Helical Antennas And Leaky Feeder: Performance Comparison For Underground High-Bandwidth Applications

Executive Summary

This report aims to provide a comprehensive comparison of the performance of leaky feeder and helical antennas, based on real-world deployments across various underground wireless projects. The analysis is focused on key performance indicators (KPIs) such as signal coverage, signal strength, multipath interference, and polarization efficiency. 

By evaluating these factors, we aim to provide informed recommendations for selecting the most suitable antenna type for underground high-bandwidth applications.

Discussion

Signal Coverage

In the case of leaky feeder systems, they are known to provide uniform signal coverage in confined or irregularly shaped environments such as tunnels, mines, and underground facilities. The distributed nature of the leaky feeder system helps maintain consistent signal strength throughout the entire coverage area. However, leaky feeders have lower performance and reach on data-specific communication protocols like Wi-Fi or cellular (anything above 1 GHz) due to losses along the cable and after the signal leaves the cable. Additionally, linear polarization propagation varies greatly depending on tunnel dimensions and surfaces.

On the other hand, helical antennas are well suited not only for open-air environments and point-to-point communication but also for confined or irregularly shaped environments like tunnels, mines, and other underground facilities. Their directional nature allows for more focused signal transmission and reception, resulting in a longer communication range. All tunnel measurements show that either vertical or horizontal polarization propagates better, depending on the specific tunnel dimensions. Using helical antennas ensures that regardless of which polarization propagates better in a specific tunnel, the circular wave, with all polarizations, naturally adapts and ensures whichever polarization happens to propagate best gets to the other side more effectively, ensuring a larger reach and coverage.

Leaky feeders are generally narrow band, e.g. they work for 900MHz only or 900+1800MHz only. Helical antennas may be designed for wide band deployments as required in modern 4G/5G networks. Network expansion is becomes easier, when frequency bands are ‘farmed’, changed or optimised. A wide band antenna is more future proof and prevents decommissioning of the old system to upgrade it for the new frequency bands (e.g. adding 3.5GHz) or the need to double up on the system and add a new leaky feeder for the specific frequency band.

Signal Strength

Leaky feeders generally exhibit lower signal strength compared to helical antennas due to signal leakage along the cable. Above 1 GHz, leaky feeders require too many in-line amplifiers to keep the signal at acceptable levels. These amplifiers require multiple points of power to be added along the tunnel and cause RF complications, such as noise in the system, failures that are not easy to detect, and poor C/I. In contrast, helical antennas typically provide higher signal strength due to their focused and directional radiation pattern. The signal strength may vary depending on the antenna’s orientation and distance to the transmitter.

Multipath Interference

Leaky feeder systems send signals indiscriminately around them. For low C/I applications or low-bandwidth applications, this might be acceptable, but for higher bandwidth applications, this can be problematic, as higher signal levels (better C/I) are desired over a limited area per access point. Leaky feeders offer no real MIMO benefit, as there is no polarization diversity and almost no spatial diversity, leading to lower bandwidths and higher latencies. At a time when 4×4 radios are becoming increasingly common, this puts significant constraints on system performance. Moreover, leaky feeders remain linear and single polarized, making it challenging to combat fading and polarization shifts.

In contrast, helical antennas have been found to be more resilient when encountering obstacles such as locomotives and hauling machinery in a tunnel. Certain polarizations propagate better past these obstacles than others, meaning that circularly polarized antennas containing all polarizations ensure the best possible propagation past such obstacles. Circular polarization gets past obstructions in tunnels much more reliably because certain polarizations always propagate better than others around such obstructions.

Polarization Efficiency

Linear polarization rotates unpredictably down a tunnel, which means that there is little or no signal for the linear antenna on the end-user device (laptop, smartphone, tablet, vehicle) at certain points of the tunnel. Additionally, linear polarization experiences first reflections interference, as linear polarization changes phase upon the first reflection. This causes a reflected wave, coming off the side of the tunnel, that interferes with the direct wave, resulting in significant signal variations with changes in distance.

Helical antennas use circular polarization, which contains all polarizations, helping to maintain signal integrity and reduce the impact of polarization mismatch and multipath fading. It has been proven that, multi polarisations are able to propagate past a locomotive (with minimal impact) in a 3m diameter tunnel – and this will have similar benefits for vehicular tunnels. Circular waves change from right-hand circular polarization (RHC) to left-hand circular polarization (LHC) upon reflection, which means they do not interfere after the first reflection, significantly reducing multipath interference.

Conclusion

In conclusion, helical antennas demonstrate significant advantages over leaky feeder systems for underground wireless communication applications. Their ability to provide focused signal transmission and reception, adapt to specific tunnel dimensions, and maintain signal integrity through circular polarization make them an ideal choice for such environments. Helical antennas also outperform leaky feeders in terms of signal strength, multipath interference reduction, and polarization efficiency, which directly translates to better performance in higher bandwidth applications and improved overall system resilience.

Furthermore, helical antennas have proved to be more effective in overcoming challenges posed by obstacles in tunnels and adapting to various polarization scenarios. This adaptability ensures more reliable communication and better coverage in underground environments.

Given these substantial benefits, helical antennas appear as the superior choice for underground wireless communication systems compared to leaky feeder solutions. By selecting helical antennas, organizations can improve the performance, reliability, and efficiency of their wireless communication infrastructure in underground settings.

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