RadCom April 2024, Vol. 100, No. 4

April 2024 19 Regulars performance which matches that of a mono-band half-wave vertical antenna, or a ground-mounted quarter-wave vertical antenna. Again, using 5° elevation as our benchmark, we see a -5.7dBi figure which matches the performance of a ground- mounted quarter-wave vertical antenna for 20m with 16 quarter-wave ground radials (see Figure 3 ). 10m band Perhaps counter-intuitively, the largest compromise is presented on the 10m band. This antenna is a full-wave vertical antenna, and once a vertical antenna become longer than around 0.8 λ on a given band, its strongest lobes begin to transfer from angles below 20° to higher elevations. In this case, on 10m, the antenna produces its best performance at around 35° elevation, which is around 15° or so higher than a typical ground- mounted half- or quarter-wave vertical antenna. Moreover, at our benchmark 5° elevation angle, performance drops from the -5.5dBi or so figure, expected from its shorter vertical cousins, to -9.0dBi. The saving grace is that when 10m fully opens, it often does so without half-measures, so DX is still workable on 10m with this antenna, albeit approaching an S-unit weaker than the other verticals mentioned. It is interesting, though, to note that when on 10m, this antenna is a slightly greater low-angled compromise than it is on 40m. Anticipated VSWR bandwidth The modelled 2:1 VSWR bandwidth offered by this antenna gives us few surprises. On 10m and 20m, the bandwidth is just over 1200kHz and 1110kHz respectively. This is more than ample to give coverage across the segments of the band you require. On 40m, however, as the antenna is approximately 60% of the length of a full-sized half-wave, bandwidth is much narrower, at around 70kHz. A choice therefore needs to be made between operating on the lower portion of 40m using CW or digital modes, or moving above 7.100MHz using SSB. Further development There is no doubt that this antenna checks boxes in terms of small-footprint HF operating and convenience when portable. Can its design be tweaked or enhanced? Here are a couple of options: Firstly, changing its orientation from vertical to sloping markedly changes its characteristics. The antenna can be fed near ground level and slope up to a height of around 8m. This would necessitate to have at our disposal around 9m of horizontal space (less than a quarter of a wavelength). On 20m, the vertical antenna provides worse coverage at lower elevation angles, with the gain at 5° weakening to between -11.6dBi and -6.5dBi, favouring the slope. This contrasts with the figures we see in Figure 3. On 10m, however, the sloping antenna gives us with some very useful lobes, peaking in some directions at -2.2dBi. This contrasts favourably with the -9.0dBi of a vertical full-wave antenna. However, it is arguably on 40m that we see a real transformation. As a vertical, and typically for this polarisation, the vertical antenna suffers in comparison with a low-height dipole, for example, at higher elevation angles. The deficit we noted earlier was in the region of -21dBi (3.5 S-units). But by sloping the antenna, we can increase its performance at 75° elevation by around 12dB to 14dB to between -1.4dBi and -3.4dBi. This allows the antenna to offer almost the same omni-directional coverage as we had as a vertical antenna on 40m, but now to within 1 and 1.5 S-units of a low-height, full-sized dipole. There is also the added benefit that, if permissible, we need only 9m of horizontal space, rather than close to the 16m-20m we would typically need for a full-sized half-wave inverted-V dipole for 40m with an apex of between 5m and 8m above ground. Secondly, and perhaps ambitiously, we could look at adding two tuned traps to accommodate the 15m band as a half-wave vertical antenna, and convert 10m operation into a half-wave rather Tim Hier, G5TM timhier@icloud.com Table 1: Comparing the modelled performance of the 3-band and 4-band versions. Band 40+20+10 version (dBi) 40+20+15+10 version (dBi) 40m -7.7 -9.2 20m -5.7 -6.4 10m -9.0 -5.4 FIGURE 2: The full-sized 40m quarter-wave antenna (red) is around 2dB better than this antenna (black) at a 5° elevation angle. FIGURE 3: The half-wave vertical antenna compares favourably with a ground-mounted quarter-wave antenna (plus radials) on 20m.