27MHz AM Transmitter / Receiver for PIC communication

[heathclf]

Ok, I'm an EE by education, but never really picked up an soldering iron until recently. I only programmed and such, so some of this is very new to me, and some isn't.

I want to build a 27MHz transmitter / receiver pair. 27MHz supposedly travels through fresh water well, otherwise I wouldn't care about the frequency, and AM bc FM seems harder.

Here, I found some old schematics: RF circuits

Here is the RX https://jap.hu/electronic/rf/tx.gif
Here is the TX https://jap.hu/electronic/rf/rx.gif

I'm using these bc they seem to be the simplest around. A few questions:
1 - First and foremost, does anyone have any better schematics / ideas?
2 - Some of the caps aren't labeled, when I've run into this in the past, they've tended to be uF, but it seems that these are probably pF, given the MHz frequency...right?
3 - Why are the antennas two different lengths? (250mm, 500mm) Seems odd.
4 - Why are they both so short? With C = λf, 5.5meters = half wave, right?
5 - I don't understand the tap off the of inductor / txfmr on the txmtr L2 / L3
6 - With the receiver, what in the world is the small network to the top right that doesn't connect to anything?

I'm all about suggestions / ideas. I just want to get my pics to talk to each other via RF, and I got this off of some guy's webpage, who states up front that he won't answer any questions.

I much appreciate any wisdom imparted, as I am in no way an RF / antennas man.

 

[Mikebits]

Who told you 27MHz will propogate through fresh water?

 

[pc88]

What kind of data do you want to transmit and at what range?

I would look for an integrated transceiver module such as this one: **broken link removed**

 

[flat5]

That receiver is a challenge for me to understand.
Regenerative first stage to two stages of audio feeding a piezo
element.

The diode stuff is a mystery to me. So is the switch part but that is incomplete.

You might use crossband modified "walky talkies".
27 and 400 or 900 mhz. Each set for one way communication.
Alread made and not expensive.

 

[heathclf]

Who told you 27MHz will propogate through fresh water?

Some RC Submarine guys. Part of my project has to do with tx-ing a signal from a small submarine back to a station, where I'd like to read it in through my rx-r and into labview, if possible.

What kind of data do you want to transmit and at what range?

I would look for an integrated transceiver module such as this one: Low Power RF ICs - Sub 1-GHz - TRF7900A - TI.com

It all depends. Right now I want something that just works, so I can get text across, i.e. rpm's, temp, heading, etc. At some point, it'd be pretty rad to tx video, but that would require changing how I receive the data, which wouldn't bother me at all.

I'm not 100% against an already made solution, but part of the point is to do it myself.

Thoughts? Suggestions?

 

[jpanhalt]

Google turned up a Q/A from Argonne National Lab, and that pointed to this as a "good discussion." Attenuation per meter is given vs. frequency/wavelength:

Underwater Radio Communication

John

 

[audioguru]

The circuits you attached are extremely simple for kid's cheap toys. The performance is horrible.
The frequency drifts all over the place.
The transmitter will cause radio and TV interference.
The receiver will pick up many signals all at the same time.