RF transmitter circuit.

rf pll 27mhz transmitter

Hi there, I'm currently working on a project on RF tx and rx circuits. Currently building the transmitter circuit with frequency of 27MHz. We obtained the waveform and its oscillating, but the amplitude is low somehow(6V) when we're using 9V battery as our source.

This transmitting circuit aint working because the signal is not able to transmit out, unless the antenna of the transmitter touches the antenna of the receiver(remote control car).

We are not using any crystals for out tx circuit.

Any idea whats the cause of this incapability of transmitting the signal out?
Does the type of antenna plays a big role on the circuit?

rf transmitter 100mhz

Antenna efficiency is very important. At your frequency the wavelength is 11 meters. The smallest efficient antenna will be around three meters. The efficiency will reduce as about the square of the length ratios below this.

I would suspect that part of your problem is in the receiver having low sensitivity. Do a test where you temporarily connect your transmitter to a full size half wave dipole and do the range test.

27 mhz rf transmitter circuit

What do you mean by antenna efficiency? Is it the type of antenna we're using that is causing the problem?

If u say its the receivers' problem, the receiver we used is a normal application like remote control car of 27MHz, the controller of this car itself is able to transmit the signal. Whereas the transmitter we built is not able to...

flatulent said:

Antenna efficiency is very important. At your frequency the wavelength is 11 meters. The smallest efficient antenna will be around three meters. The efficiency will reduce as about the square of the length ratios below this.

I would suspect that part of your problem is in the receiver having low sensitivity. Do a test where you temporarily connect your transmitter to a full size half wave dipole and do the range test.

Click to expand...

rf transmitter schematic

Depend what is your link-budget (distance between TX and RX).
In 11m band you need big antennas only if you are looking for long distance communications.
As you see, all the R/C radios on the market use less than ½ m antennas for this band.
You have to check first of all if your TX and your RX are working properly. Check the conducted output power at the antenna point, and also check the conducted sensitivity of the receiver at the antenna point. If these two parameters are ok, you can start to think to antenna.

amplifier rf 100w 27mhz

you may also design an amplifier, to boost the transmitter signal. Connecting directly the antenna to the oscillator is not good, due to the bad efficiency antenna (it is too short, electrically speaking) and it loads the oscillator too.

you may try using the 2N3866 or 2N2219 transistors, they are cheap and provide about 1W up to 100MHz.

Bear in mind that your very short antenna represents a short circuit. Check Balanis book, there you find the equations by means the antenna impedance relates to its length.

good luck,

brazilian

27 mhz crystal remort controller circuits

If you can, put a copy of your transmitter schematic here so we can see what you are trying to do.

An antenna is a coupling device to match impedance between your wired circuit and free space (the atmosphere). Additionally, the path from your circuit to the antenna has to be matched or energy is reflected from the antenna back into your transmitter output stage. The power is dissapated as heat in the output stage or wiring, rather than being coupled to free space.

There are ways of using a short antenna on longer wavelengths with matching networks to help get all the impedances correct (baluns, loading coils, etc). Your circuit may need something along those lines in order to get the power out the antenna instead of back into the transmitter wiring.

27 mhz rf remote controller schematics

Hello Wrigley.
27MHz is a quite low frequency.
You can use any diode available in the market (1n34 for example) to make a simple detector (sniffer).
With a 20 cm wire near the transmiting antenna you should be able to detect something. Then you can use a digital multimeter to see the signal level.

The detector will be as follows (according to the given connections):
Node1: antenna (20cm piece of wire) and capacitor C1 (100pF).
Node2: GND (negative of multimeter), anode of D1 (1n34), return of C2(10nF).
Node3: catode of D1, return of C1, R1 (100k).
Node4: return of R1, C2, positive of multimeter.

Make this circuit using reasonably short connections. Try to make it very compact so that you can use it for other measurements.

If you are not able to detect anything at all with the piece of wire (10 or 20 cm as said before) then maybe your transmitter is not working at all.

Use the same detector to inspect the circuit. Take away the piece of wire and use Node1 as the input. Go to different parts of the circuit and try to find (sniff) some rf.

In this way you may be able to find which part of the circuit is working and which is not.

After, when you will succeed to make the circuit work you can install the piece of wire (20cm) back into the detector (Node1). Then use the detector to optimize the transmitter to get the maximum rf power out of it.

Please, if possible post the schematics of your little transmitter in edaboard so that people can give their opinion.

Hope it helps.
S.

simple 27 mhz remote transmitter circuit

The 27 MHz band is very narow if your transmitter is not presisly on frequency, say 27.025 then your receiver will not receive the the signal get a counter and set your transmitter properly but you wil need a crystal or face lock loop oselator, crystal is the cheapest.

4 quature control

chrismicro is probably right...
Since your transmitter is not crystal-controlled, it might be
transmitting very well but not at the right frequency for the
receiver (you just need to be off by 10 kHz and it won't
work. When you touch both antennas, though, the signal
is so strong the receiver can't 'ignore' it.)
One way would to fix this would be a PLL (phase-lock loop)
or crystal control (way cheaper).
If you can attach schematics of your transmitter (as someone
has proposed) it'll be easier to help you debug it.

27 mhz very short range rf transmitter

Hi
Did you make matching network for your antenna. Antenna act as a small resitance in series witha small capacitor so it will highly load your design.
Try to use a short monopole antenna. you will need lenght ~= lamda/10
I did make the same project befor at both 27 and 100 MHz. antenna matching and antenaa design is an important issue.
Also power stagee of the transmitter and receiver senstivity

Thanks

rf sniffer

you know that the performance of antenna is not stable,there are many factor to affect it.how can you make match network

rf transmitter circuit

Matching network will match input impedance to output impedance, so that the power transferring will be maximized.
But I could not see the relation to antenna's length.
Could someone please explain?

rf 27mhz circuit working

antenna lengh will determine the value of the R and C equivalent circuit of the antenna that you need to make your power matching.

Thanks
Haytham

27mhz transmitter circuit

Still I don't understand this.
Can I make the 2m. wavelength antenna with very short length, like (not in quature or half) 10cm. ?
Can I apply a matching technique for this ?

hey..can you all send the schematics of remote control car to me..send to my email.
seesy123@yahoo.com
thanks

Dear sir:
I bought 2XAA 35MHz remote control cars, remote control distance, only 4.5 feet, very short, I would like to add 12 feet, is not adding a 2n2219 amplifier circuit, I'm not familiar,, Can you give me route map, please advice, thank you

Ben
6-3-11

---------- Post added at 02:27 ---------- Previous post was at 02:23 ----------

Connected to the question
e-mail lawhk368@gmail.com

Ben
6-3-11

guys please forget about the antenna for a moment it isnt the cause of the problems ... a 3 - 4 ft telescopic antenna will work ok as it does doe ALL radio control models around that freq.

UNTIL as hinted at by a couple of others, the OP'er crystal controls the transmitter, he is totally wasting his time playing with the antenna or anything else. It MUST absolutely MUST be crystal controlled to attain the required freq for the transmitter and of course that must match the receiver freq :-D

Wrigley ... it really is time you showed us all your schematic so we can guide you in the right direction :smile:
Dave