This is an 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 transistor a 2 transistor 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 0.47µH 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.
COMPONENT LIST:
IC1 4093 (CMOS)
TI 2N918
T2 BC557C
X1 24 MHz
L1 1uH
L2/3 5 wind of 1mm wire, diameter 5mm, tapped at 1 wind from the cold end.
R1 39K
R2 1M
R3 10K
R4 330K
R5 1M
R6 1K
C1 47pF
C2 15pF
C3/4 3-20pF
C5 1n
C6 47n
C7 470n
C8 100n
The oscillator is built around 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 intermittent 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.
| supply voltage | supply current | output power |
|---|---|---|
| 15 V | 14mA | 15 mW |
| 12 V | 11mA | 9 mW |
| 9 V | 9mA | 5mW |
| 9 V | 4.5mA | 3mW |
| 9 V | 2.5mA | 1mW |
Above: 2M RX showing the fox in action on a frequency of 144.0875 Mhz NFM.
(Click on images for larger view.)
NOTES: The transmitter was tested extensively. This transmitter is ideal for a small to medium range field. This unit was tested in a 1km radius using the beam antenna for RX and a J-Pole as the TX for the transmitter. I could easily copy the signal even though there were buildings in the way. The bonus of the antenna I used for receive is that you can either use the antenna horizontally or vertically polarized. The TX emits a continues carrier with a very good audible beep. This unit is ideal for a beginner TX to promote ARDF . The bonus is that you can use several of these units in a small to medium field. The unit is small and easy to construct.
