VK3YE amateur radio pages

'Beach 40' 7 MHz DSB transceiver

 

All material on this site (c) Peter Parker VK3YE
1997-2013.

Material may not be reproduced without permission.

 

 

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.

Beach 40 Beach 40 Beach 40

Key features include:

* Long range 1 – 2 watts output provides contacts up to 1000 km
* Frequency agile covers most of 7 MHz (40 metre) band
* Simple just 6 transistors + 1 IC
* Cheap approx $50 to build from common parts (most from Jaycar)
* Compact good for home or portable

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 has a direct conversion receiver and a transmitter controlled by either a wide-swing ‘super VXO’ or ceramic resonator.

Interested? Watch the videos, draw the circuits and read the notes below. Search 'Beach 40' for others' experiences and improvements. 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.

Then 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 has a receiver RF preamp and inbuilt L-match coupler for end-fed half wave wire antennas. It is also physically smaller and nicer made inside. See Part 6 video above.

It's still under development and more improvements are possible. Eg added receiver audio and RF selectivity, more output power, better frequency stability and more. So build it, use it and tell others how you made yours better.

Component sources

The hardest to find part is the 7.2 MHz ceramic resonator. Try hamshop.cz (English website available). Also VK5EME at Minikits now carries them. Members could also try the VK QRP Club. If no luck use a 'super VXO' instead from two parallel crystals as described above. Crystals for 7.122, 7.159 or 7.2 MHz are available off-the-shelf from some suppliers. Expanded Spectrum Systems sell ham band crystals.

Other parts should be common. If you don't have the binocular style TV balun formers, other ferrite toroids (eg T50-43) should be OK.

Publication

A two-part article on the Beach 40 appeared in Lo-Key September and December 2013 (Issues 119 & 120), from 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:

* DJ0HF
* Soldersmoke introduction
* WB6TNL's circuit drawing (first version before LM386 added)
* VK2TPM
* VK5SRP
* W6IEE