80m OCF Dipole
A Simple 7-Band Wire Antenna.
This antenna was built just in time for CQWW CW in 2011.
It has been my primary antenna for all bands since then.
The beauty of this antenna is its simplicity yet ability to cover 7 of the 8 amateur radio hf bands.
The downside of this antenna is you are not able to adjust the SWR curves of the bands individually; what you see is what you get!
The key to obtaining more bands from any Windom (or OCF) is the positioning of the feedpoint. Most people still believe a Windom MUST be fed at the 1/3 - 2/3 point. NONESENSE!
The classical feedpoint of the windom will give you the best SWR curves for a few of the bands, but if you are willing to accept a slightly higher SWR, it is possible to cover additional bands.
My Windom is very lopsided. It is fed about 19.5% - 80.5%.
The position is very critical and must be very carefully adjusted to obtain the best SWR on all bands. At another QTH, you may need to vary its position slightly to compensate for local difference in terrain.
This very lopsided split (approximately 1/5 - 4/5) creates much more problems with Common Mode Current (CMC) than the classican 1/3 - 2/3 split and additional steps are required to deal with the CMC.
I used a 4:1 DUAL-CORE Guanella balun with -43 ferrite, which limits my power to about 500w for the size of balun I have. Using -61 ferrite would have enabled running higher power to this antenna but it have had enough impedance to common mode courrent on 80m - which is where you need it the most!
In addition, I also added an RF Choke directly at the balun. This is the typical "Beads over Coax" (W2DU) type of choke. In my case with the coax run at a 90 degree angle away from the antenna, this type of choke sufficed. In some instances it may be required to use a 1:1 Guanella choke.
These two steps tamed the excessive CMC on the feedline and gave me a 7-band antenna; 8-bands if you use a matchbox on 17m.
17m: look closely at the SWR curve. On the lower chart, where I measured inside the shack, you will see a tiny dip on this band. This is deceptive. If you examine the top chart you won't see the dip and the SWR is around 5:1. This is at the end of 50' of coax. The computer model predicted over 10:1 SWR at the feedpoint for this band and I suspect that's what it really is!
Bottom Line (17m): The SWR is not low enough to use without a tuner on 17m, but it will tune easily with a matchbox. But it is certainlly not effecient on this band, due to losses in the matchbox, coax and balun. Indeed, the SWR may be as high as 10:1 at the feedpoint. You should limit your power on 17m . . . perhaps 200 or 300w. Otherwise you may damage the balun.
I do not recommend using this antenna on 17m, but it will work there. I have made plenty of contacts running barefoot on this band.
Bottom Line (overall): This is a very good antenna and an excellent all-band alternative to using openwire feedline and a matchbox. But it is a compromise, with the main benefit being simpllicity 7-band coverage.
An example of its performance is shown at the bottom of this page.
Theoretical Radiation Pattern
In free space, the pattern on the fundamental frequency is identical to a dipole, but different on each harmonic band on higher bands.
As can been seen on the right, the "figure 8" pattern of the dipole splits into multiple main lobes on higher bands.
Luckily, here on mother earth the nulls are not quite so deep so the antenna actually works better than what this drawing might suggest.
I have worked a lot of DX on 10m but I sometimes wonder how much DX got away becasue I couldn't hear it!
HOW WELL DOES IT WORK?
How well and antenna works is a difficult question to answer.
Answers like "I can work everything I hear", or "I have worked "X" amount of DX-stations in the first hour on the air" are nearly useless.
First of all, if you have a poor antenna, how do you know what you can't hear?
Second, "recipricol propagation" is a myth. Propagation is often similar in both directions but many times it is much better in one direction than the other.
In order to evaluate the antennna, it needs to be evaluated over a longer period of time and compared to other known results. This is very subjective unless you run "A/B" tests with two antennas all the time.
BELOW I show my contest results over the last 3 years, running this antenna, plus an Inverted-L for 160m. I was running 600w.
Unfortunately when I re-installed Windows to my ham radio computer last year, I lost my 2013 SSB log. I thought all logs were backed up to my NAS server, but that one was missing. I only remember how happy I was to just barely break 500K points.
What you see here for each contest is the total number of hours (TIME) I was on the air, then the total number of contacts (QSO) and DX Countries (DXC) worked on each band.
One thing is certain: you will never break a million points in these contests if your antenna is not performing decently.
A second thing is certain: I have a lousy QTH for working DX:
As can be seen in the picture above, my QTH is located on a mountain side, surrounded by tall mountains on 3 sides. My only clear shot is towards the south - which is not a particulary good direction for DX.
Note: I have enhanced the picture, showing the antenna wires in red. The 40m OCFD was taken down during the contests. I only use it during the summer when I operate up on the hill.
In order to work Stateside or Pacific, I have to shoot over the mountain, which means the RF must be radiating at 40 degrees or higher - not at all conducive for working DX.
Considering where I live, the contest results shown above prove that my 80m OCDF is a very good all-band antenna - at least on the traditional non-WARC bands.