The Sigma Euro-Comm SE HF-360 XP Top Band, is a multiband end-fed vertical antenna designed for operation from 160 through 6 metres with a suitable antenna tuner. The antenna is approximately 5.5 m tall, is fed with 50-ohm coaxial cable.

This version is particularly useful where wide HF coverage is required but there is insufficient space for full-size antennas. However, operation on bands such as 160 and 80 metres should be considered in the context of the antenna's physical length. At 5.5 m, the radiator is electrically very short on the lower bands, so overall system efficiency depends heavily on the installation and, in particular, on the available RF ground or counterpoise system.

Importance of RF Ground and Counterpoise

An end-fed vertical requires an RF return path. If a dedicated counterpoise or radial system is not provided, the return current may instead flow through the outside of the coax shield, the mounting mast, station grounding conductors or other nearby conductive structures. This can make antenna behaviour dependent on coax length and routing, mast dimensions and the surrounding installation.

This becomes increasingly important on 160 and 80 metres, where a properly implemented RF ground, counterpoise or radial system can help provide:

• improved radiation efficiency,
• reduced RF return-path losses,
• more stable matching and SWR behaviour,
• reduced dependence on coax and mast dimensions,
• lower unwanted common-mode current on the feed line.

The lower the operating frequency, the shorter the 5.5 m radiator becomes electrically. A low SWR achieved by an ATU should therefore not be interpreted as proof of high radiation efficiency. The tuner provides impedance matching to the transceiver; it cannot compensate for RF power lost in an inadequate ground or return-current system.

The manufacturer's instructions state that the antenna may be mounted at ground level or elevated and note that testing showed particularly good results when installed relatively close to ground, up to approximately 3 m. The 4:1 UNUN is also DC grounded, so adding a conventional safety earth does not by itself alter the transformer's tuning ratios. This should not be confused with an RF counterpoise or radial system, which forms part of the RF return-current path and can materially affect antenna performance.

Specifications

• Type: End-fed vertical antenna
• TX Frequency: 1.8–52 MHz with suitable ATU
• RX Frequency: 1.8–55 MHz
• Bands: 160 / 80 / 60 / 40 / 30 / 20 / 17 / 15 / 12 / 10 / 6 m
• Maximum Power: 1,000 W PEP / CW
• Digital / Continuous Carrier: up to 200 W
• UNUN: 4:1
• System Impedance: 50 Ω
• Connector: SO-239
• Height: approximately 5.5 m
• Construction: 3 fibreglass sections plus stainless-steel top whip
• ATU: Required for multiband operation
• RF Ground / Counterpoise: particularly important when optimising operation on the lower HF bands ManualsLib

The SE HF-360 XP Top Band offers a practical way to obtain broad HF coverage where antenna space is limited. On the higher HF bands its physical dimensions are naturally more favourable, while on 160 and 80 metres the quality of the installation—and especially the RF ground/counterpoise system—can have a major influence on real-world performance.