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Measurements on an FT-847 transmitter on 70MHzby Bo Christensen OZ1DJJ Bo, OZ1DJJ, did some measurements on his "TX open" modified FT-847. What he discovered was an output at 45MHz which was 10 dB stronger than his wanted signal at 70MHz!  When he measured the power through a broadband power meter the output was 10 watts: i.e. 9W out on 45MHz and only about 1W on 70MHz. There are also a couple of strong spurii at around 20-30MHz. We have subsequently found that Bo had selected the wrong logic line in the output BPF, which was responsible for the large out-of-band signal shown in the plot above. Although this signal may be attenuated, there still remain a multitude of spurii from 69-76MHz at around -40dBc, which can only be attenuated by a very high-Q external filter, so anyone contemplating modifying an FT-847 transmitter should only attempt it if they have expertise in filter design, and access to a spectrum analyser! See also this article for further details of the FT847 mods |
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