Sorry, but that’s illegal.
However, I’m going to describe a fully legal radio protocol that most people will have no idea how to monitor.
One of the great things about Amateur Radio, is the variety, and many ways we can communicate as a community, most folks just picture a couple people chatting into a microphone, kind of like CB radio back in the day, except the ranges are much greater, well HAM’s can use digital communication modes as well. PSK31 is one of those digital modes. Okay, so whats a ‘digital mode’? The simplest way to explain it is ‘it’s not talking into a microphone.’
That’s not fair, but I don’t want to turn this into a tech-heavy discussion that will bore you to death.
Instead, let me give you a very basic explanation of how PSK31 works, and why it’s kind of stealthy.

PSK31 is an encoded stream of audio that is transmitted over the air, to receive this stream, you’ll need a maturer radio, and a software application that can either be installed on a computer, or a smart phone. The audio stream sounds a bit like an old school modem attempting to negotiate a connection, check out out by clicking on the sample  here:

So as you can imagine, that noise doesn’t sound like anything you could understand.

Exactly.. So all the people out there with scanning short wave receivers hear the same thing you just heard, gooble-de-gook, and they keep right on scanning..
But you know better, you’ve sent a message that you have encoded with either your computer, or phone, and the person(s) you want to receive that message are aware as well, they are using a decoder that takes the encoded stream, and converts it to text.

Yep, text, not audio. A typical Encoder/Decoder application may look like this:

As you can see, there is a dialog box that outputs the decoded text, below that is whats called a “waterfall” and those brightly colors blue lines are encoded streams. highlighting the stream in the waterfall will start a real-time decoding operation, and the text will begin appearing as if someone was typing it, one character at a time.

You don’t need any special cables, or interface between the software and your radio, you can simply place the computer/phone near the speaker of your HAM radio, obviously if using a computer, you’ll need a microphone attached to receive the audio and process it, your phone will work fine with no extra bits.
Okay, to wrap this up, I’ve got good news, and bad news.
The bad news is, you’re going to want a General Class license to get the most out of PSK31, this is due to the many high-frequency operating privileges you’ll have available to you with that license, if you decide to not bother with a license, that’s your decision. PSK31 can be used on VHF/UHF but that will not be possible using a repeater, and so the range of those communications will be very very local.The second part of that bad news is, operating on high-frequency means you’re going to need a large antenna, so you’re not quite as mobile, although it’s perfectly fine to have a rolled up wire antenna stowed with your radio, just compared to VHF/UHF, the antenna will be exponentially larger.
The good news is, PSK31 can typically be transmitted more effectively with less power, transmitting voice communications benefits from having considerable power output, but with PSK31, you can regularly work long distance, which translates into being able to use a much smaller, and lighter radio. It’s a bit of a Catch-22, small radio, great long distance capabilities, but needing a large antenna to put all the pieces together for a complete solution.
Ultimately, if you are part of a group, you can use PSK31 to transmit on an organized schedule, entirely legally and likely never be monitored.

Station #1: Campsite is scouted, will be great to see you folks again this year. Any updates?
Station #2: Looking forward to it! See you there.
Station #3: Ditto, will bring some of the marmalade we just canned.
Station #4 Still unknown if I can make it, work has been tough, will update when I know.
Station #1: Thanks all stations for updates, see you next time.

Now, in the amount of time it took all 4 people to type and read those messages, it’s very very unlikely that someone located the stream, and started decoding to read what was said.
As far as I know, this is one of the easiest ‘stealth mode’ methods of long range communications, that is completely legal. If you have other methods, please feel free to share.