Electronic – good method for transmitting a syncronization pulse wirelessly to multiple devices

The modules you are using are only 0dBm (1mW) which is quite low power. The RFM12s that you are used to are normally 8dBm (6mW) and have a ready made, robust protocol. They are surface mount as well, so I'll assume you are using them mounted to a PCB.

My first thoughts would be to isolate RF issues from protocol issues. Set up a transmitter doing 101010101010 at whatever rate is required to transmit the data in your application. You seem to have a rate of about 2400Hz which is good for long distance without being so slow as to confuse any AGC in the receiver. Either use a scope on the receiver or set up something to detect this preamble like pattern, and see how far you can go with your current setup. This should make it possible to work out if it is an RF issue or a protocol one.

I don't know the HRR30 receiver, but most of the basic AM receivers have a non adjustable AGC in them. This means that it can be hard to work out exactly how long a preamble you need to bias the gain correctly, and also hard to work out what data rates are supported. Too short a preamble and the receiver will still be gained up and responding to noise. Too slow data rate and the gain will be all over the place. Your setup sounds fine, but may be worth investigating.

Bread boarding these transmitters and receivers is fraught with problems. I don't think that you need to resort to anything more than a quarter wave dipole though. Get the module onto a simple PCB and you will likely see a big improvement.

Another big assumption, but with projects like this it is often not worth implementing error correction. Just transmit several times and assume you will miss some packets. If that isn't acceptable, a send /ack system will be better then error correction.

The application SOUNDS so trivially simple that, if it isn't, there is no way to know due to the "death by 1,000 blows" approach to defining the problem. At 30 cm the way to use RF is to use RF. Any RF equipment that I have ever met would work over that range in free air in the absence of metal. You can buy transceivers and transmitter / receiver pairs. With the stated specs, if it transmits and receives RF it will be suitable. If any RF data transfer solution does not work in this context then it is because the spec is inadequate. Simple RF solutions usually have minimal latency once the data stream is synchronised. All that is required is Sync_phase - delay_until - sync_now. QED. Repeat as needed. Processor helps BUT a cheap no processor solution is easy.

More complex RF systems such as Bluetooth will also work but may have variable latency. This can be overcome by some interactive communications that establishes link parameters OR designed around known performance. BUT simple cheap off the shelf one way RF modules will work well enough. "ASK" transmission is probably fine and gives you something to search for.

This $5 receiver {315 MHz version} or 434 MHz version

plus this $4 transmitter or its up frequency cousin

will together give you a $9 solution if you are happy to talk one way.
Double that for 2 way.

Or

Two of these $7 transceivers - 434 MHz will give you a bidirectional $14 solution. ~ 16mm x 16mm x 4.2mm.
That size is known OK as the spec did not specifiy otherwise when all details were provided.
That cost is known OK as the spec did not specifiy otherwise when all details were provided.

Datasheet
Programming guide
eXCELLENT APPLICATION GUIDE
mORE OF THE SAME - MANUFACTURERS HOME PAGE {PROBABLY}
rfm12 LIBRARY

Two of these give you a WiFi link - over over over kill.

• BUT

In the absence of any real clues as to why the following are not suitable, the following may be suitable:

• IR may work well and easily and cheaply.
  Unless you are an IR expert, your reasons for excluding it may well be unsound.
  With what you have told us so far, IR would be the candidate solution of choice.
  Low cost, easy, low power. ie
  Better, cheaper, faster - choose any three!
  Tell us what you want and we'll tell you what you need :-)

• Electromagnetic "RF LIKE" "near field" coupling may or may not suit better than RF. .

• Optical

• Acoustic

• Capacitive

• ... GSM ... Iridium ... Inmarsat ... [just joking] :-) .

Pleased explain clearly & completely the environment and relevant constraints. eg:
Potted in concrete, welded in a mumetal drum at the bottom of a shallow shark pond ... :-).
Indoors, outdoors, ... .

There are many "obvious" solutions that may or may be suitable BUT as you do not explain your real need nobody can tell*.

Again: Tell us what you want and we'll tell you what you need :-)

IR sounds good to me, so far.