AM transmitted signal range

[medwatt]

Hello,
I have built an AM transmitter and now I can only receive signal within just 1m radius of the transmitter. I use a colpitt oscillator to get a carrier of 740khz. For the modulating signal I'm using a 1khz tone from a signal generator although I've also used an electret microphone. I am using the mc1496p IC with the circuit provided by the data sheet as a double sideband suppressed carrier am modulator.

I have tested every stage and observed a perfect sine wave for the oscillator with a peak amplitude of 0.5v. Similarly the modulated signal from the output of the modulator at pin 6 was about the same value.

The carrier signal was successfully modulated because I could see when the 1khz tone signal superimposed on the carrier from the oscilloscope.

My problem is I can only get the transmitted signal at a distance of about 1m which does not make sense and completely defies the who idea of the design.

What I have done unsuccessfully were to use a 1m long wire and use the areil from a radio as a radiating antenna which I connected to the pin 6. I noticed no change when I moved my radio about. I could only receive anything very close to the transmitter.

I hope someone can give me an idea what to do to improve the range so something like 300m which should be actually the minimum distance for AM signals.

Here is the modulator circuit if you wish to see it: **broken link removed**

 

[vfone]

At output pin 6 the level is about 1V in parallel with few kilo-ohms, which make the output power very low.
To get this distance range you have to amplify the signal and use a power stage matched to the antenna.
For 300m (depending by the frequency and what antenna is used) you need about 1W output power, for a steady link communication.

 

[FvM]

To effectively transmit a sub MHz signal with a small antenna, you should rather go for a magnetical antenna (ferrite rod). It's most likely the better receiver antenna option as well.

Is your receiver prepared to receive supressed carrier AM?

What's your motivation to use MW band? There are no allowed frequencies according to RF regulations in any country, I think.

 

[betwixt]

I agree with previous comments but would add one other point: Are you modulating the transmitter while listening on your radio? If you are just listening for the carrier it will be very weak because you are supressing it in the modulator.

Brian.

 

[medwatt]

I have no whatsoever motivation using medium band. I'm just doing it for experimental reasons. As I mentioned I can receive the signal within 2m radius clearly even with no antenna. I have tried using a radio set antenna as the transmitting antenna but to no avail.
I myself have been thinking of a power stage to boost the signal power but couldn't get one to work.

I also have to build a receiver at the end and would really like to ask if the circuit I'm using for modulation will cause me problems to demodulate the AM signal. Will it be a better (for experimental purposes) to use instead another AM modulator circuit where the carrier is not suppressed. If so, if anyone has some circuit that he's tried for AM and is sure of a definite output, then I'll be glad if I can have a chance to see it.

 

[FvM]

You easily change the operation of your modulator to regular AM by applying a DC offset to the modulating signal. Because you are apparently using a normal radio receiver, as we already suspected, this would be much better.

 

[jiripolivka]

A strange part of this exchange is that the AM receiver was never mentioned or described. Also both antennas, only once the questioner mentioned he used a "1 m wire".

AM receivers are usually quite sensitive; the problem may also be in overdriving a receiver by the modulated carrier.
I would start with a simple diode detector connected to an oscilloscope, and use a 20-30 dB attenuator between the transmitter output and the detector. By this simple system , the modulation can be clearly seen.
If the DC component of detector output can be also monitored with a DC voltmeter, you should see how well it is rejected against AC modulation.
Then either a ferrite antenna or a really long wire (>10 m) can be used at both ends for a successful transmission.
Radio amateurs use QRP mode, with ~1...5 mW transmitters and good AM receivers for shortwave communication, often around the globe. For continental links, 160 meter band can also be used, with ionosphere reflection.
Maybe the ground around your experiment is not conductive, and you located the receiver in the "skip" zone. Then there can be no signal through.

 

[vfone]

1mW transmitter for a shortwave communication around the globe !!?? I never heard about this record.

I used just for fun 1W (a thousand times more than 1mW) in 10m, doing a long pass communication with radio stations on the other side of the globe, but this was many years ago during a peak of propagation which seems that will never come back (according to the predictions).

If check the top-level R/C radios (on 27MHz or 46MHz) they use about 1W (even more) for transmissions no longer than few hundreds of meters, because they are looking for a pretty steady link communication.

 

[jiripolivka]

Dear Vfone,

pity you have not heard how the global communication with 1 mW was done. It was a secret project but nothing miraculous . As usual in a RF communication, what you need is not the power but a good signal-to-noise power ratio. And a good ionospheric propagation conditions.
The system using one mW operated at ~13 MHz. The transmitter was a crystal-controlled oscillator with a SLOW FSK, stabilized by being held in armpit.
The receiver antenna was larger and the receiver as good as it can be. The secret was a narrow-band filter, approx. 1 Hz wide. In fact, there were two such filters and outputs connected to a logic circuit. With the signal under noise, a long integration time constant was used to regenerate the signal from noise. The data rate was said to be 0.1 bits per second, but it worked. Maybe these systems are still in operation.
What is true that the QRP radio amateur records are inter-continental, and the art is to use a stable RF frequency, and a narrow filter in receiver. Read something about EME- radio amateurs communicate by reflections from the Moon, using dozens of Watts at 144 or 432 MHz. The S/N ratio is often -30 dB but it works.

I am still wondering what did our friend at AM so his signal cannot get through....

---------- Post added at 18:08 ---------- Previous post was at 18:01 ----------

I just remember what I did with my first PNP transistor made by Czechoslovak Tesla, in 1957. After making an audio amplifier, I used an old medium-wave coil set and a variable capacitor, and I made my transistor oscillate at ~ 1 MHz, medium wave. My oscillator ran on a "flat" battery, 4.5 V, and took ~1-2 mA. Its signal ( a beat with an AM transmitter) I can hear on my two-tube receiver over all range of my home, > 10 meters, and my grandfather confirmed he heard it when I went on the street, a hundred meters away. My transmitter had no antenna but the coil, and the receiver used a ~1 m vertical wire.

Then the other guy's system is so much poorer.

 

[FvM]

27/40 MHz model RC is limited to 100 mW in the ETSI region which gives still a large margin for using sub-optimal antennas. UHF small radio devices are fine with 10 mW (ETSI) or even 1 mW (FCC) for several 100 m range.

 

[vfone]

Transmitting on the other side of the world using 1mW, a 1Hz filter for RX bandwidth, and 0.1 bits/sec data rate...I think the discussion degenerate in a KGB old story

Real life communications use more and more power, even we don't want to admit this thing.
In a moon-bounce communication at least one of the two correspondents have kilowatts of power and very high gain antennas. Of course some exceptions exist, but not as a regular basis.
After many years of receiving weak signals, I did my first CW moon-bounce radio contact in 2m using "just" 300W and a 16dB gain antenna. After that I found what the corespondent used: 1.5kW and a big dish with more than 30dB of gain. So, actually he "worked" my station, and not in the other way.

R/C power is limited by standard. For sure. But a lot of manufacturers used at the end of 27MHz ages modules for increasing output power, 1W or more.
Now all the high-end R/C manufacturers use 70MHz, 2.4GHz, or even 5.6GHz.

To go back to the original post, can try using a low power PA followed by attenuators, to see exactly what is the minimum output power necessary to get the link connection.