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RF remote control circuit

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Post by zsolt Tue Jun 04, 2019 8:45 am

First topic message reminder :

Hi,
I'm also in search of  a RF control circuit . The circuit should activate a relay while the transmitter is transmitting . 
Recently i added a motor to my gate. Now it's operating from IR remote control and working fine , just that i need to wait until the gate opens . With a RF control i could open the gate from about 100m so when i get there the gate is open . At least in my theory .  
I do have some RFM12 B radio modules and i could operate them with an arduino , but anybody can do that . 
I would like an analog circuit instead , the transmitter would be installed in my car. So the supply is 12V and the antenna would be shared with the existing antenna from the radio , if possible. The receiver would be installed at the fence , supply and antenna is not a problem . Also security related problems are not an issue . (as i see now days it's enough to put an analog circuit and only a few know how it works Smile  )
I did try this circuit https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/27MHz_RX-TX_Remote.gif  , i built the circuits. I can't make it work more than 10 cm . 
I tried to tune the circuits to 2.5 Mhz , but i am not shore what i did is ok . I think the receiver is on neutrodyne principle . So i put the scope before the sfr choke and adjusted the coil until 2.5 MHz , then the same with the transmitter . 
Simply i am not able to make it work . Maybe someone knows better circuit ?

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Post by Ivan Sat Nov 23, 2019 5:01 am

Hi,
12 m is quite enough if the TX antenna is inside a car. If both antennas are in free space, far (more than 10 lambda i.e. 7 m) from conductive bodies and there are no obstacles between them, the range may be up to 100 m or even more. Fog naturally increases the atmospheric absorption, decreasing the range.
You will not be happy with possible solutions: use a more sensitive receiver (limited by external, not own noise), transmitter power up to the legal limit for license free ISM, directive antennas with as much gain as you can afford and maybe math processing of the received signal to receive the information below the noise level.

VBR from Ivan

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Post by zsolt Fri Nov 22, 2019 4:05 pm

hi 
the rf meter seems to be useful so far. 
I modified code so that the Tx is always on . I have 60 mv @ 50 cm away and about 20 mV @ 1 m away from tx antenna. Beyond that the meter is not able to show something . 
Now what i do to increase range?
I observed that when is fogg outside the system is not working at all. When is clear i can open the gate ~12 m away

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Post by Ivan Fri Nov 22, 2019 7:13 am

Hi Zsolt,
I would try a stub of coax (or maybe a whole comb structure of stubs) as a filter. The indicator itself is really wideband, ranging from LW to microwaves.

VBR from Ivan

P.S.: Do not worry that the device looks ugly. Get it to work first, tidy it up later (or maybe never tongue )!

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Post by zsolt Thu Nov 21, 2019 8:56 pm

I did a little modification to the display after studying the LM3914 datasheet and a lot of absorption rf meters from the net. 
A lot of the schematics use 100 ....250 uA needle instruments . In analogy with that i modified the LM3914 circuit that i found to measure from 0 to 114 mV.
For this i modified  the pcb ... now the whole circuit looks so ugly that it hurts eyes , but it seems to work  lol!
The schematicus i ended up is pictured like this:
RF remote control circuit - Page 2 Fsm1010
I say it seems to work because meantime during a phone call i saw (almost scared ) that all LED's light up and flicker.
I found that i can even trace wires in the wall with this thing. Tomorrow i will make some time to see  what is happening with the TX from the car. So far so good.

Since  this circuit seems to react to everything i would make it to react to 433 MHz only . Some schematics where using some kind of resonator instead of the diode which goes to ground. I do have 433Mhz cristal from an other transmitter
The eye hurting product looks like this
RF remote control circuit - Page 2 Img_2015

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Post by zsolt Wed Nov 20, 2019 10:15 pm

looking at the LM3914 block diagram i see a 20k rezistor on the signal input. From the whole document i can't find what is the input impedance. With this i will deal later. I plan to use 2 glass case diods. If not working i will try schotky. Meanwhile still searching for parts

I did a little test with 2 cooper rods as antenna.
Of course it des not work.
If i take away the potentiometer completely the leds light up when i get close to a monitor or even the soldering iron
RF remote control circuit - Page 2 Img_2014
The LM3914 is more like a voltmeter measuring the voltage drop on the potmeter . On the original rf meter the indicator is directly an ampermeter. Maybe that's the difference.
If i put a DVM on the potentiometer i get 40 mV when i approach the thing to the monitor. Maybe an amplifier is needed . Or the needle instrument needs to go..
Anyway now i know that the transmitter is rally week.

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Post by Ivan Wed Nov 20, 2019 9:18 pm

zsolt wrote:Hi,
i started building the rf meter ....
Probably no microammeter has 500 Kohm internal resistance. I expect somewhere about 1 Kohm. The trimmer in your device should have similar value.

VBR from Ivan

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Post by Ivan Wed Nov 20, 2019 9:12 pm

They are pretty old. The upper right is almost surely a germanium one. Germanium diodes have forward voltage drop about 0,3 V only. Check them with your meter.

VBR from Ivan

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Post by zsolt Wed Nov 20, 2019 8:06 pm

i think these are the oldest diode s i will ever find
RF remote control circuit - Page 2 Img_2012

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Post by Ivan Wed Nov 20, 2019 7:11 pm

Yes, that looks nice.

VBR from Ivan

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Post by zsolt Wed Nov 20, 2019 11:39 am

Hi,
i started building the rf meter .... i think this will be a separate thread for me. I can't break a 10uA indicator . I will use a scale with 10 led's to display signal strength . I found a ready made display like that with a LM 3914 on it. 
I found some diod's in glass case . No inscription on them . One has a yellow marking near to a terminal . They might be rf since i have them from a radio receiver .The schematic i made up looks like this.
RF remote control circuit - Page 2 Fsm10
I also found a broken cable tester so i will be using that case to build  this device in it. 
I thought to make the dipole from 2 telescopic antennas which i can take of from broken radio receivers .In this way i could adjust the dipole to other frequencies also

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Post by zsolt Mon Nov 18, 2019 9:44 pm

with or without, it seems to make no difference.
ps the plastic case has a little ferrite ring inside.

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Post by Ivan Mon Nov 18, 2019 7:09 pm

zsolt wrote:meanwhile should i put this ferrite on the rg174 cable?
Hi,
it is impossible to guess the magnetic properties and losses at 433 MHz from a photo. Try it and you will se.

VBR Ivan

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Post by zsolt Mon Nov 18, 2019 6:00 pm

meanwhile should i put this ferrite on the rg174 cable? RF remote control circuit - Page 2 20191112

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Post by Ivan Sun Nov 17, 2019 8:43 pm

zsolt wrote:"i did not use any shielding or filtering."    Wrong!

"i might have impedance mismatch!" At my first guess it is not critical.

VBR Ivan

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Post by Ivan Sun Nov 17, 2019 8:31 pm

zsolt wrote:the germanium diodes are quite obsolete. I found some germanium tranzistors. MP40. Do those do the job instead of the diods if i connect base to collector
Yes, they are, but there are loads of them in hamshacks. A pinpoint germanium diode is needed. The transistor may have big parasitic capacitance, I do not know the MP40 type. If you cannot find a germanium diode, use a silicon Schottky one, they should be available with no problem.

VBR Ivan

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Post by zsolt Sun Nov 17, 2019 8:25 pm

i did not use any shielding or filtering. Everything gets power through a 5 v regulator. Looking at the tx schematic i might have impedance mismatch https://www.google.com/search?q=fs1000a+schematic&oq=fs1&aqs=chrome.1.69i59l2j69i57j69i60j69i61.2910j0j8&client=tablet-android-huawei&sourceid=chrome-mobile&ie=UTF-8#imgrc=PL7aZOlHypbECM:

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Post by zsolt Sun Nov 17, 2019 8:21 pm

the germanium diodes are quite obsolete. I found some germanium tranzistors. MP40. Do those do the job instead of the diods if i connect base to collector

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Post by Ivan Sun Nov 17, 2019 8:21 pm

zsolt wrote:i forgot to say how my tx works. I have a spst button. When i press it  the  circuit gets supplied. Then the microcontroller waits a little, sends some ascii characters and after that it turns on a beeper. When i hear the beeper i let go the button.
I don't know is this affects range or not
No. To evaluate the radio channel, disconnect the MCU and test the bare TX - RX pair, with proper antennas of course. After it works O.K., add the MCU.
Btw the MCU on the TX side may be negatively affected by the EM field and/or RF on power supply rail. Do not forget proper shielding and filtering.

VBR Ivan

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Post by Ivan Sun Nov 17, 2019 8:14 pm

You compare the value shown on the meter at a certain distance from the antenna under test (AUT), when the AUT is in free space / on a metal roof / inside a car, turned at various angles etc. Or you can compare various types of AUT and what you like. The measurement is comparative only, not absolute.

VBR from Ivan

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Post by zsolt Sun Nov 17, 2019 8:10 pm

i forgot to say how my tx works. I have a spst button. When i press it the circuit gets supplied. Then the microcontroller waits a little, sends some ascii characters and after that it turns on a beeper. When i hear the beeper i let go the button.
I don't know is this affects range or not

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Post by zsolt Sun Nov 17, 2019 7:56 pm

how do i use that instrument? What to look for?

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Post by Ivan Sun Nov 17, 2019 7:50 pm

zsolt wrote:why does the car not steal the rf signal from the key when i open/close the door?
It probably steals some decibels. The range needed for a car lock is quite small. And I have no clue where the lock receiver is, its placement may be optimized. My car has no radio lock...

I do have a microampermeter with needle. I wil search for diode but how does that circuit look like?
I found an improved version with voltage doubler here https://ok1fcb.webnode.cz/konstrukce/jednoduchy-vf-indikator/

VBR from Ivan

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Post by zsolt Sun Nov 17, 2019 7:37 pm

why does the car not steal the rf signal from the key when i open/close the door?
I do have a microampermeter with needle. I wil search for diode but how does that circuit look like? The diode in parallel with the uA and al that in parallel with the 17 cm +17cm cooper rod?
Also i think i found the drawing of the tx that i have https://www.google.com/search?q=fs1000a+schematic&oq=fs1&aqs=chrome.1.69i59l2j69i57j69i60j69i61.2910j0j8&client=tablet-android-huawei&sourceid=chrome-mobile&ie=UTF-8#imgrc=PL7aZOlHypbECM:

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Post by Ivan Sun Nov 17, 2019 7:28 pm

Hi Zsolt,
"the car steals from the rf power" -exactly so! Absorption and shielding take place inside a (metallic) car. Placing an antenna outside the car body is much better.
"some way to quantify how well my antenna is emitting and how well the other is receiving" - try a simple RF indicator. It should be a wire dipole tuned into the band attached to poles of a microampermeter (an old taperecoder indicator will do) and bridged by a RF detector diode. A Schottky or germanium one will give you more sensitivity. Use the reciprocity principle to measure the receiving antenna, use it as a transmitting one temporarily.
"an amplifier is at 433 mhz" - be careful not to exceed the power and spurious limit for ISM devices!

VBR from Ivan

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Post by zsolt Sun Nov 17, 2019 6:52 pm

i found out that the coax between the tx module and antenna is 50 ohm , it is called RG174, has 30.8pF/foot, 17 db /100 feet attenuation. I use only about 20 cm so i don't think it's a problem.
The system works sometimes, sometimes not. At short distance works always, then i when i go further away not at all and if i go more it works at a distance that i would not expect. If i move about 50 cm from that spot it doesn't work again.
When i was holding the module in my hand and tested the whole thing i had all most 100m, after i mounted the tx module in the car everything got bad. Somehow the car steals from the rf power. The car has also a radio for the lock system. I don't know at which frequency but when i put new battery in the key it works more than 50m.
i think i need some way to quantify how well my antenna is emitting and how well the other is receiving.
Some on the internet write that the tx module could operate from 12v to increase range. It did not work at all, i suspect that there is an issue regarding voltage levels with the microcontroller so i got back to 5 v.
i wonder how doable an amplifier is at 433 mhz, what tranzistor could do that, or should i buy one.


Last edited by zsolt on Sun Nov 17, 2019 7:30 pm; edited 1 time in total

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Post by Ivan Thu Nov 14, 2019 9:19 pm

Well, improving an antenna is usually better than pumping big power into a bad one.

VBR Ivan OK1SIP

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Post by zsolt Thu Nov 14, 2019 9:04 pm

Ivan wrote:Hi,
first, what do you name as "1/4 dipole" ? Do you mean a dipole with 2x 1/4 wavelength (lambda) radoiators? Its overall length is then 1/2 lambda and THAT is its usual name.
Its impedance is real, cca 72 ohm balanced at resonance. I expect the TX output impedance is somewhere in that region, but unbalanced, suitable for a coax feeder. If this is your case, quite good and simple matching can be achieved by a ferrite bead(s) on the coax, just at the dipole. The bead brings inductance into the outer surface of the shield, stopping unwanted currents in it.

VBR Ivan OK1SIP
i don't know about this things. I found on the internet that i have to use a 1/4 dipole antenna. Then i googled just like that how to make one. I found a page that says that 2 pieces of 17 cm tube make a dipole like that. 
Then i built one and put it on the roof. I used tv cable to connect it to the receiver.
Today i mounted the same antenna in the car also
RF remote control circuit - Page 2 20191111
the results are slightly better.
I believe that there is no more to get out of this modules. 
It did not work while the car is under the roof so i connected both, this antenna and cars radio antenna. 
Now the remote works inside of the garage and about 50 m outside.
There may just not be enough rf power, in the end the tx works only from 5v. Maybe a tranzistor  before the antenna would increase tx power

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Post by Ivan Thu Nov 14, 2019 8:07 pm

Hi,
first, what do you name as "1/4 dipole" ? Do you mean a dipole with 2x 1/4 wavelength (lambda) radoiators? Its overall length is then 1/2 lambda and THAT is its usual name.
Its impedance is real, cca 72 ohm balanced at resonance. I expect the TX output impedance is somewhere in that region, but unbalanced, suitable for a coax feeder. If this is your case, quite good and simple matching can be achieved by a ferrite bead(s) on the coax, just at the dipole. The bead brings inductance into the outer surface of the shield, stopping unwanted currents in it.

VBR Ivan OK1SIP

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Post by zsolt Thu Nov 14, 2019 7:11 am

Yes , i will try the 1/4 dipole also .
And an other interesting fact i observed that the remote works even if the car is in the garage . Shouldn't the metallic roof stop the radio waves ? 
I have no matching circuit . Do i need that for the 1/4 dipole?

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Post by Ivan Wed Nov 13, 2019 10:03 pm

zsolt wrote:made some progress. I had to try the antenna of the car. So i hooked everything up and tested the system. I don't even have 50 m range now.
i wonder why. The antenna is 75 cm . For 433mhz i have 69.24 cm wave length.All most the same. How could the range go down so quick. Because one antenna is a 1/4 dipole and the other is vertical whip? I will try to rotate the rx antenna with 90 degree to see what happens.
Hi,
the antenna is more than one wavelenth long. You must expect high and not purely resistive impedance at the feedpoint, which is a bit hard to match to the TX output. What matching circuit do you employ?
And there is a cross-polarization issue (I have already warned you about that!). A horizontal dipole is horizontally polarized, while a vertical whip is vertically polarized. Turning the dipole to the vertical position solves this issue.
On the other hand, different profiles of antennae (wire vs. tubing) makes no problem.

VBR from Ivan

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Post by zsolt Wed Nov 13, 2019 6:29 pm

made some progress. I had to try the antenna of the car. So i hooked everything up and tested the system. I don't even have 50 m range now.
i wonder why. The antenna is 75 cm . For 433mhz i have 69.24 cm wave length.All most the same. How could the range go down so quick. Because one antenna is a 1/4 dipole and the other is vertical whip? I will try to rotate the rx antenna with 90 degree to see what happens.

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Post by zsolt Tue Nov 12, 2019 8:50 pm

Ivan wrote:hi,
I opt for the copper tape, for the sake of safety. IMHO the rods would exceed the frame of the mirror and they could cause a serious injury in the case of an accident. Soldering the tape is nearly the same as soldering a PCB. To avoid overheating the glass (and possible rupture), you can solder first, attach to the glass later.

VBR from Ivan
cooper tape peals off because the sun. I had issues with some led strips. 
if i put the antenna right on the back of the mirror it exceeds only about 5 cm.. I found a better place anyway. close to the glass. I already have a flipping support for a tablet on the ceiling( like knight rider, but only for waze or google maps). I attach the antenna in front of that. So it will be way behind the rear view mirror almost touching the window It does not take anything from visibility. That part of the glass is already masked with some kind of black paint from the factory. Actually the whole top region of the window is darkened. Till now i did not even realised that. I think is something design related
Anyway i hope i will not have to rebuild the receiver antenna with the 3mm rods instead of the thick cooper pipes, that's my biggest concern with this for now.

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Post by Ivan Tue Nov 12, 2019 7:48 pm

hi,
I opt for the copper tape, for the sake of safety. IMHO the rods would exceed the frame of the mirror and they could cause a serious injury in the case of an accident. Soldering the tape is nearly the same as soldering a PCB. To avoid overheating the glass (and possible rupture), you can solder first, attach to the glass later.

VBR from Ivan

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Post by zsolt Tue Nov 12, 2019 6:50 pm

hi, 
i opt for the cooper rod antenna. Soldering wires to that paint on the glass i think will not hold much (i don't do off road on the mountain like mr Harry..but still)
Also i found some cooper like metal with 3mm diameter that i think is good. 
The 2 antennas wil not be the same so i probably redo the antenna from the receiver (the one made of cooper pipes) if results turn out bad.

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Post by zsolt Sun Nov 10, 2019 8:20 pm

Ivan wrote:Hi,
"both antennas are not on the same axis" - it does not mind, the radiation pattern of a dipole is wide enough.
"the TX antenna is cca 10 cm under the roof" - maybe this is a problem, maybe not. The car roof is in the antenna's near field.
You may also try to glue two thin wires forming a dipole directly on the front glass or draw the dipole on the glass with silver conductive glue. Experiment and have fun!

VBR from Ivan
well, i finished the receiver for now. 
RF remote control circuit - Page 2 20191110
the antenna is made of 2 cooper pipes 17cm long each of them. i did not find information about the distance needed between the 2 pipes so i made a 5mm spacer. Actually the spacer is an axis having different diametral sections so that the two pipes go on it but not closer then 5mm. i used a plastic rod to make the spacer. In the end i put the whole thing in a plastic tube which i fixed on the roof and painted black.
I don't know about conductive paint. As i heard for an antenna the surface is important due to skin effect so drawing the antenna on the glass seems to be a good alternative. 
When i first tested the modules with no antenna they worked just a few meters. When i soldered 2 pieces of wire range increased a lot. So i believe that matching the antennas does worth the effort. I already  have a range of about 100m and the transmitter is still a piece of wire.

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Post by Ivan Sun Nov 10, 2019 6:49 am

Hi,
"both antennas are not on the same axis" - it does not mind, the radiation pattern of a dipole is wide enough.
"the TX antenna is cca 10 cm under the roof" - maybe this is a problem, maybe not. The car roof is in the antenna's near field.
You may also try to glue two thin wires forming a dipole directly on the front glass or draw the dipole on the glass with silver conductive glue. Experiment and have fun!

VBR from Ivan

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Post by zsolt Sat Nov 09, 2019 8:23 pm

Ivan wrote:Hi Zsolt,
the picture looks like you want to mount the TX dipole vertically,while the RX one horizontally. That would be bad, cross-polarization decreases the range significantly. But maybe I misunderstood the picture.
Nearby conductive objects cause loss of power, impedance changes and deformation of the radiation pattern of any antenna. Placing an antenna inside a car may make it useless due to the shielding effect. Perhaps the trial and error method is the best.

VBR Ivan
hi, 
my drawing is not very accurate... but both antennas are horizontal. The rx antenna i built is about 10 cm above the metallic roof.I will make a photo in the morning. 
I experimented with the tx which now has just a piece of cable instead of an antenna. It seams to work.
The antenna inside the car is not ready yet. The inside rear view mirror is horizontal. I plan to build the tx antenna on the back side of that. So the 2 antennas see each other directly through the front window (like i see the road or the rx antenna on top of the roof).  The antennas are not on the same axis because for that i would need to put the rx antenna above the middle of the road. That is not possible, but they are both horizontal.

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Post by Ivan Sat Nov 09, 2019 2:26 pm

Hi Zsolt,
the picture looks like you want to mount the TX dipole vertically,while the RX one horizontally. That would be bad, cross-polarization decreases the range significantly. But maybe I misunderstood the picture.
Nearby conductive objects cause loss of power, impedance changes and deformation of the radiation pattern of any antenna. Placing an antenna inside a car may make it useless due to the shielding effect. Perhaps the trial and error method is the best.

VBR Ivan

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Post by zsolt Sat Nov 09, 2019 11:42 am

i made a dipole antenna found on the internet. My construction is like a stick .
RF remote control circuit - Page 2 Rx10
The orientation would be like this
RF remote control circuit - Page 2 Orient10
I din't know how the metallic roof affects the antenna. The same antenna would be in the car . I found a place on the top side of the window on the back of the inside rear view mirror . Also i don't know how that affects the antenna.
Yagi i found hard to use because i don't know where to hide it on the car .

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Post by Ivan Sat Nov 09, 2019 8:00 am

Hi,
433 MHz has wavelength 69 cm, so many types of antennas are feasible. I would start with a 1/4 lambda vertical whip for all-direction coverage. If you want directivity (e.g. forward direction at TX and the direction of the way to your garage at RX), use a short yagi. Keep the polarization on both sides identical.

VBR from Ivan OK1SIP

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Post by zsolt Fri Nov 08, 2019 10:50 am

hi,
27 Mhz is much better , 433Mhz is even better.
Doorbell circuits don't last long . For some reason the chip burns out on tx side. 
I bought a cheap tx rx pair for arduino , they run on 433Mhz . 
My problem now is with the antennas. 
What types of antennas could i make for longest distance?
For the beginning i plan to mount the receiver on the top of my garage because it's the highest place . I am afraid to put it higher because of lightning .

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Post by zsolt Mon Jun 10, 2019 9:07 pm

I will try to adjust my existing circuits to 27Mhz to see what happens

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Post by Ivan Sun Jun 09, 2019 6:52 pm

Generally, two parameters of any transmitting antenna are crucial: matching to the transmitter last stage and so called radiation resistance. If the antenna is badly matched, the RF power reflects at its input and cannot be radiated. Short antennas (short relative to the wavelength in use) are usually hard to match. They are very capacitive, which requires big inductance in the matching network to compensate. Making a big inductance with big Q is often a hard problem. Ferrite cores are no good for power transfer on HF and higher.
The ratio of the radiation and loss resistances corresponds to the ratio of RF power radiated and converted to heat. Short whips (again relative to the wavelength) and thin wire loop antennas have a low radiation resistance, so the thermal losses dominate. These antennas may do for receiving at good conditions, but not for transmitting.

VBR Ivan

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Post by Ivan Sun Jun 09, 2019 3:11 pm

zsolt wrote:So now if i get it right and have that little transmitter connected to the antenna of the car radio, transmitting that audible tone , it should work if the receiver at the gate is  sensitive enough. Just that for bigger distance that car radio antenna is not enough. 
What if that little transmitter would have more rf power with same antenna? There are CB radios for cars which have also small antenna ( don't know about frequency) 
i did not even tought about cooper tubing loop antenna, that reminds me about 'fox hunting' receivers. I did use a receiver like that
Yes. Improving the TX antenna for 3 dB is equivalent to doubling the TX output power. Improving it for 10 dB is equivalent to increasing the power 10 times. Therefore several kilowatts connected to a short whip makes the same range as a single transistor working into a fair matched antenna...
CB in Europe works in the 27 MHz band, generally the same as old radio remote controls. Other bands are possible on different continents.
When MW broadcasting was common, car radios used cca 2 m long whips as antennas. Such antenna could be well matched on 27 MHz. Antennas designed for FM only are probably too short.
A good ste about transmitting magnetic loops is e.g. at https://www.btv.cz/mla-t

VBR Ivan

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Post by zsolt Sat Jun 08, 2019 9:48 pm

Ivan wrote:You are right,
although the arragement with two full size antennas has the best efficiecy, when you do not need a two-way communication, you can use a small receiving antenna and a sensitive receiver with good results. But, unfortunately, this does not work on the transmitting side. A magnetic transmitting loop made from copper tubing is possible, but still clumsy.
A wifi based solution is well possible, if your AP covers the area around your garage. Thanks to much higher frequency, the wifi antennas can have simply the form of meander on the PCB. To transfer the commands via internet is IMHO an overkill.

VBR Ivan
hi,
it's not such an overkill at all, for example one needs an arduino uno, a wifi shield and all sw can be found in various instructables on the net. The sw for an android operated smartfone can be found on googleplay. ( and so much about security) There are also aplications for those 'smart lightbulbs' and 'smart switches' that can be used.. 
So now if i get it right and have that little transmitter connected to the antenna of the car radio, transmitting that audible tone , it should work if the receiver at the gate is  sensitive enough. Just that for bigger distance that car radio antenna is not enough. 
What if that little transmitter would have more rf power with same antenna? There are CB radios for cars which have also small antenna ( don't know about frequency) 
As i see the receiver, the rellay will clamp only when that audible tone is detected. 
Yes the dorbell is at 433.9 Mhz.

i did not even tought about cooper tubing loop antenna, that reminds me about 'fox hunting' receivers. I did use a receiver like that

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Post by Ivan Sat Jun 08, 2019 5:08 pm

You are right,
although the arragement with two full size antennas has the best efficiecy, when you do not need a two-way communication, you can use a small receiving antenna and a sensitive receiver with good results. But, unfortunately, this does not work on the transmitting side. A magnetic transmitting loop made from copper tubing is possible, but still clumsy.
A wifi based solution is well possible, if your AP covers the area around your garage. Thanks to much higher frequency, the wifi antennas can have simply the form of meander on the PCB. To transfer the commands via internet is IMHO an overkill.

VBR Ivan

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Post by zsolt Sat Jun 08, 2019 4:18 pm

hi,
wait a moment , now i see 
"1/ To use a tuned full-length vertical at 2,5 MHz, it should be 30 m tall. Three or more radials should be about 30 m long each, too. The same antenna should be both at the tramsmitter and the receiver. Not much practical, am I right? With some sacrifice in efficiency, several metres long whip loaded by the proper coil may do the job. RF remote control circuit - Page 2 Icon_rolleyes 
"
why should i have 30m of antenna ? Remember my radio with the loop antenna? that is 20 turns on two 50 cm sticks in X form .That receives well a local station on 1330 kHz (1.33MHz) 
that is the max i thought about , and maybe 2 m of wire stretched on the fence .
And why must be identical antenna on the rx and tx if both operate on same frequency ? Isn't a radio signal a radio signal for each? 
For example my loop antenna again , i bet that the local radio station doesn't transmit with an antenna like my loop antenna but i am still getting the signal.
And anyway i just need between 50-100m range . 
 Don't get me wrong, i could attach a wifi module to the existing circuit and operate the gate with my smartphone over internet in 30 minutes (i have mobile data on that) ...... but a gate operated with a crappy radio ? how much people has that  bounce
( maybe james bond  lol! )
I don't know much about what i call "analog radio" because on the wifi and rx/tx modules i buy and use i don't get to that part . The only thing i ever looked up in a datasheet is how the information is serialized , timing and which address a few special registers have .

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Post by Ivan Thu Jun 06, 2019 8:58 am

Hi,
I recommend you to use the doorbell including the coder. The receiver selectivity is fairly low. Most doorbells work in the 433 MHz ISM band, so any ham station working at 70 cm, any PMR or remote control module can provide the carrier. The encoder/decoder pair can prevent most cases of false triggering.

VBR from Ivan

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Post by zsolt Thu Jun 06, 2019 7:00 am

hi 
could this circuit work on 27 Mhz if i change only the LC ?
I did open up a doorbell and i found 2 digital cips , one encoder at the button and one decoder at the bell, both with TTL input/outputs as i saw later in the datasheets. 
The radio circuit interfacing the encoder is made with just one tranzistor in a topology that looks like the simple FM bugs that can be found on the internet . 
At the receiver side the decoder cip is interfaced with 3 tranzistors. The first one has a topology just like the transmitter on the encoder side (where the button is placed) and the second 2 tranzistors look like an amplifier or a trigger , probably it adjusts the signal level to TTL for the decoder.
The doorbell is a good idea .

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Post by Ivan Wed Jun 05, 2019 4:27 pm

Hi Zsolt,
did you consider adapting a wireless doorbell for your purpose?

VBR Ivan

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Post by Ivan Wed Jun 05, 2019 12:35 pm

Your transmitter has a small ferrite antenna only? Its efficiency for transmitting is very low. No wonder you bridged a small range only.

VBR Ivan once more

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Post by Ivan Wed Jun 05, 2019 12:31 pm

zsolt wrote:1/ i did not try 27 mhz only 2.5mhz
2/ security is absolutely not an issue here where i live. I can even leave  without closing the gate, nothing will hapen.
3/ I wonder what frequency those toy boats have.
Hi,
1/ To use a tuned full-length vertical at 2,5 MHz, it should be 30 m tall. Three or more radials should be about 30 m long each, too. The same antenna should be both at the tramsmitter and the receiver. Not much practical, am I right? With some sacrifice in efficiency, several metres long whip loaded by the proper coil may do the job. Rolling Eyes 

2/ Be really happy! bounce 

3/ Old RC cars and boats use 27 MHz. The channel is selected by changing the crystals on both sides. Airplanes use an exclusive 40 MHz band. Modern RC models and toys usually work in the 2400 MHz band with automatic free channel selection (a Wi-fi similar technology).

VBR from Ivan

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Post by zsolt Tue Jun 04, 2019 10:33 pm

i did not try 27 mhz only 2.5mhz because i have now just a bad scope. As i mentioned security is absolutely not an issue here where i live. I can even leave  without closing the gate, nothing will hapen. Also i never bothered with ciphering and deciphering. Usually i send over some text with a cyclic redundancy check attached to it. (and don't bother with that neither at the receiver side.. if the first and last characters match it s ok.. ) The modules i have work well about 20 m. I want at least 100m. And the soil is not flat.  Down the lake i saw a fisher with RC boat, it could go from one side to the other.  I wonder what frequency those toy boats have.
Also i'm not in hurry to make the circuit work. As i built the circuit from the link below i can build an other one if it works better.
There is no documentation provided for the circuit  but i am shore it should do much better then 1 m range. I checked components and everthing is fine
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Post by Ivan Tue Jun 04, 2019 9:59 pm

Hi,
everybody can know the SPI protocol of the modules, but nobody knows the clever ciphering/deciphering algorithm you implement into the attached MCU. The TRXes provide a serial communication channel up to 115,2 kb/s. The safety can be really high! Using these or similar modules is faster than making a transceiver from scratch.

27 MHz has the wavelength more than 10 m. How is your antenna and counterweight long?

VBR Ivan

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Post by zsolt Tue Jun 04, 2019 2:48 pm

Ivan wrote:Hi Zsolt,
what do you mean saying "analog"? AFAIK any radio transmission by itself, i.e. generating and receiving a radio wave, is analog. Modulating and processing of the signal can be digital, anyway.
I recommend you to make use of ready-made modules in the ISM band 433 MHz or 2,4 GHz. There are loads of them on the web at moderate prices. Use either a specialized chip or a MCU (Arduino etc.) to implement a floating code on both ends for safety.

VBR from Ivan

Hi,
i have RFM12B modules , those communicate with a microcontroler  by serial peripheral interface (SPI) that's what everybody has. I also have modules which comunicate over I2C . I will not use them. A circuit like that from the link will do the job . I don't know why i can't make it work.
The relay clamps if the distance less then 1 m

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Post by Ivan Tue Jun 04, 2019 9:12 am

Hi Zsolt,
what do you mean saying "analog"? AFAIK any radio transmission by itself, i.e. generating and receiving a radio wave, is analog. Modulating and processing of the signal can be digital, anyway.
I recommend you to make use of ready-made modules in the ISM band 433 MHz or 2,4 GHz. There are loads of them on the web at moderate prices. Use either a specialized chip or a MCU (Arduino etc.) to implement a floating code on both ends for safety.

VBR from Ivan

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Post by zsolt Tue Jun 04, 2019 8:45 am

Hi,
I'm also in search of  a RF control circuit . The circuit should activate a relay while the transmitter is transmitting . 
Recently i added a motor to my gate. Now it's operating from IR remote control and working fine , just that i need to wait until the gate opens . With a RF control i could open the gate from about 100m so when i get there the gate is open . At least in my theory .  
I do have some RFM12 B radio modules and i could operate them with an arduino , but anybody can do that . 
I would like an analog circuit instead , the transmitter would be installed in my car. So the supply is 12V and the antenna would be shared with the existing antenna from the radio , if possible. The receiver would be installed at the fence , supply and antenna is not a problem . Also security related problems are not an issue . (as i see now days it's enough to put an analog circuit and only a few know how it works Smile  )
I did try this circuit https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/27MHz_RX-TX_Remote.gif  , i built the circuits. I can't make it work more than 10 cm . 
I tried to tune the circuits to 2.5 Mhz , but i am not shore what i did is ok . I think the receiver is on neutrodyne principle . So i put the scope before the sfr choke and adjusted the coil until 2.5 MHz , then the same with the transmitter . 
Simply i am not able to make it work . Maybe someone knows better circuit ?

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