The "N2CX Choke Balun"
Here is a handy project that illustrates many of the principles we've covered this week ... the Choke Balun. When coax is used as a feedline for a dipole, or other types of balanced antennas, it is ideal to maintain the e-field inside the coax, between the center conductor and the inner side of the coax shield. If some mechanism is not present to prevent the signal from also traveling along the outside of the coax shield, that signal will radiate and in general present an unintended/undesirable match to the radio-antenna system.
This simple project provides a way to limit, or "choke" off the the signal flowing along the outside part of the coax shield, thus preventing the feedline from radiating, and thus you get more of the intended signal to/from the antenna.
The N2CX Choke Balun consists of a toroid wound with 10-turns of RG-174 coax, located right up at the feedpoint of the dipole antenna. The coax feedline connects to one side, and the legs of the dipole antenna are connected to the BNC connector on the other side of the box (one side of the antenna goes to the center conductor; the other side goes to the "ground" side of the BNC connector.)
The schematic couldn't be simpler ... just strip and connect the ends of the coax to the BNC connector on either side of the toroid.
We used a small plastic box from radio Shack ... any small, lightweight enclosure would work equally well. (It'll be helpful to ultimately make the enclosure water-tight to reduce the effects of changing climates.
When connecting a BNC to a panel of some sort, a handy trick to do is to use an old BNC adapter on the busy-end of the BNC connector being attached so you don't bend/deform the barrel of the connector. This also provides a handy way to grasp the connector while tightening.
Here's what the enclosure looks like with the two BNC connectors installed.
This FT-114-43 core is 1.14" in outer diameter, and it has a permeability mix of 43, which is quite suitable for our use at HF.
RG-174 coax is very handy to have on the workbench, as we make use of about 20" here. It's lots easier to prepare the ends before you start winding, rather than afterward.
When putting the 10 turns of the coax on the toroid core, you count just as with standard single-wire windings - each time through the core counts as a turn.
You'll need to keep a firm grasp on the coax as you make the windings.
Then, when complete, use a tie-wrap or a piece of electrical tape to hold the ends together.
Next, simple connect each end of the coax to the BNC connectors as you would normally expect: center conductor to center pin, and shield to the "ground" of the connector.
Tuck the toroid down inside the box in preparation for buttoning things up.
But before you do close up the box, let's measure the actual inductance of the choke.
IMPORTANT: Measure from the "ground/shield" sides of each BNC connector ... after all, the coax's shield is actually the issue here, right?!
With our handy-dandy AADE L/C Meter II (you DO have one, don't you?!), we measure a shield inductance of about 44 uH.
Lastly now, let's look at the result of using this choke balun in a coax feedline to a dipole antenna being used on 80m.
We see that the reactive impedance resulting from the choke is 967 ohms ... which happens to be enough to stifle the RF current flowing all along the outside of the coax
thus making our antenna system perform a whole lot better!
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