10 July 2014

Like a child on 70 MHz

The day after Italian HAMs were allowed to use 70 MHz band again for a few months in 2014, I reconnected my transverter and erected the dipole on the balcony. I tuned the band checking the local beacon with my ears and looking at the DX cluter with my eyes: the band was open.
All of a sudden I heard someone distant having a QSO. Disappeared. QSB was very fast. Then another voice, very strong, GM4JTJ, that came back to my 5W SSB balcony signal! Hooray!

I felt like 25 years ago, when as a child I worked my first DX'es on CB channels. I kept tuning, calling CQ, tuning, trying to ignore the female voice(s) calling me for dinner. But this time it wasn't my mother's voice, it was my youngest daughter's reminder that dinner was on the table.

I walked to the kitchen, explained that a unique event was going on, an event that happens 4-5 times a year and doesn't last long. When returning to the shack I got the familiar "don't complain if there won't be food left for ya", this time thrown at me by the older daughter.

It was a funny parent-child-parent role inversion.

In the following days I came up with a quick way to explain propagation. You must have seen Stargate movie to understand it: sporadic-E looks like a wormhole opening to some random part of the world.

Now waiting for another randomic wormhole...

08 July 2014

My Moxon beam for 70 MHz

I like the compact design of Moxon 2-element directional antennas, even if they are harder to build than a normal Yagi. Given the fact that 4 metres in Italy are allowed year-by-year, I do not want to invest in a large antenna, and I believe the gain of a Moxon should be enough to take advantage of sporadic-E openings.

Last year I had started building one and almost forgot. Then yesterday I had a pleasant surprise when I realised that everything was ready to erect the antenna and measure SWR! The only "problem" was how to hold the two wire ends pointing at each other.

Problem solved with two rubber bands cut open and secured to the wire with simple sticky tape!

The H-shaped frame is made of 20 mm PVC pipe screwed together with slightly modified Tee joints.

A huge storm was approaching, but wind was light so I could lift the antenna on the fiberglass telescopic pole on the balcony, measure lowest SWR at 66 MHz (1.6:1 at 70.2 MHz) and take the picture above.

Both local beacons were off-the-air, so I could test the extra gain.

Then the storm came and it offered a good opportunity to experiment with the camera:

26 June 2014

Rebuilding a dual-band mobile antenna

Until few months ago in the car I used one of those ultralight dual-band magmount antennas. They come with such a long RG174 coax that all the antenna gain at UHF is lost in the cable. Day after day, closing the car door, the little coax got pulled at the plastic base and the antenna stopped working.

A direct repair was hard, because the strong magnet is firmly glued to the small base ("E") blocking access to a solder point and the antenna itself ("A") does not screw or solder to a PL259 connector.

So I decided to destroy the original base ("E") and recover the screw ("B"): cut the plastic and pull out. The other end of the screw ("B") fits into a good old banana ("C", "F"), which plugs smootly into SO239 ("D").

I am not too confident the simple contact pressure would hold the antenna at 130 km/h, so I am still looking for a way to get everything into a PL259.

21 June 2014

Newly built memory keyer (based on K3NG work)

This is the 400th post on this blog!

One year and half after discovering my old memory keyer had been dipped in battery chemicals, I finally managed to assemble a replacement. This time I reproduced K3NG's excellent work based on Arduino. I did my little code adjustments to get it working with an Arduino Leonardo clone (ATmega32U4), and now it works the way I need it to.

The circuit pictured still misses the internal buzzer for audible feedback, otherwise it is 100% operational.

Now it requires proper housing and power supply. I do want to check its current consumption, just to know how long a battery will last.

Even though the box will need to be "large" to house all those controls, this keyer is a perfect companion for FT817, which lacks memories in its internal keyer.

18 June 2014

Talking AT commands to GSM cell/smartphone (over Bluetooth)

Today's discovery, by chance, was that some cellphones/smartphones expose a "standard old fashioned" serial modem when connected over Bluetooth to a computer. This means that it is possible to interact with the phone using the standard and extended AT command set.

What for? Well, any Bluetooth enabled embedded system (Arduino + HC-05, Raspberry PI + USB-BT dongle, ...) can have access to wireless telephony services such as SMS, dialin/out and potentially GPRS/Internet. All this without the requirement of a special modem device and an extra bulky cable in between. And probably you already have a suitable device laying around.

Not all cell/smartphones support this mode. For example an Android 2.3.3 Samsung Galaxy S i9000 does, while a Windows Mobile 6.1 Samsung SGH-i780 does not. I have two more cellphones with Bluetooth to test.

I think this is a simple(r) way to enable remote control of embedded systems, even though a lot of experimentation is needed.

Edit: Nokia 6233 exposes a modem over Bluetooth. Hooray!

03 May 2014

First time in Japan

This morning on 12 m I got my first reply from JA. OM JF2IWW was 539 with deep QSB, but he copied my 5 W and gave me a 439. Then fading took him away and I missed most of his message, but I heard his final greetings.

I would like to know more about his power and antenna, but there is nothing online.

02 May 2014

How cellphones vibrates

Vibration alert in mobile phones is accomplished with a small but powerful electric motor that spins a small asymmetrical weight. In the picture below the motor is the "horizontal cylinder", while a 50 €cent coin gives an idea of actual size. The third object is a 1F 5V supercapacitor.

Small motor, 1F 5V supercap and a 50 €cent coin.
What do these have to do in my blog? Well, I bought two rubber band powered airplanes as a gift, and as soon as I saw them flying I immediately wanted to "improve" them with a longer flight time. How? My idea is to use the small motor and the supercap. The total weight is about 5 grams and the motor runs longer than 10 seconds off the supercapacitor. I need to figure out a way to easily power the motor after launch: a normally closed tactile switch could be handy.

Nevertheless, a bigger challenge is how to connect the motor to the propeller...