Inside my 23 cm transverter based on LMW parts
Schematic for the 23 cm transverter
The transverter has a common double balanced diode ring mixer for RX/TX. The use of small helical filters gives good spectral purity and good IMD performance.
The IF is 144 MHz. My Icom IC202S is often used for this purpose. I use about 500 mw of drive on transmit from the 2 m rig. This reduces heating effects in the TX attenuator, and less 144 MHz RF radiation.
TX output from the board is about 30-40 mW at 1296 MHz; this is further amplified with another BLU98, which is on a separate PCB, up to 400-500 mW, before finally the signal is fed to the linear amplifier.
Inside the 23 cm linear amp, two Mitsubishi 20 watt modules
A Pair of Mitsubishi linear power modules, M57762's make a good platform for the linear amplifier. They are ideal devices to use when only low drive power is available; they have about 13 dB of gain. Another advantage these units have is they operate from a 12-14 V DC supply, ideal for portable operation.
They are a little bit inefficient, around 30%, so they do require quite a high current, 10A at the rated output. With a single module, an output power of between 15 and 18 watts is achievable with 250 mw of drive. This also is the most linear operating region for these modules.
The two modules are used in parallel. Each module will match into 50 Ohms without any need for further components. Wilkinson four port splitter/combiners are used for both power splitting on the input, and combining on the output.
The splitter/combiners are made using 'Sage-wire' transmission lines. The fourth port is terminated with a 50 Ohm load. This gives good impedance matching and isolation between the ports.
As the DC input power is about 140 watts, the modules do need to be mounted on a substantial heat sink.
I switch the bias voltage off during receive periods to reduce any noise generated by the modules. This is done with a simple relay circuit controlled by switching voltages sent from the transverter.
There is no RF switching in the amplifier. I use a single mast head mounted relay, with separate RX and TX coaxial feeds. This reduces loss and the cost of expensive relays for 23 cm use.
Diagrammatic connections for the 23 cm station
Feeding DC up the coax
I feed the DC power for the Pre-amp and change-over relay up the receive coaxial cable, with suitable DC and RF isolation as shown below.
The dc supply is fed via a l/4 stub that is a short-circuit to rf at its lower end. The short-circuited stub, made with mini PTFE coax, (l/4 x vf) functions as a trap circuit with a high rf isolation between the dc supply and the rf power, but provides for a low loss of rf energy to and from the antenna. C3 prevents the antenna short-circuiting the dc supply.
The dc supply for the pre-amplifier and the antenna coaxial relay are available at point A. If the station is transmitting, or switched off, the relay is de-energised and the pre-amplifier is disconnected from the antenna. This fail-safe method has its advantages during thunder storms as it protects the GaAs FET amplifier from damage by static (EMP).
23 cm Pre-amp
The mast head pre-amp uses a low noise GaAs FET (ATF10135) with a 0.5 dB NF, 13 dB gain @ 4 GHz.
Set the quiescent current to 25 mA. Adjusting VR1 until a voltage drop of 250 mV measured across R1 can do this.
With a signal present, tune VC1 and 2 for best signal-to-noise ratio.
Schematic for the 23 cm Pre-amp
LMW Electronics Ltd
Leeside
Merrylees Industrial Estate
Desford
Leicester
LE9 9FS
Tel. 01530 231141
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2000