Using the RTL-SDR as a transmitter

OK, this is very low power but it shows you what can be done. A QRPP transmitter or signal generator anywhere between 500MHz and 1.5GHz.

Please note the image below is not on my blog (it is linked as shown) and will be immediately removed if this is a problem.

http://rtlsdrblog.rtlsdrblog.netdna-cdn.com/wp-content/uploads/2015/06/transmit_hack_rtlsdr-500x375.jpg

See http://www.rtl-sdr.com/using-the-rtl-sdr-as-a-transmitter/

See also http://www.steila.com/SDR/RFgenmod/index.html .

Legal local AM broadcasting in the USA

On http://www.amateurradio.com/ there is a link today to the SSTRAN ANT3000 MW AM transmitter kit. This is legal in the USA for broadcasts around the home or with an FCC Part 15 compliant base loaded vertical for greater ranges.

Very many years ago my very first ever TX was an AM transmitter for MW using a crystal earpiece  a microphone. This was one of the circuits in my Heathkit Electronics Workshop which I got for Christmas 1961. It only got around the house. Another circuit was a MW receiver and I recall DXing with it that same holiday.

This kicked off my interest in radio and amateur radio that has remained with me ever since. I am not sure this TX was strictly legal in the UK. Years later I have reached the other side of the planet with less power!

QRP 5W CW transmitter from SM0VPO

Many years ago Harry SM0VPO worked at Pye Telecom. I had no idea back then that he was such a prolific and good designer.  This is his design for a simple HF 5W CW transmitter. His website has loads of great ideas.

See http://www.qsl.net/qrp/tx/5wtx.htm .

Spark transmitters and modern simple rigs

In the early days of wireless, spark transmitters were all they had. Selectivity and bandwidths were not major concerns.  These days it is all so different with sometimes very crowded bands and the bandwidth of both transmitter and receiver being of major importance.

See http://en.wikipedia.org/wiki/Spark-gap_transmitter for some interesting reading. I recommend you do not try a spark transmitter as you would be very unpopular because of the wide signal and interference caused.   By all means read about them though.

A modern challenge is to see just how simple rigs can be made yet still function credibly on the amateur bands today.  Rigs like the Pixie are fun, but such ultra-simple transceivers are let down by deaf or easily overloaded receivers. To my mind, the receivers have to be selective, sensitive, have netting, and not be easily overloaded. The challenge is to overcome these issues! Often TX power is not the deal breaker.

UPDATE 1740z:  W5OLF has just shown me a photo of his tiny 1 inch ferrite rod antenna on which he has had some success with on 10m WSPR.  I tried some WSPR experiments with ferrite antennas some years ago on 40m, 30m and 20m. As long as the ferrite does not saturate they do work. See www.g3xbm.co.uk .

OXO on 14MHz

An OXO transmitter on 14MHz

This afternoon I decided to build a 14MHz OXO transmitter. As is often the case, I like to re-use old boxes and project parts, so the FETer box was photographed for posterity, then gutting and used for the OXO transmitter. In this version I did not go for full break-in and instead used a simple changeover switch, but added a push button net function to allow me to net the VXO without the PA being keyed. A small toroid in series with the crystal allows about 15kHz VXO range around the 14.060MHz QRP frequency. I used a 2N3904 for the oscillator and 2N3906 for the keying transistor and a 2N3866 for the PA, but may change this for 4  x 2N3904s in parallel for lower cost. My output is a little low at around 500mW.

It works fine with reverse beacon reports from Iceland and Slovenia and a nice 2-way QRP QSO with IK2RGV who was running 5W.

The OXO really is a classic circuit: you just build it and it works.  Another version is on the QRPkits page.

481THz AM transmitter

This morning I took a step further towards making a complete 481THz lightbeam transceiver by breadboarding and testing a simple AM transmitter for use with a high brightness red LED. The circuit could hardly be simpler but works very well. The bias on TR2 is adjusted to give around mid-rail so the LED has a standing bias before audio is applied. Listening (without optics) on my modified KA7OEI receiver head and looking on a scope, the modulation looks good with around 150mA current into the LED.

Simple 500kHz TX design by GW3UEP

A very neat and simple MF TX called the E-QTX has appeared on the Google UK500khz group website. This design looks so straightforward that even I should be able to make it in the back of the garage. It comes from GW3UEP (via G3XIZ) and is aimed at encouraging more NoV holders to come on the band. It produces 8-10W with a 12V supply and more than double this from a higher supply voltage.

See http://groups.google.com/group/uk500khz/web/gw3uep-cw-tx.