Electronics 49MHz transmitter/reciever

I'm investigating making a, well, walkie talkie transmitter of a sort. In a perfect world, I would like to amplify the signal coming too and from a walkie talkie. How ever, transmitting is really the most useful purpose. I plan on making something like this/ however the problem with this circuit, is that it only transmits on the FM band and wont go down too frequencys as low as 49MHz.

The transmitter doesnt have to be powerful, it only has to transmit around 500 metres. A big aerial isnt a problem. I dont mind about weight as it will be stationary most of the time (Weight as in batteries...) The reason I want to use a regular cheap walkie talkie, is that it seems my felow airsofters fancy pants walkie talkies that only have "channels" to select from dont seem able to tune into my ol' walkie talkie.

Help always appreciated

 

I'm no radio guru, so this is theoretical, but if you saying that circuit can't be used is based only on the components in the diagram, then it might just be possible.
If so, he says on that circuit page that the Radio frequency is set by L1, and C2 in parallel with C3 (add them together).
You can buy small chokes (1uH and up) from electronics suppliers. If you go with 1uH, then you can pick your capacitor(s) to give about 50MHz - this works out at somewhere between 10 and 11pF
For an LC circuit:
F = 1/(2*pi*sqrt(LC)) = 1/(2*pi*sqrt(1uH * 10pF)) = 1/(2*pi*sqrt(1e-6*10e-12)) = 50.329mHz
F = 1/(2*pi*sqrt(LC)) = 1/(2*pi*sqrt(1uH * 11pF)) =1/(2*pi*sqrt(1e-6*11e-12)) = 47.987mHz

So in theory you can use that circuit if you replace these components - L1 with a 1uH inductor, and C2/C3 with 10pF ceramic cap and a 1pF variable capacitor, for example.
If you want to pipe in audio from another walkie talkie you could use an audio op-amp with a trimmer pot for gain set so that the maximum output voltage is 100mV (which is what the site says is the biggest realistic input).

If your walkie talkie is AM only, then you can probably get away with a simpler circuit. Again you'll have some combination of R/L/C components to tune it. Something perhaps like this - the resonant components are clearly visible by the antenna. The components left of Q2 up to P1 and Q2 itself are just a [fixed] audio gain stage.
this might also come in handy.

 

This sounds like a good plan. I pretty much understand your maths, which is good going for me I recon the 50.329mhz will be close enough to the walkie talkie's 49.860MHz to operate.

I have absoloutley no idea wether it is AM or not. Its just sort of there transmitting in its own little world

Im still going to have a go at making this circuit. But I still would quite like to try amplifying the signal that the walkie sends and recieves. I could try and mod something like this as it operates on a frequency between 47-230MHz. Im not sure how I would power it though. Thoughts? I certianly think its going to be a far easier approach.

 

bumpity bump!

 

OK I'm not entirely sure I know what you're after still... Do you mean you want to amplify the radio signal the walkie talkie sends, i.e. make the walkie talkie more powerful and have longer range (in which case short of 'upgrading' all the others as well, there isn't much you can do about the signal it receives) or do you mean the audio output signal (if there is one, e.g. a jack socket).
The TV Booster has an internal low voltage supply and runs from the mains, so you could just snip the leads off and solder wires in to the appropriate places then use a battery (i.e. bypass the mains side of it entirely). It will probably flatten most batteries fairly quickly though, but a gel-cell would last substantially longer.

 

Yeah, I want to amplify the signal it sends. Because effectively if both of them are sending a more powerful signal, they are going to be recieving A more powerful signal.

But I did have a brainwave regarding the FM transmitter. What I could so is incorporate A very cheap FM radio circuit into the project box. So effecitvely you are going to have a 2 way communication system. This would probably be an easier method, but the walkie talkie idea is still open of course.

 

Well firstly try a larger antenna. I know from my radio instructor (from my time in the ATC) that it is possible to send/receive AM radio from a much longer range if you remove the rubber covering on the antenna, and uncoil the aerial wire by chucking it down the side of a tall building. I'd imagine many walkie-talkies have removable aerials (with some kind of BNC connector underneath) so you might even be able to buy a longer aerial for it. This would be similar to buying one of those huge wifi antennas so you can pick up faint signals from right across the house.
As for making the broadcast stronger, your only hope really is to add something in place of the antenna - something like that TV signal booster - and then put the antenna on the other side of that. The TV thing may not work as it is not designed to transmit so the power output is likely not very high.
You'd possibly be able to design/build something specifically for the purpose - an RF linear amplifier that can amplify the input (from the antenna socket) with a large transistor or something, and then transmit it again. This would be a lot more complex than this in practise (look here). A class A1 (single-ended) amplifier will be the simplest to design and build, B1 (push-pull) would be more efficient, but trickier to design, and will add a bit of distortion.
Essentially in the case of A1 you have one transistor driving the output load (probably a transformer), in B1 you have two transistors which are only driving the load for opposite halves of the cycle, which allows more power. (For completeness, there's also class AB1 where there are again two drivers which are on for just over half the cycle). The thing I just liked two looks like Class B, as you can see TR2 and TR3 both drive the load (T2), and are fed from opposite ends of a centre-tapped transformer (T1), so if you fed a sine wave in, each will only see the top or bottom half of it respectively.

So, quite an undertaking... if it were me, I'd give it a go after a bit more research (parts cost shouldn't be too bad, most expensive thing would be shipping. You may have to wind your own transformers on a ferrite rod or ring).

Edit: if you want to rebroadcast on AM and FM (assuming your transmitter is AM) you'd have to get the audio signal out of the AM signal first -- so you'd need an am receiver and an FM transmitter (& linear amp?) as well as the AM linear amplifier. Much easier to just amplify whatever signal comes out at the moment (AM or FM).

 

just out of curiosity, is this even legal to do? i've heard of some guys at school who had a (homemade) FM station for their house (for the same music all thruout the house) and the FCC gave them a few warnings...

 

Well this sounds like a ker fuffle to say the least! The problem with these walkie talkies is they are honestly just a cable going straight to the antenna. It does'nt have any fancy connectors. WHich could be a god send, but also a nightmare. I have tried taking off the plastic, which didnt seem to do much at all. Its just steel (I assume) coiled up very tightly all the way to the top. Nothing amazing it would seem. I cant uncoil ithis down the side of a building, however, I may be able to mount it in a vertical collum. Would this achieve the same result?

But if I can get this FM transmitter to work on the frequency I want (Check your PM's Cinnader please ^_^) and then perhaps make a reciever, based around a crystal of what ever frequency I want (Well I will have 4 49.860MHz crystals If i need them ) I could just have the two circuits in the same box. Then it would be effectively a two way radio.

I dont suppose you could direct me too a tutorial on making an antenna from acratch. I have found loads telling you how to design these antenna, but none telling a real begginer what to actually do...

The frequency I will be operating on Is allowed, because it is the frequency for walkie talkies. And because I am operating on such a low power, I couldnt be bugging anyone or be picked up.

But I am currently considering taking part in the course required, so I can legally transmit on a higher power. (10Watts I think) Plus they will teach me how to make an antenna

 

You could use the 49.860MHz crystal for an AM transmitter but for FM you need to modify the frequency which will be so stable that it wont bend
Apparently however you can use a lower frequency crystal then multiply up the frequency several times, and then just 'nudge' the multiplication factor a bit to give you your frequency modulation, like this (see the table under 'Introduction'). Unfortunately when he starts mentioning Varicaps (http://en.wikipedia.org/wiki/Varicap) in phase locked loops and such, I start to get out of my depth (Incidentally, when/if I mentioned vari-caps in my PM, I meant variable caps, as in ones that you adjust with a screwdriver or knob, not those diode things )

 

How about an FM reciever that is capable of going down too 49.860MHz or set at 49.860 MHz? (perhaps using the crystals) Just wondering because then I could use that FM transmitter with the capacitors you suggested, and it would be alot less hassle than amplifying the signal too/from the walkie talkies.

Also, any suggestions on the antenna tutorials, I am really struggling to find a very basic one...

 

You may be able to do a Xtal based receiver by filtering the antenna "input" through a band pass filter (possibly attainable with only R, L and C components) of about 49.860 MHz, then establish the frequency difference [I don't know how, sorry] between the reference signal and the input, which would I think be the audio signal + whatever else got through the filter.
I also know little about antennae and also can't find much on the net (perhaps a book would be better [old WW2 training manuals should start pretty basic?]) but I'd guess that a length of wire should be OK, perhaps a dipole if you are feeling adventurous , they look like the next simplest thing. Don't forget you need to ground (earth) the circuit properly whatever you use.

 

Most likely the guys with the new walkie-talkies have modern UHF phones working at 446- or 466Mhz.
Modifying a 49Mhz walkie-talkie for use on those frequencies would require extensive work, pretty much replacing the innards with a UHF radio.
The modulations are also completely different, the modern ones use FM the old AM.

ps.
Sorry for necropost, just had to add this to people who might stumble on this via google (like me).

 

My apologies for resurrecting an old thread but I am currently sitting with a similar project.

This is the circuit diagram:



There is no rating on this diagram for the crystal denoted as X1. The description that came with it states:

The text with the circuit states that the two units operate on 49.860MHz for one unit and 49.410MHz for the other. Is this the frequency of the crystals I need to get - and does this mean the one unit runs on 49.860MHz and the other on 49.410MHz?

Thanks for any help with this

 

Duplex means two way communication [on one band]. I would assume you should need to use the same crystal value in each but the accompanying text seems to contradict that. You might want to pick up a pair of each value though in case you run into interference or tuning problems - you can also mix and match to get it to work.

Oh, and you'll possibly have problems finding ZN414s - they're about 35 years old if I remember correctly (granted, the 555 is about 40 but timers are way more popular than AM radios are!). A TA7642 may work as a suitable replacement.

You've also got an unlabelled electrolytic capacitor between your AM radio IC and your amp. My guess would be something in the range of 10uF-100uF.

I should however point out that while 49MHz falls within the citizen's band (and 49MHz walkie talkies have always been license exempt), equipment must conform to specific equipment approval requirements. If you haven't got your RSGB Foundation Licence or above, your equipment is illegal to operate.