Super-J – As good as it gets?
The collinear J-Pole, often known as the Super-J, does improve the behavior over a regular J-Pole.
As many attest, there is an advantage when vertically combining 1/2 radiating sections to have a bit of separation between the half-wave end points.
The Super-J has very little separation between the two half-wave radiators.
Some web sites suggest the Super-J has a full 3 dB gain over the plain J-Pole which is a pretentious claim obvious to anyone with a copy of the ARRL Antenna Book and other references. Indeed, the Super-J will never approach the gain advantage of a properly separated two element collinear antenna, but we can get closer to this goal.
Below you will read about how to get 0.8 dB more gain out of the trusty Super-J by replacing the traditional phasing stub with a long coil.
The Collinear J-Pole Phasing Stub
The all important phasing stub ensures the two main radiators of the Super-J are in phase. Without the stub you have a version of the erroneous 5/4 Wave J-Pole with its almost perfect cancellation of energy towards the horizon. To make the Super-J work, the conducting portion of the antenna (above the J-Feed) is made 3/2 waves long where the middle 1/2 wave is shaped to reduce it’s far-field net radiation. This leaves the two remaining radiators in-phase and contributing towards horizon coverage.
Focusing on the phase stub, we have the following possibilities in Figure 1…
Figure 1A – Franklin Array Stub  – Simple 1/4 wave stub
My original version of Hiking 2m J-Pole uses a simple bent pipe phasing stub 1/2 wave in total length or about 1/4 wave “stub” length. Normal operation places equal and opposite currents on the top and bottom pieces reducing the effective radiation of the stub assembly. Simulation and practice confirm this system works quite well.
Figure 1B – Simple stub bent into circle (known as ‘Curved Stub’ from now on)
Many builders take the stub above and bend it into a tight circle around the waistline of the antenna. This reduces the stub’s protrusion markedly and yields the same performance in a visually more appealing assembly.
Not Shown – 1/2 wave of wire bent into 1 turn coil
We can take the above stub and simply wrap it once around the waistline of the antenna to provide the same phasing benefit. The “single turn inductor” provides some cancelling of this middle 1/2 radiation, but not as well as the above techniques as will be seen below.
Figure 1C – Increasing the gain with more separation
As mentioned above the best vertical collinear antennas are realized with some separation between the two 1/2 wave radiators. This is the only way to approach the “perfect world” 3 dB gain of a collinear over a simple half-wave. One way to do this is to take the single 1 turn coil and stretch it to the proper separation distance.
Figure 1D – Using a multi-turn coil for the phasing component
The above 1 turn coil can reduce its diameter by increasing the number of turns. The resulting assembly is very sleek.
Figure 2 shows the above stubs as they appear in the entire antenna assembly…
Current magnitudes are shown to reveal what proper antenna phasing looks like.
Figure 2C and Figure 2D reveal the superior separation between the two primary radiators. Quite a bit of personal EZNEC simulation went into confirming the ideal distance. The right “mechanical length” of the coil for a two element array is about 1/4 wave. In the end 16 inches was “my” best compromise between additional gain and development of unwanted sidelobes of energy.
My primary goal was the antenna in Figure 2D. Figures 2A, 2B and 2C phase stubs are pretty much just a half-wave of wire bent to fit. You might think the same can be true of a 22 turn coil of wire. The problem is this coil starts to behave more like an inductor with more turns and tighter diameter.
There is nothing new about little coils on antennas. Any collinear antenna you purchase for your mobile installation has them. Cebik discusses how to properly model them in his paper “The Case of the Curly Collinear”.
While taking a class on antennas from Steve Best, I asked him about modeling coils on collinear antennas. He indicated these types of coils are part inductor and part simple wire. A perfect inductor will have constant current through its entire length. Coils like these will actually vary the current just like a straight wire. His advice was simply to model the helix shape in NEC to account for the various effects.
I did just that and modeled the coil with a fixed diameter appropriate for #12 solid copper wire wrapped around 3/4 inch PVC material.
Examining Collinear Antenna Vertical Element Separation Distance
My first task was to figure out the best vertical separation. I used EZNEC to try different vertical spacings with the goal of increasing the gain before the sidelobes get too large. Figure 3 shows some of the results…
The traditional curved stub, in red, has a very nice pattern with good control over high angle elevation confirming Cebik’s findings in his J-Pole research “Some J-Poles I Have Known”. The other patterns reveal the onset of sidelobes at 16 and 24 inches. In the end, I choose 16 inches as the proper compromise between gain and lobes.
One painful thing to notice in these freespace simulations is how little extra gain is available using more separation. It amounts to 0.8 dB difference. However, this does bring gain advantage over a half-wave (not shown in the graph) closer to the hopeful 3 dBd.
A 16 inch coil made by wrapping 22 turns of #12 solid bare copper wire around a piece of 3/4 inch PVC was the best answer. However, let’s explore the design space a bit. Let’s see what happens to the antenna pattern if we keep the same 16 inch spacing, but trade between coil diameter and number of turns.
The single turn coil provides the least amount of “middle half-wave” radiation mitigation so you can see some side lobes. This improves quickly with 2 or more turns. For all practical purposes, the pattern for all coil-turns is identical. This NEC simulation tells us the pattern is controlled almost entirely by the separation distance no matter how you manufacture the phasing coil.
Comparison of “Coil-Super-J” to the Super J-Pole Antenna
Figure 5 compares four items:
- Our final design with 22 turn coil wrapped around 3/4 inch PVC,
- The 16 inch collinear with a 1 turn coil,
- The Super-J-Pole with the curved stub,
- Traditional J-Pole
The 22 Turn Coil Collinear shows about 2.6 dB over the half-wave regular J-Pole. The traditional Super-J shows 1.8 dB.
Wire length in Collinear J-Pole Phasing Coil
Many think the wire length in the coil should be exactly 1/2 wave long. However, it took about 80 inches of wire to make the 22 turn coil. This is about 1 full wavelength of wire. I had to double check the currents along the segments in NEC to be sure I wasn’t fitting a full wave in the coil. I wasn’t.
The following table lists the resulting coil lengths after optimizing each coil design to achieve resonance.
|# Turns||Diameter||Conductor Length|
Table 1 – Phasing Coil Parameters
As the coil behaves more like an inductor the magnetic fields retard the changes in currents thereby forcing the actual length of wire longer for the same total phase change.
The following graph illustrates how wire length changes with coil dimensions.
Despite the extra wire, all the coils above are 1/2 wave in “electrical length.”
Results during the AT Golden Packet
With no time for testing, we took the Hiking Super-J to Stony Man in Shenandoah National Park for the ATGP event. We brought the new phasing coil along with the older stub in case it didn’t tune up or hear signals. The good news it did tune up and it did hear quite well. We got the same good APRS signal reports as the previous years. This, however, is not much of a test since mountain top to mountain top, despite the 100 mile distance, is just not that difficult. All the signals were S6 or better.
I think you will agree the coil collinear J-Pole is more sleek than the my traditional ugly-stub version used on previous ATGP outings.
Real measured results of the Collinear J-Pole
NEC simulation, when used correctly, is a proven tool for antenna design. However, real measurements certainly give the designer more comfort.
I had good fortune to test this new antenna configuration using a real measurement system. I tested the Hiking J-Pole in both the old and new configurations. The base of the J was about 18 inches above ground. I also tested a simple three radial quarter wave mono-pole for reference with its feedpoint 48 inches above the floor.
The Hiking Super-J using the coil is certainly the best of the three antennas. Both Super-J configurations beat the Monopole handily. Since there is a real “ground” in this test environment, the taller Collinear J has a clear advantage.
Here is the coil made with #12 solid copper wire wrapped 22 times around 3/4 inch PVC resulting in a coil diameter a bit over 1 inch.
My target length was 16 inches, but wound up being 16.5 because of measurement creep. 2m is pretty forgiving of errors this small. Figure 10 shows how the wire is secured.
This coil should work as the phasing stub in any 2m collinear antenna, but I can only vouch for the one I made. Dimensions for the entire antenna are forthcoming.
One thing to keep in mind is this coil should be the thing you adjust when bringing the antenna into resonance. I used this approach in NEC and came out alright. If you insert your coil into your collinear project and the tuning changes, check if your are too high or low in frequency and remake the coil longer or shorter (electrically not mechanically) respectively by changing the number of turns across the 16 inches.
Observations of the Collinear J-Pole
Using a small diameter coil for proper phasing and spacing of collinear elements in the Super-J rewards the builder with higher gain and relatively sleek look. Any version of the Super-J will be quite a bit better than a standard J-Pole or simple mono-pole. However, at 2.6 dBd this new version gets tantalizingly close to the theoretical 3 dB over a half-wave goal.
Phasing stubs are always cumbersome and the coil does a fine job making this a simpler mechanical assembly. As the coil is wound around PVC, the entire antenna can be made from home center components with ease.
Designing the coil is not at all straightforward using traditional inductor calculation techniques. I used NEC to design the one above and it was spot on target. EZNEC or any electro-magnetic design tool is essential for proper design of phasing coils. Otherwise the only method left is trial and error prototypes.
Improvements to the Super-J are possible and practical by replacing the phasing stub with a phasing coil of proper length. Simulation, testing and real measurement reward the builder with gain better than maverick J-pole designs such as the well meaning, but lackluster Collinear 5/8ths wave j-pole.
What should we call this new configuration?
- Coil Super-J?
- Collinear J-Pole
For now “Collinear J-Pole Antenna” makes sense.
- “Stacking Broadside Collinear” – Charles Rauch (W8JI)
- “The Case of the Curly Collinear” – L. B. Cebik
- Steven Best – Antenna Instructor
- “Some J-Poles That I have Known – Part 4” – L. B. Cebik
- Johnson, Richard C. and Henry Jasik (1961-1984). Antenna Engineering Handbook. (2nd ed., pp. 27-14). New York: McGraw-Hill.