URGENT: Need help on an AM transmitter design

[teedee]

Hi,

I'm building an am transmitter according to the following schematic. However, my circuit could not transmit any signal for more than 1 meter. I originally used two of 2N2222 transistors instead of the BC109C but even when I switched out the 2N2222 transistors with the BC109C ones, the transmitting range didn't improve at all. I suspect that the transmitting power is too low but don't know how to make it higher.

I'm a newbie for RF circuits. If anyone knows how to improve the transmitting range of the circuit, please let me know. I would appreciate any of your ideas.

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Thanks!

 

[audioguru]

Changing the transistors doesn't make any difference because any small silicon transistor will work up to hundreds of MHz but this circuit works at only 1MHz.

The frequency is so low that a half-wave antenna must be about 80m long and the circuit should have an earth ground.

Reduce R2 to 220 ohms and the range will be farther. Then try 100 ohms.

 

[RadioRon]

I recommend putting a 1000 pF capacitor in series with the antenna, just to protect your output circuit from DC faults.

If this thing is oscillating at about 1 MHz, your range will be very dependent on the length of your antenna. The longer the better, especially if it is going straight up. You might try using an alligator clip on the antenna lead and hooking it to metal things on hand, like a metal gutter downpipe, or maybe a window frame that isn't painted for example. Unfortunately, your oscillator tank circuit has the antenna tightly coupled to it so every time you try a new antenna, you will probably have to retune the frequency back to where you want it.

Do you have an oscilloscope available? If you do, you can put the probe tip near your oscillator or near your antenna just to see if you are getting much depth of modulation. Perhaps you AM modulation depth is just too low.

 

[teedee]

Hi, thanks for your tips.

I reduced R2 to 220 Ohms and the quality improved a bit but the range is almost the same. I tried to reduced more and it stopped working. I also removed R1 and C6, and shorted R8 to improved the gain of the CE amplifier. It seemed to have the signal transmitted a bit further, almost 2 meters.

I have an oscilloscope and I clipped the probe to the antenna. It showed that the signal is modulated. I actually changed the LC tank to 100uH and 100pf to get a higher frequency. As I understand, if it transmits at a high frequency, the length of the antenna can be reduced. However, I don't know if changing the frequency of the LC tank will require changing other stuff as well or not. Should I change it to a higher frequency? I have an am radio. Can I just change the value of the LC tank of the radio to pick up the new frequency of the transmitter?

Thanks.

 

[stevez]

Teedee - I don't know where you are located but most countries have regulations that limit what you can do. If your plan is to broadcast without a license you should take the time to become aware of the rules.

I took a quick look and for the US, unlicensed operation is permitted on the AM and FM bands (more or less 530 to 1700 kHz and 88 to 108 mHz) with at least the following restriction - that the input power to the final amplifier be limited to 100 mw. There are other restrictions. If you are here in the US then you should plan on limited the power input to your final amplifier to 100 mw. You might simplify this and assume that the power consumed by the preceeding stages is negligible - and simply check to see that the power supplied doesn't exceed approximately 100 mw under operating conditions.

 

[TheVictim]

Isn't there a rule that states the antenna including the lead wire needs to be less than 10' tall? Coils can get by this ruling, which gave birth to the antenna known as the "British Joystick". Barely legal, it allowed for amazing range while still being within the rules.

 

[teedee]

stevez: thanks for the info. I didn't know about those rules. I am located in the US. I'm actually trying to make a wireless audio transceiver for my guitar amplifier, which is also a project I'm doing for one of my classes. I just want it to work within 10 meter radius. I thought within that range, the device would not interfere with my neighbors' radios and I should be ok. So I have tried to get the transceiver work up to that range and didn't pay attention to those regulations.

TheVictim: I'm using coils as the inductors and antennas for both the transmitter and receiver but somehow the operation range is still very limited, under 2 meters (about 6ft.) :-(

 

[Nigel Goodwin]

Well for a guitar link you don't want an AM transmitter, the quality wil be pathetic! - if you really want to use a crude transmitter like this?, look at the various FM bugs on these forums, with Audioguru's version highly recommended.

 

[audioguru]

Geetar players like AM radio and too much fuzz from their geetars because they are deafened to frequencies above only 3kHz.

 

[stevez]

I took a few minutes last night to look up the rules for the US. Part 15 is clear about the input power and it mentions the requirements for compliance with other rules including other areas in Part 15 as well as 73 and 74. On the pages that describes the input power limitations there is an estimate of expected range (200 ft) but it's not a rule. The other parts may provide some guidance or regulation on the antenna but it would take time to find. There is likely to be some requirement related to harmonic/other emissions, bandwidth, etc - worth knowing but for experimenting and extremely low power I don't think it's a big concern. Over the years I've heard things about not extending past the property line but I've never seen anything in the rules to support that.

Keep in mind that in the schematic it seems that your antenna is very much a part of the tuned circuit. Anything that's near it - including you - is likely to adjust the tuning and unless your receiver can follow it you'll have to sit perfectly still.That may not be a problem if all you intend to do is show that it works.

There are some pretty good 88 to 108 mHz FM transmitter schematics out there as well as some modest cost kits. Keep in mind that construction techniques are somewhat critical and that simpler circuits will drift easily or be very sensitive to vibrations.

Best of luck and keep working on it.

 

[TheVictim]

There is no rule about the location of the transmitter. You can mount your British Joystick on the top of your car and drive it up a mountain if you want. Check out some sites on legal pirate radio setups. Some of these are quite impressive.

 

[Hero999]

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