FIRST TRANSMITTER (2020)
Following on from Low Power Transmitter build (c1966) which shows a design using germanium transistors; modern technology for that time! As an exercise, I have tried to fashion an equivalent design for 2020 (50+ years later!) I have used mostly (recycled) components that I have readily at hand.RF oscillator: FET Kalitron type circuit, from: Amateur Radio Magazine (Aust') March 2003 (kalitron dip meter by VK3XU) Modulator: prebuilt cheap LM386 audio module Microphone is an electret (capacitance) type. Very sensitive, outperform a carbon mic! The electret microphone diaphram is an insulating plastic film, polarized with a permanent electric charge. It is made by heating the film in the presence of a strong magnetic field. The microphone is effectively a small capacitor, with a high impedance at audio frequencies. The o/p is buffered by a FET source follower inside the microphone capsule. This acts as an impedance changer with o/p impedance of several hundred ohm. Coupling b/w oscillator and modulator is by 1:1.5 audio transformer and serves as a form of high level modulation of the +ve supply rail to the oscillator. The power supply is 2 series'd 9V batteries, regulated to 12 volt by simple zener diode. Initially without any regulation: the frequency drifts away with ever reducing battery voltage. For bench experimentation purposes; the tuneable oscillator is set at ~40.8 MHz, which falls within a LIPD spectrum assignment. (LIPD = low inteference potential device) see AMCA document:
Radiocommunications (Low Interference Potential Devices) Class Licence 2000
LIPD frequency assignments (from ACMA documentation)
| Frequency range (MHz) | Max' Power | Notes |
| 29.7 - 29.72 | 100mW | |
| 30 - 30.0625 | 100mW | |
| 30.3125 - 31.00 | 100mW | |
| 36.6 - 37.00 | 100mW | |
| 39 - 39.7625 | 100mW | |
| 40.25 - 40.66 | 100mW | |
| 40.66 - 41.00 | 1 watt | 40.8 MHz in this band |
Mounted in box.Coil/caps changed so as to resonate @ ~21.3MHz (15 metre band Amateur Service)
Performance is good, volume setting pot on audio board functions as mod' level, and is FM (although still discernable with an AM detector) No 2nd or 3rd harmonic radiation is observed.
FIRST TRANSMITTER Version #2
As an experiment I have used a compressor/compander type modulator circuit. For full explanation of cct action see this page Basically a compressor is an amplifier circuit which exhibits gain, but continually adjusts that gain to maintain a reasonably constant amplitude output signal, regardless of the amount of signal coming from the microphone itself. This circuit also gives good results!
© 2Q 2020