144MHz ARDF transmitter

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PartlistCircuit diagram
Functional description

IC1 : CD4093IC2 : 78L05IC3 : ICS501T1 : BC557c
T2 : BLU86T3 : BC557cL1 : 2 x 8 wind. *L2 : RFC **
L3 : 8 wind. *L4 : 8 wind. *R1 : 220k ***R2 : 180k
R3 : 100kR4 : 1kR5 : 220kR6 : 47k
C1 : 10n ***C2 : 10nC3 : 1µC4 : 1n
C5 : 100nC6 : 100nC7 : 5-60pC8 : 100n
C9 : 330pC10 : 10nC11 : 10nC12 : 5-60p
C13 : 5-60pC14 : 5-60pC15 : 10µ/25VC16 : 100n

*:L1, L3 and L4 = 0.2mm CuL wire on Amidon T20-12
**:L2 = broadband VHF choke
***:for internal keying : R1 = 1M, C1 = 1µ/25V

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Functional description

For ARDF competitions on 2 meter a transmitter is needed that has :
With a supply voltage of 12 to 13 Volt this transmitter has a HF power of about 0.5 Watt.

The hart of the transmitter is the integrated circuit ICS501 that allows it to generate a stable a clean signal on 144MHz using just a few components. The ICS501 contains a complete PLL oscilator of which the divider ratio can be set via pins 4 and 6. Between pins 1 and 8 a crystal is connected, the exact frequency can be adjusted with C7. At pin 5 a 20mW signal on 144MHz is available, with T2 this signal is amplified to 400-600mW. With this power and a supply voltage of 12V the output impedance is about 200 Ohm, L1 transforms this to 50 Ohm. The filter C12-L3-C13-L4-C14 takes care of the final impedance matching and for a sufficient harmonic surpression. With IC1D a 800Hz oscillator is built that is used to modulate the transmitter via T3. With R3 the carrier level can be set. Depending on the posistion of S1 the transmitter can be internal or external keyed. T1 is used to switch the oscillator and IC2 supplies the ICS501 with 5V.

Components and construction
Start the construction by cleaning the PCB and drilling all holes with 0.8mm (30mil). Enlarge the holes for R3,C7, C12, C13 and C14 to 1.2mm (50mil) and the mounting holes to 3.5mm (0.15 inch). Now check if all holes are drilled with the right diameter (this is your last chance to drill holes !). Next mount and solder the 2 SMD components (IC3 and T2) on the solder side of the PCB. Fixate the component with a small screwdriver (will also reduce the heating) and solder all pins. Avoid heating the components too much, if required ask a friend who has expercience with soldering SMD's. Be sure that IC1 in placed correctly (pin 1 is bottom/left). Now mount and solder all remaining components.
L2 is a wideband VHF choke (ferrite bread). L1, L3 and L4 are wound with 0.2mm (32AWG) CuL equally spread over a T20-12 toroidal core (Amidon). If you want to use other toroidal cores you will have to experiment with the number of windings. Take into account that the material of the core not only determines the inductance but also has an influence on the harmonic surpression. If you cannot measure the harmonic surpression you are advised to stick to the T20-12 cores.
L1 is a transformer with 2 x 8 windings, take 2 pieces of 0.2mm (32AWG) CuL wire of about 10cm (4 inch) length and twist them. Use this to put 8 windings on the toroid. Use an ohm-meter to determine which ends belong to the same winding and solder 2 different ends of 2 different windings together, this is the central tap of the transformer.
All fixed resistor are 1/4W metal-film or carbon. All fixed capacitors are ceramic, except C3 (polystyrene) and C10 (tantal, take care of polarisation and voltage). In case of internal keying C1 is also tantal.

Settings and adjustment
With S1 internal (A-C) or external (A-B) keying can be selected. With S2 the divider ratio of the ICS501 is selected, the various devider ratios allow a wide choise of crystals to be used. Most practical (and cheap) are 18MHz or 24MHz crystals. Due to the nature of the oscillator the crystal will work on a slightly higher frequency, as a result of this a 18MHz crystal will produce a signal between 144.060 and 144.090MHz (adjustable with C7).
Before starting the adjustement : Connect 12V to 'Vcc' and adjust C12, C13 and C14 for maximum output power while monitoring the supply current, that should never exceed 200mA. Repeat the adjustment cycle (C12 - C13 -C14) a few times. You should be able to reach a output power of at least 300mW with a supply current of about 110 to 140mA. Now disconnect 'Key' from ground and adjust R3 in order to get the supply current back in the range of 110 to 140mA. The output power (without modulation) should now be about 200mW. Connect 'Key' again to ground and adjust C12, C13 and C14 again for maximum output power. The output power should be at least 400mW. Take care that the supply current never exceeds 200mA.
If required adjust the frequency with C7.

Output power and harmonic surpression
The output power and harmonic surpression measured with Rohde&Schwarz FSEA (13V supply voltage) :

Use as ARDF transmitter

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Circuit diagram

A high resolution TIFF file of the circuit diagram is included in the download package.

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Printed Circuit Board

The exact dimensions of the PCB are 3.075 x 1.30 inch (78 x 33 mm).
A high resolution TIFF file of the PCB is included in the download package.

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