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!