Article "The DX Prowess of HF Receivers" by Tadeusz Raczko, SP7HT, focuses on comparing the performance of HF receivers designed for amateur DXing and competition. The author points out that the classic laboratory tests of equipment manufacturers often use wide signal frequency spacings (50-100 kHz), which leads to optimistic results that do not correspond to real conditions in crowded amateur bands. In practice, weak DX signals are often received simultaneously with very strong local signals at a distance of only a few kHz.Therefore, SP7HT recommends testing at narrow spacings of 2–5 kHz, which better reflect actual conditions.
The two key evaluation parameters are blocking dynamic range (BDR) a intermodulation dynamic range (IMD DR). BDR expresses the receiver's ability to handle strong signals without loss of sensitivity, while IMD DR shows resistance to intermodulation interference arising from the combination of strong neighboring signals. Both BDR and IMD DR values drop significantly in real-world tests with close pitches, revealing how the devices perform in tough DX conditions.
Several models of ICOM, Kenwood, Yaesu, Ten-Tec and Elecraft brands were compared. They achieved the best results Elecraft K2, Ten-Tec OMNI-VI+ and heavily modified Drake R-4C, while these receivers maintained a high dynamic range even with closely adjacent signals. In contrast, modern transceivers with multiple mixers and wide first IF filters showed higher interference rates and worse results.
The Elecraft K2 and Ten-Tec OMNI-VI+ are designed exclusively for the ham bands and use simpler designs with one or two mixers. The main selectivity is already in the front part of the receiver, which significantly reduces interference. The K2 has a one-shot conversion, relay controlled narrow preselectors and adjustable crystal filters for both CW and SSB. The OMNI-VI+ uses double conversion with low intermediate frequencies (9 MHz and 6.3 MHz) and a double filter for even better selectivity.
The author also points out the importance low phase noise local oscillators. Too much noise causes a reduction in dynamic range and the appearance of cross-mixed signals that can completely cover a weak DX signal. In this respect, K2 and OMNI-VI+ significantly exceed most modern devices.
In conclusion, the SP7HT notes that if the goal is to receive weak DX signals in narrow bands, receivers designed exclusively for amateur bands provide better durability and performance than general-purpose devices with wide coverage. According to the author, the discovery of the K2 and OMNI-VI+ models represents a positive shift in the design of HF receivers and hope for the DX community that other manufacturers will follow suit.
Everyone makes their own conclusion. Here is the original article in PDF format 020910qex036.pdf.
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