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The 'Beach 40' is a simple yet practical voice transceiver for the 7 MHz amateur band. Go any simpler and it's hard to get contacts. And anything more complex often doesn't get finished. So if you're not quite ready to assemble an SSB transceiver, give this simpler DSB rig a shot.
Key features include:
* Long range 1 – 2 watts output provides contacts up to 1000 km
The Beach 40 became a hit as soon as the YouTube video came out. It appeared on Soldersmoke and got picked up by the Minimalist_QRP_Transceivers yahoo group. Radio club members are also getting involved.
It’s a project for the tinkerer. The results are good for the number of parts but refinement is both possible and encouraged. It features a direct conversion receiver and a transmitter controlled by either a 2-crystal wide-swing ‘super VXO’ or supplied ceramic resonator.
Interested? Watch the videos, draw the circuits and then read the notes below. Search 'Beach 40' for others' experiences and improvements. Also I'll add links to this page as I find them.
Part 1: Beach 40 introduction, circuit description and demonstration
Part 2: Substituting an LM386 audio amplifier
Part 3: Substituting a 'Super VXO'
Part 4: Boosting the LM386 audio amplifier
Part 5: Adding a fine tuning control
Part 6: A smaller case, L-match antenna coupler and RF preamp (Beach 40 Mk II)
Beach 40 construction notes
Printed circuit boards discourage experimentation so no layout is provided. Instead assemble it ‘dead bug’ style on unetched circuit board material, using glued pads to support parts if necessary. Other possibilities include ‘grid board’ (PC board with copper sawed into pads approx 5mm square with hacksaw and mitre box) or matrix board.
First build the oscillator/buffer using the ceramic resonator. Check it tunes a useful range of 40 metres. The outer leads of the tuning capacitor are bridged to maximise capacitance and frequency range (I get approx 7.050 – 7.200 MHz). Or use parallel crystals as a wide swing VXO. Two cheap 7.122 MHz crystals can tune as low as 7.060 MHz with a carefully selected series inductance (made from several RF chokes in series).
Then make the balanced modulator. Use a 2 hole TV balun former for the output coil and make sure the windings go to the right places. Keep leads short in this and other RF-carrying stages.
The microphone amplifier is next. The circuit is suitable for an electret microphone. If using a dynamic unit leave out the 22k resistor and possibly raise the 100n coupling capacitor value if insufficient or thin audio.
With microphone connected, 12 volts applied and a short lead on the balanced modulator’s output, hear yourself on a nearby 7 MHz SSB receiver. Stop talking, tune off to hear carrier note and slowly adjust the balance trimpot near centre position for minimum signal. Do likewise for the trimmer capacitor. Reset the trimpot for the deepest null. This process eliminates the carrier, which is not needed for DSB.
Proceed to build and test the transmit amplifier, driver and power amplifier stages in turn. Output power increases with each added stage. The collector circuit of the first two stages uses a twisted pair of enamelled copper wires threaded through two hole TV balun formers (like that in the balanced modulator). The collector circuit of the final amplifier stage is simpler, using a single thin wire wound through a 6 hole ferrite.
The low pass filter is after the relay, which is controlled by the microphone’s PTT. This uses off-the-shelf 1uH RF chokes. Disc ceramic capacitors will work but use better types (eg polystyrene or mica) if you have them for less loss.
The local oscillator and balanced modulator (used as a product detector) is shared with the receiver. All that’s needed is an audio amplifier. The high gain LM386 circuit uses gives sufficient gain for headphones and will drive a speaker on strong signals.
Testing involves transmitting, checking output power and voice clarity on a receiver. Tune around the frequency for any undesirable spurii, instability or broadband hash. Layout, shielding and decoupling are all important. Don’t build the transceiver so small to make component access and troubleshooting difficult; you will almost certainly have to do it or even change values for best results with your components.
The Mark II version includes a receiver RF preamp and inbuilt L-match antenna coupler for end-fed half wave wire antennas. It is also physically smaller and nicer made inside. This is documented in the Part 6 video above.
This is still a project under development and many improvements are possible. These include added receiver audio and RF selectivity, more output power, quieter transmit/frequency switching, better frequency stability and more. So build it, use it and tell others how you made yours better.
The hardest to find part is the 7.2 MHz ceramic resonator. They used to be stocked by Rockby Electronics but their website now shows no stock. Some have obtained them from hamshop.cz (English website available). VK5EME at Minikits has advised me that he now carries them. Members might also wish to try the VK QRP Club.
Other parts should be fairly common. If you don't have the binocular style TV balun formers, other ferrite toroids (eg T50-43) should be OK.
A constructional article on the Beach 40 appeared in Lo-Key September 2013 (Issue 119), published by the VK QRP Club. Please add a 47uF electrolytic capacitor in series with a 22 ohm resistor between Pin 1 of the LM386 and earth on the circuit (page 19).
Beach 40 links
Visit the following for others' experiences in constructing this rig: