pilar's projects

17/9/2015 Pilar's Projects

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Pilar's PageA stable Radio Microphone With Loop Antenna

Superregenerative Air Band Radio

A Stable Radio Microphone With Loop Antenna

Pilar Ortega

This article began as a design guide for hyper cranial erogenic field generators, however,your primitive earth brains are completely unable to respond to erogenic fields in the100MHz region. I personally had a brain upgrade (for purely medical purposes of course)so I am able to respond to these fields. But for you poor primitive earthlings we mustresign these devices to the only use you are able to find for them – short rangetransmitters for your FM broadcast band. Oh – and the rumour that I bought a house nextto an FM broadcasting station is completely false. Well, mostly.

IMPORTANT NOTICE:

We, as engineers, do not build miniature transmitters for the actual purpose of usingthem. They are engineering challenges intended to illustrate principles. We do notadvocate their use for a couple of reasons.

Firstly, their use probably contravenes radio regulatory and licensing legislation ofyour country. In other words using one, for whatever purpose, is probably illegal.

Secondly, if used as a covert listening device, they probably contravene theprivacy laws of your country.

Lastly, it is simply WRONG to use them for clandestine purposes.

Readers build and operate this circuit entirely at their own risk and it is the reader’sresponsibility to determine the legality (or otherwise) of the use of this circuit in theirown country or state. If the reader chooses to ignore this warning, we hope they will atleast take seriously the nuisance they can cause when operating illegal transmittingcircuits, and heed the suggestion for ways to improve stability and reduce the risk ofinterfering with legitimate services.

THE PROBLEM POOR STABILITY

Anyone who has constructed any of the numerous transmitter circuits on the web willhave found most of them to be unstable. The reason – they lack crystal control. Widedeviation FM (as employed on the FM broadcast band) is not amenable to crystalcontrol the frequency deviation is too great and a crystal oscillator simply cannot bemade to deviate that far. The answer – a phase locked loop – is complex and too big anundertaking for most hobbyists.

The stability of these circuits is made worse by the usual choice of antenna – a piece ofdangling wire. But even a rigid whip antenna doesn’t improve matters very much. Thesimple presence of an antenna causes the immediate RF environment to become part



of the oscillator circuit with the consequence that any change in that environmentresults 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 itfound application in the large number of MP3 transmitters on the market. The loop isfairly 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 destructivelyinterferes 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 stabilitydividend it is almost independent of the RF environment. As a result, surroundingobjects (including you) are no longer part of the radiating process. This yields atransmitter that doesn't shift transmit frequency if you come near it or pick it up! Youcan even touch the 'cold' side of the loop without significant frequency shift!

To be effective, a loop antenna needs to be resonant. But if the loop is substituted inplace of the transmitter's usual tank inductor, then resonance is assured regardless ofthe actual transmit frequency. The result is a very compact transmitter that is stableAND has useful range. Something of a dream come true really!



Radio microphone using loop antenna

The core transmitter components (those inside the dotted box) are mounted on a subboard. This is for no other purpose than to move the critical transmitter componentsout to a middepth position in the case. Note too how the ‘hot’ end of the loop (thatconnected to the second transistor collector) is more central than the ‘cold’ end (thatconnected to the supply). These two stratagems serve to put distance between thesekey components and the user’s hand.

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

Superregenerative Air Band RadioPilar Ortega

Civil aviation still uses AM communications between 108MHz and 136MHz. It isfairly easy to build a receiver that operates on these frequencies, however, itwill also be found easy to build a receiver that won’t tune high enough! Thetrick to getting the tuning range high enough is to use a high frequencytransistor and VERY short leads. I used a BF199 transistor but a 2N2369 willprobably work well too.

By far the most reliable construction method is ‘ugly’ construction on a 1 inchby 2 inch piece of copper clad board (I built this circuit on perforated boardmany years ago and it only just tuned high enough to receive my local airportsignals). By using the copper foil as a ground plane we can mount componentsin mid air. This is more robust than it might seem because many componentsconnect to decoupling capacitors that provide an anchor to the board. Keepingleads short reduces stray inductance and capacitance, and helps ensurestability and good tuning range. The circuit is based on an old one advertised



in a hobby magazine many years ago, modified slightly.

The tuning coil ‘L’ is simply four turns of 0.8mm diameter wire with a diameterof 5mm. There is no former. The coil is compressed or expanded to adjust thetuning range. We need to arrange that we can just tune the upper end of theFM broadcast band when the tuning potentiometer is at minimum setting(wiper at the ground end). This will bring the Air Band into the main tuningrange of the receiver. The antenna is a short telescopic one and couples with a¾ turn link turn. Note that, if the coupling is too tight (the link too close tothe main coil) then the regenerative stage will be too loaded and won’toscillate.

The varicap diode ‘D’ was an unmarked component in my receiver, however, theBV409 should work admirably here. All capacitors with the exception of theelectrolytic are ceramic. The transistors are best mounted upside down withtheir leads bent sideways.



No switch was used in my receiver. Instead, a stereo 3.5mm socket was usedand the third contact used to switch the supply. Note that this trick only workswhen a mono plug is inserted! I mounted the earphone socket such that ittraps the battery in one corner of the case. I expect this apparently coolsolution is going to come back to bite me when I drop the radio, and thesocket gets smashed off by the momentum of the battery!

In operation the regeneration control is increased until a rushing sound isheard. This noise indicates that the circuit is oscillating but will diminish whena signal is received. At this time the tuning and regeneration potentiometerscan be adjusted for best reception. The receiver will be found to be mostsensitive when the regeneration is set so that the circuit is just oscillating.

Purists will add an RF gain stage prior to the regenerative section, as thisminimises coupling of oscillations to the antenna as well as adding sensitivity.In practise the circuit is sensitive enough but one should be aware that RFradiation from the antenna can interfere with nearby equipment, in particular,televisions.

If the circuit were operated on a plane it might well interfere withcommunications so on no account should this circuit be used near an actualplane. In any case, you would rightly be barred from bringing such items on aplane, let alone be permitted to use one!!!

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