40 Meter OCF Dipoles
(This chapter has a surprise ending.)
The 40m OCF Dipole, also called FD-3, is one of the most popular multi-band antennas in Germany. It competes in popularity with the "Half Size G5RV" from England.
Most people believe the 40m OCF cannot work 15m.
"By design it has a good SWR only on 40/20/10m."
THIS IS WRONG! THIS ANTENNA CAN WORK ON 15m!
Many people use this antenna in its classical 1/3 - 2/3 form on other bands, such as 17 and 12m by matching it with an antenna matchbox. YES, this works, but NO it does not work very good. The reason is that the high SWR on those bands causes excessive loss (in the form of heat) in the balun. Some are even so silly as to try and use it on 15m with a matchbox. Now that is really a "space heater".
STOP AND THINK ABOUT IT: We have been using 40m dipoles on 15m (3rd Harmonic) for nearly 100 years, so why shouldn't a 40m OCF work on 15m?
The answer is simple: It won't work if you feed it wrong. But if you feed it right, it works GREAT! Most people feed it wrong.
"HOW YOU FEED IT" makes all the difference in the world.
There are several ways to feed it and get good performance on 15m as well as 40, 20, and 10. Some are better, some are worse, and each has its own advantages and disadvantages. I will explain these in the examples, which for now are labeled Version 1, Version 2, Version 3.
I did not uploaded details of Version 1.
Version 1 worked fine, but it was too heavy.
As a result I redesigned the antenna (mechanically), calling it the "Ultra-Light" and built Version 2.
Version 2 worked great and was basically complete, but I decided to improve on the balun. As a result, Version 2-2 was created. Ver. 2-2 is complete and is a very usable antenna.
With Version 3, I began to experiment with another split ratio, in an attempt to achieve better SWR on 40 and 20m. I accomplished this, but it had a price: considerably more issues with common mode current.
Version 3 encorporated a new balun design which was hyped to be better than the traditional Guanella, but I was unable to substantiate that.
So I moved on to Version 4, which encorporated a dual-core 4:1 Guanella balun. I was quite satisfied with the results, but like Version 1, it ended up being too heavy for use with thin fiberglass poles. I needed to trim at least 100 gr.
Never say die! I pressed on with Version 5, which looks like it has great potential. I am currently investigating ways of reducing the weight of the addiditional 1:1 current balun required to tame the common mode current. This antenna is usable but I am not finished with the development.
In the meantime I have postponed further work on the design in order to conduct a study on common mode current in this type and similar antennas. Once I have completed that, I expect I'll be able to perfect Version 5 in short order.
If you care to follow my CMC Test, which will take ALL summer to complete, you will find it here:
<-- THE DETAILS OF ALL OF MY 40m OCFD ANTENNAS
<-- ARE SHOWN ON THE LEFT.
<-- Simply click on the version you wish to see.