MHz.
Figures 2a-2e. Showing CL-OFCD SWR curves on 40, 20, 15, 10 and 6
Table 2. Voltage and current minima for the center loading capacitor, based on anticipated power
This shows the dilemma facing an OCFD designer. With the fundamental tuned to the bottom of 40 meters, the resonant frequencies for 20 and 15 meters are still above the top of the band. The designer’s task then is to move the feedpoint and / or alter the length to change the SWR and the resonant frequencies. Fortunately, this “Whack-A-Mole” exercise has a reasonable solution.
Many OFCD designs place the feedpoint at 33% of the length, but this does not resonate on 15 meters. By moving the feedpoint to 20% or less, we get a nice 15- meter resonance at the expense of slightly higher SWRs on some of the other bands. This is a small price to pay to get 15-meter coverage.
The Center-Loaded Off-Center-Fed Dipole (CL-OCFD)
The typical OCFD can be improved using a technique called “center loading” invented by Serge Stroobandt, ON4AA. His website has extensive information about building an 80-meter CL-OCFD.2The basic principle is to place a capacitive load at the center of the antenna where the peak current occurs for the fundamental frequency. This raises the resonant frequency for the fundamental and odd harmonics. For even harmonics, the current is at a null at the center, so the capacitor has no effect on their resonant frequencies. In the 40-meter case, the capacitor affects 40 and 15 meters and has no effect on 20 or 10 meters. The effect on 15 and 6 is smaller because the capacitive reactance is lower at the higher frequency. With this tool, we can lengthen the antenna to bring the high- band resonances within the band, then bring 40 up to mid-band with the capacitor.
The 40-meter CL-OFCD design is shown in Figure 1. The dimensions shown are for #14 insulated wire such as THHN or Davis FlexWeave™.
Optimizing this antenna resulted in some departures from the assumed positions for the feedpoint and load. The 20-meter SWR was reduced by moving the feedpoint from 20% of the antenna’s length to 16.4%. The 10-meter resonance was moved a little higher by moving the load from 50% (the center) to 61%. These compromises placed the load on the current peak for 10 meters closest to the center.
In our testing of both the capacitivelyloaded OCFD and a more conventional 40- meter OCFD with a 20% offset feed, SWR on the lower bands is comparable but the capacitively-loaded OCFD is significantly better on 10 and 6 meters. Measured SWRs on 40 through 6 meters with 50 feet of RG-8X coax are shown in Figures 2a through 2e.
The antenna is built with Davis RF FlexWeaveTM wire, but electrically it behaves like THHN from a home improvement store. Cut the wires a little bit longer than the design dimensions so they can be trimmed to your working environment during tuning. (See sidebar for construction details–ed.)
The capacitor is 330 pF and must be able to handle the voltage and current requirements for the maximum power shown in Table 2. For 300 watts, a Cornel Dubilier CDV16FF331J03F 330-pF silver mica capacitor will suffice. For up to 600 watts, a low-ESR, low-inductance ceramic capacitor such as the Knowles- Syfer 330-pF, 3-kilovolt ceramic is required. For power levels greater than 600 watts, 150-pF and 180-pF Knowles-Syfer 3-kilovolt ceramic capacitors in parallel are needed.
It is necessary to protect the capacitor from static charge buildup. A bleeder resistor is wired in parallel to the capacitor. For low power, a 1-megohm, 1-watt low-inductance resistor is used. For higher power, a 2.7-megohm, 2-watt high voltage resistor is required (see materials list, Table 3).
Photo A shows the silver mica capacitor and bleeder resistor mounted on an insulator and encased in a mixture of silicon caulk and corn starch (see <https://tinyurl.com/97t29d4d> for more on that). Photo B shows the surface- mount ceramic capacitors mounted on a 1-inch-by-1-inch PC board with the resistor. Photo C shows the PC board mounted on a 1-x 2.75-x 1/4-inch piece of CorianTM and encapsulated in thermally conductive epoxy. The PC board copper is divided into two by cutting it down the middle with a Dremel® tool or hobby knife.
Photo C. Entire
Photo D: Baluns at 90°.
One of the key things we learned in our work with OCFD antennas is the importance of adequate feedline isolation. Without good isolation, the feedline becomes part of the antenna and will negatively affect tuning on the lowest frequency band, in this case 40 meters, while bringing RF into the shack on multiple bands. A simple test to check feedline isolation can be run before you run the coax into the shack.
With the antenna installed with some of the coax still on the ground, measure the SWR profile on 40 meters. Then lift the coax off the ground and measure SWR again. If the SWR profile changes, you have inadequate feed line isolation.
We achieved good isolation with a combination of a 4:1 balun connected to a good 1:1 choke balun in two separate boxes.3 In order to minimize coupling and increase isolation when putting both baluns in one box, the two toroids must be separated by at least one inch or placed 90° apart as in Photos D and E.4 Without additional isolation either at the antenna or along the feed line, we found some commercial baluns were marginal for use on extremely unbalanced antennas like OCFs.
Interestingly, there is an online article about a 40-meter OCF antenna by W3AZ (formerly K1POO) with nearly identical dimensions but without the load capacitor.5 The author also found that a 1:1 balun / choke following the 4:1 balun was needed to get the expected 40-meter resonance.
The performance of this and any antenna is best up high. However, this antenna will work well starting at 20 feet. As with all HF antennas, the resonant frequency will go down as the antenna gets closer to the ground. We have been using and testing this antenna at 40 feet and Dirk Hart, KB1HKN, has had one at 30 feet on- the-air for several months. Launch the antenna and measure the SWR on each band. When trimming, only shorten the wires at the ends. Shorten 1 inch on the short end for every 6 inches from the long end to preserve the feedpoint ratio. Do your shortening in small increments of no more the 3 inches on the long end and only change the short end when you have shortened the long end 6 inches. Placement of the capacitor is less critical, so you don’t need to worry about changing its position.
Photo E. Baluns separated by at least 1 inch.
Forty-meter OCFDs are good for portable operation because their length is manageable and they permit operation on 40, 20, 15, 10, and 6 meters with a short feedline. There are numerous articles about OCFD antennas that choose the 20% feedpoint over the 33% feedpoint. They conclude that it is a good tradeoff to get 15-meters for slightly higher SWR elsewhere. We began with that and produced an acceptable design that places resonance within each of the usual
four bands plus 6 meters using capacitive loading with less than 3:1 SWR using 50 feet of coax on all bands up to 6 meters.
1. Multiband Off-Center Fed Dipoles for 160 & 80M, Bob Rose, KC1DSQ, and Bob Glorioso, W1IS, CQ magazine, June 2020, p. 42
2. Multiband HF Center Loaded Off-Center Fed Dipoles, Serge Stroobandt, ON4AA, <https://tinyurl.com/77ju92e>
3. Understanding, Building & Using Baluns & Ununs, Jerry Sevick, W2FMI, CQ Communications, <https://tinyurl.com/2umv63db>
4. Balun Basics: Why a Balun? What’s a Balun? How do I make a Balun?, Bob Glorioso, W1IS, and Bob Rose, KC1DSQ, CQ magazine, January 2021, p. 30.
5. K1POO 4-Band Off-Centre-Fed Dipole (40, 20, 15, 10), Richard Formato, W3AZ (formerly K1POO) <https://tinyurl.com/4akzmu6h>
• 75 feet – #14 Insulated wire, THHN or Davis RF Flex WeaveTM • 4:1 Voltage Balun followed by a 1:1 Current Balun / Choke
• 2 – End Insulators
• 1 – Insulator to hold the R-C circuit
• 1 – 1-inch x 1-inch single-sided PC board
• 1 – 2.7-M, 1-watt, 3-kilovolt resistor, 5% Vishay VR68000002704JAC00, Digi- Key VR68J2.7MCT-ND
• 1 (<300W) – Silver Mica Capacitor, 330-pF, 350-volt Cornell Dublier, CDV16FF331J03F, Digi-Key 338-3106-ND
• 1 (<800W) – Ceramic Capacitor, 330-pF, 3-kilovolt, Knowles Syfer, 222523K00331GQTAF9LM, Digi-Key 1608-1588-1-ND
• 1 (<1500W) – Ceramic Capacitor, 180-pF, 3-kilovolt, Knowles Syfer, 222523K00181GQTAF9LM, Digi-Key, 1608-1589-1-ND
• 1 (<1500W) – Ceramic Capacitor, 150-pF, 3-kilovolt, Knowles-Syfer, 222523K00151GQTAF9LM, Digi-Key, 1608-1588-1-ND
• Potting Silicone – see text, or MG Chemicals, 834FX, Black Flexible, Thermally Conductive, Potting Compound, Amazon
• Kits with pre-mounted and potted capacitors and resistors are available on the author’s website <www.ocfmasters.com>
Step-By-Step Construction Guide
Here is a step-by-step guide to building the antenna described in this article, after assembling the RC load (see main article text) and baluns (see main text plus W1IS and KC1DSQ’s January 2021 CQarticle, “Balun Basics.” See Figure 1 for antenna schematic and Table 3 for parts list).
1. Cut 15 feet 7 inches of wire and connect one end to the balun. Use about 6 inches of wire to secure the wire to the balun ring bolt for strain relief. Connect and solder the other end to the load pigtail, making sure that the length of wire between the balun and load is 15 feet 7 inches including pigtails.
2. Cut 12 feet 6 inches of wire and connect it to the balun using 6 inches or so of wire to secure it to the loop and connector to the balun.
3. Connect the other end of the wire to the end insulator making the length between the balun connection and end insulator 11 feet, including pigtails. Wrap excess wire back on the end of the antenna to be used for tuning.
4. Cut 41 feet 6 inches of wire, connecting one end to the load and soldering it to the load pigtail.
5. Connect the other end of the wire to the end insulator while assuring the length between the load, including the pigtail, and the end insulator is 40 feet 8 inches.
6. Connect the 100 feet of RG-8x to the balun.
7. Loft the antenna at least 40 feet (lower heights will work if this is not feasible; see main text) and connect the coax to an antenna analyzer or a rig with an SWR meter.
8. Adjust the lengths of the ends in the ratio of 6:1 long to short end to maintain the 16% feed point — 1 inches on the short end for every 6 inches change in the long end — for best SWR at 7.15 MHz. The length of the center wire can remain the same.
9. Secure the ends, cut off excess wire and heat-shrink or securely tape all soldered connections.
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