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.

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.

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!

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.

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...

Not much gong on (on 10 metres)

As usual, Spring decreases my will to build circuits (that's why there were no updates for a while). But I feel the urge to radiate some RF too. So I threw out of the balcony a 1/4 wave radiator for 24/28 MHz and last Monday I could work W1AW/1 in PA with 5 W SSB. After that ... just two continental QSOs in PSK31 throughout the week. A WSPR session brought in some North America reports, one night.

Sadly, the culprit is not a deaf or noisy antenna, but closed (and empty) bands. Will keep monitoring 10/12 m, then maybe I will lengthen the radiator wire to 15 m.

AD9850 board and temperature

My AD9850 DDS chip and the 125 MHz canned oscillator run pretty warm to the touch. No info were found online, so here you are my values in room temperature (23°C): both stabilise between 45 and 48 °C.

I let it run for few hours and it didn't break nor catch fire.

Chip temperature is not output-frequency dependant, but output-frequency depends on the oscillator temperature. Touch the canned oscillator and it will change of few Hz. I am wondering if I should add a small heatsink to the oscillator. Is it worth?!

Arduino controlled AD9850 DDS signal generator

Been blog-silent for a while, but my AD9850 DDS is now complete. The Arduino (Nano) code evolved day-by-day while assembling the circuit. I re-assigned some pins to simplify wire routing and implemented an easily extensible menu. My code is based on AD7C work and it is available on request.

I have added a scan function with user settable lower/upper frequency limits.

I have added a menu driven with a resistive potentiometer, which requires only one button for confirming the action and theoretically can host dozens of functions. While it is not an innovative idea, it is very comfortable to use as long as there are 5-to-10 total choices.

I am now using a TWI/I2C interface to the display, which uses 4 instead of 8 total wires.

DC is provided with a step-down regulator. I will add DC filtering capacitors near the Arduino and DDS boards.


That's two knobs and one display that now require a suitable housing.