INTRODUCTION:
The Off-Center-Fed (OCF) Dipole is a antenna design that uses the variation of resistance along a resonant dipole to provide a feed-point resistance that can be matched to a 50 ohm coaxial transmission line with a 4:1 balun. The measured SWR as a function of frequency is given for configurations that were used at the home station. The inverted "V" configuration used at the 1998 Reading Radio Club Field Day exercise is currently used at the home station.
ANTENNA DESIGN AND TEST EQUIPMENT:
SWR measurements were made using an MFJ-259 analyzer. The antenna was a 136 ft length of #14 seven strand copper antenna wire with the feed-point located 45.5 ft from one end. A Radio Works model B4-2KX current balun provided a balanced signal to the antenna and a 4:1 impedance transformation to match the 50 ohm RG-213 transmission line to the antenna. The transmission line length was 134 ft. These conditions approximate the actual use of the antenna at home and field day.
"V" ANTENNA:
The first antenna configuration tested used at the home location was a "V". The ends were above ground about 35 ft and supported by ropes from nearby trees. The weight of the balun and coaxial cable lowered the feed point to about 25 ft above ground. This antenna has been used on 80M, 40M, 20M, 17M, 12M, and 10M with the internal antenna tuner of a Kenwood TS 450 and TS 690.
The Table 1 shows 2:1 SWR frequency above and below the resonant frequency.
| Freq. (Mhz) | 80M | 40M | 20M | 17M | 12M | 10M |
| Below (Mhz) | 3.53 | 6.91 | 13.99 | 17.51 | 24.61 | 28.20 |
| Resonance (Mhz) | 3.64 | 7.14 | 14.28 | 17.79 | 24.94 | 28.38 |
| Above(Mhz) | 3.75 | 7.41 | 14.60 | 18.09 | 25.25 | 28.64 |
| BW (Mhz) | 0.22 | 0.50 | 0.60 | 0.58 | 0.66 | 0.44 |
These results show the antenna is usable on:
80M CW
40M Full Band
20M Full Band
17M Low end of CW
12M Full Band
10M Portion of CW and Full Novice and Technician SSB
INVERTED "V" ANTENNA:
The next antenna tested was an inverted "V" with the feed point supported by a single 40 ft mast. The feed point was 38 ft high with the ends 8 ft above ground. The data in Table 2 gives the SWR measured at 0.1Mhz steps across the 80, 40, 20, 10 meter bands.
| FREQ.(Mhz) | SWR |
| 3.5 | 1.5 |
| 3.6 (resonant) | 1.2 |
| 3.7 | 1.5+ |
| 3.8 | 2.0 |
| 7.0 | 1.6+ |
| 7.1 | 1.5 |
| 7.2 | 1.2 |
| 7.22 (resonant) | |
| 7.3 | 1.3+ |
| 7.4 | 1.6 |
| 14.0 | 1.2+ |
| 14.1 | 1.1 |
| 14.2 (resonant) | 1.1 |
| 14.3 | 1.2 |
| 14.4 | 1.5+ |
| 28.0 | 1.6 |
| 28.1 | 1.5+ |
| 28.2 | 1.5 |
| 28.3 (resonant) | 1.5 |
| 28.4 | 1.5 |
| 28.5 | 1.5+ |
| 28.6 | 1.6 |
| 28.7 | 1.7 |
| 28.8 | 1.7+ |
| 28.9 | 1.9 |
| 29.0 | 2.0 |
The bandwidth is slightly better for the inverted "V", however the usable portion of each band is similar. Table 3 shows the effect of changing the height of the ends above ground and the effect on the resonant frequency and SWR.
| Long End (ft) | Short End (ft) | Resonant Freq. | SWR |
| 8 | 8 | 3.62 Mhz | 1.6 |
| 8 | 3 | 3.64 Mhz | 1.6 |
| 2 | 3 | 3.60 Mhz | 1.5+ |
| 2 | 14 | 3.58 Mhz | 1.5 |
The resonant frequency of the antenna changed approximately 0.6Mhz by changing the heights of the ends. The lower resonate frequency was observed with the short end at 14 ft while keeping the long end 8 ft above ground. Changing the end heights can be used to tweak the resonant frequency.
The resonant frequencies for the other bands are shown in Table 4.
| Freq. (Mhz) | SWR |
| 7.0 | 1.7+ |
| 7.1 | 1.5+ |
| 7.2 (resonant) | 1.5- |
| 7.3 | 1.6 |
| 7.4 | 1.8 |
| 14.0 | 1.2+ |
| 14.1 | 1.1 |
| 14.13 (resonant) | |
| 14.2 | 1.2 |
| 14.3 | 1.5 |
| 14.4 | 1.8 |
| 28.0 | 2.0 |
| 28.1 | 1.7 |
| 28.2 | 1.5+ |
| 28.3 | 1.5 |
| 28.33 (resonant) | |
| 28.4 | 1.5 |
| 28.5 | 1.6 |
| 28.6 | 1.7 |
The data demonstrates the antenna is still usable on the higher frequencies.
BALUN SELECTION:
The balun used to feed the OCF dipole is an important factor in the performance of the system. A current balun is used to minimize the flow of current on the outside of the coaxial cable feeding the antenna. The bandwidth is another factor to consider when selecting a balun for an OCF antenna. The bandwidth was measured using the SWR bridge and a 200 ohm resistor connected across the balanced output terminals of the balun. Caution, the 200 ohm resistor must electrically look like a resistor over the frequency range of interest for the characteristics of the balun to be observed. The data shown in Table 5 presents the SWR and resistance of the balun used in these tests.
| FREQ.(Mhz) | SWR | RES.(ohms) |
| 1.8 | <1.1 | 46 |
| 3.5 | <1.1 | 48 |
| 7.0 | <1.1 | 48 |
| 10.0 | <1.1 | 49 |
| 14.0 | 1.1 | 50 |
| 21.0 | 1.2 | 55 |
| 28.0 | 1.5 | 55 |
| 30.0 | 1.5+ | 55 |
| 50.0 | 2.2 | 30 |
These results show good response below 21Mhz and only a small increase in SWR through the 10M band. Sweeping the frequency to higher frequencies did not show any abrupt change in response until 138 Mhz. Between this frequency and 148 Mhz the bridge oscillator stopped operating. It is believed that this was caused by the resonance of the balun.
BRIDGE CHECK:
The bridge was tested for resistance calibration by connecting a 50 ohm load to the bridge RF connector and sweeping the full frequency range of the generator in the bridge to determine if there were any crazy resonance, and if the SWR scale indicates 1:1 over the band of interest. If the 50 ohm load is good, the response should be flat across the full operating range of the bridge. The SWR calibration was checked using a 100 ohm and a 150 ohm resistor connected to the RF connector of the bridge. The 100 ohm resistor should produce a 2:1 reading and the 150 ohm resistor a 3:1 SWR reading. For these tests a lower frequency of the bridge is used to avoid resistor stray parameters.
CONCLUSIONS:
I have found the OCF antenna to be a good performer on several bands.
The logsheets for the 1998 Field Day CW station indicated that stations
from all sections of the USA were worked.
Try it.