How to Set Up an Android Tablet for Use in the Field for Burst Digital RF Communications, Part Two: Apps, Radio Preparation, and Final Test, by 4nt1p4tt3rn

If you missed Part One, you can find it here:

Step 4: Installing apps

In order to install apps on this tablet, we’re going to sideload them. Strictly speaking, this means transferring them onto the tablet from another device rather than using the common method of installing them over a network from the app store.

To do this, we’ll need a micro-SD card. I’m partial to SanDisk Ultra cards, but anything that the device will read will work. To begin the process, you’ll once again need to remove the tablet from the case, in order to access the SD card slot. Do so, and insert your SD card. Be sure to push it in until there’s a faint click, and the card sits flush inside the slot. You may need to use a fingernail or knife blade.

If it’s new, you should see the a small icon in the upper left of the screen, near the time display. Stroke the screen down from the top, and you should see a notification about the card, with a “Tap to set up” option:

Tap it, and you should see this:

Choose “Use for portable storage” on this screen. When you do, you should see this screen:

Remove the card by pushing down on it until it pops out partway, then pull it the rest of the way out of the device. Do not lose it! Now, you’ll need some way to read and write to the card from a laptop or desktop computer. I use this device:

It appears that these aren’t available any more, but any similar device that connects to your computer via USB and has a micro SD card slot should work, such as this one: https://www.amazon.com/Reader-uni-Adapter-Aluminum-Memory/dp/B08P1T8R46/.

Insert your micro SD card into the reader/writer, and connect it to your computer. It should appear as a drive in your File Explorer (yes, I use Windows. If you’re using a Mac or Linux, you should be familiar enough with it to figure out where it mounts an external storage device, and how to access it):

Now, we’re going to download several APKs and copy them over to this drive. For convenience’s sake, we’ll store them in the Download folder on the micro SD card. There’s no right or wrong here; it’s just personal preference. APK is the file format used by Android apps. An Android app file, therefore, usually has a “.apk” file extension. I’ll provide you with a list of URLs for the APKs we’ll be downloading, and you should download them, then copy them to where you want them on the micro SD card.

First, we’ll be using andFLmsg. This is the primary app we’ll be using on the tablet, and its purpose is to send and receive digital data over RF. You can download it here: https://sourceforge.net/projects/fldigi/files/AndFLmsg/AndFLmsg_V1.5.0-20210812.apk/download

Next, Drone Scanner. This is why we left the wifi and Bluetooth antennas intact – this app uses them to listen for nearby drones, and provides you with the exact latitude and longitude of both the drone, and the drone’s operator. This can be highly useful in the field. Visit this page: https://apkcombo.com/drone-scanner/cz.dronetag.dronescanner/download/apk and there will be something near the top of the page that looks like this:

Click on the blue down arrow on the right, and it should download the APK for the app. This method will be the same for all links to APKs on apkcombo.com.

Now, let’s download SDRTouch from https://apkcombo.com/sdrtouch/marto.androsdr2/download/apk.

You should now have these in whatever folder you’ve set for your browser to download files into. I use the default Downloads folder in Windows:

Here, you can see the three files we downloaded. Don’t worry about the times they were downloaded; yours will likely all be near the same time.

Open another File Explorer window, this time with the contents of your micro SD card displayed, so you can see both at the same time:

Drag and drop each of the three files you just downloaded into the “Download” folder on the micro SD card. This will copy the files over to the card. Once you’re done, if you double click on the Download folder on the micro SD card, you should see the three files there:

You should now be able to remove the micro SD card and reinsert it in the tablet. In Windows, you should unmount the card first by clicking on the circled icon here:

And select “Eject USB drive”.

Once the card is back in the tablet, turn it on. On the main screen, there should be four icons across the bottom of the screen: Camera, Files, Chrome, and Settings. Click the Files icon. You should see something like this:

Click on the “SD card” item at the bottom, then click on the “Download” folder that appears on the next screen. You should now see a screen listing the three files you put on the micro SD card:

Click on the first file, and this should appear:

Click on “Settings” in the dialog box, and allow the OS to install files from Files by Google:

Once you’ve enabled “Allow from this source”, click the back arrow. You should now see this:

Click “INSTALL”, and after a few seconds, you should see a message at the bottom of the screen that the app was installed. If not, try again.

Do this for the other two apps as well. Since you’ve already told the OS to allow file installs from the Files app, you shouldn’t need to change that setting again.

Quit the Files app, and you should be back at the home screen. Swipe up to access all the installed apps, and you should see the three apps that you just installed, along with the rest of the apps on the tablet. If you like (and I do), you can choose to place them on your home screen for faster access:

Congratulations! You’ve now sideloaded three apps to your tactical tablet. You may decide you want to load some other files, such as PDFs of topological maps from https://topobuilder.nationalmap.gov/ or another source, or frequency charts.

What I strongly recommend AGAINST adding is anything mission-specific, or that could help identify you or your team. So, no PACE notes. No map annotations. No personal info. Nothing other than vague, general info that cannot be used against you, your team, or your organization. If you wrote it, it’s probably not safe to keep on the tablet. Not even encrypted. Assume the tablet will fall into enemy hands, and any information on it will be exposed. Ensure nothing is useful.

To that end, I also strongly recommend removing the SD card after you’ve installed what you’re going to install. This limits you to onboard storage capacity, and keeps you from wanting to store a bunch of potentially damaging information on the card. Remove it and keep it someplace safe back at base. Do not carry it with you in the field. You do not need it.

If you suspect capture or death, I also strongly advise destroying the tablet. Folding it in two is an excellent step. As is puncturing the Li-Ion battery with a knife. It’s that big, shiny rectangle taking up half the tablet guts you saw when disconnecting the speaker and mic at the beginning. Be aware that this will cause the battery to combust, possibly explosively. It may also rupture when you fold the tablet in half, with the possibility of a similar result.

Step 5: Physical radio preparation

Now that that tablet is prepped for use, it’s time to prep the radio. Just like with the tablet, we’ll start with physical preparation. On Baofeng radios, there is a very bright, multicolor LED present between the volume knob and antenna:

While use of this LED can be partially disabled in software, it will still occasionally light up. This can be extremely problematic in the field, particularly at night. While some people are content with using electrical tape, 100MPH tape, or similar to mask off the light, we’re going to take a bit more permanent approach.

While I’d enjoy nothing more than to remove the LED entirely, after researching this, it turns out it’s rather annoying to accomplish, and also compromises the watertight case seal. Instead, I’m going to recommend a two-pronged approach: grinding and painting.

Grinding down the dome of the LED accomplishes two things: First, it changes the shape of the LED, which changes the light emitting characteristics, making it less omnidirectionally radiative. It also lowers the LED to a point at or below the surrounding plastic.

Second, it roughs up the surface of the LED. This further changes the light emitting characteristics of the LED, diffusing the light being emitted. It also provides a better surface to which paint may adhere.

By using a Dremel with a small grinding attachment, you can easily and quickly grind down the dome. Be careful not to grind down so far that you expose the diode itself (the two metal legs embedded in the acrylic). We’re just trying to deform the dome and rough it up. Just going to the same level as the top of the surrounding bevel should suffice. When you’re done, it should look like this:

As you can see, the dome is now gone.

Now that we’ve ground down the LED dome, it’s time to paint it. The best approach for this is going to be multiple coats. I’ve tried nail polish and paint with little success, but black Plasti-Dip works very well. Here’s the finished product, after two passes with Plasti-Dip using a hobby paintbrush. In this picture, the light is on, and bright white (using the MONI side button under the PTT):

There is also a small flat LED on the front face of the radio, below the VFO/MR button. You may also wish to coat that with some Plasti-Dip, to be on the safe side. Otherwise, it may light up when you least expect it. There is a similar small flat LED on the AR-152, but it’s on top of the radio, near the volume knob.

Make very sure you use a couple of coats, and test your handiwork. Otherwise, you may be surprised that you missed a spot, or did not adequately coat one small area. Even something that seems dim in room lighting can be quite bright on a dark night.

Also make sure you cycle through the always-on, flashing, and off cycle of the MONI button, because once you have successfully covered the LED, you’ll have no indication whatsoever that the LED is lit. You don’t want to just leave it on, because it will drain your battery charge faster, and if for some reason the coating gets scratched, you’ll suddenly and unexpectedly be emitting light.

Now that we’ve made some progress with light discipline on this radio, let’s talk antennas. I know everyone wants to be all high-speed operator cool and have a folding whip antenna on their comms.  Don’t. What you need is something that:

1. Will not break easily
2. Will not get in the way
3. Does its job
4. Is the absolute least amount of antenna you need for the mission

Most of the time, you don’t need 5-10 miles of range for what you’re doing. You likely only need a mile or two. To that end, there’s a perfect stubby antenna you can use: https://brushbeater.store/products/bnc-vhf-uhf-stubby-silicone-antenna. Buy this, along with these SMA female to BNC female adapters, in black: https://brushbeater.store/products/sma-female-to-bnc-female-adapter-2-pack. They’ll fit perfectly on these UV-5Rs, don’t take up a lot of room, and will get the job done. You’ll also find them invaluable, because you’ll now have a more common, less fragile connector attached to your radio, and one that’s better suited to connecting to antenna wires as well as antennas themselves.

The goal of digital burst comms is to transmit a lot of information quickly, so you aren’t transmitting for more than a couple of seconds. This is to avoid detection and direction finding. The goal of using a short antenna and minimal power is much the same: the less distance you radiate, the less risk you take. You only need to radiate far enough for your receiver to pick up your signal. Likewise, anyone transmitting to you should use just enough power to reach you, and no more.

You will also need an APRS-K1 data interface cable: https://brushbeater.store/products/k1-data-interface-cable. This is what we’ll use to connect the radio to the tablet, via the tablet’s headphone jack. That may seem odd to you, since it’s called a “data interface cable”. However, all it’s doing is carrying analog audio. For those of you old enough to remember modems and dial-up internet, this is literally the same thing: a modem (MOdulator/DEModulator) takes digital information converts it to an analog audio signal. Likewise, it can take an analog audio signal and convert it to digital data. In the 1970s through 1990s, modems were connected to a computer and your phone line, and you’d dial into a service where another modem on the other end would answer the phone, and the two modems would use sound to establish data transfer protocol parameters, then send and receive data. AndFLmsg acts as the modem in this scenario, and the radio is the stand-in for the phone. Together, they can send and receive data with another radio and tablet configured to connect to it.

I would also recommend physically removing the PTT button. Thankfully, this is both easy and reversible. Just take a knife and slip the point down into the side of the button, between the button and the case, and pry upwards. The PTT button should just pop off, and you can always put it back on whenever you want. You can even still manually operate the PTT by pressing the exposed metal pad inside the recessed area where the button sat.

I recommend doing this because I’ve noticed that, in the field, it’s sometimes easy to knock or pull the K1 data cable loose from the radio, or pull it out entirely. To that end, I recommend buying these: https://www.amazon.com/dp/B0CBFS5PM5. These heavy-duty “tactical” rubber bands are perfect for wrapping around the radio with the cable in place, and will hold it tight without pressing the VFO/MR button accidentally, even if the band covers the button entirely. It would, however, trigger the PTT if the button was left intact, so we removed it. In case you’re wondering, these same bands also serve to hold a headset/mic cable in place on the AR-152, though you will need a placard or similar providing a standoff for the AR-152’s PTT, or the band will trigger it. And since the AR-152 is my primary for voice comms, I very much do NOT want to remove its PTT, in case I have a cable or headset failure in the field.

Step 6: Radio configuration

Now that the physical side of the radio is sorted, we need to adjust multiple settings in the radio’s software to prep it for field use. Turn the radio on, and unlock the keypad by holding down the “#” key. You should always lock the keypad after any interaction with it, to ensure you don’t accidentally change anything in the field.

Now, hit the “MENU” button, and make sure you’re on the “SQL” item (squelch). If not, use the UP and DOWN arrow buttons to get there. The settings I’m about to give you are straight out of NC Scout’s “The Guerrilla’s Guide to the Baofeng Radio”. I’m using them because I agree with the settings as the best for each option, and I’m referring you to the book because it is a must-have for anyone who’s reading this manual. You can purchase it from him directly at https://brushbeater.store/products/the-guerrillas-guide-to-the-baofeng-radio-authors-verison, or from Amazon at https://www.amazon.com/Guerrillas-Guide-Baofeng-Radio/dp/B0BNV21F4P/.

I will list the menu options in order, and the value each should be set to. If I do not list an item in the menu, ignore it and leave it at whatever it’s set to, assuming it’s set to the default value. To change a menu item, press MENU again, then use the UP and DOWN arrows to find the correct value, and press MENU again to save the change.

SQL (squelch): 1

STEP: 6.25K

TXP (Transmit power): LOW

VOX (Voice-activated transmission): 1

TDR (Transmit/dual receive): ON

BEEP: OFF

TOT (Time out timer): 60

R-DCS: OFF

R-CTCS: OFF

T-DCS: OFF

T-CTCS: OFF

VOICE: OFF

BCL (Backlight): OFF

SFT-D (Standard offset): OFF

WT-LED: OFF

RX-LED: OFF

TX-LED: OFF

ROGER: OFF

With these settings, your radio is now properly configured for use in the field as a dedicated transceiver for burst digital communications. You have disabled the LEDs, the annoying voice prompts, the “roger” beep, the backlight, the button beep, made sure it’s configured to send and receive correctly, set the power to the lowest setting, ensured the squelch function isn’t going to clip any received messages, and made sure you won’t accidentally transmit longer than 60 seconds. You’ve also enabled VOX mode, which means you do not have to push the PTT button to transmit: any audio received over the data cable will cause the radio to transmit.

This is another large reason why I like using these “tactical” rubber bands to hold the data cable in place. If that cable comes loose, you’re potentially running around in the field with a hot mic because you need VOX on. And, if the radio’s on, it’s also going to surprise you with unintended noise, usually when you least expect it and most want to avoid it.

Having said all that, get NC Scout’s book. Read it, cover to cover. While it does cover this radio, its true focus is proper use of such radios in situations where you might need them. Which is so much more than just how to configure and use the radio.

Step 7: Putting it all together

Back in Step 4, we installed some software on the tablets, including andFLmsg. Now, we’re going to use it. If you have two tablets and two radios, all properly configured, this is easy. Otherwise, you’ll need to find someone else with an identically-configured radio connected to a device running andFLmsg in order to test your send and receive capabilities.

For the purpose of this guide, we’ll assume you have two identically configured tablets and radios. Connect each radio to a tablet using a K1 data cable. Turn on each radio, with the volume about ¾ of the way up. Set the first radio’s first frequency to 145.500, and the second frequency to 146.500. For the second radio, set the first frequency to 146.500, and the second to 145.500. Effectively, the reverse of the first radio. Each radio will send on a different frequency. Because we turned on TDR in the previous step, both radios will be able to receive on both frequencies. Make sure the first frequency is selected on both radios.

Now turn on both tablets, and start up andFLmsg. The first time you start it, it will ask you a bunch of questions regarding permissions. You should choose “only while using the app” for all of them. Denying any of them will cause the app to quit. It will then ask you about a folder to use. Choosing “USE DEFAULT” should be fine. You should now see a screen similar to this:

This is the screen you’ll send messages from. If you touch the line on the lower part of the screen, a keyboard will appear onscreen, and you can type your message. When you hit “SEND TEXT” in the lower right corner, your message will be transmitted over the frequency the connected radio is set to.

If you swipe right, you’ll see the screen any received messages will appear on. It should look like this:

Once you’ve got both tablets connected to a radio, the radios on as instructed, and andLFmsg running, it’s time to send your first message. Set one tablet to the first screen for sending a message, and the other tablet to the second screen, for receiving a message. Then, type a message into the first tablet, and press “SEND TEXT”.

The message should appear after a few seconds on the second tablet, assuming all went well:

If not, it’s possible your radios are too close together. Try putting them 10-20 feet apart, and re-send. You may also want to switch to the receive window on both tablets, and press “SQLCH DOWN” until garbage starts appearing on your screen, and then hit “SQLCH UP” once to stop the garbage. You may even need to experiment with the volume on each radio. Sometimes, if it’s too loud or too soft, messages won’t be received properly.

That’s all there is to it! If you have another radio handy, you can tune it to the first frequency on your transmitting radio, and you’ll be able to hear the audio of the message being transmitted. You’ll also get a stark reminder as to why we physically disabled the speakers on these tablets, and why we’re using rubber bands to hold the data cables in place on the radios. That is very much NOT a sound you want coming from the treeline while trying to surprise an enemy force. Either way, having a third radio tuned to your transmit frequency is a great way to ensure that the messages are, in fact, going out over the air, as is checking the displays on the radios themselves to make sure the transmit and receive graphics are present in the upper left corner, and activity is indicated on your transmit channel.

If you’re curious, here’s what that transmission looks like on a spectrum analyzer (this is the TinySA Ultra, which is an invaluable field tool for SIGINT. You can get one here: https://brushbeater.store/products/tinysa-ultra-spectrum-analyzer). Anyone who knows what they’re doing might spot this transmission. However, it’s not on the air long enough for them to direction-find (DF) you. And, as long as you don’t transmit over and over and over from the same position, you’re safe.

In this image, you can see the transmission beginning, exactly at the transmit frequency set on the radios: 145.500 MHz.

In this image, you can see what the entire transmission, that lasted about three seconds, looks like on the waterfall plot. Very tight around the transmit frequency, dense, and without a lot of frequency modulation. Which is as expected, since the data is encoded in different audio frequencies, not RF frequencies. This signature is typical of transmissions using MT63_2000_LG encoding. Each encoding has its own unique signature, though for the most part, they look a lot alike, because of the data encoding taking place via audio, not the RF itself, as is common with things like AM and FM broadcasts.

Anyone seeing this when monitoring a frequency band would quickly be able to determine what frequency you’re transmitting on, and have a pretty good guess at what you’re doing. Some of you may be wondering what’s preventing the enemy from having a radio and tablet set up and configured to receive these messages as well, assuming they use something like a spectrum analyzer to detect the transmission and determine what frequency you’re using. The answer is simple: Nothing is stopping them. Anyone who is set up properly can receive and decode these messages. That’s why it’s also important to learn to use One Time Pads (OTP) and Trigrams. That way, even if the message is intercepted, the enemy will have no idea what was said, and no way to crack the code being used. Trigrams and OTPs are beyond the scope of this guide, however, and I’ll once again refer you to NC Scout’s excellent “The Guerrilla’s Guide to the Baofeng Radio” as a resource to learn about these, and how to properly use them in this scenario.

Some of you may also be wondering about the various protocols available for transmission in andFLmsg, the ability to send data other than text, etc. NC Scout also covers this in his book. I will state that choosing a different protocol just because it’s not the default protocol is not going to provide you any additional security. In the profession, this is known as “security through obscurity”, and is very seldom effective. Protocol analysis against a captured signal is fairly easy, as is simply running through all available protocols until the captured signal decodes properly. You should only choose a particular protocol for sound technical reasons that also make sense in mission context. Both for the purpose of this guide and for use in the field with text messages, the default protocol in andFLmsg is more than adequate to the task, and there’s no need to change it.

If you’re interested in a decent IP67-rated case to carry your setup in and don’t want to spend Pelican money, the Harbor Freight “Apache” cases are a decent substitute, as are the Eylar cases on Amazon. I have this one specifically to carry both radios, cables, and tablets: https://www.amazon.com/gp/product/B081DGR5QY/. Make sure you also take an adequate recharging solution with you to your base camp, along with any necessary cables, charging bases, etc. And ensure everything’s fully-charged and functional before setting off on a mission. While gear failures happen, failures due to inappropriate preparation are completely avoidable.

Night operation

When using a setup like this in the field at night, you will obviously have a light discipline problem. Your tablet will be emitting a strong light, even with the brightness cranked way down (which I recommend you do at night). You will need to carry something to mitigate this. If you have a quiet poncho, I recommend using it, covering yourself and the entire setup while it’s in use to contain the light leakage. However, I’m going to stress the word quiet. If it’s noisy, you’re going to be breaking noise discipline just getting it out of your pack and in position over you. The entire point of this activity is to prevent the enemy from determining your location. Hiding the light does no good if they can hear you doing it. Otherwise, something like a soft poncho liner, blanket, or other large, thick fabric will work. A shirt or jacket will work in a pinch, but they’re not really large enough to allow you to see and operate the tablet and block the emitted light.

Conclusion

This guide has hopefully helped you understand the process to build and configure a workable digital burst communications platform for field use, and how to do so securely. Once again, please pick up a copy of NC Scout’s “The Guerrilla’s Guide to the Baofeng Radio” if you haven’t already. He goes into much more detail about certain aspects of this topic, including quite a bit more on proper usage once everything’s set up.