Hi,
In my work, I usually go to remote fieldwork areas (glaciers) and deploy multiple instruments (GNSS receivers, cameras, weather stations), often several hours away from the camp. I want to monitor battery voltage and power consumption of each instrument to know if they are working correctly and if batteries need replacement.
While I can use LoRa, everyone involved in the fieldwork always carries a VHF/UHF radio, so I would like to use the handheld VHF radios as receivers instead of having multiple dedicated LoRa receivers.
So I want to put together a system in which an Arduino reads the voltage and current of the instruments and "reads it aloud" by playing audio files from a microSD card that contains the numbers. This audio output would be connected to a VHF radio that broadcasts the information so everyone can hear it.
I know how to do all this, but to save power, I would like to keep the radio off except when transmitting. Then, I would like to trigger the transmission remotely by just making a short transmission in a specific (or not so specific) frequency.
Then my question is: Is there a simple way to detect a power peak in a specific VHF frequency with Arduino?
To detect a specific frequency, in the VHF or UHF range (you did not say which) then you need a receiver such as the type in the VHF\UHF radio so that with all the other VHF\UHF radio activity there appears to be you have a receiver thats capable of being selective enough to not respond to 'noise'.
Why not just trigger the transmission via a LoRa link ?
So what is the difference between Lora Receivers (more than one)
and
Radio Receivers (more than one)?
All the Lora Transmitters can send to 1 Receiver.
Sorry I missed this line.
What does this mean?
HI, @crada
Tx timer would be easy to accomplish.
Have you looked at the Rx standby current of most modern handheld VHF/UHF transceivers.
I have a couple of Quansheng UV-5K units and can get almost 2 days of continuous Rx from them.
But you need to do research into other brands.
I did a quick check of my unit, after turn on, and the nice lady speaks the standby drops to less than 4mA, peaking to 27mA at about 3 second intervals.
The battery is 1600mA, 7.2V Li-ion.
I would have to look at a scope trace of the load current to get definitive values.
Check the battery performance at glacier temperatures too.
Do you have an Amateur Radio License?
Tom.... 
No, A "front end" capable of receiving radio signals is always required.
An SDR (Software Defined Radio) can scan a wide range of frequencies and the computer steering it, usually a PC, can detect a peak.
if you have a schematic of the receiver, find the squelch signal and use that as the trigger. the squelch is the circuit that mutes the receiver if there is no RF on the frequency you are tuned to.
If these are all commonly used frequencies, how will your system know when others are using the frequency and will not interfere with the communications?
This is not at all difficult to do, using the same VHF radio that will perform the data (as voice) transmission.
Leave it on in receive mode and to trigger a data transmission, transmit specific tones which the Arduino recognizes as commands.
"Ham" operators use this technique to control repeater stations.
The OP knows full well he is NOT dealing with ham radio. If he was, the voice would also have to identify the call letters of his license. Likely GMRS radio and the voice transmission is required as data transmission is not legal.
That's an option. But I would like that anyone with a VHF radio could trigger the transmission.
It is higly depend on communication type you guys use. If it is modern digital walkie-talkie, then it will be hard to do.
Those small walkietalkies use frequency modulation with quite small frequency deviation. That means your transmitter must be precise.
May be you can get away with AM, providing your radios can receive it. If it is AM then it is much easier to do
mmm....
Then you need a simple superregen receiver with noise suppression. Those are normally used (were used) by tiny RC models. What frequency your radios work? Do you use digital subchannels? Or may be you remember radio model name
Superregens are simple and very sensetive. You can receive signal, from a radio and use it to trigger your transmission.
This one: BH1RBG RF Lab - SuperRegen: AirCraft band receiver , can receive both AM and FM signals and can be tuned (i hope) to 433MHz
I know there are RF modules with all the features of a handheld radio (like the ones using in this project), but I only need the feature of the squelsh, which is measuring the power received in a specific frequency. Then, for example, if I detect a spike in the power received at 145.350 MHz, it means someone just transmitted at that frequency, and I can use that as a trigger to answer with the sensor data.
Thanks @jremington and @TomGeorge , it seems that is the best way to go, use the transmitting radio as a receiver and just try to get one with low RX power consumption.
I thought maybe there was a simple circuit or module that could do the trick.
The SuperRegen suggested by @vvb333007 is great, and closer to what I was looking for, but I'm not sure it is worth the effort without any guarantee that it will have a lower standby power consumption than the handheld radio.
Not only is using the handheld radio as the receiver the simplest solution, manufacturers of quality handheld equipment go to extreme lengths to maximize battery life.
Superregens are very low power devices with extremely high sensetivity.
But they have one problem: they produce white noise if there is no signal. But it is a minor problem.
Around 2007 I had simple RC helicopter which had very simple superregen working at 27MHz.