144MHz low power ARDF transmitter

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PartlistCircuit diagarm
Functional description

IC1 : 4093 (CMOS)T1 : 2N918 (*1)T2 : BC557cX1 : 24MHzL1 : 1µ L2/3 : 5 wind. (*2)
R1 : 39kR2 : 1MR3 : 10kR4 : 330kR5 : 1MR6 : 1k
C1 : 47pC2 : 15pC3/4 : 3-20pC5 : 1nC6 : 47nC7 : 470n
C8 : 100n

*1 : if you have problems finding the 2N918 you can try any other NPN transistor with a Hfe > 20 and a transit frequency of at least 400MHz. A quick check in a databook gave following transistors that fullfill these requirements and are also pin to pin compatible with a 2N918 :
BF155, BF166, BF255, BF271, BF272, BF290, BFJ77, BFJ78, BFJ79, BFW30, BFW41, BFW70, BFX18, BFX19, BFX20, BFX21, BFX47, BFX59, BFX73, BFX89, BFY66, BFY79, BFY90
Be aware that I just compared this types in a databook. They will most likely do the job, but no guarantee.
*2 : L2 and L3 = 5 wind. of 1mm CuAg wire, diameter 5mm, tapped at 1 wind. from the cold end

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Functional description

This is a easy to build and low cost 144MHz mini transmitter that produces a remarkable 'clean' 144MHz signal up to 15 mW. It can also be used as exciter to drive a PA. In combination with a good UHF transitor a 2 transitor 200mW TX can be build or in combination with one of those power modules that are used in handies several Watts can be produced.
With a low cost 24MHz xtal the oscillator runs on the 3rd overtone (72MHz). The collector circuit acts as a doubler. In this type of oscillator the xtal oscillates well above it's natural frequency, resulting in a end frequency of +/- 144.070MHz for a 24MHz xtal. Any other 72MHz xtal can be used, but a lot of these xtals are 5th overtone. In this case L1 should be reduced to 0.47ÁH to avoid that the xtal will run on the 3rd overtone.
The oscillator is modulated with a mixture of AM and FM, so both AM or FM receivers can be used.

The oscillator is built arround T1, with a 24 MHz xtal used on the 3rd overtone (72 MHz). The collector circuit acts as a frequency doubler and the critical coupled double band pass filter (L2, L3, C3, C4) makes sure that all (sub)harmonics are at least 40 dB below the carrier. C3 and C4 have to be adjusted for maximum output power.
With R2 the output power of the oscillator can be adjusted between 0.1 and 15 mW.
The 2 Schmitt-Trigger NAND gates of the 4093 produce a intermitted LF tone that is used to modulate the oscillator. The remaining gates are disabled by pulling their inputs to plus.
With R6 the modulation level can be adjusted.

Output power vs. supply voltage and current
supply voltagesupply currentoutput power
15 V14mA15 mW
12 V11mA9 mW
9 V9mA5mW
9 V4.5mA3mW
9 V2.5mA1mW

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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 2.0 x 1.5 inch (50.8 x 38.1 mm).
A high resolution TIFF file of the PCB is included in the download package.

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