*Introduction: *

Off Center Fed Dipoles are popular multi-band antennas. They work pretty well, but the resonances don’t line up in all the bands as well as we would like. If you cut the length for good coverage of the high bands, the lower band resonances are lower than ideal. If you cut the length for good coverage of the low bands, the resonances for the higher bands are above the top of the bands. The OCF Masters antennas solve this problem by cutting the length for the higher bands, then selectively raising the resonances of the lower bands using capacitance loads placed at specific locations along the length. This discussion will cover our 160 m and 80 m OCFD designs.

*Statement of the Problem:*

Typically, an 80 m or 160 m OCF antenna is tuned to the bottom of the fundamental band. This is necessary to get the high band resonances any where near where they need to be. Since 80 m and 160 m bands have a high percentage bandwidth, the SWR at the top of those bands is higher than desired. But the resonance frequencies for the high bands is still higher than ideal. This is the great compromise necessary for OCF antennas.

There are two problems responsible for this. First, the center of the bands themselves don’t line up well. The center of 160 m is 1.9 MHz. Harmonics of that for the 80, 40, 20, 15, and 10 m bands are 3.8, 7.6, 15.2, 22.8, 30.4 MHz respectively. Thus, it is necessary to tune 160 m to the bottom of the band. The integer harmonics of 1.8 MHz are 3.6, 7.2 14.4, 21.6, and 28.8 MHz for 80, 40, 20, 15, and 10 m bands. This would produce a high SWRs for the top of 160 m and 15 m. However, it’s worse than that because of the second problem.

The second problem is that the harmonic resonances occur at frequencies a few percent higher than the integer multiples of the fundamental. This exacerbates the problem. This occurs because of the end effect. The end effect causes a dipole to be electrically longer than its physical length. This depends on the ratio of wire diameter to length. For #14 wire and 160 m or through 10 m half wavelength, the difference ranges from 2% to 2.75%. Dipole designs are routinely shortened to compensate. When operating in harmonic modes, the end effect is progressively smaller for each successively higher harmonic. This increases the spread between the high and low bands. A 160 m OCF antenna cut for resonance at 1.8 MHz has harmonic resonances at 3.65, 7.29, 14.63, 22.0, and 29.31 MHz for 80, 40, 20, 15, and 10 m. The SWRs for the top of 160 m and 20, 15, and 10 m are too high.

** Solution: **The OCF Masters’ antenna solves that problem by cutting the length for low SWRs on the high bands. The low band resonances are then selectively raised by inserting load capacitors at specific locations along the length. For the 80 m design, the load capacitor is placed at the center of the antenna. The figure below shows the current distribution for 80, 40, 20, 15, and 10 m.

Note that at the center, the current is a maximum for 80 m and at a null for all the even harmonic bands. That means that a capacitor placed at the center will shorten the electrical length for 80 m but will not affect any of the even harmonic bands. We discovered this principle independently, however we later found that it had been discovered and documented by Serge, ON4AA 13 years earlier. His excellent web site covers this material in great detail.

As ON4AA points out, the 80 m solution does not scale to 160 m. A capacitor at the center can fix the 160 m resonance, but 80 m and 40 m resonances are still too low. The 160 m solution developed by us follows the same playbook of cutting the length for the high bands and selectively raising the resonances of the low bands using capacitive loading on current peaks for low bands. However, our implementation requires two capacitors, neither of which is placed at the center. The length is cut to place the 20 m resonance at the bottom of the 20 m band. This places 20, 15, 12, and 10 m well for good coverage of those bands. The capacitors are then used to raise the resonance of 160, 80 and 40 m.

The first capacitor is placed on the current peak for 80 m. This occurs 1/4th of the length from one end. This capacitor impacts the resonant frequency for 80 m and 160 m but does not impact higher even harmonic bands because of the current distribution. So, 160 and 80 m can be fixed, but the 40 m resonance is still too low. The second capacitor is placed on the current peak for 40 m located 1/8th of the length from the opposite end. This raises the resonant frequency for the 160, 80 and 40 m bands without affecting the higher even harmonic bands. Extensive simulation and experimentation determined that by using 330 pf capacitors in both locations, the 160, 80, and 40 m resonances are raised by the correct amount for good coverage of those bands. By luck, the 60 m and 12 m SWRs are also under 3:1. The 17 m and 30 m SWRs are above 3:1 but less than 4:1. Operation on 30 and 17 m may require reduction of power to prevent damage to the balun.

For the 80 m design, extensive simulation and experimentation found a design for which the center loading capacitor could also be 330 pf. This allows for the 80 m design to use the same load capacitor as the 160 m design. The capacitor chosen is capable of handling the voltage and current for 800 W for both the 80 m and 160 m antennas. The load for higher power uses a 180 pf capacitor in parallel with a 150 pf capacitor. This can handle the voltage and current for 1.5 kW for both antenna designs. The capacitors must be protected from static charge buildup with a bleeder resistor. A 2.7 Mohm 2W low inductance resistor rated for several thousand volts is used.

*Balun Selection:*

The balun transforms 200 ohms at the antenna to 50 ohms for the coax feed. This is an unbalanced antenna so it is necessary to deal with common mode current. A Guanilla current mode balun provides some level of common mode suppression. A homebrew balun used two type 43 ferrite cores to build a 4:1 Guanilla balun followed by a 1:1 Guanilla balun also using type 43 material. Type 31 material would be a better choice for the 1:1 as it provides more choking impedance. Our reference design uses a Balun Designs 4116 hybrid balun. This uses a voltage mode 4:1 balun followed by a current mode 1:1. Both designs provide common mode suppression at the feedpoint of the antenna and work well. Our practice adds a 1:1 isolation balun just before entry to the shack to suppress RF pickup from the outer sheath of the coax from entering the building.

*Conclusion: *

Off Center Fed Dipoles are popular multi-band antennas, but the resonances do not line up as well as we would like. This can be corrected by cutting the antenna length to provide good coverage of the higher bands, then fixing the lower bands with capacitance loads at the appropriate positions. This design is the result of extensive simulation and experimentation. It served well in the local club’s 2019 Field Day and 2020 Winter Field Day. The 80 m design serves well at KC1DSQ and the 160 m design serves well at W1IS and KA1ULN. The design is robust and reasonably easy to build. The loads offered by OCF Masters simplify the construction of this antenna.

See our article on End Effect and Multi-Band Antenna Design for more detail.

]]>