How'bout Am Transmitter ?

[joeypc]

Hello Everybody !
Has anyone here made a simple AM Transmitter rage form 530Khz-1600Khz successfuly ? I've tried many schematics on the internet but it didn't run ! So If anyone here has made an am transmitter successfuly can share the schematic with me ?
Thank you !

 

[Papabravo]

You do realize that operating such a transmitter is legally risky. Even at low power levels the signals can propagate a long way an interfere with commercial broadcasts. I think a frequency shift would be a good plan.

 

[joeypc]

Well... I would like to say that the range is only about 15m-20m and that's ok.

 

[Papabravo]

Well... I would like to say that the range is only about 15m-20m and that's ok.

Except if conditions are right even a flea powered transmitter might propagate a considerable distance. Do you really want to risk it? Do an FM tansmitter, then you're pretty much guaranteed that LOS is the maximum proagation distance.

 

[joeypc]

You meant "propagation" ? Well I just need low power AM Transmitter will it be difficult ?

 

[RadioRon]

I think that PB has overstated the danger of interfering. At these frequencies you are likely to have a pretty low radiated power, mainly because your antenna will be so small, and skywave propagation really isn't much of an issue until you get up towards 2 MHz and beyond.

As for legality, well there are several issues, but first of all here are some critical excerpts from US law, FCC Part 15 (of course, if you are not in the USA this doesn't apply, but Canada is very similar):

Section 15.23 Home-built devices.
(a) Equipment authorization is not required for devices that are not marketed, are not constructed
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 not possess the
means to perform the measurements for determining compliance with the regulations. In this case, the
builder is expected to employ good engineering practices to meet the specified technical standards to the
greatest extent practicable. The provisions of Section 15.5 apply to this equipment.


The above means that when you build something, it should meet all the rules, but you don't have to prove it to anyone, as in get certification, or do verification of any sort.

Here is the category that such a homemade thing is in:

15.3 Definitions
(o) Intentional radiator. A device that intentionally generates and emits radio frequency energy
by radiation or induction.

And here is the basic law:

15.5 General Conditions
(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions
that no harmful interference is caused and that interference must be accepted that may be caused by the
operation of an authorized radio station, by another intentional or unintentional radiator, by industrial,
scientific and medical (ISM) equipment, or by an incidental radiator.

and here is the technical requirements:

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

So, I think if you meet all these rules, and you don't interfere with your neighbor's enjoyment of their favorite talk-radio program (like Coast to Coast AM) then go ahead and experiment. You might learn something.

 

[stevez]

joeypc - if you are capable of staying well below the requirement you can learn quite a bit by experimentation, as suggested. The challenge of course is knowing enough about the subject to be sure you stay within the required boundaries. My advice seem to be split between Papabravo's and RadioRon's - if you don't know with some degree of certainty that you are compliant then don't do it - however if you are willing to put the effort in to doing this well its certainly an excellent learning experience.

An AM signal is generated by modulating a continuous carrier with audio. In generating the continuous carrier you need to do it in a way that puts all of the energy on the primary frequency - easier said than done. The next problem is modulating the carrier in a way that keeps all of the energy within a reasonable audio limit - that's not easy to do with a simple circuit. It requires that enough audio drive be applied to actually modulate the carrier - and that the drive be limited so that energy is not radiated outside the intended frequencies.

I'd suggest that many of the simple AM circuits that you find are probably not truly compliant - but they also cause little or no problems if power input is kept low and they are used only on occasion for experimentation. If your plan is to put something on the air continuously then it would be wise to be very careful.

If you are interested in Amplitude Modulation you might take a look at some older amateur radio literature. While it might be a challenge for beginners it is certainly is within the bounds of what a hobbyist can handle.

 

[joeypc]

Well thank you all for your advice(s). Seems to me the AM transmitter is more difficult or more complicated than FM transmitter(I've made some FM transmitters before).Well... For Ex I would like to make an AM transmitter the freq is about 1200KHz (of course ignore the tolerance) and the range is about 30 meters. In my country it's legal to broadcast If the range is within 100 meters. The distance is not far enough huh ? Wanna more ? Ok. If you want to transmit with the range more than that (as a radio amateur experimenter or hobbyist only) just have a register with the Ministry of Radio Frequency Management. That's the rule about Radio Amateur in my country.Well.. to tell the truth I don't need such that range, just 30 meters across the romm and that's ok. I just do it for fun. I've found this AM transmitter schematic on the internet. Has anyone built this before ? I've tried this circuit many many time with all of my patience and this goddamn thing will never work ! The oscillator stage doesn't work ! So could you all here give me any advice on this circuit ? Thank you ! Here is the link to the article:**broken link removed**
PS:Has anyone here got some experiences in building\making AM transmitters ? So could you share the experiences with me ? Thank you for those experiences !

 

[crusty]

I feel your pain i've tried about 3 different schematics online, and i could never get a good oscillation out of them, usually it seems the audio inputs are never isolated enough and cause the tank to not oscillate. 2 of them i built I could get great stable oscillation out of them without any type of audio source, but as soon as you connect the audio, there goes the carrier. All this done on simple breadboards with no traces was also pain when it came to diagnosing. I finally gave in bought a kit, since i have no inexpensive or easy way to construct a pcb with traces. Not to mention the kits being easy as pie to construct since you don't have to go searching for individual parts, though you don't learn as much as breadboarding something, but it is less frustrating when you want something to work the first time you solder it together. Do you all use the same frequency band as US? 535-1705khz? I would think amateur am broadcasting would be a more reasonable entry since you have wider bandwidth and less broadcasts than fm, especially with low power and small antenna.

 

[audioguru]

Radio circuits don't work on a breadboard. Its inductance of each connecting wire and capacitance between adjacent tracks is too high.

This extremely simple circuit has the antenna connected directly to the RF oscillator. So its frequency will change if anything gets near it or moves away and the oscillator might even stop.

 

[RadioRon]

I don't have a link to a schematic, but just thinking about this idea, it seems to me that it would be easy to build an AM transmitter as follows. First, build an oscillator using one of the many logic IC RC oscillator circuits. To get variable operation up to 1.5 MHz you would need to build this on a pcb protoboard not a solderless breadboard. You would use a potentiometer to vary the frequency. You need to use HC logic to get the high frequency of oscillation, don't use 74C or 4000 series.

The oscillator would run off of, say, 9 volts power supply (like a battery for example).

Next, you use a logic buffer IC as an output amplifier. For example, the 74HC240 might be a good choice. You can put more than one gate in parallel to get more output power. The power supply for this chip would not be run directly from the battery. Instead, it would be passed through the secondary winding of an audio transformer (something like 600 ohms to 8 ohms type, where the 8 ohm winding is the secondary). The primary side of the audio transformer would be driven by your transmit modulation audio. You would probably need an opamp to go between a microphone and this winding.

It will be necessary to put a series resistor between the oscillator output and the buffer input, since they are operating at different supply voltages. A 10K ohm would be OK I think.

The output of the 74HC240 gate(s) would then be passed through a low pass filter, with a configuration of series L, shunt C, series L, shunt C, then the antenna. This filter would smooth the waveform and so reduce the harmonics to an acceptable level. The values of L and C would be chosen to have a low pass cutoff frequency at 1.8 MHz.

Now, who wants to draw up the schematic for this?

I've seen this done for an HF transmitter in the ARRL Handbook, 2001 edition, page 17.96 showing the project for "an experimental 1/2 W CW transmitter". I'm adding the idea of high level AM modulation by simply modulating the DC power supply to the output amp using the transformer.

 

[crusty]

I don't have a link to a schematic, but just thinking about this idea, it seems to me that it would be easy to build an AM transmitter as follows. First, build an oscillator using one of the many logic IC RC oscillator circuits. To get variable operation up to 1.5 MHz you would need to build this on a pcb protoboard not a solderless breadboard. You would use a potentiometer to vary the frequency. You need to use HC logic to get the high frequency of oscillation, don't use 74C or 4000 series.

The oscillator would run off of, say, 9 volts power supply (like a battery for example).

Next, you use a logic buffer IC as an output amplifier. For example, the 74HC240 might be a good choice. You can put more than one gate in parallel to get more output power. The power supply for this chip would not be run directly from the battery. Instead, it would be passed through the secondary winding of an audio transformer (something like 600 ohms to 8 ohms type, where the 8 ohm winding is the secondary). The primary side of the audio transformer would be driven by your transmit modulation audio. You would probably need an opamp to go between a microphone and this winding.

It will be necessary to put a series resistor between the oscillator output and the buffer input, since they are operating at different supply voltages. A 10K ohm would be OK I think.

The output of the 74HC240 gate(s) would then be passed through a low pass filter, with a configuration of series L, shunt C, series L, shunt C, then the antenna. This filter would smooth the waveform and so reduce the harmonics to an acceptable level. The values of L and C would be chosen to have a low pass cutoff frequency at 1.8 MHz.

Now, who wants to draw up the schematic for this?

I've seen this done for an HF transmitter in the ARRL Handbook, 2001 edition, page 17.96 showing the project for "an experimental 1/2 W CW transmitter". I'm adding the idea of high level AM modulation by simply modulating the DC power supply to the output amp using the transformer.

sounds like a good idear, using logic osc. would help solve some of the drift issues when connecting different loads for sure, and give it a pll lock type deal, though i'm not sure how bad the noise would be? i would assume if wouldn't be too bad if filtered with some caps.

 

[joeypc]

Well.... take a look at this page, take notice of the title "Micro Power AM Broadcast Transmitter"
**broken link removed**

 

[RadioRon]

Well.... take a look at this page, take notice of the title "Micro Power AM Broadcast Transmitter"
**broken link removed**

That looks pretty good, but hard to know unless you build it.

I am a bit puzzled why they would recommend a schmitt trigger inverter rather than an unbuffered regular inverter like the 74HCU04. Since you want linear operation, a schmitt doesn't seem like the easiest oscillator to get running when you're pushing the limits. For high frequency I would select the HCU04.

 

[whiz115]

Well thank you all for your advice(s). Seems to me the AM transmitter is more difficult or more complicated than FM transmitter(I've made some FM transmitters before).

you can make a very simple AM transmitter...

https://scitoys.com/scitoys/scitoys/radio/am_transmitter.html

see how easy it is? i wouldn't worry for anything other than locating
the crystal oscillator and the transformer.

 

[crusty]

I've seen that, just never ordered the part and put it together, but yeah, I love concept, simple, plus isolated inputs, nice. very KISS

 

[audioguru]

How many 9V batteries does the AM transmitter have?
It shows a single 9V battery that has 18V!!!

 

[joeypc]

I feel your pain i've tried about 3 different schematics online, and i could never get a good oscillation out of them, usually it seems the audio inputs are never isolated enough and cause the tank to not oscillate. 2 of them i built I could get great stable oscillation out of them without any type of audio source, but as soon as you connect the audio, there goes the carrier. All this done on simple breadboards with no traces was also pain when it came to diagnosing. I finally gave in bought a kit, since i have no inexpensive or easy way to construct a pcb with traces. Not to mention the kits being easy as pie to construct since you don't have to go searching for individual parts, though you don't learn as much as breadboarding something, but it is less frustrating when you want something to work the first time you solder it together. Do you all use the same frequency band as US? 535-1705khz? I would think amateur am broadcasting would be a more reasonable entry since you have wider bandwidth and less broadcasts than fm, especially with low power and small antenna.

Hello Crusty, Could you share two scematics that you've built ?
Thank you !

 

[crusty]

Hello Crusty, Could you share two scematics that you've built ?
Thank you !

Thing is i've used other people's schematics, and i've not had any luck out of building them from scratch, also it's been a couple years I forget which ones i've tried. I would take a suggestion from someone whom has already built a working model, and they should be able to point you in direction of a good schematic to start out with. i'm just a lil n00b.