Many thanks to Brian (K3ES) who shares the following guest post:
Building and Testing the VK160 Antenna
by Brian (K3ES)
The ability to set and achieve long- and short-term goals keeps me interested and active in the Parks on the Air (POTA) program. Often these goals are associated with POTA awards. Currently, I am working slowly to complete the activator version of the James F. LaPorta N1CC award, which requires an activator to make QSOs on 10 amateur bands from 10 different parks. With my operating style, I have found it achievable to make QSOs on the 9 available HF bands (80m, 60m, 40m, 30m, 20m, 17m, 15m, 12m, and 10m), and this has become easier with the rising solar cycle. I have completed QSOs on non-HF bands using 2m and 70cm simplex. The other options to pick up 10th band QSOs include the 6m band and the 160m band.
I have found it difficult to make unscheduled POTA contacts on 2m and 70cm, and scheduled contacts can be difficult to arrange in parks that are remote from population centers. I have built a 6m antenna, but contacts are seasonal (and for me very elusive). So I started looking for a way to add 160m capability to my portable station. Ultimately that resulted in homebrewing a new antenna that I now call the VK160, and here is its story.
I needed a field-deployable 160m antenna. My operating style requires that the antenna system be both light and compact. QRP power levels are sufficient for my purposes. I am very comfortable deploying wire antennas in the Pennsylvania woods, and QRP wire antennas can be both light and compact. I have found that end-fed antennas are simpler to deploy in the field, because they can be configured as an inverted V or as a sloper, using only one point of support.
An end-fed half wave (EFHW) antenna would be naturally resonant, but would need to be over 250 ft (76m) long. A wire antenna of that length would be challenging to deploy, even in more open areas. So, I decided to pursue a 9:1 unun-based end-fed “random wire” (EFRW) antenna. In fact, I have two commercial EFRW antennas available, but have never been successful in tuning them for 160m using the ZM-2 tuner in my field kit. So, I concluded (probably incorrectly, but more on that later) that I needed to build a 9:1 random wire antenna with a longer radiating element than the 71 ft wire built into my largest existing EFRW. I also wanted to build this antenna myself, using available components, so that it would be both inexpensive and customized to my needs.
I broke the task into four parts:
First, I needed to build a 9:1 unun suitable for use at QRP power levels. The 9:1 unun is an autotransformer that reduces antenna feedpoint impedance by a factor of 9, hopefully a level that a wide-range tuner can match to the 50 ohm transceiver impedance.
Second, I had to design and build mechanical elements of the antenna system, incorporating the electrical components needed for the feedpoint.
Third, I needed to select a suitable non-resonant wire length for the radiator.
Finally, I needed to deploy and test the finished antenna on the air. If successful, testing would culminate in completing an on-air QSO with the antenna being driven at 5 watts or less.
Building the 9:1 Unun
While I have built successful 49:1 ununs as the basis for EFHW antennas, I had no experience building 9:1 ununs. Accordingly, I started with the ARRL Antenna Book, then a web search. VK6YSF’s excellent web page provided very detailed instructions for 9:1 unun construction. His 9:1 Unun design was based on a FT140-43 toroid wrapped with heavy gauge magnet wire, with design power rating around 100 watts. My application was focused on 10 watts maximum, and I wanted a lighter-weight solution to the unun design.
Looking at the components I had available, I found FT50-43 toroids and 24 AWG magnet wire in my inventory. I had used those during construction of successful 49:1 EFHW antennas. The VK6YSF design, built with the smaller toroids and lighter magnet wire, seemed to be a good (and cheap) starting point.
The next problem that presented itself was a problem with translating the winding technique to smaller wire and a smaller toroid. Put simply, my fingers do not have the dexterity to wrap three parallel 24 AWG wires around a ½ inch OD toroid without getting them crossed, twisted, or worse. So, why not twist the three conductors from the start, and wrap the toroid with “trifilar” windings? It would be simple enough to identify the mating wire ends after wrapping, just with a set of continuity tests. That would facilitate proper connection of the wires to yield the final auto-transformer configuration.
I posed the “trifilar” winding question to my friends over on the QRPer.net discussion board. Nobody identified a significant flaw with the proposed method, but neither did anyone have experience that would assure success. So, I decided to use the “trifilar” winding technique to construct my 9:1 unun, with the full recognition that its success would be uncertain, and only proven by testing the finished product.
In the end, I found that 10 “trifilar” turns fit neatly on the FT50-43 toroid, so that is how I assembled the final component.
One of the antennas I purchased commercially is a Packtenna 9:1 EFRW. I have always marveled at KJ6VU’s elegant design, where the circuit board for the electrical components is also shaped to serve as a wire winder. Without the sophistication to build my antenna feedpoint directly onto a circuit board, I still recognized the wisdom of attaching feed point circuitry to a wire winder for ease of deployment, retrieval, and transport of the antenna. I planned to repeat the successful format of using a combination feedpoint/winder (FPW) with the new antenna build.
With my first antenna projects, I searched for suitable and easily-formed construction material from which to assemble the FPW.
Among the materials in my shop, I found some pieces of 1/16” transparent acrylic sheet left over from a previous project. With a bit of experimentation I have found that this material serves quite well in constructing antennas. It can be cut, sanded, drilled, and solvent-glued, and it can also be formed into convenient shapes by softening with a heat gun, and bending while soft. I retrieved some acrylic sheet and set to building. I built FPW longer normal to minimize the number of wraps needed to store the radiating wire, since I expected the wire to be more than double the length of any of my other antennas.
The FPW body is about 12 inches (30 cm) long. It has U-shaped notches at each end and a relief cut along one side to simplify winding the wire in a figure-8 pattern, which prevents tangling during antenna deployment.
I attached the electrical components to the FPW. This included mounting the female BNC connector to an acrylic bracket that was heat-formed into an L-shape, then solvent-glued to the winder. This bracket also served as an anchor point for the 9:1 unun and for two miniature banana jacks used to electrically connect the radiator and counterpoise wires. Once the wiring of all components was complete, I applied a generous amount of hot melt glue to secure and weatherproof the electrical components and connections.
In attaching the radiator to the antenna feed point, I consider it essential to provide strain relief. Drilling 3 or 4 small diameter holes through the acrylic sheet structure, then stitching the wire up and down through those holes, supports any load on the wire without stressing the electrical connection. I used the same method to attach the far end of the antenna wire to a small link cut from acrylic sheet that supports it when deployed. The FPW and the link have drilled holes holding split rings with small carabiners for quick attachment to ropes, trees, or other support structures during deployment.
I broke the FPW when I deployed the antenna for Winter Field Day (WFD). I tossed it across a 10 foot (3m) section of frozen pond to avoid walking across the thin ice, and it hit the base of a sapling on landing, and the side of one of the winding notches broke off. The break resulted from impact combined with the acrylic being brittle from the cold.
The damage prevented wire from being wound onto the FPW, so mechanical repair was necessary. This repair consisted of gluing additional layers of acrylic sheet across the broken end, reinforcing it and providing sufficient material to rebuild a notch. A bit of hand-work with a file created the new notch, so that the FPW could again be used for storing the radiator wire.
Selecting the Radiator Length
The “random wire” to be paired with a 9:1 unun can be better described as a non-resonant wire. Recall that a resonant wire fed at its endpoint has very high impedance, the reason that EFHW antennas use an unun with 49:1 or greater impedance transformation ratio. For a non-resonant end-fed “random” wire (EFRW) the length must not be resonant on any of the bands of interest, or the 9:1 transformation ratio would be too small to permit matching with most available antenna tuners. This means that the selected length must be neither a half wavelength, nor an integer multiple of a half wavelength, on any of the bands of interest.
My intent was for this antenna to function on all amateur bands from 160m to 10m. To improve the likelihood of success, I restricted my focus to the CW portions of those bands. Table 1 (below) presents the key elements of the analytical method, along with the analysis results. The first column identifies the band of interest. Columns 2 and 3 specify the maximum and minimum frequencies in MHz. Wavelength calculations are based upon the standard dipole formula: λ/2 [feet] = 468 / f [MHz]. Columns 4 and 5 present the minimum and maximum half-wavelengths associated with the CW frequency interval (all lengths in Table 1 have units of feet).
Further analysis is based upon a target radiator length range of 130 – 150 ft. Column 6 presents a series of integer values (n) for each band for which nλ/2, (n+1)λ/2, and sometimes (n+2)λ/2 bound the target range for radiator length. Column 7 and 8 present the minimum and maximum, respectively, of nλ/2 for each resonant interval being examined for that band. Finally, Columns 8 and 9 present the usable minimum length [nλ/2max] and usable maximum length [(n+1)λ/2min] for that band in the target range (130 – 150 ft).
By taking the maximum (across all bands) value of Usablemin and the minimum (across all bands) value of Usablemax, an all-band non-resonant interval of 139.0 – 148.8 ft emerges. I selected my radiator length as 144 ft, the midpoint of that interval.
I fabricated the radiator from 26 AWG PolyStealth antenna wire, a strong, light-weight material which has served me well with past antenna builds. I cut a 144 ft length, and soldered on a miniature banana plug that mates with the radiator jack built into the FPW. Using a banana plug provides the option to easily replace the radiator wire with one of a different length, if necessary for this experiment, or to meet a future need.
I provided the option to add a rapidly-deployable counterpoise system to the antenna, enabling adjustment of the antenna tuning characteristics, if necessary. I arbitrarily cut three 17 ft lengths of 26 AWG PolyStealth wire and soldered them together into a miniature banana plug that mates with the counterpoise jack on the FPW. During testing, this counterpoise system was always installed, with the three wires spread out on the ground, radially from the FPW.
Table 1: Analysis of Non-resonant Length Windows Between 130 ft and 150 ft
(f in MHz, and lengths in feet)
Testing the Antenna System
With the antenna build complete, I made two attempts at testing. The first, which was not fully successful, involved deploying it in the back yard, and attempting to tune it using my Emtech ZM-2 QRP tuner. I used my conventional inverted-V configuration with the apex up about 30 ft (9m), and I deployed three-17 ft counterpoise wires on the ground around the feedpoint. I set up my Lab599 TX-500 Discovery with the ZM-2 in the feed line, and went through the following tuning routine on each amateur band from 10m to 160m.
I first adjusted the ZM-2 controls to maximize the volume of received noise. Next, on a clear frequency, I switched the ZM-2 to its tune mode, sent a continuous tone signal from the TX-500, and adjusted the ZM-2 controls to minimize brightness of its tuning LED (ideally the LED went fully dark). Following adjustment, I switched the ZM-2 back to operating mode, and noted the SWR level indicated on the TX-500, and stopped sending the tone. On all bands, the indicated SWR was 1.5:1 or less (often 1:1), with one exception. I was unable to achieve an acceptable tune on 160m.
The ZM-2 performed flawlessly in its specified frequency range, comprising amateur bands from 80m to 10m. Using the ZM-2 to tune the 160m band was a bridge too far, so an alternate solution was required. I had hoped that the ZM-2 might achieve an acceptable tune on 160m with the longer 144 ft radiator wire, where it had been unsuccessful tuning 160m with my 9:1/71 ft random wire antenna.
I thought the experiment was worth trying, because the ZM-2 is light, robust, and requires no power beyond the transmitted RF signal for operation (all features that are beneficial for portable operations). But alas, it was not to be.
For the next test of the new antenna, I brought out my LDG Z-11Pro II autotuner. It is a wide-range tuner rated for 100 watts on amateur bands from 160m to 6m. While not as light as the ZM-2, the Z-11Pro II works well for portable operation. It requires power for operation, but can run on an internal AA battery pack, or can be powered by the same 12V LiFePO4 battery that powers the radio.
With Winter Field Day coming up, I decided to take a risk and use it as an opportunity to deploy, test, and hopefully operate with the new antenna. Once again I set it up again as an inverted-V, but this time with the apex only 20 feet (6m) in the air. Again, I used three-17 ft counterpoise wires arranged on the ground around the feedpoint. This test was more than successful, ultimately giving me cause to name the new antenna VK144. The details of my Winter Field Day operations were reported in a previous QRPer.com Field Report, linked here: Brian puts a new antenna to the test during Winter Field Day!
Here, suffice it to say that I successfully tuned the antenna for operation on 10m, 15m, 20m, 40m, 80m, and 160m. Further, I made successful WFD contacts on all bands except for 160m. Due to the concurrent CQ 160m contest, I made a successful 160m contact with a contest station rather than a WFD station.
I am very pleased with the performance of the VK144 / Z-11Pro II autotuner antenna system. I will be looking for opportunities to put it on the air to get 160m contacts from POTA parks, furthering my slow pursuit of the James F. LaPorta N1CC Activator award. I really hope my story has demonstrated that building your own antenna is achievable, rewarding, and fun.
If I can do this, you can too!
Following is a list of the specific materials that I used to build the VK144 antenna. I have successfully used it to make CW contacts on HF and 160m amateur bands at QRP power levels. Other methods and solutions are numerous, limited only by your imagination and ingenuity.
- 1/16 inch (1.5mm) transparent acrylic sheet (Plexiglas®) as FPW structural material
- Solvent for acrylic welding (use with caution, according to manufacturer’s instructions)
- female BNC connector
- 2-miniature banana plugs and 2-mated jacks
- FT50-43 toroid
- 24 AWG enameled magnet wire
- 26 AWG PolyStealth antenna wire
- Flux-core solder suitable for electronic use
- Hand tools for measurement, fabrication, and soldering
Best 73 de Brian – K3ES