Various ramblings – Electronics, radio, old computers, cars & other junk…

Monthly Archives: December 2010

40m/7MHz Loaded Dipole

My first project for the radio section of my website, and a nice easy one at that! This is a loaded dipole for the 40m/7MHz band, this design was chosen mainly due to the fact I don’t have enough room to hang a full size dipole. I created the design using a great calculator I foundĀ here. There are many great calculators on that website (I also used the air-core inductor calculator for this antenna), well worth bookmarking!!.

I punched in a maximum length of 7m into the calculator and started fiddling with the values until I got something useful. 7 meters happens to be the longest length on the roof where I can easily hang a dipole (relatively) straight. Saved me having to put up a tower/supports, and also meant it was very discrete and out of the way.

The antenna is very basic in its construction, made from .75mm enamled wire I pulled out of an old transformer. Loading coils are formed on 1″ ID PVC pipe, and the center insulator holding the BNC connector and balun is a piece of plastic packaging I found in the bin in the garage. This is one very low cost antenna as pretty much all materials were recycled, even the bolts in the loading coils which recycled from the transformer I pulled apart. The only thing I purchased was the BNC connector.

Total time spent building and hanging it was probably around the 2 hour mark. While I was finishing the balun, I had the antenna strung up about half a meter off the ground. Thought I’d do some quick testing with my VR-500 receiver (not very good on HF…), and managed to listen in to a very clear SSB transmission from Tasmania (VK7), which is around 400km’s from where I live in Victoria! Since hanging it up on the roof I’ve been able to listen in on several SSB transmissions, and plenty of CW too. Just need to learn morse code…

Hammerhead 586 slate PC

Ruggedised slate PC – AMD “586” @ 133MHz, 32MB RAM, 1GB CF boot drive

I picked this computer up off eBay for $26. This beast is machined from a single block of aluminium (years before Apple!!). It is a bit of a slug, especially running Windows 98, but it’s more than capable of running solitaire which is great with the touch screen. Because it uses a magnetic touch screen it only works with the stylus, I’ve been spoilt by capacitive touch screens and I still get caught out trying to use it with fingers…

Not long after I got it up and running the hard drive died. This was a big deal as I hadn’t yet copied the contents of the drive, and after trawling the web couldn’t find a great deal of information about it nor any drivers. Luckily I managed to revive the drive for long enough to copy the contents using the freezer trick. I replaced the drive with a 1GB compact flash card, as a bonus it runs a little faster and the battery lasts a bit longer.

Sharp PC-7100

Portable/luggable PC – Sharp 8088 @ 10MHz(??), 640kB RAM, 20MB hard drive, 360kB floppy drive

I found this machine many years ago while walking home from school (around 1998-1999 as a rough guess). Sitting on the nature strip left out for the hard rubbish collection, covered in dust and looking very sad. I was curious what sort of machine it was having never seen a clamshell portable computer, I picked it up and took the poor little guy home. After a quick clean I turned it on and up came a long forgotten DOS 3.3 install, including the previous owners documents and programs.

Today it’s looking a little bit tired, it could do with a clean and perhaps a few sessions of retr0bright. Sadly I think the hard drive has died as the last time I powered it up it hung on the BIOS POST screen for a long time and eventually showed a hard drive error. A few power cycles did eventually manage to get it booted but trying to run a program or do any file manipulation on the hard drive resulted in an error. The floppy drive also seems to be playing up a bit with the typical repetition of seeks and errors, hopefully only requiring a clean?

Micro-Professor MPF-1P

Z80 practice board – Z80 @ 3.8MHz, 4kB RAM, 20 character VFD display

My brother saved this computer from the bin at work. It’s a small “practice board” for learning Z80 assembler. Not terribly useful to me as far as learning assembler goes but it is an interesting addition to the collection.

This computer was made by a company called Multitech Industrial Corporation, in 1987 they changed their name to Acer. After some digging around on the Internet I managed to dig up a copy of the instruction manual. Even after flicking through it for 30 minutes it’s tricky to use, it has multiple modes and the 20 character display is very cramped.

The manual also mentions a bunch of accessories that I don’t have (a printer, an EEPROM programmer and an I/O board). Another key part of this computer I am missing is the plastic shell that keeps it all together. When folded away in this vacuum formed case it looked like a thick book. This “book” held the main board, one add-on board and the instruction manual. I made my own box out of MDF to keep the board safe, much better than the cardboard shoebox it was found in.