A Stable Radio Microphone With Loop Antenna

A Stable Radio Microphone With Loop Antenna Anyone who has constructed any of the numerous transmitter circuits on the web will have found most of them to be unstable. The reason – they lack crystal control. Wide deviation FM (as employed on the FM broadcast band) is not amenable to crystal control - the frequency deviation is too great and a crystal oscillator simply cannot be made to deviate that far. The answer – a phase locked loop – is complex and too big an undertaking for most hobbyists.

T

he stability of these circuits is made worse by the usual choice of antenna – a piece of dangling wire. But even a rigid whip antenna doesn’t improve matters very much. The simple presence of an antenna causes the immediate RF environment to become part of the oscillator circuit with the consequence that any change in that environment results in a change in load and a shift in transmit frequency. We present here the loop antenna - something of an outsider until quite recently when it found application in the large number of MP3 transmitters on the market. The loop is fairly unique amongst the antenna types in that it doesn’t depend on a ground ‘ballast’. The circulating currents in the loop radiate RF energy, most of which destructively interferes for a loop of a practical size, but radiate it does nonetheless. The loop's independence of a ground or counterpoise brings a very considerable stability dividend - it is almost independent of the RF environment. As a result, surrounding objects (including you) are no longer part of the radiating process. This yields a transmitter that doesn't shift transmit frequency if you come near it or pick it up! You can even touch the 'cold' side of the loop without significant frequency shift!

Plastic pillars and tie wraps serve to secure the loop to the main board, thereby preventing movement of the loop in the event that the radio microphone is dropped.

To be effective, a loop antenna needs to be resonant. But if the loop is substituted in place of the transmitter's usual tank inductor, then resonance is assured regardless of the actual transmit frequency. The result is a very compact transmitter that is stable AND has useful range. Something of a dream come true really! The core transmitter components (those inside the dotted box) are mounted on a sub-board. This is for no other purpose than to move the critical transmitter components out to a middepth position in the case. Note too how the ‘hot’ end of the loop (that connected to the second transistor collector) is more central than the ‘cold’ end (that connected to the supply). These two stratagems serve to put distance between these key components and the user’s hand.

Advanced Electronics Projects

Stable Radio Microphone With Loop Antenna

10K 47K

22p

47K

1-22p

2K2

**

0.1 on/off

0.1 1n 0.1

1M

LED 10p 56K

2N2222

BC547

9V

MIC 1n

470

** 35mm square loop of ideally 2mm diameter wire

PARTS LIST Transistors

BC547 2N2222

1 1

Resistors

470Ω 2K2 47K 56K 1M

1 1 2 1 1

Variable resistor

10K

1

Capacitor (All are ceramic capacitors)

10p 22p 1n or 1Kpf 0.1μF

1 1 2 3

Button trimmer

1-22p

1

Condenser MIC LED - Red On/Off Switch Battery 9V (6F22 type) 9V Battery clip

1 1 1 1 1

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