27 MHz Amplification Query

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the problem is this pair i build is giving a range of less than a feet to me! What could be wrong with it? Please provide some solutions to increase this range!
 


I once tried to make a homebrew receiver to replace the one lost in my radio-controlled plane that flew away over a nearby hill, never to be seen again.

I still had the transmitter which worked fine (on 72 MHz I think).

I managed to construct an inefficient receiver. I built a proper-sized LC tank and tuned it to 72 MHz. This alone was an achievement.
I amplified it through one stage.
I split the pulse train with a 4017 IC.
The pulses were able to control servos.

It worked, but only a few feet away. I realized my radio reception needed more amplification.

I had seen schematics of radio receivers which cascade the incoming signal through 2 or 3 stages. These are coupled by LC tanks. They respond to the resonant frequency, while suppressing noise.

All components would need to be miniaturized, and lightweight. My oscilloscope was not designed for that high a frequency. The task was beyond my knowledge. I decided it would take someone who had more radio expertise, to make a receiver which was sensitive enough.

If you wish to try, it's best to consult radio schematics for bringing in 27 MHz. All components must be custom adjusted. They can be normal size and weight, being in a ground vehicle.
 

the receiver i built (circuit diagram is attached in the previous page) also includes a tuned LC tank and 2 stages of cascaded amplifiers.......
 

If you are only achieving such a short range it suggests the transmitter and receiver are tuned to different frequencies. You should get reasonable range even without any antenna at all. I suggest you try this:

Add a capacitor of 10pf in series with the receiver antenna, this will reduce the signal strength slightly but more importantly it will reduce the capacitive effects of the antenna wire on the tuned frequency. Then change C2 for a variable capacitor instead of a fixed one. 100pF would be a good starting value if 47pF is supposed to be the value for 27MHz. Turn the transmitter on and adjust the capacitor for best reception.

If you want to increase the range further, I suggest increasing the receiver gain by adding a preamplifier stage between the antenna and it's present connection point. Note that this is NOT the same as the gain provided by the existing two transistor stages. They are there to recover the regeneration voltage, not to improve it's sensitivity.

Brian.
 

with no antenna i am getting a range on 6 inches! and weirdly with a 4 inch transmitter antenna the range increased to some 2 inches more...... but when i added the length of transmitter antenna to some 7 inches; the range from the tip of the transmitter antenna to the tip of the receiver antenna was only 2 inches now! And I have employed a tuned circuit with 2.2uH fixed axial lead inductor and a trimmer capacitor of (6 to 20pf)!

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and i am supplying 6 volts to transmitter from a 1.35Amps dry battery and 9 volt battery to the receiver......transmitter is built on a breadboard and the receiver is on a veroboard......
Is this range i am getting is it the full capability of this pair or is it that i have done something wrong in the construction?? If latter is the case is their a way to check the same?
 
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RMMK

A bit of basic testing is required

If you are going to try and build transmitters, you should have some way to determine what frequency it is producing
A frequency counter is almost essential. Failing the ownership of a counter you should at least have a commercial radio receiver
that well beyond either side of the frequency you expect your transmitter to be operating on

Without this basic method of testing, you have no idea if your oscillator is even oscillating and if it is, what frequency its oscillating on

It doesn't matter what anyone on here tries to tell you what to do, if you have no methods for determining freq. we are all almost wasting our breath

GET a freq counter or a radio receiver that covers 25 - 35MHz or wider range, and determine where you osc. is working
Without that basic info, we have no idea if we should be suggesting what component values to change to get the TX onto 27MHz

regards
Dave
 

ok here is the problem...... even if i remove the crystal from the transmitter, the receiver still receives the same signal. And the signal goes out when i remove power from the 555 IC.
 

This is what I suspected was happening when I suggested seeing what came out of the receiver when the transmitter was powered down.

If the crystal is 27.0MHz and the oscillator is running, you should be able to pick it's harmonic up on 108MHz, right at the top edge of the VHF broadcast band.

The effects of different antenna wires is probably causing the transmitter to oscillate at different frequencies, some nearer the receiver frequency, some further away, that's why the apparent difference in range. When everything is running at the correct frequency you should find the range increases dramatically.

Brian.
 

And not just that but the receiver also picks signal from my power supply if i place it too close with it!

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Is it the capacitance issue with the breadboard b/c i also tried removing the antenna wire from the pair..... and yes i checked it from a borrowed oscilloscope that it seems like the crystal s not oscillating! Is it that bad to work on RF over a breadboard? Should i build this as well on a veroboard (making pcb's is quite tedious)? i am also sceptical about these RFC's because i have used axial leaded inductor of the same ratings...... is it okay??
 
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Interference is always an issue with AM systems, particularly at relatively low frequencies like 27MHZ. It's one of the reasons VHF is normally used because as the frequency increases the number of interference sources and their harmonics is less.

Breadboard isn't really a good idea although you might get away with it at 27MHZ. Consider the number of turns on the coils you use in relation to the length of connecting wires and you see how other construction factors can make a big difference to performance. Veroboard might be better as long as you keep it as small as possible and cut off unused sections of track. It has the disadvantage that placing tracks parallel to each other produces highest coupling between them and that is exactly what you don't want in RF circuits.

2.2uH in needs 15.8pF across it to resonate at 27MHz so the inductor and capacitor you are using should be OK.

Brian.
 
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    RMMK

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and sir what could be the solution for the crystal to be not working?
 

It could be for several reasons, including the layout.
Another possibility is the 'cut' of the crystal may be wrong for that oscillator circuit, try connecting he crystal between the base of the transistor and ground instead of base and collector to see if that helps.

Brian.
 

i tried placing the crystal as you said but it didnt work! I also tried a new crystal but i got the same problem......

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my worry is why is the receiver picking up the signal with the same intensity even when the crystal is removed from the transmitter....... Sir I have a 49MHz transmitter should i try tuning this 49MHz transmitter with the receiver and see if it works?? but i am not sure if its an AM transmitter..... it houses a TX-2B IC.
 
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I tried building the transmitter again on a veroboard but without any success....... I even clustered the components so close that the dimensions are roughly 1.5-inches X 2-inches..... Same issue again!
 

my worry is why is the receiver picking up the signal with the same intensity even when the crystal is removed from the transmitter
Click to expand...

There will be more puzzlements like this, as you construct your homebrew receiver and transmitter units.

It's a case of trying to make a Futaba quality system on your own, without having the instruments available to Futaba's R&D team.

Since you have a project to finish, you may be better off finding a radio-control truck/boat/plane, and cannibalizing its radio system.
 
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hahahaha...... Seriously I was thinking the same! Well i have interfaced my 433MHz RF Modules with my Arduino and started working with it but still i need to complete this 27MHz thingy just for the sake of learning....... though the 433MHz pair will be getting employed on the land based setup! I have also worked around with a broadband RF Pre-amplifier..... i will upload this schematic in a day so that it may get verified.......
 

umm i placed the probes of my DMM across the terminal of the first and second transistor on the transmitter and whenever i do the same one at a time in either of the case; the receiver seems to pick up the signal from a greater range...... what is happening?
I placed a 27pF capacitor in series with the receiving antenna and tried to tune it....... amazingly it got tuned to some extent and gave me a transmission at a range of 15 feet. But unfortunately it was for a very small time!
I have noticed a very weird thing happening on which i have no clue is that the resistor R2 of value 100-ohms on the emitter of Transistor T1 in the transmitter is getting hot! Plz help..........
 
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When dealing with RF you have to take account of the effects the test equipment has on the circuit. Your DMM probes probably have more inductance than the coils in the circuit and they pull the frequency very significantly when you apply them.

If the emitter resistor gets hot it indicates the transistor is probably oscillating although not necessarily at the right frequency!

Take a look at this similar thread for ideas: https://www.edaboard.com/threads/247752/

Brian.
 

In the above thread he replaced the crystal with an equivalent of RLC...... should i do the same??
And its not just the Emitter Resistor heating up but the R6 on the transmitter is also heating.......
 
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A crystal can be modelled as an RLC network to do a simulation but I would strongly recommend you use a real crystal in your transmitter. A crystal will resonate at the frequency marked on it, if you use another tuned circuit you introduce a lot of new variables which will only confuse matters.

I can't see why R6 should get hot, please confirm we are taling about the same schematic. On the transmitter diagram I'm looking at, R6 is part of the timing network on the NE555 modulation generator. Even placed directly across the supply it would only dissipate 81mW.

Can you post a photograph of your transmitter please so we can get an idea of the physical influences there may be on the circuit.

Brian.
 

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