Building Coax Choker Baluns & How to Measure The Resonance Point
Over recent months we have seen many articles on the subject of capacitor-type antennas. All of these antennas have the bad habit of making the coax feeder radiate as well. To evaluate these antennas properly we do need efficient current or choke baluns at their feed point to “choke off ” the current flowing on the outside of the snake cable coax braid.
Actually one type of antenna called the “MicroVert” antenna makes full use of this phenomenon and deliberately puts the coax feeder to work by using a choke balun at a distance close to a quarter wavelength from the feed point. This way the coax feeder acts as a kind of efficient counterpoise which greatly enhances the performance of this little giant.
There are many types of “chokers or current baluns”. The more frequently used types:
– Two bifilar windings on a large ferrite toroid
– Coaxial cable wound on a large toroid
– A number of ferrite beads placed over the coax cable or some clamp-on ferrites
– Rolling up a length of coax into a coil of a certain diameter
– Making a resonant coax coil balun
The choking action of the first four types relies on the increased inductance on the braid of the coax cable which chokes the current flowing on the exterior side of the coax. The currents flowing on the inner conductor and on the interior side of the coax remain unaffected. If you can measure the inductance on the braid you can calculate the choking reactance using the formula:
where F is the frequency in MHz and L the inductance in microfarad (μF).
There are numerous articles and books describing how to make these “chokers” but it is extremely difficult to find any data on the “choke resistance” which these baluns bring to the table. So, we are left a little bit in the dark and just hope for the best. On the other hand, we all know that a parallel resonant circuit does show a high impedance of up to several kilohms at its resonance point so that is the reason why I choose to make this type of balun.
Making a 10 and 20 Meter Resonant Chocker
There is absolutely nothing new in making these baluns. For the convenience of the readers here are the basic details for a 10- and 20-meter balun.
Figure 1 is a drawing of the 10-meter balun with resonance point at 28.4 MHz.
As shown, use 7.5 turns of RG58 tightly wound on a 2-inch diameter PVC form. The capacitor is an 8 picofarad made of a pair of tightly twisted gauge 15 enameled copper wire. It uses PVC end caps with female 259 connectors. Capacitors can also be made of a piece of coax or can be of any type provided the voltage breakdown is sufficient. Total length of the balun including the end caps is 4.5″.
Figure 2 is a drawing of the 20-meter balun with resonance point at 14.240 MHz.
Use 16 turns of RG58 tightly wound on a 2.5″ diameter PVC form. Capacitor of 14 pF made again of a pair of twisted wires. Total length of the balun including the end caps is close to 7″.
In order to get some idea about the required value of the capacitor needed I measured the inductance of the coils and calculated the required capacity to get resonance. This is pretty straightforward but how do we now measure the exact resonance point where the highest impedance is supposed to be?? Using an Autek antenna analyzer I could measure the choke impedance on the braid of the coax but apparently the resonance point was several MHz lower than expected. To quote another GARDS member “Good Old George” – George Sharp, KC5MU”, what is going on here? I soon found out that hooking up the antenna analyzer caused more stray capacitance which in turn shifted the measured resonance point downwards substantially. So the question remains: where is the optimal point?? Actually, the measured impedance values were some 2300 ohms at the highest point.
I still had an old “solid state” Grid Dipper as well as an adaptor from MFJ to be used with the MFJ-259 antenna analyzer converting this piece of equipment into a grid dipper as well. These two Grid dippers would allow me to measure the resonance point without making galvanic contact with my balun. Great idea, but it just did not work. I found that both dippers missed the sensitivity to show a resonance dip even when placed very close against the balun. Only when I opened up the baluns and put the coil of the dipper inside the balun could I measure a correct dip, and yes, I can confirm that they were spot on the calculated frequency. The resistance values as indicated on my Autek antenna analyzer are very imprecise in this range and should only be looked at as indicative at best.
The Old Proverbial Dutchman Inside me
Putting the end caps back and closing the baluns again I felt like the old proverbial Dutchman trying to find out whether or not the light in his fridge would still be on after closing the door. There must be a way to find out so I decided to measure the resonance point differently. Thinking more about this problem I finally saw the light! I would use the MFJ analyzer/dip adaptor as a signal source of RF energy and the parallel resonant circuit of the coax balun as the receive antenna. The FS meter should give maximal readings at the point of resonance.
I also realised that by now most antenna affectionado’s do have an antenna analyzer and hopefully also a Field Strength meter. If not, Radio Shack sells one for less than $20 USD.
Making a Grid Dipper Adapter for the MFJ-259 or Other Antenna Analyzers
Figure 3 shows the homemade grid dip adaptor to be used with the MFJ-259 antenna analyzer. It is made of a 4″ long thin-walled piece of 3/4″ PVC form, PL-259 connector and 27 turns of 15-gauge enameled copper wire. Note the coil is open ended.
The PL-259 connector is sold by Radio Shack and is of the soldering type allowing for type RG8 coax. First we have to prepare the connector by soldering the inner core or body to the outer coupling ring. To do this, put the connector in an upright position with the coupling ring in the forward position. This can be done by making a small hole in a piece of wood and inserting the center pin of the connector into this hole. Another way is to screw the PL-259 connector into a female connector. This works well but after the soldering and cooling down, some pliers will be needed to free the PL 259 from its counterpart. For the soldering, use a high power soldering iron or a “pencil torch”. My wife’s “creme brulee” burner (after all we live in New Orleans, USA) worked very well. Be careful using your fingers as it takes quite a while for the connector to cool off.
Figure 4 shows the PL-259 with the core soldered to the coupling ring.
Now, drill four little holes in the PVC winding form as indicated in Figure 3. The first two should be 1/4″ apart. This is not essential but it allows the turns of the coils to stay firmly in place. Take a piece of enameled copper wire (gauge is not so relevant) and thread it through hole # 3 straight into hole # 2 and then back into hole # 1. Do not pull the wire tight yet. Now solder the wire to the center pin of the PL-259 connector and push the connector into the PVC form. This is a very tight fit and some gentle hammering or tapping with a small tool may be necessary. Pull the wire as taut as possible and start winding the coil. During the winding, use your foot as a restrainer on the remaining wire on the ground to help make a very close and tight winding. After some 25 to 27 turns, put the wire through hole # 4 and fold back a short piece of about 1/2 inches to keep it in position. You might wish to use a rubber end cap which you can find in Home Depot or Lowes‘ stores (or similar hardware and plumbing stores). Caps used on the legs of chairs to protect wooden floors work well.
The grid dip adapter for your MFJ-259 is now ready. Note that the original has a coil with a return to the chassis or ground of the meter. I do not know whether it is better or not, but for sure, it is easier to keep the coil open ended. Check for any possible short circuit at the connector by measuring the resistance between the center pin and the coupling ring. Also check for the basic resonance dip of the coil. In my case a large dip occurred at 50 MHz. This dip has to be well outside the measuring range of the adaptor, if not, increase or decrease the turns of the coil accordingly.
Measuring the Resonance Point of the Choke Balun with End Caps Mounted
The proof of the pudding is eating it! The moment of truth had come to prove that my theory would also work in practice. My FS meter has a short-sensing antenna and with a short piece of wire, I made a connection between this sensor antenna and the ground side of my balun connector. A huge off-scale deflection of the meter occurred close to where my resonance point was supposed to be, but again, the value was lower than calculated. I was introducing stray capacitance by hooking up the FS meter. I found three solutions to avoid this problem and still get a clear deflection on my FS meter. During these measurements the grid dipper is placed as close as possible to the coax windings.
1) Make a small coil of 3 to 4 turns of enameled copper wire on the end cap of the coax balun. Leave one end free and connect the other end to the FS meter. Keep this coil as far as possible from the coax windings. You should still get an appreciable reading on the FS meter.
2) Take a piece of straight copper wire and connect it to the FS sensor antenna. Place this piece of wire horizontally at about 1/2″ above the coax windings and this again should allow you to get a clear reading.
3) Use a small capacitor with a value of 1 to 3 picofarad to link the FS sensor antenna to the ground side of the balun connector. The lower the value of the capacitor the more precise the reading.
I found that all three methods give coherent readings of the resonant point and that this method is far more sensitive than doing it the classic way. Once you have established where the dip is you can increase the accuracy of the measurement by increasing the distance between the balun and the grid dipper.
The Adjustable Coax Choke Balun
While exchanging ideas among the fellow GARDS’ members during these projects, a prototype of an adjustable coax choke balun was conceived and built by GARDS member Pascal Veeckmans, ON4CFC who deserves all of the credit for this particular device. I asked Pascal if he would write up (in Flemish) the details of his design so it could be used in conjunction with this article and for the benefit of the reader. Pascal is a fellow countryman from Belgium and thus, I have translated Pascal’s design and construction details below.
Pascal to the Rescue
During my struggle with the building and measuring of the coax baluns I communicated several times with Pascal who had experienced the same difficulties as I did during this project. However, Pascal came up with the bright idea of making the coax balun without further need for opening the balun. In other words, the capacitor was made to be variable without a need for opening the balun.
As may be read about in an article on magnetic antennas for the 10 and 6-meter bands by Pascal, he had developed a new kind of variable trombone capacitor (the article by Pascal is in this same issue of antenneX for August 2001). To apply the same principle to the coax balun is a piece of cake but you got to conceive the idea first! As usual, everything seems so simple once someone dreams up a concept and explains it.
While the pictures and drawing speak for themselves I will give a short explanation on how Pascal built this device. You can either use the standard end caps for PVC pipe or alternatively use some homemade end closures of the type Pascal cut from a piece of polyethylene breadboard using the kind of drill for installing doorknobs. I myself could not find a tight fit for the PVC pipe diameter I used so I had to go for the standard end caps. If you choose to use end caps, consider the black variety made from ABS material as they do have a much flatter surface for mounting the connectors.
Cut two small “coupling” plates from copper flashing and drill all the holes in them as per the drawing and the pictures. They should give a good fit on the end caps. One of these plates will be mounted on the inside of one end cap as per the picture, but first solder a length of small diameter copper tubing which will form one side of the variable trombone capacitor. The total length should be the length of the balun but watch out for a short circuit at the other end of the balun! The construction must be solid enough so the tube is self-supporting and cannot move loosely once installed inside the balun. The other coupling plate is mounted on the outside of the other end cap but an extra hole must be drilled in this plate and a brass nut soldered on it.
We can now mount the balun and check all connections with an ohmmeter for possible shorts. The end caps should be mounted so that a treaded rod or bar (which forms the other plate of our variable capacitor) can be screwed in and out of the copper tubing. The threaded rod itself is insulated with one or two layers of shrink tubing or any other insulation material available. A piece of Teflon tubing would be ideal but, in most applications, I do not expect a high voltage. Once the balun is adjusted to the correct frequency, the rod can be secured with an extra nut and washer. If desired, cut off any protruding part of the rod as well.
After building the prototype, Pascal reported that is was easy to adjust the resonance frequency in a very precise manner using a field strength meter and the dip meter as described above. Those who want to build traps for dipoles with fine-tuning capabilities can of course use the same technique and tuning can be done using the described method.
As usual it was a pleasure to work and exchange ideas with Pascal. His practical approach to solutions and construction techniques are to be admired!
Originally posted on the AntennaX Online Magazine by Jef Verborgt
Last Updated : 21st May 2024