Rf Am Transmitter 433.92mhz, How to increase "the power

[zundapp]

Hi all!
I'm not sure that a new topic was necessary (if not, please correct this and let me know...).
I have "a problem" with RF key of my car...
It works only about 5-7 meters around the car (the battery is OK) and I want to buy/build a car alarm which will use as a remote control the RF car key.
So, I need to increase the distance of transmitter action (I made some tests and I'm sure that the transmitter is to poor and the receiver is OK).
The RF schematic/photo of the TX attached.

Thank you!

 

[biff44]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

You can try to make the 47K resistor lower. It will increase the current to the transistor, and should increase the transmit power.

You could go to a higher battery voltage, although you would need to keep the microcontroller from over-voltaging (check the data sheet).

It is possible that the antenna loop is not tuned right on frequency. You could cut the line and add a small series reactance (series inductor or serice capacitor) to slightly retune it.

I would NOT screw much with the two capacitors near the transistor, as the thing will probably stop oscillating.

 

[zundapp]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

About the resistor... I tried and nothing good happens.
About battery... I used the 6v only for RF block (the controller ramains at 3v and the data range also remain between 0-3v, the controller accept max 3.6v!) Results: nothing good.
I'll try the last suggestion. Few pF or few nH could be OK?
The (pcb) antenna is in series with the collector of the transistor!
Thank you!

 

[pranam77]

The 47K resistor is the modulation input and has nothing to do with the transmission power. I strongly guess the receiver antenna is not connected or the same is cut somewhere. 90% there is nothing serious wrong with the Transmitter. (Provided it is a commercial one not mishandled) Check the front end of the receiver part. Good luck.

 

[pauloynski]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

1- Check if the it is running at the right frequency
2- If the RF transistor has an emitter resistor, decrease its value (this will give you more power)
3- Check if the receiver is at the same frequency.

 

[biff44]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

The 47K resistor is the modulation input and has nothing to do with the transmission power. I strongly guess the receiver antenna is not connected or the same is cut somewhere. 90% there is nothing serious wrong with the Transmitter. (Provided it is a commercial one not mishandled) Check the front end of the receiver part. Good luck.

Really? Gee, I guess you learn something new every day. I assumed that when the input to the 47K ohm resistor was high (5 V or 3.3 V), that the base current in the oscillating transistor was determined by Ib = 5V/47K (modified slightly by the small emitter resistor). Thanks for the correction.

 

[FvM]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

Seriously, most of the suggested points can't be checked with some RF tools and the knowledge, how to handle it. You should also check, what's the specified range of remote control, it's not necessarily above 10 m.

Regarding discussed technical details, the 47k resistor supplies the modulation, but also the bias current for the self-oscillating output stage. So it of course matters for the output power. But I don't know, if it allows to increase it. A measurement would help.

 

[zundapp]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

The receiver works OK.
I made an "experiment": with a "domestic" receiver planted inside the car, which has a LED on output (which "was read" by a friend) I tried to transmit with the transmitter described above.
Both receivers, this and the car receiver works identically (~10m). After that I used another transmitter (**broken link removed**) with this one the distance is around 50 meters and both receivers works ok.
I know that the car transmitters (RF Key) are not so powerfull and almost all works under 20m.
The reason for my request is to use the RF key to drive the car alarm and I need to operate from a "decent" distance.
1- Check if the it is running at the right frequency
The frequency is ok - a lot of RF Key tested works the same.
2- If the RF transistor has an emitter resistor, decrease its value (this will give you more power)
Done... the distance is the same... With 20k instead 47k, the distance decrease!
3- Check if the receiver is at the same frequency.
It is...
So, the tx was designed to operate from low distance. I only need to modify it for a "decent" area covered. I have no ideea... All I tried was not hopeful...

 

[pauloynski]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

As I´ve posted above, check for emitter resistor (not base). This kind of transmitter usually has a 22Ω emitter resistor. Decreasing the value will increase the power.

 

[zundapp]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

I modified this resistor to 20Ω and... nothing. But I found something interesting:
With a simple external wire (34cm lenght) connected in A point (see attachments) the power increase absolutely amaizing!!!
If I reduce the lenght of this antenna to 17cm the power return at the initial bad value.
Any ideeas?
____________________
I can't upload the files (Upload Error: Could not upload Attachment to ./files-eboard/tx_6575.jpg.).
The point "A" described, is in the schematic attached in the first post, between PCB Antenna, R(collector)=56Ω and the capacitor to the emitter!

 

[zundapp]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

Obviously "the problem" is the antenna.
The antenna is like in photos (one is flipped!). I added a series inductor (33nH) with the antenna (at the collector) ... it looks the same, nothing hopeful

 

[vincent_yuang]

agree biff44's point, add a few nH to increase power and the emitter resistor 47K decrease 47. i had make the same product,the distance is up to 12m.of course you must decrease the hole on PCBA.try more...and you will be success!

 

[biff44]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

The antenna is an important issue. It was probably designed to:
1) fit in a specific (small) package size
2) meet the RF emissions requirements (when matched to the transmit power of the oscillator) by not being "too big". If an antenna is smaller than it should be, it loses radiation efficiency.

You can dramatically increase the range of a key fob type transmitter by
a) using a large antenna, like a quarter wave whip
b) adding a large ground plane for the antenna to work with
c) boosting the oscillating power, or adding a power amplifier

However, you will probably violate the emissions rules of the country you are operating in.

 

[zundapp]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

@vincent_yuang
The emitter resistor is 56Ω and the base resistor is 47k.
I modified both to 20Ω and 20k and I added a 33nH in series with the "original" antenna (nearby collector) and... nothing!
You're right, the distance is up to 12m. I need about 30-40m.
You said "...you must decrease the hole on PCBA...".
I don't understand what you mean (I suppose that PCBA mean PCB Antenna...):
= decrease the number of holes? I mean to replace the vias with simple holes (no contact between top and the bottom of PCB?
= to fill some/all existing antenna vias?
= to reduce the loop dimension?
I saw on the other type of remote (433MHz) (almost identical) from a car allarm, a smaller antenna (almost a half of dimension loop) only on top of PCB, which works around 40m. So... is possible!

@biff44
"However, you will probably violate the emissions rules of the country you are operating in."
I am far away of this. In Romania the max. power admitted for these devices is 10mW.
A lot of "domestic" devices sold here has up to this power and it works up to 100m.
Thank you!

 

[vincent_yuang]

try the attached...,it is my customer project,maybe useful to you.

 

[zundapp]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

Thank you!
About my questions in my previous post? Have you an answer?
Have you some "building details" about your antenna?
Schematic is here:, or in first post of this topic.
Thanks again!

 

[vincent_yuang]

the antenna's impedance must be 50Ω and 1/4 wavelengh dipole.it can supply the best power and the best channel stability.

 

[Abhishekabs]

1/4 wavelength Dipole!!??
i think you want to say monopole, right?

 

[vincent_yuang]

not monopole,it is dipole antenna.

 

[zundapp]

Re: Rf Am Transmitter 433.92mhz, How to increase "the p

I found a remote control (433.92MHz) which has a smaller pcb antenna and it works around 50m.
I don't understand how this transmitter has more power than my original key fob?
I'm confused...
Some photos/schematic attached: