RF Transmitter

Hi! I am wanting to make a vhf fm transmitter for an arduino fox-hunt. I have seen many simple circuits for a fm broadcaster. I was wondering why there are so few and so difficult circuits for a vhf transmitter. Most fm circuits put out a couple milliliters of power but that's all I need.

Thanks,

Noah (KM4VXF)

More information needed.
Whats going to be used to receive the transmissions?
What range and frequency will the transmitter require.

Noah517:
Hi! I am wanting to make a vhf fm transmitter for an arduino fox-hunt. I have seen many simple circuits for a fm broadcaster. I was wondering why there are so few and so difficult circuits for a vhf transmitter. Most fm circuits put out a couple milliliters of power but that's all I need.

Not clear what you are saying here.

You have seen many simple circuits for a fm broadcaster, but there are few (and difficult) ones for vhf ?

Most fm broadcasts are on vhf I thought.

In the Arduino world using vhf is very un-common as it is normally illegal.

Definetly more information needed.

Hi Noah, i see you have an amateur radio call sign. You might get a better response from an amateur radio forum.

Also you need to be aware or what is legal in your territory.

Noah517:
I was wondering why there are so few and so difficult circuits for a vhf transmitter. Most fm circuits put out a couple milliliters of power but that's all I need.
Thanks,
Noah (KM4VXF)

It's because the parasitic inductance and capacitance in the physical layout of the circuit contribute increasingly with frequency. By the time you get to 100MHz, a short wire doesn't behave like a perfect short wire any more. So the actual component layout and circuit design is a whole art in itself for VHF and UHF. The physical layout becomes a lot more important.

The fm broadcast are around 100 MHz and I would like to transmit around 144 MHz. From what I understand, I can transmit Morse code automatically. There are 100's of easy circuits for 100 MHz band but the only ones for the 144 MHz band are extremely difficult. I would just be receiving it with my hand held.

Thanks,
Noah

What I'm trying to ask is can I somehow convert this circuit to 144 MHz

Hi,
That circuit is for a FM bug, that would work on the FM broadcast band, its frequency stability is not to critical as it is WIDE band FM.

The amateur 144MHz, is NARROW band FM, and so stability is of greater perfomance, you do not want your hounds having to continually retune thier foxhunt receivers.

Your Tx will need to be stable, so crystal or PLL or DDS based design is needed.

The circuit you have linked will be sensitive to component layout and vibration.
A LC circuit at 144MHz will be very unstable.

Did you google foxhunt 144MHz transmitter

http://www.open-circuit.co.uk/tro2.php

Tom... :slight_smile:

Noah517:
The fm broadcast are around 100 MHz and I would like to transmit around 144 MHz. From what I understand, I can transmit Morse code automatically. There are 100's of easy circuits for 100 MHz band but the only ones for the 144 MHz band are extremely difficult. I would just be receiving it with my hand held.

Well most handhelds these days are dual 2M/70cm.

Does it have to be 2M ?

Setting up a frequency programmable and stable 70cm Morse FM beacon is really quite easy, devices are around $2 each.

Noah517:
put out a couple milliliters of power but that's all I need.

Milliliters of Powers are what you have after a fox-hunt

...R

srnet:
Well most handhelds these days are dual 2M/70cm.

Does it have to be 2M ?

Setting up a frequency programmable and stable 70cm Morse FM beacon is really quite easy, devices are around $2 each.

That circuit will produce such a low output power, it will be legal to experiment with. Change the 39pf capacitor to a variable capacitor of any type and tune to see it you can reach 144MHz. IF not, reduce the 1uH inductor a bit. Experiment!!!

Now about Morse(CW). You will not be able to receive CW on an FM receiver. Will need to be MCW or modulated CW. So you will need to replace the microphone with a tone generating circuit and key the tone generator off and on to created the MCW.

Paul

Paul_KD7HB:
Now about Morse(CW). You will not be able to receive CW on an FM receiver. Will need to be MCW or modulated CW. So you will need to replace the microphone with a tone generating circuit and key the tone generator off and on to created the MCW.

I understood that the OP wanted to do FM Morse so it could be picked up on the average VHF\UHF Handheld.

How's would I go about making a 70cm transmitter?

Semtech has lots of parts that will do that. The SX1239 SX1276 will also do 2m ham band. These are all digital modulation though -- OOK and FSK for example. You could send a 101010 pattern using FSK at an audible baud rate which would sound like a square wave tone on FM receiver. Don't know if that will work for your needs.

TI and many other vendors also have these sorts of parts (e.g. CC115L from TI) that will do 70cm.

I've used both Semtech and TI parts before.

There are probably some eval boards available from Semtech/TI/others if you don't want to gin up a custom PCB. If lucky you might find one for under $100.

Have you considered the legal aspects? in England it would be totally illegal -even a few mW, which won't get you very far.

I suggest you buy an approved transmitter /receiver - there are may 433MHz devices available, or the the NRF24 series which will give you a range of a 100 metres or so. For longer range consider LoRa devices.

RF design is not trivial.

Allan

Noah -- what sort of background do you have in RF design? It's hard to make recommendations w/o knowing that.

I think Noah is in the US -- and emission limits for a home-built intentional radiator at 70cm are likely not enough for a transmitter hunt.

I'm assuming that the ham license is required because of that, and then the power limits are much higher. There's a wider band of frequencies available (420-450MHz).

How does a RF foxhunr work???

Puzzled

Allan

It would help a lot if the OP could tell us what range is required.
ie are the foxhunters on foot or in cars.
Many years ago I used to do foxhunts and both cars and on foot .
The transmitters used then were from 5 to 10 watts on both 144 and 432 Mhz , but the transmissions were
AM as its much easier to make simple hand held receivers which are needed when you get close.
FM for foxhunts doesnt work all that well unless its a short range fox hunt , and very low transmitter power is used.

Noah517:
How's would I go about making a 70cm transmitter?

Use a RFM22B which is based on a Si4432. Its an FSK data module, but put into direct mode does a reasonable FM Morse. You can set the frequency and power level between 100mw and about 2mW. There is a 1W version to. Available in 443Mhz, 868Mhz and 915Mhz versions, but can actually be programmed from 240Mhz to 930Mhz.

I never wrote any Arduino code for it but the PICAXE version has been in orbit twice now, on one ocaision I got a accurate RDF fix with a 7 element yagi at 2000km using a RFM22B.

allanhurst:
Have you considered the legal aspects? in England it would be totally illegal -even a few mW, which won't get you very far.

For the general public, but surely a licensed Amateur can operate a remote transmitter as long as there are arrangements in place to turn it off, if requested.