The off-center-fed (OCF) dipole or inverted-V seems like a natural choice for POTA.
Theory:
By feeding the dipole off-center, usually with a percentage ratio between 25/75 and 33/66, the antenna can resonate on multiple bands, like 40m and 20m at the same time, both with very low SWR.
While the normal impedance of a typical center-fed dipole is approximately 73 ohms, the impedance of an OCF dipole is higher, closer to 200 ohms. A 4:1 transformer can be used to match the antenna to your 50 ohm coax and transmitter.
Many think that a 4:1 current-type balun is required, but I’ve had good results with a 4:1 voltage-type balun. I like the SPI-RO shown below because it has very nice brass screw terminals. This greatly simplifies connecting wires in the field and even permits changing out wire sets to cover other bands.
click image to enlarge
Advantages:
• excellent on-air performance
• omnidirectional as inverted-V
• no radials = ground-independant
• covers 40m and 20m at same time
OCF dipoles are not without a few issues. First off, some baluns are heavy, requiring a strong center support. If you operate in parks with some tall trees you’re probably all set. If not, you will need a very sturdy mast. An OCF dipole also needs a fair amount of room as the overall length is a half wavelength for the longest band it’s designed for.
The 40m OCF dipole seems to make the most sense for POTA. By choosing different ratios, you can get SWR dips on several other bands. I chose a ratio of 30/70 to optimize operation on both 40m and 20m without needing an antenna tuner.
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Example 40m SWR |
Example 20m SWR |
Here are relevant details of my 40m/20m OCF antenna:
• long element = 47 feet 6 inches
• short element = 21 feet 6 inches
• approximate height of feedpoint: 40 feet
• long leg rope extender = 14 feet
• short leg rope extender = 41 feet
• angle at apex between legs = 100 degrees
OCF Inverted-V
You may choose a different ratio to add 15m coverage, but the SWR on 20m will likely be higher and you may need a tuner. An alternate design for 30m might also cover 15m with a 20/80 ratio and element lengths of 9.5 feet and 38 feet.
Try an online calculator to design your OCF dipole, for example:
portable-antennas.com/ocfd.php |
