WHAT'S WRONG WITH THE OCFD ?
WHAT’S WRONG WITH THE Off-Center-Fed-Dipole?
The Off-Center-Fed-Dipole – also called a “Windom” by many companies selling them – has a somewhat blemished reputation amongst amateur radio operators. Although thousands of “hams” use and love these multi-band antennas, especially here in Europe, it seems just as many people, including several well-known antenna experts, view it as a harmonic generator, a Common Mode Current (CMC) generator or even an outdoor dummy load. The experts condemn it for its nasty CMC problems and many ex-users claim “never again.”
I believe, if built properly, the OCFD is a great multi-band antenna, fitting the needs of thousands of hams. Unfortunately I have rarely seen it built properly – not even by the commercial OEMs charging premium prices for these antennas.
- How is it possible that the opinions of this antenna can be so diverse?
- Which opinion is correct?
- The main reason for this was answered in the sentence immediately preceding the question. Most OCFD antennas are simply not equipped to cope with problems they are ‘sometimes’ faced with in the field. (NOTE: “sometimes” – not always).
- Both opinions are correct! The explanation for this is complex and follows below.
Brief Explanation: Whether or not the OCFD antenna works satisfactorily depends on a combination of many technical and environmental factors which vary from one installation to the next. If we had to name a single cause, it would be “the balun,” though that’s not the only reason. It also depends on how the antenna is installed as well as the understanding of the word, “works.”
Detailed Explanation: There are numerous factors contributing to the potential for the OCFD to be a good working antenna in some cases and a disaster in others:
- The BALUN.
- The Design of the OCFD.
- The effect of Common Mode Current (CMC) on the OCFD.
- The Environment; identical antennas behave differently at different locations.
- The Power Level of the transmitter.
- The User’s experience and expectation level.
Let’s look at these individually:
- The Balun: Most OCFD antennas I have seen (or heard of) are using a balun that was either designed wrong from the beginning, or built with one or more toroid(s) or ferrite rods, having the wrong ferrite mix and/or the wrong number of turns of wire. In addition they are often under-sized which causes unnecessary power loss. In some cases the loss is so great that it is understandable why some people think these antennas are dummy loads. In all cases it can lead to severe CMC problems in installations with less than ideal environmental, depending on the power level used. See Sidebar on Baluns for OCFD Antennas.
- The Design of the OCFD: The classical OCFD antenna places the feedpoint at 1/3 the overall length, for a feedpoint split of 1/3 – 2/3. The reasoning is that this is the point where the impedance on most of the bands is equal. In outer space this might be correct. It was originally thought to be around 300 Ohms but in the meantime we have learned that here on Earth, it varies with height and is typically somewhere between 230 Ohms (lower heights) and 280 Ohms (higher heights). We have also learned that the “equal impedance” is only theoretical. Due to the different affects of end effect on the different bands, there is no point where the impedances are equal. Finally, in our quest to obtain “all band” operations (specifically bring in 30 and 15m) we have seen several suggestions for moving the feedpoint away from its classical 1/3 – 2/3 position. Different feedpoint splits have different CMC characteristics; in general, the farther away from the physical center of the antenna, the worse it will be. More important, the closer to the ground (in wavelengths), the worse the CMC will be. For this reason CMC is almost always greatest on the OCFD’s fundamental frequency.
- The Effect of CMC: The effect of common mode current (CMC) on the OCFD is significant. In field tests run by the author in 2013 comparing the effects of CMC on a 40m dipole to the effects on a 40m OCFD showed huge differences in the way the antennas behave in the presence of high amounts of CMC.1 CMC affects the shape of the SWR curve, the relative SWR level itself, the “apparent point of resonance” (point of minimum SWR…which is usually not really the exact point of resonance), and even the ability for the meter or analyzer measuring all of the above to respond correctly. Suffice it to say that with OCFD antennas care must be taken to do all we can to prevent and suppress CMC. It is here that 95% of the commercially available baluns FAIL miserably! “That’s the biggest problem.”
- Different Locations: Every antenna is subject to influence by surrounding objects. The OCFD antenna is especially critical of this because of its significant ‘potential’ problem with CMC. Unless the antenna is mounted perfectly in the clear of all objects for several wavelengths, we should not expect it to behave the same at all locations. Indeed, things such as one leg running over a rooftop of a house or garage, especially over a metal building or structure, running next to several parked cars or other antennas can play havoc with an OCFD antenna. How we run the coaxial feedline away from the antenna can also have a significant effect on the CMC characteristics of the OCFD.
- The Power Level: Power Level plays a very important role in whether or not CMC will be an issue. In my 2013 test of CMC with OCFD antennas, it was found that low levels (i.e. 30mA or less) had insignificant consequences. As the level of CMC rose, bad things began to happen to the antenna’s SWR curve. Of course the feedline was radiating as well, which is bad in itself.
With traditional modes (i.e. SSB/CW), at QRP power levels one should never have any problem at all. Running 100w, under normal circumstances, we don’t experience much negative impact unless environmental influences cause a rise in CMC. At power levels of 500w, CMC can quickly do a lot of nasty things not only inside our own shack but also to our neighborhood. Running 1kW or more into an OCFD in a typical city location is simply a bad idea. The antennas are never in the clear and extremely high levels of CMC can be present.
With the increasing popularity of digital modes, even small amounts of CMC can cause major problems with the computer being used. This can be in the form of crashes or corruption of data or software.
- Users’ Experience: Users’ experience varies, to say the least. If one OM previously ran a 3-element Yagi and another previously had a mobile whip ground mounted in the back yard with a few radials, obviously their opinions of how well the OCFD antenna works will vary significantly! When it comes to antennas, “Everything Works” – as pointed out in the enjoyable article by N6BT in the July 2000 issue of QST. In this article, Tom pointed out how he worked all continents on a single weekend using only a light bulb as his antenna. Sure it “works”, but this is not the kind of “works” that most of us are seeking for our antennas.
ALL OF THE ABOVE helps explain why different users have different opinions of this antenna.
It is important to point out that OCFD antennas which were not built as they should be may still be excellent performers in many cases. Indeed, thousands of users of commercial and home-brew OCFD antennas are quite satisfied with their results, even though their antennas have inadequate baluns.
The balun is like the safety belt in our cars. If we never have an accident, it makes no difference whether we wear the safety belt or not. If we do have an accident, it can potentially make the difference between life and death.
If we are lucky enough to have an OCFD installation that is free and clear of all obstacles and we are not running high power, the balun plays a lesser role. When our OCFD antenna is erected in a less than perfect environment, a good balun may likely make the difference between having a good working antenna, or one we want to replace at the first opportunity.
BEST PRACTICES FOR OCFD ANTENNAS:
- Always mount your OCFD as high as possible to minimize the potential for CMC problems. This is especially necessary with 80m versions of the OCFD.
- Use a balun suitable for the rigorous CMC challenge of the OCFD antenna. Until the balun OEMs change their current portfolios, it is unlikely that you can purchase this.* Luckily these baluns are not difficult to home-brew.
- Do not depend on the person wanting to sell you a balun for your advice on which balun to use with OCFD antennas. The person may tell you most people use this or that balun but remember, 95% of the OCFD antennas are built with inadequate baluns. Do your own due diligence.
- Run your coax, whenever possible, equal distance away from both legs of the OCFD antenna for as far as you can.
- Place an additional RF choke in the coax line, just before it enters the house.
- If you are bound and determined to run 1.5 kW of power, you are asking for trouble. Perhaps you should choose a different, more symmetrical antenna.
1 DJ0IP’s 2013 “Common Mode Current Test”; http://www.dj0ip.de/cmc-test/
IF I HAD TO PICK JUST ONE REASON
why the OCFD has a bad reputation,
clearly it woud be the BALUN!
Over 75% of the baluns used in OCFD antennas
are either bad or wrong!
AND THAT'S WHY THE BAD REPUTATION!
VERY IMPORTANT NOTE:
At my home QTH I currently have 2 hf antennas installed; both are OCFD antennas.
PLEASE DO NOT interpret the title of this page to imply that ALL OCFD antennas are bad.
A WELL DESIGNED OCFD IS A GREAT ANTENNA (as my contest results show!)