STL propaganda indeed: QW vertical – dipole – STL model pattern comparison

STL propaganda indeed: dipole – STL pattern comparison compared the patterns of a Inverted V dipole and STL, both configurations typical of SOTA deployments.

Seeing some pretty wild extrapolations to a vertical quarter wave with elevated radials, again typical of SOTA deployment, this article presents a comparison of all three using NEC-4.2 models.

See STL propaganda indeed and STL propaganda indeed: dipole – STL pattern comparison for details of the models for the STL and dipole.

The QW vertical is modelled using 2mm dia copper wire for vertical and radials, the radials are elevated 0.5m over ‘average ground’ (0.005,13).

Bear in mind that these are models that are based on some assumptions like ground parameters for example, and results may be different for other scenarios. Likewise, the results at 20m cannot simply be extrapolated to other bands, and practical modes of propagation utilised vary from band to band.

Key differences

Polarisation

Polarisation is a significant difference. Vertical ground waves are attenuated more slowly than horizontal waves, though ground wave propagation is not so commonly exploited on 20m due to its very short range. Because vertical ground waves are attenuated more slowly, a vertical polarised receiving antenna is likely to capture more ‘local’ noise that a horizontal one, but in SOTA context, local noise is not such an issue on mountain tops.

The QW vertical is vertical polarisation.

The STL is vertical polarisation.

The Inverted V dipole is horizontally polarised broadside to the dipole, and tends to vertical polarisation off the ends.

Radiation pattern

Radiation pattern is a 3 dimensional characteristic, often selectively plotted in two dimensions in the most favorable plane… which is fair enough but the reader needs to keep in mind the bigger three dimensional characteristic as it applies to their own application.

The radiation patterns of the antennas are quite different, the vertical is omnidirectional in azimuth whereas the others are not. So, it is challenging to produce a single general figure of merit comparing all antennas.

Above is a comparison of gain in the plane of maximum gain of the STL and dipole. 

The gain of the QW vertical might be surprising initially, but keep in mind that it is a QW vertical with four radials mounted 0.5m above real ground.

At the zenith, the dipole is a standout winner so for NVIS it is the obvious choice… however NVIS is not such a common mode for SOTA on 20m.

 

Above is the polar pattern at 30° elevation. At some azimuth angles the dipole is better than the QW vertical, and vice versa at others… on average there is not a lot in it. The consistent and large loser is the STL.

Hams wax on that a QW vertical is the best thing for DX. Broadside, the QW vertical is superior at elevations less than about 13°, and some DX is at such low elevation but some is well higher. End on and the picture is quite different, the chart above shows that at 30° elevation the QW vertical is around 4dB better.

How do you reduce this to a single dB figure of merit without making crude assumptions that discard important detail of the differences between the antennas. A field trial will provide comparative results, but they will depend on the stations contacted, including their direction and path elevation.

Individuals will have their own ideas about which is more convenient for deployment, anecdotes about what works and what doesn’t but there is a scarcity of methodical formal scientific field experiments, it just isn’t the amateur’s way.

Richard’s (G3CWI) experiment Comparing the performance of an inverted vee dipole with a small transmitting loop on 20m is an exception though it excites defensive criticism of hams with certain beliefs (aka prejudices). The results Richard obtains are quite consistent with valid NEC models of the two antennas, an experiment comparing a QW vertical (typical of SOTA deployment) with the same STL would be interesting.