The OCFD MAX above is shown with the optional 15 meter element kit installed. This is an optional feature that can be factory installed or installed at a later date by the OCFD MAX user. Click here for full information on the 15 meter element kit.
Over the years, several antennas have had their time in the sun. One of the most enduring is the G5RV. Of course, number one is the dipole. Currently, the off-center-fed dipole is enjoying a third round of popularity.
It all started with the Windom, an 80 meter dipole with a single wire feeder. The single wire feeder is tapped into the horizontal element at a point about 14% off center. This point permits using the Windom on all even harmonic bands. So with one horizontal wire, you get 80, 40, 20, and 10 meters. Since the feedline is a single wire, it will radiate and does so vigorously starting directly at the output of your antenna tuner. The antenna has to be fed against a good RF ground system. If done correctly, it's possible to generate a nice radiation pattern on several bands. Unfortunately, it has a bad reputation as an RFI generator, but that's another story.
In the 60's, the Windom's favored feed system became twin-lead or other balanced feedline. A 4:1 balun was used at some convenient length down the balanced line to convert from balanced line to coaxial cable. One thing wrong with this system is the fact that there are problems feeding an unbalanced antenna with balanced feedline. Still, the antenna worked for many users.
The latest OCFD versions
Currently, the OCFD or Off-Center-Fed Dipole enjoys increasing popularity, but there is a lot of controversy. Most of the controversy is over the balun type that is used at the off-center-feedpoint of the antenna.
On the positive side, the OCFD has a lot going for it. If the horizontal element is a half-wave on 80 meters, the antenna will cover 80, 40, 20, and 10 meters. If implemented correctly, it's possible to make the antenna work on 17 and 12 meters, too. 15 meters is a problem with most designs. There are claims by some makers of OCFD's that they will work on all bands or most bands, but it's important to consider the system loss involved in picking up those extra bands. In my research, experimenting, and computer modeling of the OCFD, 15 meters is a problem. Fortunately, the problem is easily remedied.
The Balun and Feedpoint Offset
One interesting point that needs to be made is the use of a balun with an unbalanced feed point. You have to be very careful in picking the balun's characteristics. Actually, it's best to use a matching device that is unbalanced and yet achieves the necessary input-to-output impedance ratio on each band. The most popular ratio is 50 ohms to 200 ohms or a 1:4 (typically called a 4:1) balun. A 6:1 balun also enjoys significant popularity. A very good case can be made for other ratios, too. It all depends on where to tap into the main element of the antenna. A rule of thumb for the feedpoint is one-third of the way from one end. This is a good generalization, but optimization of the exact offset from center results in improved performance.
|Optimizing the OCFD
If you're going to optimize the feedpoint, you just as well optimize the balun or preferably an asymmetrical device. There is a point where just the right winding inductance, core type, ratio, among other factors, results in a good match on all covered bands. You also achieve the necessary feedline isolation to keep the feedline from radiating. Uncontrolled feedline radiation is a major shortcoming with most examples of the OCFD on the market. Feedline radiation can be a wonderful asset if properly controlled. After all, that's been the secret of the CAROLINA WINDOM® for over 25 years. You can read all about the CAROLINA WINDOM® in other sections of this web site.
The 6 meter surprise
Speaking of matching units, our matching unit works well into the lower VHF region, so you get 6 meter coverage.
The OCFD, properly implemented, deserves a place in the antenna arsenals of hams who want a single, simple wire antenna that works on several bands. The RADIO WORKS is now making a fully performance optimized OCFD antenna available in four models. As with all of our antennas, proper construction insures a long useful life of your OCFD. Our newly designed matching device is optimized for the best compromise in matching, feedline isolation, and efficiency. Oh yes, there is also the issue of high power. Our OCFD is rated for 1500 watts (SSB/CW duty cycles).
The 15 meter solution
Since 15 meters will be important as the sunspot cycle progresses, I have added a 15 meter parallel element that provides a low SWR and a great radiation pattern on 15 meters. The 15 meter element is available as a kit that you can add to your OCFD MAX antenna at a later date or you can order the antenna with a factory assembled 15 meter element. One nice thing about the element design I use is that the 15 meter element doesn't have to be routed away from the antenna at some angle that requires more supports. A length of Kevlar line is supplied with the RADIO WORK'S OCFD MAX that ties the end of the 15 meter element wires back to a specific point along the horizontal element of the OCFD. This provides the proper geometry for the 15 meter element while avoiding the need for additional supports or other complications. All the parts are supplied and the horizontal element is marked so all you have to do is to tie a couple of knots and apply some cold-shrink tape to hold the Kevlar line in place.
Outstanding performance and attention to detail yields an excellent value.
All things considered, the RADIO WORKS OCFD MAX takes the OCFD antenna to the next level in performance and construction. On top of all that, the price is right. You get a lot of performance on a lot of bands for you antenna investment.
|OCFD MAX 80||OCFD MAX 80 + 15||OCFD MAX 40||OCFD MAX 40 + 15|
|80, 40, 20, 17, 12, 10, 6m||80, 40, 20, 17, 15, 12, 10, 6 meters||40, 20, 17, 12, 10, 6m||40, 20, 17, 15, 12, 10, 6m|