29 October, 2013

Elecraft K3 modifications

There aren't that many modifications that you can do to the Elecraft K3. This is very different from the K2 as in my list I now have 138 different modifications for it. But Elecraft does have a few K3 enhancements and mods on their home page and here are two additional modifications that I have done to my K3.

Plug-in roofing filters on
main RX board
The first one is to add a wideband LC-filter (roofing filter). The filter was inspired by ideas from W5DHM with three tuned sections at the IF frequency of 8.215 kHz. It is to the right in the image. It is not the best of filters, and probably compromises performance somewhat, most likely because of its low image rejection 30 kHz away. It has however served me well as a receiver filter for the latest version of K1JT's software WSJT-X. That software processes a 4 kHz band for both the JT65 and JT9 digital modes, and the LC-filter has demonstrated to me the utility of having a wide roofing filter for reception of those modes. The filter also works well for listening to broadcast AM which was what W5DHM designed it for in the first place.

21 October, 2013

Studies on Morse code recognition

Nice drawing of Morse key by Dutch
radio amateur 
Dick Kraayveld, PA3ALM
In the early 90's there were some interesting studies performed on Morse code recognition and the effects of pitch frequency, signal to noise ratio and code speed. They were part of the PhD work of Peter Montnémery who is a medical doctor and also radio amateur SM7CMY. I reread these papers now since there was a discussion on the pitch frequency (CW pitch resolution) just recently on the Elecraft mailing list. This discussion comes up from time to time, as it did for instance in 2006 also (Sidetone questions -- copy speed vs sidetone pitch).

The two first papers are probably the most interesting ones for radio amateurs, so therefore I have posted their abstracts and a key figure from each of them also.

13 October, 2013

The simplest possible AM transmitter

Here's a design for a 1 MHz amplitude modulated (AM) transmitter. I've been looking a while for something like this, a simple short range AM transmitter for the medium wave band, as I needed something for demonstration of my collection of old radios.

The result is the AM transmitter shown here in an Altoids tin on top of a Radionette Kurér radio. This is a portable tube radio from the 1950's. Several hundred thousands were produced, and it was exported from Norway to 60 countries. It is still popular among collectors.

The transmitter is as simple as it gets. The heart of it is a 1 MHz crystal oscillator in a can. Its 5 Volt power is modulated via an audio transformer, one taken from the output of a transistor amplifier (primary 147 ohms - secondary 3 ohms). I drive the modulator from my cell phone into the low resistance side of the transformer and get good audio when the phone's volume is set to maximum.