The Wheel is a DSB Transceiver for 80, 40 or 20 Meter
The transceiver got its name during the Dutch ‘night owls’ net. We all know the expression ‘re-inventing the wheel’. People thought Fred (PA0MER) was re-invented the wheel. In this case a transceiver using techniques of the past.
It all started because of a receiver design, the DC-80, by Wim (PA0WDW) in CQ-PA, 1980. Fred made a transceiver out of it and published it named ‘The Wheel’ in CQ-PA in October 1991. Later, the design of a ‘4-wheeler’ was published in CQ-PA (1992). Albert (PA3EKN) helped a great deal in it’s development. The first design was promoted and published by Henk (PA0GHS) in the BENELUX QRP CLUB and many PCB’s were produced for the home brew radio amateurs. The latest design has never become a success, however. I’ve also spend a couple of hours on the project and made numerous changes. The current design has a whole new print-layout with a minimal size. The print contains a full transceiver for 80, 40 or 20 meter band with an output power of about 3 Watt and carries the name ‘MINI-WHEEL’. The print itself is 9 by 4,5 cm and the components are ‘wired’ (no SMD). The print is one-sided and has a ground plane on the components side.
The heart of the transceiver
The transceiver has a TCA440 IC as its main circuitry, which is also known as A244D. Siemens had sold the rights to a producer in the former Eastern Europe. That’s why we amateurs can still use the chip. The TCA440 contains all active components needed for a complete receiver. Only the frequency regulating parts are needed to complete. This IC is seen in many amateur designs.
The concept of this transceiver is a DC (Direct Conversion) receiver and a DSB (Double Side Band) sender. That means that during receiving and transmitting, both side bands are used. This has some disadvantages but there are plenty of advantages. One is that expensive filters aren’t needed, and a simple balance mixer combined with LF-filtering does the job while only one oscillator is needed for both sending and receiving.
The TCA440 IC contains all of these functions. The IC has a HF pre-amplifier with a controllable gain, a balance mixer, a controllable MF/LF amplifier with 100 dB gain, an AGC amplifier, a Hartley oscillator and a connection for an ‘S’- meter.
Click on the image to enlarge the diagram. Or download the file by clicking here.
How it works
The oscillator circuitry is built with a Toko-coil, two polystyrene capacitors and a varicap-diode. The varicap voltage is supplied by a 10 Kohm 10 turn variable resistor. You should use a good variable resistor for this; the expenses are nothing compared to the fun and the advantages of it.
The coil needs some more attention. It should be a so-called ‘pink’ VRZA-coil and has been named because it was frequently used some years ago in designs of this society. Since the oscillator circuitry in the TCA440 only oscillates with a fully symmetric coil construction, we have to use this coil. The coils can be bought from the BQC. The Band filter for receiving is very simple and is build with the same Toko coils.
The low-frequency filter system is an RC filter 3k3 + 1mF near the output of the balance mixer and the input of the LF amplifier and an LC filter with the 100mH coils connected to the output of the LF amplifier. Because of this we can get an low pass up to 2,4 kHz.
The receiving path is provided with an AGC, so that loud stations are reproduced as loudly as a weak station. The amplifier and the V9/V10 rectifier circuitry do their part of the job. The transmitter part is a simple N6RY amplifier stage with a gain of 16 dB. The 2SC1696 stage has a gain of some 20 dB and brings the output power to more-the-less 3 Watt DSB. For the harmonic suppression we use the well known PI-filter. This two trap filter system gives enough suppression for minimal 40 dB. There are also some known circuitry implemented such as RF indication, a microphone amplifier and a TX/RX switch.
How to use it
If you connect a good dipole antenna, you can make easily qso’s on the 80-meter band. The controls are very simple. There is only a tuning control for the frequency and a volume control. The signal report is not always S9+ 20dB but they will hear you and we can live with that HI.
The modulation is very characteristic. To be exactly, it’s not easy to prevent a little ‘FM-ing’ on the signal. The reason for that is that the oscillator is tuned on the same frequency as the relative strong transmitter signal. By using good protection and de-coupling this instability is minimised during modulating. While I was designing the print layout I tried to avoid as many problems as possible, of course.
The holes in the PCB must be drilled with 0.8 mm drill. The holes for the controls, filters etc, have to be a little bigger and will be treated later. The component side of the PCB acts as a ground-plane and we need to free all holes on this site except for the ones indicated. We need to solder tree wires on the PCB:
On the solder side, a thin coax-cable needs to be soldered from the Relais to junction R7/V5. This cable needs to be grounded on both sides!! ( 1. From relais +T to R9, 2. From relais + R to R7 and 3. 13.8V to 78L05 in = N1. )
Remove the little capacitors the TOKO-coils. We can easily break them with a sharp needle. Don’t try to remove them completely because it’s likely you will destroy the coil itself. The coils should be soldered as first components on the PCB and must be grounded on the component side.
Click on the picture to get an enlargement of the layout. Or download the file by clicking here.
Click on the picture to get an enlargement of the components layout.
Click on the picture to get an enlargement of the pcb layout. !! In reality the pcb measures 89 x 46 mm !!
Or download the file by clicking here.
Select all components to mount on the component side. The capacitors need to fix on a 5 mm grid. Mount the resistors vertically. The layout act’s as a guide to mount the resistors in an optimal way. The TCA44O is mounted using a socket, the LM386 can be mounted without socket. Solder the metal screen at several places to the PCB. You have to choose for which frequency band you want to operate our ‘Wheel’. The frequency dependent components are summarised in below tabel.
The coils you have to wind yourself:
|L4||– 10 wdg trifilair 0.2 mm enamel wire on FT37-61 ringkern|
|L12||– 24 wdg single 0,5 mm enamel wire on FT37-61 ringkern|
|L13||– see table, on T50-2 ringkern, 0,5 mm posynwire|
|L14||– see table, on T50-2 ringkern, 0,5 mm posynwire|
The transistor in the Final stage have to be cooled. Use a piece of metal or make use of the case. The transistor must be mounted in an isolated way – the collector is also the housing! – and must be produced by MITSUBISHU. All other brands do not fulfil the specifications needed to give enough RF power.
Connect all user controls and don’t forget to switch a 470 ohm resistor across the S-meter/RF connection. Elsewhere no power output is available. Or leave out C45 for the time being. Finally connect the 13.8 V supply.
Tuning and testing
There are only a few tuning spots. At first the VFO needs alignment to cover the correct frequency band. Turn the tuning resistor fully anti clockwise and listen with a monitoring receiver of which the antenna or pickup coil is near to the rig. Monitor the lowest frequency of the chosen amateur band. Adjust Coil L3 till a carrier can be heard on the monitoring receiver. If you have properly build and adjusted the rig, it must now be possible to cover the whole band till the upper frequency limit with the tuning knob. If the frequency goes lower, the tuning resistor is wrongly connected. Now tune Coil L1 + L2 for maximum signal (received). Occasional L1 for max. signal lowest band end and L2 max. signal on highest frequency. Herewith the receiver has been tuned.
The transmitter only needs alignment of the two variable resistors. Place both resistors in their mid position and modulate by means of the microphone. The power output should be about 1 to 3 watt, depending on the chosen band. Tune P3 using a monitoring receiver for minimum signal during no speech. De-tune the monitor receiver a couple of KHz to be able to hear the carrier. Tune for maximum carrier suppression. Remains the microphone gain. Adjust P2 in such a way that the transmitter is correct modulated. Take care!, not to much gain because then side-band splatter, also with QRP, will increase!! Arrange a proper 50 ohm matching for the transmitter, as otherwise FM-ing and /or distortion will increase dramatically.
There are only two controls: one for frequency tuning and one for audio gain. A 13.8 volt, microphone, PTT en speaker connections. Use shielded cable to the helical-resistor.
Solder parallel to microphone entry a 100 nF capacitor to prevent LFD. The microphone type must be dynamic with an impedance of about 600 ohm.
It’s possible to connect a meter to read the frequency, receiving signal strength and RF power. You can choose a instrument for about 100 mA to 1 mA. Experiment with the serial resistor (R26) to match. For RF-indication it can be necessary to increase C45 to 10 pF. If you are not using a ‘S’ meter,a 470 ohm resistor must be soldered in stead, because the transmitter will not produce any power. This is because of the influence to pin 3 of the TCA440 (AGC). This is also the case if the meter is switched from ‘S’meter to frequency read-out.
Preferably a metal housing is used. If space permits, an amplifier can go in the same box, but be prepared of FM-ing. A proper shielding must be made between amplifier and transceiver.