10 GHz Amateur Radio Beacon GB3SCX now has DFCWi

From 1100z on Wednesday 19th September, the 10GHz beacon GB3SCX located on Bell Hill in Dorset, IO80UU59, carries a new modulation type - DFCWi - to assist in evaluation of the mode.

The beacon modulation still carries a normal on/off CW ident followed by 15 seconds of carrier on its GPS locked frequency of 10368.90500 MHz (nominally!). This is then followed by a DFCWi transmission with tones 400 Hz higher than this for a Dot and 600 Hz higher for a Dash, ie "." = 10368.9054 and "-" = 10368.9056. (All subject to +/- 11Hz error due to DDS resolution issues)

Dual Frequency CW, with a third idle tone, or DFCWi. is described in more detail at http://www.scrbg.org/g4jnt/ and is briefly summarised here again.

DFCWi is coded as normal CW, but instead of different length dot and dash symbols separated by breaks in the carrier to indicate intersymbol gaps, the dot and dash symbols are made of equal length, but instead occur at different tone frequencies. The gaps between the symbols are greatly shortened and replaced by a third tone frequency rather than a break in transmission. Consequently DFCWi is
a 100% duty cycle transmission mode.

DFCWi is primarily aimed at reception/decoding using a Spectrogram, or colour waterfall display, but with a bit of practice can be resolved by ear.

The parameters used on GB3SCX are :
Symbol Interval 0.5 second
Idle period, 10% of symbol = 50ms
Recommended tuning tones :
Idle 600Hz (or 400Hz)
Dot 1000Hz (or 800Hz)
Dash 1200Hz (or 1000Hz)

Both of these tuning-points 'sound' quite pleasent to listen to with their tonal harmonic relationship, but make no different to their appearance on a waterfall display.

There are several suitable pieces of display software around, but the most versatile is Spectrum Lab by DL4YHF
http://freenet-homepage.de/dl4yhf/spectra1.html
This can be quite complex to drive, and there are a host (nay, quintillions) of settings. A good starting point is :

11025Hz sampling rate
1024 FFT
50% overlap (this makes the display move faster and CW symbols look better) Black/White palette (that's my prefernce, anyway!) Horizontal scrolling (unless you want to get a cricked neck)

The 0.5 second symbol length is optimised for an FFT of 5 - 10Hz bin size, so should give teh ability to decode at a few dB below what is copyable by ear. Part of this experiemnt is to see if these parameters are suitable for microwave use such as whether longer symbols lengths might help, optimum tone spacing, etc.

A sample plot from the new beacon received and plotted with Spectrum Lab is available on the website.

Andy G4JNT
http://www.scrbg.org/g4jnt/