Bonner County Amateur Radio Club   bcarc

Simple 80 M CW Texting Net

AF7YQ , November 09 2017
A
Re:Simple 80 M CW Texting Net
AF7YQ
I have been researching simple receiver designs on the internet. There are designs called "direct conversion" which are essentially amplified crystal radios... believe it or not, and people have used these simple designs, along with similarly simple QRP transmitters, to communicate over long distances. Regenerative receivers is another option.

Using some integrated circuit chips, one can increase the gain of these designs. However, one issue is selectivity. I have an idea that we can use the same signal generator - a direct digital synthesis chip - to drive the transmitter (under Arduino control) sending "perfect" CW at 30-60 WPM, and then offsetting the signal generator's frequency by some small amount - say 3000 Hz - and using that to feed a mixer. The mixer is also fed the preselected and amplified 80M signal, and the transmitted signal from the other unit - at the exact same frequency this unit transmitted on - mixed with the 3000 Hz offset will of course generate a 3000 Hz difference signal. This signal can be "detected" via a diode, and amplified.

This amplified signal of course will include noise and other signals at other frequencies other then 3000 Hz, but we feed it to a tone decoder integrated circuit, which will only "hear' from 2700 to 3300 Hz - a fairly narrow range of 600 Hz width. This will be turned into a digital "on/off" signal by the tone decoder chip, and fed to the Arduino to convert back to text.

So we have an Arduino - with a Bluetooth Keyboard, and a small LCD display - to transmit our fast and accurate CW. The Arduino displays what you are typing, as well as what you receive, so both conversion to CW and conversion back to text is done on the Arduino. It is easier than most CW decoders because we ASSUME the other end is sending PERFECT CW.

One issue might be time between keystrokes - there is no "mark" frequency, such as in many digital transmission protocols. We might decide to transmit a "space" character whenever there is nothing to send, thus holding the channel "open" while waiting for the operator to type something...

We would also like to define a header and footer for each transmission, that would be detectable and allow transfer of information. For example, automatic collection of sender call letters, automatic insertion of user call letters, perhaps even automatic logging. Maybe even store and forward message handling could be incorporated. But that is down the road.

Anyone interested in playing around with some simple circuits to transmit and receive "perfect" CW on 80 meters using Arduino and some transistors, caps, resistors, diodes, coils, let me know..

Eric Anderson, AF7YQ
A
Re:Simple 80 M CW Texting Net
AF7YQ
I should mention what the goal of this effort is:

1. To create an inexpensive communications system to keep us connected and especially in emergencies, including updates on conditions locally and message passing
2. Low power, probably less than 10 watts
3. Small tuned 80M whip antenna that we build
4. Price tag under $100 for entire setup (including keyboard, antenna, but maybe not the battery)
A
Re:Simple 80 M CW Texting Net
AF7YQ
After much study and consideration, it becomes clear that for a QRP transmitter, we need a fairly efficient antenna design. Most "compact" antennas - such as those designed for automobiles, are extremely inefficient for 80 meters. Say 1%? Yup. Or worse.

I think 50 mw is a little lean for getting out.

Thus, we are going to switch to 40 meters, as we have discovered an antenna design that maximized radiation locally, and, in effect, gives about 10 db gain locally (within 200 miles during the day, within 1000 miles at night). Please check out the replacement forum "CWTXN: Computer CW Texting for Low Cost Local Communications"