Back on the 10m waves

This antenna was easy to tune, since I have enough support to hold a 10m quarter of wave radiator (4m long fiberglass telescopic fishing pole).

I had a couple of hours to tidy up the shack-lab while putting out some CQ's.
First I tried on 10m WSPR at 0.5W, receiving report from DU and VK lands. Then, not satisfied with the gratification coming from an automated system since I was in the shack, I remembered of PSK31. I tuned down a little and managed 8 QSO's, including a new one OD Lebanon that came back to my call.

The antenna is fed with RG58 coax coiled up to form an RF choke. The choke is held in place with a clip-on ferrite bar. I have no idea if RF is flowing back to the shack. At the indoor coax connector the SWR is 1.5:1 on 10m and 2.5:1 on 12m, so the antenna is somehow a dualbander.

The fishing pole is long enough for a 15m quarter of wave radiator without needing loading coils, so I might give it a try.

How it all began

After my initial interest in electronics (will be in another post), sometime in 1980's my brother and I were given a pair of 27 MHz walkie talkies. I could have been 8 or 10 years old.

Those walkies worked on 27.145 MHz, an "alpha" CB channel used also for remote controls. The microphone was the loudspeaker itself. The receiver was broad, very broad but sensitive: I could hear truck drivers on channel 5 (27.015 MHz) from kilometres away.

With no understanding of frequencies or channels, my mother and grandparents took me along the truckers' road trying to talk with them. No success, of course.

I don't remember if it was the very same Summer or the next, I got a 3 channel portable CB, then followed by a CTE Alan 48 which gave me my first sporadic-E DX on channel 34 FM with an Austrian station.

Yesterday my mother asked me what to do with those walkies, "Gig Bravo 2000": no way they're going to the recycle bin!

I am tempted at replacing the XTAL and try some extreme HF AM portable operation...

HVPP fusebit doctor for AVR's

Great job did these Polish guys that created the High Voltage Parallel Programmer (HVPP) for Atmel AVR microcontrollers. HVP programming is the only way to regain control of an AVR uC that has received wrong "fuse" settings, like I did with RSTDISBL=on.

An OM asked me to program a couple of ATmega32, he sent them over but they were not reacting to my serial programmer. A quick check with the Engbedded AVR Fuse calculator swapping high and low fuse bytes suggested that the previous owner had mixed them up, locking the chip to HVPP.

So I built the Polish circuit using an ATmega168 as the fusebit doctor on the protoboard. Pictures show the mess of wires, but it worked nevertheless. While I was at it I also recovered my locked 2xATmega168.

Green LED = chip rescued!

Adding contact information to digital storage media

Following my own year-old post about backing up digital contents (pictures/videos) while on the go, I want to share a simple idea against the physical loss of digital storage media.

Unlike a mobile phone, when a digital (video)camera is forgotten somewhere, there is usually little or no means of getting it back to the owner. While the electronic device itself can be easily replaced, it is not the case for the digital content that has been stored on it. So, how about adding your own contact information to your digital storage media? If it falls in friendly hands then there are good chances it will be returned.

I came up with these ways of making our digital belongings:

• handwrite an email address on the storage card (SD/SDHC, CF, TF, xD, ...) with a permanent marker, if there's room for it [your mobile phone may have been lost/stolen together with the camera, so just a phone number is not a safe idea]
• create a README.txt or OWNER-INFO.txt file in the root folder of the storage card containing contact information, return info, reward, ...
• create a digital picture containing contact information (the same of the README.txt file) and place it amongst your other pictures

The latter suggestion comes in handy if who rescues your lost device decides to browse through the content instead of plugging it to a computer. Imagine this: you find a digital camera on a bench, you check the content and find a picture showing owner's phone number, you call/text that number and return the device right away.
Oh, by the way, you don't have to edit the picture on the computer: write everyting on a large piece of paper and take a picture of it ;-)

Writing your details on the digital camera may make it less attractive for a future sale, but it can work as well as long as the card is not removed (i.e. for a sale). Remember that your main concern is to recover your memories, not the device itself.

RasPi from RS delayed

Not that I am longing for another Raspberry Pi, but I got an email from RS that my order with them will be shipped about 4 weeks later than expected. Weird, since Farnell was weeks early! Maybe there is an unbalance of orders they receive?

This is a good chance to cancel my order with RS, unless someone in a hurry wants to buy it from me (how much I paid it + S&H to your doorstep).

How to measure coax cable velocity factor

A recent (2010 AD!) discussion on an Italian forum tried to answer the question: "how to measure coax cable velocity factor?" Depending on the cable dielectric and size, vf is between 0.6 and 0.9.

If you have an antenna analyzer, such as the MFJ-259, it's easy, since the device will do it for you. Just follow the instructions on the user's manual.

If your instrument doesn't allow that, then you will have to proceed with trial and error. For example connect a known non-inductive impedance (resistance) on one end and look for the frequency where the same impedance is shown on the other side of the unknown cable. This condition occurs at multiples of lambda/2, so choose your frequency accordingly. The resistance must be different from the line impedance.

Edit 2013-10-22. OM Don adds that due to the intrinsic feed line attenuation, the resistance seen at the generator's side will be less than the face value. Obviously "how much?" depends on line length and test frequency, but could be in the order of ohms or fractions.

With a two-channel oscilloscope another comparison can be done, on two sections of the same cable, same length. Feed cables with a power splitter and observe the signal at both far ends: if waveforms are out of phase, then your two cable sections have different vf.. From this point you can use your imagination to work out other measures the o'scope can provide...

Raspberry Pi arrived and tested

My RasPi from Farnell arrived 3 days after the shipping notificaton. It came with regular mail from UK.

Setting it up was not so straightforward:

• pushing the operating system ISO to the SD card was not so easy with MS Windows
• I had no USB keyboard at home
• I have no HDMI-capable screen, at home
• I had no DHCP-enabled network to connect it to

With the help of a colleague I got a working SD card and put my RPi on the net. I ran few apt-get's to update it and install gcompris and childsplay packets. Unfortunately there isn't much choice of software ported to the ARM processor. No problems to get a vncserver running at a decent resolution or 1280x900 over the 100 Mbps network link.

At home I used the composite video out to test how functional it can be, but the result is not encouraging. Both childsplay and gcompris require a minimum screen resolution that is larger than what is available on TVout, while they run OK on the existing home computer + LCD screen.

On the other hand it may work as a digital media center with OpenELEC, but I haven't had the time to test it in the real world. But I can imagine the burden of wires around/behind the TV if an external hard-disk is added, then a USB-to-WiFi adapter, a powered USB hub ... not charming at all!

My RasPi test drive impressions are that it has enough computing power to do interesting things, with an excellent performance/price ratio. I am a bit skeptical about its usefulness in bringing youngsters closer to in-depth computer science and programming. And I doubt I will have much use for it. :-(

Raspberry PI from Farnell

Yesterday I received an email from Farnell stating that my RPi had been shipped (order #ORP051xxx). Since I ordered it the same day I submitted my order to RS, I was expecting a similar 11 weeks delivery time. But it got on my way about 50 days early. Could the recent increase in production capacity have something to do with it? Or most people ordered their copy from RS, since the communication was a tad better?

There is apparently no way to know which express courier Farnell used, nor to get a tracking number. On the other hand, RPi @ RS was paid right away, while Farnell charged my card on the shipping day.

Still no shipping news from RS, though.