Simple Fm Repeater

Hello to all...

I'm trying to search the internet for any FM repeater plans...
It's just that here in my place, the cops and rules are not so enforced so me and my friend is trying to put up a communications system, using simple fm transmitters... And the repeater does the job of 'repeating' the signal and even lengthening the range for it...
Has anyone a schematic for this?

Polter said:

Hello to all...

I'm trying to search the internet for any FM repeater plans...
It's just that here in my place, the cops and rules are not so enforced so me and my friend is trying to put up a communications system, using simple fm transmitters... And the repeater does the job of 'repeating' the signal and even lengthening the range for it...
Has anyone a schematic for this?

Click to expand...

No, because such a project ISN'T simple, nor is it legal. For a start you would need high quality equipment, if you want to retransmit in the same band - I'm presuming you're talking about toy bug transmitters?.

Yep...
But i'm also thinking about much more complex, duplex systems... the one that can be used to connect a modem to a phone line wirelessly...

communications company in the phils sell wireless modems, i'm using one to connect to the net where i can also listen to fm radios.

what's the speed?

The problem with a repeater that doesn't change the frequency is preventing feedback. There are ways of doing this electronicly but thay are very complex. You can do it mechanicly be ensuring the transmittin aerial is adiquatly sheilded from the recieving aerial and both have to be very directional yagi arrays. You also need to be very careful with the phasing, i.e. the transmitting aerial needs to be an even number of wavelengths away from the reciever or some of the transmitting radiation will cancel out the existing radiation.

hello hero...
do you mean that a repeater is just like an amplified receiver transmitter which has a circuit that prevents feedback? much more like a radio and a feedback filter device then a transmitter? how could that be?

Polter said:

hello hero...
do you mean that a repeater is just like an amplified receiver transmitter which has a circuit that prevents feedback? much more like a radio and a feedback filter device then a transmitter? how could that be?

Click to expand...

As I said, it's VERY difficult to do well, and requires expensive and complex equipment - and it would be a GREAT deal more complicated to do it on the same frequency!.

As already explained, to do it on the same frequency means you have to prevent any feedback from the transmitter to the receiver - extremely directional arials help - and so does putting a mountain between them, and running a screens cable over the mountain.

Amateur radio repeaters use a 600KHz shift on the 144MHz band, and VERY well designed and built equipment. They commonly use a single aerial, connected (if I remember right?) via a 'circulator' - which is essentially a piece of plumbing, giving a VERY, VERY high Q circuit.

We used to have a radio telphone system at work, using low band VHF, and this had a talk-through system so you could talk from van to van - it used a 10MHz split, and seperated arials at the base station - even so sensitivity dropped considerably on talk-through.

The simple examples you gave use wildly different frequencies - analogue cordless phones used 2MHz and 49MHz for example!.

As Nigel has mentioned, your repeater needs to transmitt and recieve on different frequencies. A simple way to build one is to take two portable radios a connect them as follows:
Radio1<-->Radio2
COR------>PTT
RxAudio-->MicAudio
Gnd<---->Gnd
This will make a good poor mans repeater system suitable for voice communications.

The only way to do it on a single frequency is to use the store and forward technique. With this method, the reciever COR line is muted when the transmitter is active. When the reciever COR is active, the recieved audio is recorded in a memory chip until COR drops. After the recievers COR drops, the Transmitter is keyed and the recorded audio is played back and rebroadcast.
This type of system works well for extending the range of a simple paging network, but is too awkward for realtime voice communications.

Duplexer is one term for a device that allows transmission and reception on frequencies that are close (600kHz) but not the same. As Nigel said, they are tricky to build and tune. The physical size is proportional to the wavelength.

Cross-band repeat is a popular amateur radio arrangement. Some of the more expensive mobile tranceivers have a cross-band repeat feature - 2 meter/70 cm . Multiple is close but not exactly integral and with that it seems that isolation is less challenging. I do not know for sure if an amateur can use a single dual band antenna in a cross-band repeat scheme.

Save your time and money.... I think the US is still experimenting with detonation EMP weapons in that part of the world!

Oh... that would be REALLY difficult to do...

I've come up for another plan...

But, for example I would transmit to a certain frequency, say 104mhz and then my friend will do the same, and at the same time, another is listening to the same frequncy, we will cut each other's signal, right? how can i sort of 'combine' them?

Polter said:

Oh... that would be REALLY difficult to do...

I've come up for another plan...

But, for example I would transmit to a certain frequency, say 104mhz and then my friend will do the same, and at the same time, another is listening to the same frequncy, we will cut each other's signal, right? how can i sort of 'combine' them?

Click to expand...

You don't!.