Wednesday, April 25, 2012

Indoor wire antenna for WSPR

WSPR is a wonderful tool for comparing HF antennas. I "spot" and get "spotted" all around the globe running just 2W into my outdoor wire dipole on 20m. A few stations are often very strong and I wondered if I could be active with just an indoor end fed wire.

About 10m of wire running around the book shelves tunes up nicely with the ZM-2 Z-match tuner. A 1.5m counterpoise runs around the floor.

Despite all sorts of local electrical noise in the room from fluorescent lights and computers I'm hearing Ross, VK1UN (in Victoria) pretty well.

VK4BV and VK1UN are both decoding me 700km away in opposite directions north and south.

Note that previously VK4BV had me at +10db SNR and now I'm -9 so that's much worse but due to the huge dynamic range WSPR can handle it's great to get a reading at all.

Later it's getting better:

WSPR provides a good platform for evaluating indoor antenna options.

Saturday, April 14, 2012

Progress on the ARNSW Home Brew 40m AM challenge

In spare time I've been working on my entry for the ARNSW home brew group challenge project which is to build a low power AM transmitter for 7.125MHz.

There have been a couple of false starts here. First I ambitiously tried to make a pulse width modulated transmitter but couldn't get it working. Next I tried high level AM modulation using an audio transformer but I think it saturates and I could get only very small modulation with it.

Now I'm trying low level modulation of the IRF510 power stage. This looks distorted but seems to be working pretty well.

The AF board uses a pre-amp with AGC picked up from a recent trash and treasure at Dural (thanks John VK2ASU). For the power stage is a module from Jaycar - I know, a bit slack on my behalf, but an audio power amp is the least interesting bit for me.

The RF board starts with parallel 7.122 crystals pulled as close to 7.125 as I can get, a buffer stage, a driver, an IRF510 final and a low pass filter.

Along the way, the bias on the IRF510 got out of control making a big bang and releasing its smoke. I now use a current meter at all times so I can spot thermal runaway before catastrophe strikes.

Happily they are cheap.

Lots of work still to do and I'm very much looking forward to having a contact with this tangled mess.

Friday, April 06, 2012

Element14 taking Rasberry Pi orders

Now that Rasberry Pi has passed electrical compliance certification I went to take a look and see if I could order one. Previously I've only been able to register interest.

Turns out Element14 will take orders with of up to 1 per person. The supplier lead time is listed at 143 days.

The other bad news is that to get one of these in Australia adds a $12.95 shipping charge to the $38 price of the board taking it to $56.05 once GST is added. Not so cheap, but not so bad for a unix capable single board computer.

Sunday, April 01, 2012

Improving the wspr map

I've been improving my version of the wspr map.

The reddit amateur radio subreddit has been supportive and I'd really like to see this contributed to the main site.

Latest improvements are thicker spot polylines for stronger reports and if you roll over a trace it pops up details of that spot. Any feedback most welcome.