Folded Antennas; An Important Point of Clarification

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This technical paper highlights the ambiguity in the antenna technical literature regarding the radiation resistance of folded antennas, such as the half-wave folded dipole (or quarter-wave folded monopole), electrically small self-resonant folded antennas and multiple-tuned antennas. The feed-point impedance of a folded antenna is increased over that of a single-element antenna but does this increase equate to an increase in the antenna’s radiation resistance or does the radiation resistance remain effectively the same and the increase in feed-point impedance is due to transformer action? Through theoretical analysis and numerical simulations, this study shows that the radiation resistance of a folded antenna is effectively the same as its single-element counterpart. This technical paper serves as an important point of clarification in the field of folded antennas. It also showcases Plextek's expertise in antenna theory and technologies. Practitioners in the antenna design field will find valuable information in this paper, contributing to a deeper understanding of folded antennas.

This paper presents a comparison of Chilton ionosonde critical frequency measurements against vertical-incidence HF propagation predictions using ASAPS (Advanced Stand Alone Prediction System) and VOACAP (Voice of America Coverage Analysis Program). This analysis covers the time period from 1996 to 2010 (thereby covering solar cycle 23) and was carried out in the context of UK-centric near-vertical incidence skywave (NVIS) frequency predictions. Measured and predicted monthly median frequencies are compared, as are the upper and lower decile frequencies (10% and 90% respectively). The ASAPS basic MUF predictions generally agree with fxI (in lieu of fxF2) measurements, whereas those for VOACAP appear to be conservative for the Chilton ionosonde, particularly around solar maximum. Below ~4 MHz during winter nights around solar minimum, both ASAPS and VOACAP MUF predictions tend towards foF2, which is contrary to their underlying theory and requires further investigation. While VOACAP has greater errors at solar maximum, those for ASAPS increase at low or negative T-index values. Finally, VOACAP errors might be large when T-SSN exceeds ~15.

This paper presents the antenna G/T degradation incurred when communications systems use very inefficient receive antennas. This work is relevant when considering propagation predictions at HF (2-30 MHz), where it is commonly assumed that antennas are efficient/lossless and external noise dominates over internally generated noise at the receiver. Knowledge of the antenna G/T degradation enables correction of potentially optimistic HF predictions. Simple rules of-thumb are provided to identify scenarios when receive signal-to-noise ratios might be degraded.

This paper describes the design and characterization of a frequency-scanning meanderline antenna for operation at 60 GHz. The design incorporates SIW techniques and slot radiating elements. The amplitude profile across the antenna aperture has been weighted to reduce sidelobe levels, which makes the design attractive for radar applications. Measured performance agrees with simulations, and the achieved beam profile and sidelobe levels are better than previously documented frequency-scanning designs at V and W bands.

Signal power measurements for a UK-based network of three beacon transmitters and five receiving stations operating on 5.290 MHz were taken over a 23 month period between May 2009 and March 2011, when solar flux levels were low. The median signal levels have been compared with monthly median signal level predictions generated using VOACAP (Voice of America Coverage Analysis Program) and ASAPS (Advanced Stand Alone Prediction System) HF prediction software with the emphasis on the near-vertical incidence sky wave (NVIS) links. Low RMS differences between measurements and predictions for September, October, November, and also March were observed. However, during the spring and summer months (April to August), greater RMS differences were observed that were not well predicted by VOACAP and ASAPS and are attributed to sporadic E and, possibly, deviative absorption influences. Similarly,the measurements showed greater attenuation than was predicted for December, January, and February, consistent with the anomalously high absorption associated with the “winter anomaly.” The summer RMS differences were generally lower for VOACAP than for ASAPS. Conversely, those for ASAPS were lower during the winter for the NVIS links considered in this analysis at the recent low point of the solar cycle. It remains to be seen whether or not these trends in predicted and measured signal levels on 5.290 MHz continue to be observed through the complete solar cycle.

This paper shows that the Q-factor and VSWR of a monopole are significantly lowered when made resonant by reactive loading (as is used in practice). Comparison with a self-resonant Koch fractal monopole of equal height gives similar values of VSWR and Q-factor. Thus, the various electrically small monopoles (self-resonant and reactively loaded) offer comparable performance when ideal and lossless. However, in selecting the optimum design, conductor losses and mechanical construction at the frequency of interest must be considered. In essence, a trade-off is necessary to obtain an efficient, electrically small antenna suitable for the application in hand.

The design of a 16-element waveguide array employing radiating T-junctions that operates in the Ku band is described. Amplitude weighting results in low elevation sidelobe levels, while impedance matching provides a satisfactory VSWR, that are both achieved over a wide bandwidth (15.7-17.2 GHz). Simulation and measurement results, that agree very well, are presented. The design forms part of a 16 x 40 element waveguide array that achieves high gain and narrow beamwidths for use in an electronic-scanning radar system.