Ive been told, that the FT-857D had a tendency to drop some of its firmware alignment values. It seems that this has been known to cause the poor receive issues im having by corrupting the first few parameters. So, I have a list of 'typical' alignment values, and later today I will run through the alignment menu and see if what my radio has in its memory is close to the values to be expected.
With any luck, the problem will turn out to be a software corruption that can be rapidly sorted out, and not a hardware failure necessitating fiddly SMT rework.
Amazingly, the Discone antenna was awaiting me in the Goods In/Out room when I arrived at work yesterday evening. Exceptional, since it was free delivery, and all in cost over £15 less than any other sellers, to have arrived literally 'next day'.
Inspecting the antenna, it seems reasonably well built, with a stainless steel hub, but the 5mm diameter elements are aluminium, as is the support mast. Since i'm intending using this in the loft, this is of no real concern. For the time being it will feed down to the shack by RG-58 coax, not ideal at UHF over the length of run, but I already have it in position. I 'may' have some RG-214 with pretty manky sheathing that can be used instead, if its long enough. Another possibility is to use a run of some of the CATV coax I have, but i'll have to consider the impedence mismatch in that case.
It will be interesting to sweep this antenna with the analyzer. The manufacture claims a bandwidth of 25-1300MHz. Doubtful. Putting the dimensions into an online calculator, I get a minimum frequency of 90Mhz. However, I dont know the parameters for the bandwidth, it may be in that calculator a VSWR <1.5:1, whereas i'm happy to work on a 2:1 bandwidth, even more for receive only purposes. If the VSWR is 1.5:1 or less at 70MHz and 145MHz then i'll be happy with that for transmission purposes. I'll put my findings up on here.