Original AM Micropower Transmitter

8/8/2019 Original AM Micropower Transmitter

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Original AM Micropower Transmitter

The picture to the left is a high quality radio transmitter for the A.M.broadcast band. The transmitter legally operates with "micro-power"and will not set any distance records but, unlike simpler designs, the

frequency stays put and the fidelity is excellent. Although theschematic looks somewhat complex, the circuitry is easy to build andadjust for experimenters with a little "tweaking" experience. A simpleoutput meter confirms proper signal level and checks antenna tuningwhile "on the air". Add an audio mixer, tape recorder, and perhaps a CDplayer and have a near-professional micro-power station.

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Most values are not critical but a few choices must be madecarefully for best results. The output tank is tuned to thecrystal frequency by selecting the values from the chart above.For example, for a 1 MHz transmitter, the chart indicates 500pf and 35 uh. A 33uH and 550pF (470 + 82, perhaps) would bea good start. This chart assumes that a 220 pf capacitor isalready connected between the collector and base of theoutput transistor as indicated in the schematic so the indicatedcapacitance is in addition to the 220 pf. A variable inductor or

capacitor will allow the tank to be fine-tuned for the maximummeter reading with no antenna connected (a few volts with a10 megohm voltmeter or about 50 microamps with a currentmeter). After the antenna is connected, the loading inductor inseries with the antenna is selected for the minimum meterreading (best antenna loading). (A 3 foot antenna will needabout 820 uH for a 1.6 MHz output frequency.) Longer

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antennas or higher frequencies need less inductance andshorter antennas or lower frequencies will need more. Themeter reading should drop by more than half with a reasonablygood antenna but the reading can be ignored if sufficienttransmit range is achieved. The antenna, which is shortrelative to the wavelength, is hard to match well because it hasa very low radiation resistance in series with a very smallcapacitor. (The power dissipated in the radiation resistance isthe power that is transmitted.) The loading coil helps toresonate out some of the series capacity resulting in moreantenna current and thus more radiated power. Some retuningof the tank may be desirable when the loading coil value ischanged. A remote radio playing back through a baby monitoror walkie-talkie makes a good signal quality monitor for

antenna tuning and positioning.

Note: The antenna in the picture above is just a short metal rodfrom an old fireplace screen stuck through an important-looking insulator strictly for appearance. It's really too short foroptimum range.

The crystal can be practically any surplus crystal with afundamental frequency between 530 kHz and 1.7 MHz in 10kHz increments but the higher frequencies work best. Choose a

crystal frequency away from strong local stations at or above800 kHz for best transmit range. Proper operation of theoscillator may be verified by probing the junction of the two1000 pf capacitors with a high impedance oscilloscope probeconnected to a scope or frequency counter. Full modulation isachieved by applying about 2 volts peak-to-peak to the base ofthe current source transistor in the differential amplifier. Themodulation voltage varies the current in the diff. amp. awayfrom the nominal 20 ma. setpoint and this modulated current isconverted to a clean, high voltage sinewave by the output

tuning circuit. The modulated signal may be observed with anoscilloscope connected to the antenna terminal if desired.

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The photo above shows a prototype built with metal transistors

(just for looks!) and with a few additions like the variablecapacitor in series with the crystal for fine tuning and thevariable inductor in the collector of the output transistor.Circuit construction is mostly non-critical but a few pointsshould be observed. Ground-plane is not mandatory but ithelps control parasitic feedback elements when less thanperfect layout techniques are used. The two capacitors acrossthe base-collector leads of the diff-amp transistors should haveshort leads. Bypass the 15 volt supply well, perhaps withadditional 1 uF capacitors not shown in the schematic. The 100ohm emitter resistor in the modulator may be bypassed with a22 ohm resistor in series with a 470 uf capacitor to increasethe modulation sensitivity to about 1 volt peak-to-peak which istypical of many sources. Eliminating the 22 ohm resistor willincrease sensitivity to under 100 mv but the linearity will suffersomewhat.

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An amplifying audio mixer may be added as shown in fig. 2 if

more than one audio source is to be used. The gain resistormight be near 2.8k for typical 300 mv sources or considerablyhigher for lower level sources. If the signal level is different foreach source then vary the 600 ohm resistors to compensate. Alarger resistor will reduce the gain. Set the main gain resistorfor the weakest source then increase the 600 ohm resistors inthe other channels for the proper balance. A fancy mixer panelcould be constructed with potentiometers in place of theresistors. Remember that some op-amps are not sufficientlyfast to amplify high fidelity audio. For simplicity, choose an

internally-compensated audio op-amp such as the LM833.Since the LM833 is a dual op-amp the second amp could beused as a separate pre-amp for a microphone or other low-level sources using the same schematic as the mixer. Theoutput of this amp simply feeds one of the mixer source inputs.

Applications:

A continuous-loop tape could give sales information to passing cars.Place a sign that says, "tune to xxxAM for information," next to thehouse or car that is for sale.Transmit special seasonal music at Christmas or Halloween toenhance your decorations. (Use a similar sign.)Transmit a cassette player or other audio source to the car radio forbetter sound.Make a pair of toy AM band two-way radios by adding inexpensive

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AM radios. Or talk between cars on a trip using the car radio forreception.Make a baby monitor that works with any AM receiver.Transmit control tones to a number of cheap AM receivers forunusual remote control applications.

Build a fully functional radio station for the kids - complete with vumeters, slide faders, and an "on the air" light.

Besides making a nice general purpose radio transmitter thePersonal Radio Station is suitable for some nice practical jokes:

Hide the transmitter with a cassette tape player in yourpersonal effects as you ride in the back seat of a friend's car.(Leave out the meter circuit to keep the size down.) Ask yourfriend to tune in that new radio station - since your transmitter

is crystal controlled it will be at the right place on the dial.What your victim hears is up to you. The circuit will workreasonably well with a single 9 volt battery instead of 15 volts.How about a less than desirable school lunch menu for thekids. Or, if you are younger, an unexpected school closing forthe day. (I didn't really suggest that one, did I?) A newsannouncement of your marriage proposal will get results. Localnews personalities will probably be delighted to help make atape.

The Law

Part 15 of Title 47 of the Federal Code of Regulationsaddresses the construction of homemade AM bandtransmitters. The three most germane paragraphs follow:

15.5 (General conditions of operation)

(a) Persons operating intentional or unintentional radiators shall not be deemed tohave any vested or recognizable right to continued use of any given frequency by

virtue of prior registration or certification of equipment, or for power line carriersystems, on the basis of prior notification of use pursuant to 90- 63(g) of thischapter.

(b) Operation of an intentional, unintentional, or incidental radiator is subject to theconditions that no harmful interference is caused and that interference must beaccepted that may be caused by the operation of an authorized radio station, by another intentional or unintentional radiators by industrial, scientific and medical

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(ISM) equipment, or by an incidental radiator.

(c) The operator of a radio frequency device shall be required to cease operating thedevice upon notification by a Commission representative that the device is causingharmful interference. Operation shall not resume until the condition causing theharmful interference has been corrected.

(d) Intentional radiators that produce Class B emissions (damped wave) areprohibited.

15.23 Home-built devices.(a) Equipment authorization is not required for devices that are not marketed, are notconstructed from a kit, and are built in quantities of five or less for personal use.(b) It is recognized that the individual builder of home-built equipment may notpossess the means to perform the measurements for determining compliance withthe regulations. In this case, the builder is expected to employ good engineeringpractices to meet the specified technical standards to the greatest extentpracticable. The provisions of 15.5 apply to this equipment.

15.219 Operation in the band 510-1705 kHz.(a) The total input power to the final radio frequency stage (exclusive of filament orheater power) shall not exceed 100\milliwatts.(b) The total length of the transmission line, antenna and ground lead (if used) shallnot exceed 3 meters.(c) All emissions below 510 kHz or above 1705 kHz shall be attenuated at least 20 dBbelow the level of the unmodulated carrier. Determination of compliance with the 20dB attenuation specification may be based on measurements at the intentionalradiator's antenna output terminal unless the intentional radiator uses a permanentlyattached antenna, in which case compliance shall be demonstrated by measuring theradiated emissions.

In this circuit, the final radio frequency stage is the transistorconnected to the output tank. This transistor conducts one-halfof the bias current flowing through the modulator transistorwhich is set to 20 ma in the circuit as shown. This current maybe determined by measuring the voltage across the 100 ohmresistor. The output transistor drops about two-thirds of thepower supply voltage which is 10 volts with the 15 volt supply.

The power dissipated in the output stage is therefore 10 matimes 10 volts which is the legal limit of 100 mw. An antenna

9.8 feet long is the legal limit and is more than adequate if aproper loading choke is selected. In fact, an antenna only a fewfeet long is more manageable and may be adequate in manyapplications. Harmonic content of the circuit as shown wasmeasured at the output terminal to be 27 dB below the carrierwhen tested at 1.6 MHz. If the tank values are selected nearthe values suggested by the chart, similar performance should

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be achieved. The connection of a properly loaded antenna willfurther filter the radiated signal so the device should be wellinside the technical requirements.

Copyright, 1995-2002

Charles Wenzel

Improved Circuit

The following circuit is an improved version of the transmitterabove. It features a high Q pot core autotransformer thatprovides a very high voltage to the antenna, greatly improving

the range and an improved crystal oscillator section. (Alsoseephono oscillator for a tunable version.)

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The pot core is an 1811 size high-Q ferrite core with an AL of250nH/T2 . Two complete turns are wound on the bobbin, a loopis brought out for the tap, and 28 more turns are wound tocomplete the coil. Notice the knot in one end of the wire tohelp identify the ends after assembly. The bobbin is inserted inthe core halves, the core halves are held together with a weakclamp (a strong clamp can break the core), and a couple ofdrops of epoxy or hot melt glue are applied to the outside of

the core halves. Do not get glue on the faces of the core, thehalves must be held tightly together BEFORE glue is applied.

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The transformer Q combined with the turns ratio is selected togive high antenna voltage without clipping on the peaks of themodulation and without excessively limiting the bandwidth ofthe transmitted signal. The output tuning capacitor is adjustedfor the maximum field strength and will be near 20 pF at thetop end of the band and near 80pF at 1 MHz. The diode/metercircuit in the first design may be connected to the collector of

this new design but the tuner is adjusted for the maximummeter reading.

The prototype is built on a piece of Vectorboard with a nicemulti-turn trimmer for tuning the antenna. Longer antennas orantennas that have more capacitance may require lessinductance and a few turns may be removed from the pot core.

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If you prefer to have a tunable transmitter, considerthe oscillator circuit below (second schematic). The AM radiooscillator coil will give excellent stability. Simply connect theoutput directly to the base of the leftmost NPN in thedifferential amplifier and adjust the 500 ohm potentiometer toget about 1 or 2 volts on the collector of the oscillator. Thecircuit will work fine as-is at 15 volts but raising the emitterresistor from 470 ohms to 1k will save a little power.

Field Strength Meter

Here is a simple field strength meter that is helpful whentuning the output stage:

The circuit draws less than 10 uA with no signal so no switch isrequired. The variable capacitor is adjusted to tune the meterto the desired frequency by adjusting for the highest meterreading when held near the transmitter's antenna. If the meterreaches full scale during tuning or use, move the meter further

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from the antenna. (As you peak up a transmitter, you mayneed to move the meter several times to keep it on scale.) TheMPSA18 may be replaced by other high gain NPN transistors, ifdesired. This meter has an "expanded scale" in that it goesfrom a zero reading to full scale over a fairly small signal levelchange making fine tuning easy.

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Point-to-point wiring is fine for this low frequency circuit.Remember, the meter DOES draw current when there is a

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meter reading above zero so don't leave it near the transmitterfor long periods of time if you value you battery!

Note about part 15 AM band transmitters:

The above circuits do not represent the absolute optimumsetup for maximum legal signal strength but they are closerthan some think! For example, some people object to the 470ohm resistor across the output coil in the first circuit on thegrounds that it "wastes" power that could have beentransmitted. Although another type of matching might give abetter output, virtually all of the power is going to be wasted,anyway. A "matching network" that can provide the required

loading when connected to a legal antenna without anyresistors and gives a good quality signal must be lossy. You

just can't see the resistor. I can state this with confidence dueto the tiny "radiation resistance" and capacitance that a legalantenna has. If you somehow manage to efficiently match tothese values, the Q of the resulting network will be tens ofthousands and the resulting bandwidth would be a few tens ofhertz. All the listener would hear is a low rumble!

In order to get the bandwidth high enough for music, the

natural Q of the short antenna must be lowered by a factor ofseveral hundred (at least) and there goes all that preciouspower! Basically, you want the highest possible voltage on theantenna but that value is limited by the maximum acceptableQ (lowest tolerable bandwidth) and the allowed power. Oneway or the other, you must include resistive losses. Thesecircuits (especially the first) "over do it" a bit to make themmore forgiving. (A poorly tuned matching network may notexhibit the necessary loss causing the modulation to bedistorted.)

Having stated that, you can do somewhat better than theabove circuits. If the transmitter were 100% efficient, youshould be able to get antenna voltages above about 200 voltsRMS and still have good audio bandwidth. More realistically,100 volts RMS is practical and the second circuit does fall a bitshort of this value. Playing with the transformer might yield

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better output but it is hard to pick up significant rangeimprovements when only a few dB are a stake and the tuningbecomes more and more critical; major improvements justaren't possible. I have read suggestions of different circuittopologies and higher quality reactances to reduce circuitlosses but when you are done you will just have to add aresistor to get the Q back down!

Now, if you don't care about the bandwidth, that's a differentmatter! I wonder how far low speed data could betransmitted...

I should point out that starting with a higher supply voltage willdirectly give more antenna voltage for a fixed Q. And, it maybe practical to make a low-loss non-resonant transformer toget a higher antenna voltage with a given Q. I would bump upthe power supply voltage first, the transformer would be achallenge! A high voltage mosfet or even a vacuum tube wouldmake an interesting output device for a high voltage version.

The same circuit might work fine simply by connecting theoutput inductor to the higher voltage, leaving the rest of thecircuit connected to 15 volts and reducing the current byincreasing the 100 ohm in the emitter of the bottom transistor.Adjust the current down to keep the transmitter legal.

Richard de los Santos , Jack Dofelmire and Phong Nguyenteamed up on their ECT final project entitled " Micro Power

Transmitting Station" using this AM transmitter. Nice lookingreport!

Tunable Phono Oscillator

A "phono oscillator" is a simple, short-range AM bandtransmitter that was typically used to send the signal from aphonograph to a nearby radio, eliminating the need for anamplifier and speaker. This version uses only one transistor

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and can be tuned to any desired frequency near the top of theAM band. Fidelity is surprisingly good considering the simplicityand is suitable for transmitting "Golden Age of Radio" typeshows to a restored tube set.

Instead of a crystal, this transmitter uses an oscillator coilintended for AM radios. These coils usually have a red adjustorand the winding with the highest resistance is used, leavingthe other pins unconnected. The 33 pF places the optimumfrequency near the top of the dial but it may be increased to68 pF for operation near 1 MHz. The input and collector chokesare millihenry values (not microhenry). The transistor isn't

critical and just about any NPN small-signal type will work fine.The collector output may be used to drive other circuits (likethe differential amplifier in the previous circuit in place of thecrystal oscillator) but it may be desirable to add a fewthousand ohms in series to prevent excessive loading of theoscillator (see circuit below). The Ant. output has a loading coil

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in series to achieve a much higher voltage on a few meterlength of wire but you won't get much range from this circuit.

My circuit board was designed to plug into those whiteprototyping boards. If you count parts and find one is missing,it's on the back due to a slight layout boo-boo.

Tune up is easiest with an oscilloscope. First tune the oscillatorto the desired frequency with no modulation. Then apply a 1kHz, 100 mV p-p sinewave to the input and adjust the 500 ohmpotentiometer for the most symmetrical waveform. Looking atthe Ant terminal with a few feet of wire may give the best view

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of the actual waveform since the audio signal can make the RFlook a bit distorted at the collector even though it isn't. Simplyplacing a scope probe near the antenna is usually sufficient.Play with the input level and 500 ohm pot to get the bestwaveform but keep the input a little short of 100% modulation,perhaps 90%. The1N914 (or similar diode), 0.1uF capacitor and1 megohm resistor allow a digital voltmeter to monitor theoscillator level. The best operating point will produce a meterreading near 0.7 volts with no modulation. These parts may beleft out if the circuit is to be adjusted with a 'scope.

A simple oscillator to replace the crystal oscillator in the firstproject above may be constructed as follows:

The circuit will operate on 15 volts without modification but the470 ohm emitter resistor may be increased to save a littlepower.

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Unfair Radio Transmitter

It may come to pass that AM radio stations begin to disappear,switch to a music format, or, gag, switch to local programming.

The process will probably be slow and those interesting talkprograms will be available for some time over the Internet. Forthose caught in such a state of Limbo, the Unfair Radio

Transmitter comes to the rescue. The Unfair Radio Transmitteris a personal radio station that broadcasts an Internet audiofeed throughout the house so that those AM radios scatteredabout remain useful.

Those who have experimented with electric field transmitterslike those above have probably discovered that they only workwell when the antenna is free and clear and there is little solid

matter between the transmitter and receiver. The electric fieldis easily "shorted out" by even slightly conductive materials. Incontrast, this loop transmitter generates a magnetic field thatcan cut right through the thickest walls. I live in an old rockhome filled with concrete and reinforcing wire mesh and thistransmitter's signal can be easily picked up in the bomb shelter

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at the opposite end of the long house. I'm sure it can bereceived in the cave next door!

The circuit is nearly identical to the ones above with theexception of the loop components and a few of the biasresistors. The dimensions of the loop were chosen to adhere tothe most strict interpretation of the FCC rules regardingantenna length. The simple square design yields an inductanceof 2 uH when constructed with 1/2" copper tubing. Thatinductance determines all the rest of the antenna values. First,

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the series combination of the two capacitors (shown as 8000pF) must resonate with the inductance at the selectedfrequency. The Q must be kept down to about 225 foradequate bandwidth for typical talk radio programming (evenless for music). That Q value means that the losses in theantenna circuit must be near 4,000 ohms, shunt. The 10 kresistor shown is representative of a value that might be addedto bring the Q down to the right value. Don't worry about the"loss" of signal power; it is unavoidable if the proper bandwidthis to be maintained. Tuning and selection of loadingcomponents will be discussed below. It turns out that themaximum voltage swing that can be achieved at this Q with aclass-A driver is about 70 volts p-p at full modulation, so acapacitive tap is used to cut that voltage in half, just right for a

24 volt power supply. 35 volts p-p swing on the collector willbring the collector voltage down to about 6.5 volts, leavingabout 4.5 volts from collector to emitter.

The 180 ohm emitter resistor sets the current in the outputstage to about 10 mA or 5 mA per side. The stead-statevoltage across the output transistor is about 20 volts so thepower in that last transistor is 20 V x 5 mA = 100 mW, thelegal limit. You can't simply lower the 180 ohm resistor formore power, the voltage swing will be too big.

One nice thing about this circuit is that it operates at a fairlylow impedance. A 10X scope probe may be directly connectedto the output collector to directly observe the voltage:

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This image was taken with an oscilloscope probe connecteddirectly to the collector of the output transistor. The 22 uHchoke that provides bias to the collector of the outputtransistor may be a higher value, typically 100 uH; I justhappened to have a large 22 uH at hand and the value isn'tcritical. The low 22 uH value just reduces the effectivecapacitance of the capacitors slightly, perhaps 5 %. The twoseries 8000 pF capacitors are selected to resonate the loop atthe desired frequency. Tack in candidate values then add atrimmer capacitor across the coil to see how you did. You canalso leave a trimmer capacitor in the circuit, if desired. Idiscovered that my capacitors were slightly too big so I simplyparalleled a small 100 uH molded choke to lower theinductance a bit. It turned out that this inductor had just theright amount of loss to eliminate the need for a shunt resistor.But, earlier, I used a trimmer capacitor and about 10 k to get

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the proper Q. If the loop is not sufficiently loaded, the sinewavewill begin to flatten on the top and bottom at full modulation,at about 70 volts p-p. You can increase the Q (by reducing theloading of the coil) to get more swing but your signal will soundmuffled. Just add a resistor across the coil until the waveformlooks like the one above. The voltage above suggests that thecirculating current in the coil is over 3 amps!

I found it handy to have a trimmer capacitor across the coil forpeaking with my field strength meter when prototyping. Onceeverything is working perfectly, the capacitor may be replacedwith a fixed value. I found that sticking my hand in the loopdidn't detune the circuit much so a trimmer capacitor may bemounted right across the coil and tweaked by hand at the veryend. The scope probe does detune the loop slightly but a lightadjustment of the trimmer brings the amplitude back to thepeak.

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The circuit is mounted in a standard plastic conduit box but theinterface to the loop was more difficult than might be obvious.

The copper tubing doesn't really fit inside the PVC tubing. I

heated the PVC until it was soft, then shoved it on the precisedistance I needed. I then had to sand down the end of the PVCpipe so that it would fit into the box again. The end result is avery snug fit, but it would be a lot easier to just use all PVCtubing and run some heavy wire or braid through the inside.Perhaps a piece of RG58 inside the PVC would be about right.

The smaller diameter of the conductor will increase the

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inductance a bit so expect lower value capacitors forresonance.

In order to solder to the ends of the pipe, I cut little tabs withan high-speed abrasive wheel. They're bent toward the bottomof the photo. Notice the 100 uH choke that tuned my final

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design perfectly. That was replaced by a resistor in parallelwith a 365 pF trimmer capacitor at one time. The twocapacitors are made by paralleling two old-style micacapacitors for a value near 9000 pF. Stay with high-qualitycapacitors like mica, glass, porcelain, Teflon, and NPO ceramic.Ordinary ceramic is a bit lossy but those losses might be justabout what you need to set the Q. If your capacitors are toolossy, you will be able to get a peak but the voltage will be low.

The large 22 uH could have been another 100 uH or higher. Iused some old 2N718A metal transistors instead of the 2N4401but most small-signal transistors should work fine. Using twopin connectors for the power, audio, and antenna make it a loteasier to make changes to the board. Notice the piece of wiresoldered to the ground plane in the top-right corner of the

board. That is for connecting the scope and voltmeters. Notshown is the 24 volt molded power supply and the cable thatruns to the computer speaker jack.

The audio gain is a bit high to accommodate wimpy sourcesbut the 22 ohm in the emitter of the bottom transistor is lowenough to start causing distortion. If your computer can drivethe circuit readily, increase that value until you are running thevolume on the laptop near full. There are freeware programsfor generating a sine wave that are great for looking at the

modulation level.

This was a fairly trouble-free project and it works great but letme know if you have any problems. ([email protected])

FYI: The name "Unfair Radio Transmitter" is a play on the"Fairness Doctrine", intended as humor. Well, I think it's funny.
mailto:[email protected]:[email protected]

Original AM Micropower Transmitter

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