4:1 OCFD Current BALUN
It is believed by most people, that a good 4:1 Current BALUN
is adequate for use in an 80m OCFD.
My own test have convinced me that this is a false assumption,
unless you can mount the antenna 80 to 100 ft. in the air.
With an 80m OCFD, unless the antenna is mounted at least 80 ft. or higher in the air, the very best 4:1 current balun that you can build will have insufficient Common Mode Impedance to fully choke all of the Common Mode Current on the feedline. At typical city heights (30 to 50 ft.), you must add an additional choke to remove all of the CMC. My hundreds of field measurements prove this. They’re posted elsewhere on my web.
IMO, lab measurements only tell us so much. The rubber hits the road in the field, and that’s where the final judgement of the goodness of a BALUN must be made. This is easy to do and does not require real expensive lab equipment, though it does require some test equipment. You must have a (scanning) antenna analyzer and an RF-Ammeter. This runs about $400, so maybe that is also considered expensive. Maybe not.
The purpose of a balun is to prevent RF current from flowing from the antenna, along the outer surface of the coax, back to ground. CMC is easily measurable. All you need is an RF-Ammeter. Thus, the ultimate test of the goodness of any balun is to see how well it accomplishes this task in the antenna you intend to use it in.
This is more meaningful than any lab measurement.
Testing a balun in an OCFD requires some preparation:
- Before you begin, you must cut a normal (symmetrical) dipole to its calculated length and insert a good 1:1 current balun.
- Attach an ‘electrical’ half-wavelength of coax to it and erect it to its intended height of use.
- With the analyzer, scan the entire 80m band (begin 100 kHz below and end 100 kHz above the band) and determine resonance (j0) AND then also determine the frequency of minimum SWR (I’ll call SWRmin). Write these two frequencies down.
- If possible, save the file to a computer and study the ‘shape’ of the SWR curve across the band on a larger monitor. Make a screen shot. THIS step will be important later when examining the 4:1 balun in the OCFD.
To test the 4:1 balun,
- Remove the 1:1 balun and connect the two wires together where the balun was attached.
- install the 4:1 balun to be tested at your chosen feedpoint position (e.g., 20%, 29%, 33.3%, etc.).
- The coax is still an 'electrical' half-wavelength long.
- Using the analyzer, scan the entire 80m band again, save the file, and determine j0 and SWRmin again. Write it down.
Next, add additional coax to the electrical half-wavelength to extend it to a ‘physical’ half-wavelength. (Note: actually 97% of the physical length is more correct, but a half wavelength is close enough).
- Using a transmitter, apply 100 Watts of power to it (or whatever power level you plan to use)
- With the Ammeter, measure the CMC on the feedline. Write it down.
In a perfect world:
- The j0 and SWRmin, though not on the same frequency, will be the same, or nearly the same in both sets of measurements (dipole and OCFD).
- Adding various random lengths of coax to the feedline will not significantly change the frequency of SWRmin.
- The shape of the SWR curve of the OCFD will be very close to what it was in the symmetrical dipole.
- You will measure very little RF current on the end of the [physical ½ wl] feedline when measuring CMC with the Ammeter.
In the real world of a 4:1 Current balun on an 80m OCFD at City Heights:
- You will measure a significant amount of RF current (CMC) on the feedline.
- SWRmin will show much higher in frequency when fed as an OCFD, than what it showed when configured as a symmetrical dipole. For instance, in the case of the 4115 BALUN, it rises about 300 kHz! :-(
- Adding various random lengths of coax to the feedline changes the frequency of SWRmin noticeably.
- The shape of the SWR curve will be distorted (usually elongated or flattened out).
- The absolute value of the SWR may be different. Not always.
- You will falsely believe your antenna is working correctly because you see good SWR on or near the middle of 80m, AND good SWR on the higher harmonic bands. This is impossible if your balun has sufficient CMI on 80m. (REASON)
Proving the 4:1 Balun is inadequate:
- At the feedpoint, insert a good Maxwell choke (i.e., 1000 to 1500 Ohms CMI) between the coax and the 4:1 Balun.
- Repeat the tests with the analyzer and electrical half-wavelength of coax. You will see the frequency of SWRmin begin to come down. If the balun is really poor, like the 4115 was, you can add a second Maxwell choke and see SWRmin drop again.
- Remove the Maxwell Balun(s) and repeat
tests again with a 1:1 Guanella inserted between the coax and the Balun. For 80m, use 17 or 18 turns of coax on a #43 core.
- The results will be very close to those of the symmetrical dipole.
Because the good 4:1 current balun is sufficient for a 40m OCFD, but not (not even close) for an 80m OCFD, I assume the issue is electrical height of the antenna. I assume raising the 80m OCFD to at least twice the height I used in my tests (i.e., from 40 ft. to 80 ft.) will prove the (dual-core) 4:1 Guanella balun to be sufficient.
I am very happy to discuss this with anyone who will take the time to study the results of my vast field tests (altogether about 1500 field tests, all data recorded). Please contact me directly for this.
I will not discuss this on facebook and especially not with those who won’t even state their name or call sign!
DJ0IP FIELD TESTS ON OCFD ANTENNAS:
- 2015/16/17 80m Battle of BALUNs Test (requires login)