Wednesday, 7 August 2013

Discone Diva

The discone is now mounted in its final location in the loft. Currently fed via a run of RG-58 with several splices and adapters, but still pulling in VHF and UHF air transmissions. The feedline isnt ideal, calculated loss is about 2.8dB at 300MHz. So, Sam and I spent a little time yesterday running a nice tidy new length of Times Fibre TFC-10 CATV coax. Although this is 75ohm, its loss at 300MHz is a mere 0.85dB, a considerable improvement, and anyway, a sweep of the discone with the antenna analyzer revealed its resonant much closer to 75 than 50 ohm. The cable still needs connectors installing, but it should be a great improvement.

The FT-857D looks like it might be a much harder job than I can manage. The parts that would need changing are tiny, well beyond my soldering kit, so its all down to whether my previous boss will allow me to make use of his workshops SMT stations. As a result of this job, ive not even started the show antennas. I have a feeling were going to be limited on the bands this year! Still, being indoors means we should get more operating time, and having mobile internet now means I can establish out frequency on the 'net.

Also, to compound matters, the main PC failed yesterday. Getting this back online, or replaced and its data recovered, is much more a priority than anything else im afraid.

Friday, 2 August 2013

Hard or soft?

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.

Thursday, 1 August 2013

By Heck, Its All Gone Quiet!

Its been a while since I posted on here, mostly this is due to having gone on holiday. So, not much has progressed. Both the 10m WSPR transceiver and the 4m Transverter are still in progress, but not sufficiently further to report on.

The one big, and annoying, change is that my FT-857D has gone deaf. Tested yesterday and all across HF and 6m is down at -85dBm ish. Ive already got a kit of spare PIN diodes, which im led to believe are the most likely suspects, and I now have a printed and bound copy of the Technical Suppliment, aka Service Manual. This needs repairing pronto as not only is it my mobile rig, it is also due to be the 2nd HF station at the show in under a month.

Also needed for the show is the 2nd HF antenna, in this case a Cobweb. Construction is yet to begin on this. Construction is also due to start on a horizontal antenna for 4m. Ive found that the very thin end section of the fishing poles that will form the spreaders of the cobweb, although no use in that antenna, are perfect to make a little Moxon for 4m!

Ive also finally got around to a solution for 2m FM and for general listening - a new Discone antenna that hopefully will arrive shortly. This will probably be loft mounted, so will be a little down on an outside rooftop location. When its in place i'll run the analyzer across it and report on here if it really does live up to the claims of the manufacturer!