Correctly Configuring the Flagpole, Tuner, and Balun "Feed System"
Correctly Configuring the OCF Vertical-Dipole and DX Flagpole, Tuner, and Balun Feed System
We do get a fair amount of queries starting out with, "how to feed the OCF antenna correctly?" and it's easy to understand why, as the answer certainly is not intuitive.
Most of us think we can plug a 50 Ohm coax into an antenna out of the box and we're off to the races breaking pile-ups and feeling loud! That's the goal with our 10-band vertical-dipole (aka "DX Flagpole") but feeding our OCF antenna just doesn't work that way. There is a sequence to the "feed system" that is required for higher performance. Tune in below for more information from one of the antenna sages of our time, Mr. John Portune.
Yep, here we go again asking for wisdom on the matter from John Portune, QST antenna design author and career engineer who was kind enough to share the below for all of us. Thanks, John!
"Correctly Configuring the Flagpole, Tuner, and Balun"
It is a common belief that one must “ground everything.” This is NOT true for the Flagpole antenna. It must “float” electrically above ground. The reason is basic. Like a common wire dipole, the flagpole antenna is a balanced antenna.
Most verticals work against ground radials and are fed unbalanced at the base. The flagpole must not be fed unbalanced. It is an electrical-half-wavelength vertical dipole. Both ends are insulated from ground and the off-center (OCF) feed point must be provided with a balanced feed.
Therefore, the tuner must also have a TRUE balanced output.
However, most modern tuners are unbalanced by design. The main output is grounded via the shield of the SO-239 output connector. Some commercial tuners do have an output labeled as balanced. In most cases, a balanced output is only simulated by an internal voltage balun. This method cannot be used with the flagpole antenna.
The ONLY satisfactory way to provide a true floating balanced output from an unbalanced tuner is to insulate it from ground. Then to make it “float,” a 1:1 current choke balun must but placed in the coax directly ahead of the tuner.
Figure 2: Palomar Engineers BA-2 ferrite sleeve 1:1 current choke balun installed on a short coax pigtail. (Heat shrink tubing removed)
Any true commercial or home-brew current balun of sufficient power-handling capacity is acceptable. Make sure, though, that it is a current balun; many are not marked. The Palomar Engineers BA-2 high-power 1:1 current ferrite sleeve kit balun, is shown on a short coax pigtail in figure 2.
Lastly, it is essential to mount the tuner with the balun on an insulated surface or pole nearby. Do not lay them directly on the ground or connect them to ground.
Again, thanks John for your perspective. That makes things much easier to understand.
For the rest of us (you), what experiences are you discovering with this configuration? Do you "float" your tuner or use another method? Would you share with us your findings?
Some folks have mentioned they use a matching network or remote tuner at the base of the vertical and ladder line to the shack, we've even had some say they enjoy DX just fine with their tuner in the shack as hard to believe as that may be! We maintain you must use the above configuration, of course, for high-performance on the ham radio, 80-6M bands.
We'd enjoy hearing from you.
73, Greyline Performance, RF Research
aka DX Flagpole