Electronic – Why do receivers have a choke but transmitters don’t

Solar aspects:

A 1 inch square PV cell (25mm x 25mm) using best available methods will give about 75 mW in full sun. In worst case winter conditions of 2 hours equivalent full sun per day that's 150 mW.hours/day. Energy stored to battery from that will probably be 100 mW.hr or less.
As long as tx_power x tx_hours_per_day <= 0.1 Watt.hours then you will have about enough energy on average. To allow for below average days you'd want battery capacity for many days of operation. A 500 mAh LiIon cellphone battery holds about 1.8 Wh of energy or enough for about 2 weeks of sunfree operation. That's a small cellphone battery but > typically 1 square inch so a smaller capacity one would be needed.

Whether 100 mW.hr/day is enough depends on Tx power, tx burst duration and TX repetition rate. 100 mW.h = 360 Watt seconds or 15 Watt.seconds/hour.
So eg 150 bursts x 1 second x 0.1 Watt per hour
or 150 bursts x 0.1 second x 1 Watt per hour or ...

If operated out of direct sunlight then PV panel output will be lower to much much much lower with resultant loss in transmit energy availability.

Channel reuse

All TX/RX may be on the same channel as long as you have short TX periods as a % of total time and as long as you either detect "collisions" or have a protocol which can accept some collisions.

This Wikipedia ALOHANET article provides a good basic introduction to the considerations when using multiple interfering transmitters on the same channel. Many protocols based on these principles have been developed. The original Aloha net used randomly times transmissioms. Slotted Alohanet arranged for transmissions to start on defined time boundaries and achieved a useful increase in throughput. (See article).
Note that collision detection is not an essential requirement in some cases if the statistical probability of delivery of a given "packet" is high and if multiple transmissions of the same packet is viable.

Equipment:

You have note specified country of use, power level, range and more. All these affect the answer. But, there are numerous low power TX/RX units for sale which could do this with ease. Once you provide more details it will be possible to make some specific recommendations.

Your question has far too little detail to be able to be answered well overall. Some of the parameters are mentioned above, but you need to provide a seamless 'picture' of what you intend.

Those types of transmitters and receivers only work on a single channel. The receivers usually employ a form of active gain control (AGC), so will increase the gain until something (usually noise) is received. The gain is reduced to an appropriate level when a transmission starts.

Generally best practice is to:

• Agree on a bit rate between the two systems (e.g. 4800bps). Generally receiver/transmitter pairs will give you some guidance on these.
• Transmit a preamble of 10101010 (generally somewhere between 8 and 40 bits) before starting the main transmission. This has two purposes - it allows the AGC to settle at a good point, and can be detected by the microcontroller so it is aware that a transmission is starting.
• Transmit a sync word e.g. 0xD5F7, and only listen to packets which have the correct sync word. The length of the sync word varies.
• Use the rest of the data packet to detail the address, direction, and data.

I have found that the VirtualWire library for the Arduino is actually a nice, clear example of how to go about this. Maybe take a look, even if you are working with another microcontroller family.

One further thing worth considering is the encoding of bits. You can just send a 1 as a 1 and a 0 as a 0. There are more complex schemes though. Manchester encoding ensures that each bit of data transmitted has at least one signal transition. This has two significant effects:

1. Regardless of the data sent, there will be an equal number of highs and lows sent. This means the AGC can work properly and there is no DC bias to the signal.
2. As there is a transition for each bit, it is easy to recover the clock from the signal even if it is not known. This really means you transmitters no longer have a need to use an external crystal - the internal oscillator is accurate enough.