Project Information

While sitting on an airplane during a recent trip I thought about building a radio that would be something like the famous Paraset Radio(Spy Type Suitcase Radio) that was famous during World War II. For those not familiar with the Paraset there are many references on the Internet. Basically the Paraset is a vacuum tube radio that uses a single tube as a crystal controlled oscillator putting about 6 to 7 watts in the 3 to 7 MHz range and a multitube Regenerative Receciver. The whole radio fit in a suitcase and was usually air dropped.

Well that seemed like a great idea and there have been many exact reproductions that look exactly like those radios. However once you have tried some of the newer technology, it is hard to go back. Thus the Paraset idea was dropped in favor of a small feature rich QRP radio. Basically this QRP radio is a single conversion superhet with a homebrew four pole crystal filter. The transmitter shares a common double balanced mixer and bandpass filter with the receiver. Keying is crisp and a sidetone is available by monitoring the actual transmitted signal.

This project uses many circuits from other radio designs and have been amalgamated into the final project. The break-in keying/sequencing circuit is a design by W1VT in his 7 MHz transceiver that is described in the ARRL publication QRP Power. The final amp is similar to the ones I used in several of my SSB radios. The receiver IF is at 5.185 Mhz and the VFO is 1.8 to 1.9 Mhz. At this frequency the VFO is very stable and covers the lower 100KHz of 40 Meters

The receiver has an RF amp that is located on the TX final amp board. A common bandpass filter feeds a SBL-1 Mixer that is used both on Transmit and Receive (again W1VT design). The SBL-1 is followed by a postmixer amp using a 2N3866 with 3 DB pad and the homebrew xtal filter is followed by a tuned MC1350 IF amp. The Product Detector and Carrier Oscialltor use the venerable NE602. Output from the product detector is fed to a single 2N3904 audio pre-amp and then on to an LM386. Notable is that the 2N3904 is partially muted during transmit so that some audio passes. The actual transmitted signal is being monitored at a reduced volume. There are pot controls for the RF and IF gain but these have not been brought out to the front panel.

The Frequency Display is a design by Francesco Morgantini, IK3OIL and employs a single PIC16F84. Francesco was kind enough to send me the hex code for the PIC. The display features programmable offsets so that the frequency of the VFO is converted to the transmit frequency. The counter can be used as a general purpose counter. Here is a link to his display design. It is easy to build and works like a charm!

The power output is 2+ watts and the signal is clean. I have made six QRP contacts so far and the best DX has been the mid-west a distance of about 1800 miles from me. Contacts with Iowa, Missouri, California and Oregon have been made with only 2 watts. Using the SS Amp I regularly work into the east coast. This was a fun project and while not a Paraset it does fit in a suitcase.

I upgraded the keying circuit to include a solid state switch so that a 100 watt solid state homebrew amp can be keyed directly. Five components give a dealy of about 1 second so that the amp relay is held "on" during cw transmission and returns to receive about 1 second after you stop transmitting. The 13 Volt transmit voltage that feeds the low level stages in the transmitter is sampled and passed through a 1N4148 diode where it charges a 10 Ufd 25 volt electrolytic capacitor connected to ground. The plus side of the charged capacitor is fed to a voltage divider comprised of a 22K 1/4 watt resistor in series with a 100K ohm 1/4 watt resistor which has one end grounded. At the junction of the 22K and 100K the base of a 2N3904 is connected here. The emitter is connected to ground and the collector is connected to the amp relay. The 2N3904 is functioning as a DC switch with an "off delay" hold of about 1 second.