Many thanks to Brian (K3ES) who shares the following guest post:

The VK160 Antenna packed on its Winder/Feedpoint for storage, transport, and deployment.

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.

Objective

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 “50” portion of the FT50-43 toroid designation specifies its 0.50 inch (1.27 cm) outside diameter. The “43” portion designates nickel/zinc composition that is suitable for high frequency inductive applications.

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.

FT50-43 toroid with three-10 inch (25.4 cm) segments of 24 AWG enameled magnet wire staged for construction of the 9:1 unun.

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. Continue reading In Pursuit of the Top Band: Brian describes how he built and tested a field-portable 160 meter EFRW antenna →

Field Report: Winter Field Day with a New Antenna

Winter Field Day (WFD) 2022 found me operating indoors, despite my best intentions to get out in the field. I was recovering from a bad cold, and did not want to risk having complications arise from sitting out in the cold and damp. So for WFD 2023, outdoor time with my radio was a must. I wanted to operate a 1-Oscar station, and to do that I had to set up more than 500 feet from my home QTH. So a site next to the pond in our back field, about 800 feet from the house, became my WFD shack.

I started planning and collecting kit for the operation in mid-January, with the long term forecast hinting at near-freezing temperatures with the potential for rain or snow. Let me introduce you to my WFD station.

Shelter

A shelter was needed to keep out rain and snow, and to provide a barrier against the winds whipping across the adjacent field. A few poles lashed together, and steadied by stakes, provided the frame to support the shelter. With a poly tarp secured to this frame, the resulting a-frame shelter, while not completely enclosed, did provide effective weather protection for both operator and equipment. There was about 3 inches of snow on the ground when I set up the shelter, and I was able to pile some of it along the bottom edge of the tarp to keep the wind from getting under it.

A-frame shelter constructed of a poly tarp stretched over a frame of poles

Antenna

I know that when setting up a station, the antenna is not generally the first thought, but I had recently finished building a homebrew 9:1 random wire antenna with a 144 ft radiating wire, and I wanted to test it out. There are better ways to put a new antenna on the air, but the opportunity was a good one. Besides, I had backup antennas that could be quickly substituted if the need arose. In the event, things worked well, and the antenna proved agile and capable on all activated bands. I particularly wanted to see if it would tune on the 160m band. More on that later.

The feedpoint of the 9:1 random wire antenna is built into a wire winder to support transport, deployment, and recovery for field use. The RF functional components include a female BNC connector, a 9:1 unun, and miniature banana jacks for connecting the radiator and counterpoise wires. There is no wire on the winder, because a corner was broken off during WFD deployment, and a mechanical repair is needed. (Click to enlarge)

The 9:1 unun is mounted near the BNC connector, and is electrically connected to a one miniature banana jack for the radiator and another for the counterpoise. The unun and wiring are covered with hot melt glue to provide mechanical integrity and protection from the weather.