RadCom April 2024, Vol. 100, No. 4

18 April 2024 Regulars Antennas A n antenna which allows access to the 40m band, with minimal need for horizontal space, is a prerequisite for those living with a small garden, or who wish to use a small footprint when it comes to deploying an antenna in a portable arrangement. Vertical antennas are often used in these situations, with the caveat that either elevated or ground radials are used, thus adding to the horizontal space required for operation. There is, however, a compromise solution available for consideration. This antenna requires minimal horizontal space (just for the coaxial cable) and, in its vertical configuration, a minimum height of approximately 12m, thus enabling its use at a portable location or at home by using a tree or a non-conductive 12m pole to hang the wire from. The antenna is, therefore, quick to deploy, produces some useful low-elevation-angle radiation, and provides access to 20m and 10m, as well as 40m, without using an external ATU. Antenna outline A modelled diagram for this antenna is shown in Figure 1 . It is fed using a high-impedance transformer at its base, typically a 49:1 or 56:1 impedance ratio. These transformers are readily available commercially, and plenty of guides exist online as to how to construct one for yourself. The version I used was a home- made 56:1 transformer, made by a good friend, who has researched extensively into the most- efficient options to allow for relatively-low losses from 10m through to 40m. A 50 Ω coaxial cable is then simply connected to the transformer, with the additional option of using a common- mode choke anywhere from 2m or more away from the transformer, or connecting a 2m-long counterpoise to the transformer’s ground-lug. I usually adopt the former practice, although I have yet to encounter any RF issues when choosing not to use a choke or counterpoise at 100W SSB. As Figure 1 indicates, thanks to the high- impedance transformer, the antenna provides coverage for three harmonically-related bands. The 20m band is covered as a full-sized end-fed half-wave antenna, the 40m band as a shortened end-fed half-wave antenna (around 60% of its full size), and on the 10m band there is also a good match to 50 Ω as a full-wavelength antenna. All three bands are fed at the high impedance end of the antenna, thus necessitating the use of this high-ratio impedance transformation from around 2,500 to 3,000 Ω to near 50 Ω . The coil positioned at the top of the 10m-20m portion of this antenna has a dual purpose. By achieving an inductance of around 32-35 µ H, this coil acts not only as a loading coil for 40m, but also as a resonant trap for 20m. The coil itself is easy to construct. A section of PVC pipe makes a good former for a coil. There are online calculators [1] available to help you determine the number of turns required, based on the diameter of the coil (including the diameter of pipe and wire combined), the thickness of wire used, and the inductance sought after. In my case, I used some spare 35mm PVC piping left over after doing some plumbing work, and 0.4mm thick insulated wire. By using the online calculator, I was able to determine that an inductance close to 34 µ H could be achieved by using 39 tightly-wound turns of this wire around the pipe, allowing for 4cm from beginning to end of the coil. The calculator suggests that should provide an inductance of around 34.5 µ H. I drilled two small holes approximately 5cm apart from each other into an 8cm-long piece of PVC pipe, using a 4mm drill. Having fed a small 4mm bolt and washer through the first hole, I then attached the wire to a 4mm ring terminal, and then to the bolt, using a 4mm nut to secure it. I was able to wind 38 complete turns before attaching the wire to a bolt at the other end. Attaching each of the probes from my Peak Atlas LCR45 meter onto opposite bolts, I found I had achieved an inductance of 33.4 µ h, close enough! Modelling performance Using MMANA-gal modelling software, we can obtain a good idea as to the behaviour exhibited by this antenna on the 40m, 20m and 10m bands. 40m band The omni-directional pattern associated with a vertical antenna is revealed in Figure 2 . We use a 5° elevation angle for comparison between a full-sized ground-mounted quarter-wave vertical antenna, with eight quarter-wave ground radials, and our shortened end-fed half-wave vertical antenna. The quarter-wave vertical antenna is approximately 2dB better than our shortened end- fed half-wave antenna. This is largely because of the reduced efficiency of the end-fed half-wave antenna through shortening of its length. However, the shortened half-wave antenna is within an S-unit of the full-sized quarter-wave antenna at this low elevation angle, and is a contender for working some DX contacts on 40m along the early-morning grey line, and in the late evening. Where this, and typically vertical antennas as a whole, suffers is in comparison with a low horizontal dipole for higher elevation angles. A low inverted-V full-sized half- wave dipole exhibits gain approaching 5dBi or 6dBi at 75° elevation. Our antenna, with its bias towards lower elevation angles, exhibits -15dBi at this same higher elevation angle, or around 3.5 S-units weaker than the dipole. 20m band This antenna shines on 20m. As a full-sized vertical half-wave antenna, it produces good low-angle FIGURE 1: A diagram showing the antenna’s configuration.