Help with RF Xmtr/Recv Project for direction finding

[bcollinsks]

I would like to ask for some help from anyone who has experience in Transmitter / Receiver Design. My son is working on a RF Direction finding robot for school. The project is for the design of a robot that can find and follow someone carring a small transmitter on them. Additionally, the transmitter would have a friend-or-foe signal added to the design by my son. This doesn't have to be complicated. It could be just a carrier and a carrier with a simple modulated tone for FOF. I looked and found a RF DF project on NELNICK.com. It uses a 555 and a tank circuit for a basic osc. and a tank to a 741 op amp on the receiver side. The gimic is that the receiver antennas are horiz. and would need the transmitter in front of them for maximum received signal. This would vary the drive motors. It uses 3 receivers, 2 in front and one in the rear that causes a rotation until the front antennas see the signal again. That said, it looks great but I can't get the RF section to transmit over distance. The circuit called for a 1.8mH coil and a 20-400pf trimmer for the tuned circuit. I purchased an inductance meter and built the coils. From my signal generator I can see they can be tuned in the 200Khz range. It is not a problem to vary this. I built the transmitter and it is putting out a 184khz square wave into the tank and out to the hookup wire. The signal is about 8volts peak so it should send it out fine. The receiver tank on the signal gen. looks fine and tunes in the same range but I can't see the transmitter unless it is right by the receiver. I have twenty plus years in electronics and lots of equipment but I have never tried to make a transmitter / receiver before and it is kicking my but! I have to believe that I am missing something that is right there and I don't know about it. I looked up a few other RF osc. Projects and built some. One used a 2n2222 and made a beautiful sinewave at about 6Mhz with a tuned circuit. I had the same problem receiving the signal - only about 2 feet! I'm dying but much worse is that, I am letting my kid down. I can't let that happen. I have tried everything I can think of to try. I would really appreciate it if someone would look at the site **broken link removed** and Tell me what they think. I am building these on breadboard for testing and using hookup wire for the antennas. At this point I am just trying to receive the signal. I am not even trying the DF side yet. Maybe I need some sort of ground plane to actually get the transmitter to be able to push it's signal. I can see it on the board at the antenna with my scope but that doesn't mean that it's transmitting. Since I see the same problem with oter RF circuit projects I have to be doing the same thing I guess. Please help. I can send pics and scope signals if that would help. I have to get this front end working so that my son can do the rest of the project. I am open to any ideas especially ones that will get me out of this bind. (ie buy this prefab board and it will do what you need) However, I would really like to know what I am doing wrong. It bugs the heck out of me to not get it working.

Thanks in advance to any brave sole that comes forward and tries to help. Working in Electronics most of my life, my son looks to me like I am superman and can solve this problem. I really can't let him down but I am about to have to do just that. It will kill me if it comes to that.

Bob

P.S.
I have e-mailed Robert Nalley, that posted the Project on Nelnick. (he runs Nelnick I believe) and he sent me several pages from a text book on coil formulas. I will keep trying to get in touch with him but this has proven to be difficult. He really seems to want to help but I could't get the answers from him yet. I think I need building tips that apply to general RF transmitter/receiver circuits. (like yes the circuit will work but only if you do this.....) Even seeing some pictures of a working circuit could shed light on what I might be doing wrong.

 

[Russlk]

Transmitters and receivers for 433 mHz are available and cheap. That might be your best way out. Mouser (www.mouser.com) has an antenna.

 

[Nigel Goodwin]

My immediate thought on looking at the receiver was 'what an absolutely useless thing it is'. I'm not suprised it doesn't work over much distance.

Firstly, there's no rectifier, I can only imagine it's hoping the 741 will rectify it in some way?.

Secondly, a 741 is probably the worst opamp you can buy, it's got very low bandwidth and is useless at these sorts of frequencies.

Thirdly, the input resistance is only 1K, and it's directly across the tuned circuit - this will flatten it's response, giving hugely reduced signals from the tuned circuit.

Disconnect the 1K resistor, and connect a scope across the coil, and see what sort of range you can get on the scope.

 

[bcollinsks]

Hi Nigel,

Thanks so much for the reply. When I look at just the tuned receiver tank and antenna, I still don't see much. The transmitter has to be within a foot or so to get a good signal of 10-20mv. I really thought that the tuned circuit would produce more than that and was looking to see if the transmitter was at fault. If you build and oscillator at 200Khz using a LC tuned circuit then feed it to hookup wire through a 47pf cap all mounted on breadboard, would you expect it to transmit? I am using a 9V battery and my signal is just under 9v peak. I thought that it would be overkill at that voltage and really send it out at least 100 foot. Do I need to have some sort of ground plane or something? I am in a bind on this and will greatfully accept any help you could provide.

Bob

 

[Nigel Goodwin]

Hi Nigel,

Thanks so much for the reply. When I look at just the tuned receiver tank and antenna, I still don't see much. The transmitter has to be within a foot or so to get a good signal of 10-20mv.

Is that with the 1K resistor disconnected?. With that resistor disconnected, you should be able to see a nice sharp peak on the scope as you tune the tuned circuit.

 

[bcollinsks]

Yes, that is with the resister removed. Maybe my breadboard connections are poor. It did respond fine when I injected a signal into the tuned circuit from my signal generator. It tuned fine in the 150 - 250Khz range. If I can't see it at the receiver is there any way to prove the transmitter is putting the signal out? I tried to pick up a harmonic on my AM radio but no luck.

 

[Russlk]

The transmitter really should have a resonant antenna. At these low frequencies, the antenna will be a small part of a wavelength, so the larger the better. Try making a resonant coil 2 feet in diameter as the transmitter antenna. At 200kHz, try 7 turns on 24 inch dia form and 0.01uF capacitor.