Inexpensive Handheld Transceivers in Perimeter Security
I will start out by pointing out that all of these ideas are for a world Without Rule of Law (WROL).  Many of these ideas would be illegal to implement as described here while law and order prevails.  Additionally, any attempt to outwit the U.S. Government with technology will likely fail miserably.  These ideas, as presented are intended to help defend against lawless behavior.  That said, I suspect the enemy is likely to be less than fifty individuals because of the logistics of feeding and moving larger groups when your sole source of supply is plunder.  Their grasp of technology may be good or bad, but they could acquire communications equipment as they go along.  Those that survive the first month or two are going to be the people who can adapt to the situation and capitalize on every opportunity.  Therefore, I think it likely that you will face intruders who are using radio communications of one sort or another.
The proliferation of the small, inexpensive handheld transceiver (HT) opens a world of possibilities for the prepper.  Obviously, they would be handy for communicating with other members of your Mutual Assistance Group (MAG).  They are light enough to be carried by patrols and perimeter watch stations.  The ability of some units to scan, adds the possibility of passive intelligence by listening for traffic initiated by intruders with less than honorable intentions.
The Radio Hardware:
Privacy Channel, Private Line (PL) tone, or Continuously Tone Coded Squelch System (CTCSS) are all terms used to describe the same system and are interchangeable.  For the simplicity, I will simply refer to it as CTCSS.  Privacy is a misnomer in this use.  It is a system developed by Motorola to permit multiple companies to use the same frequency for communications.  It is a low frequency tone added (encoded) to the audio on a transmitter.  A receiver set to decode the CTCSS tone will break squelch (enable the speaker) and pause the scan when it detects the correct tone.  This tone is filtered out in the final audio stage, so you never hear it from the receiver, but it is there.  This provides no privacy, but prevents you from being bothered by other company’s traffic.  If you turn off privacy tone decode on your receiver, you will hear all the traffic.  Interestingly, your receiver will still filter out the tones.  Many ham radios come with the ability to scan through and report to the operator what tones are being used.  If the RSU is capable, it should be programmed with “busy lockout” enabled.  This prevents the radio from transmitting if it detects carrier present on the frequency to prevent multiple RSUs from jamming each other’s signals.  The internal speaker should be disconnected either by cutting the internal wires or just plugging a dummy plug into the earphone jack.
For the purpose of this article, Remote Sending Unit (RSU) will refer to a ham or GMRS radio capable of transmitting on preset channels with CTCSS tones.  I don’t know if MURS or FRS radios use CTCSS tones.
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Base unit will refer to any radio configured to scan the frequencies (channels) of the RSUs deployed.  This can be a base station, mobile, or even a handheld transceiver (HT).  The base unit should be configured with a unique memory channel for each RSU.  This can be done even if the RSUs are all transmitting on the same channel if they are each using a unique CTCSS tone.  There is no real limit on the number of base units in a given system.  They are inexpensive and can be worn on everyone’s hip as well as at the watch stander’s stations.
Connecting your RSU to gather data:
Alarm sensors can be connected to the microphone and push-to-talk jacks on your RSU.  Many less-expensive RSUs you may be using will not have external connections for this, but if you are wiring alarm sensors to a radio, you should have no problems removing the outer housing of your radio and soldering connections directly to the radio’s internal connections for these functions.  In a simple application where you simply want an audible alert tone to be generated, you can do away with the push to talk connection in lieu of setting up Voice Operated Transmission (VOX) on the RSU.
When connecting external sensors, you will need to use a “pulse stretcher” that will lengthen the time an alert status is present in order to ensure that radios scanning the RSU channels will reliably detect the transmission.  For most uses, you will also want the RSU to transmit an alert tone to catch the attention of monitors.  This can be an external tone generator.  Many security devices like electric eyes already have a pulse stretcher setting because conventional alarm systems often suffer from this same problem. If you aren't familiar with them, you can google "monostable multivibrator" for plans and explanations.
In its simplest form, this system does not have any system monitoring capability.  There is no way for you to know if an RSU or sensor attached to it fails and will therefore never send an alert.  A compromise would be to have the unit send a system status OK tone (distinctly different from the alert tone) on a pre-arranged schedule.  This will involve some sort of electronic time clock.  The clock attached to each RSU should be programmed to send at a different time, but the system OK tone can be the same for all RSUs in your system.  In addition, a circuit can be added to sense faults in the alarm circuit and a third tone can be used to alert for this.  Once a sensor fault alert is received, the base units can be and probably should be programmed to skip the faulty unit on their scan sweeps until someone can respond physically to the faulty unit and report a corrected condition.  Perhaps the point here is that the more complex the system is, the more information it can provide, but even a very simple system is capable of gathering and relaying critical data.
I got thinking about what kinds of things other than voice traffic could be conveyed by these radios.  That led me to the concept of a wireless alarm system with extended range.  An obvious use would be road sensors to detect when physical traffic is present and headed your way.  An electric eye can be set up across a road with the equipment concealed at the roadside.  If equipment allows it, I would place two electric eyes, one above the other so that one is a few inches and the other one is perhaps four feet above the pavement.  With the output wired in series, both would have to be showing alert at the same time in order to trigger an alarm.  This helps to reduce the false-alarms caused by birds feeding on the insects which are attracted to the infrared light emitted or small animals traveling along the road.
If equipment and budget allows it, you might mount one RSU system like this a mile from your home and another a half mile from your home.  This way you can tell by the time lag between outer and inner perimeter alerts, whether the intruder is afoot or in a vehicle.
Electric eyes might also be useful across trails or other areas where travel will narrow down to a particular path.
If the approaches from one direction are too broad for an electric eye, a honey pot might be used.  A honey pot would be something that is attractive to the point of being irresistible to an intruder.  Care should be exercised to ensure that your honey pot is not so attractive that it rouses suspicion.  Anyone who survives the first month or so will be very suspicious of a pile of food by the trailside and will very likely discover the trap without setting it off so they can use it against you.
You might place an old rifle stock so it is quite noticeable to someone approaching.  In order to verify it is just a useless broken stock rather than an intact or repairable weapon, the intruder has to approach from a direction you have pre-determined.  Rather than place any sensors on the rifle stock, place some vibration sensors on branches the intruder will have to brush aside, or a small switch that is tripped when someone steps on a board beneath the sand.  The point is, the sensor should be something undetectable even after being tripped.
A house as a Honey Pot:
Many people will be moving to the cities for FEMA or other government aid.  Others might just be out of town when the bottom falls out and unable to return.  If there is a nearby abandoned home in the direction of likely migration, you might use it as a honey pot.
A few solar powered yard lights inside the windows will create an inviting glow in the evenings.  If the home has an existing alarm system then you are way ahead of the game.  It will already have magnetic door contacts and perhaps motion sensors as well.  Otherwise, you can add them yourself.  These can be connected to an RSU you conceal in the attic.  Along with the security system connections you might want to rig a brief tone generator along with a longer connection in which you activate a remote microphone concealed near the front door of the house so you can listen to what is being said by the intruders.  Their conversation might reveal something of their intentions, but it will also tell you the ethnicity and gender of the intruders and possibly even an idea of whether they are a small or large group or scouts for a much larger group.
Since these intruders have no reason to think they have been discovered, you now have some time to prepare for their arrival at your own door.  Anything you can do to make them linger at the honey pot will help.  Perhaps a prominent note telling the imaginary relatives of the residents about the cache of weapons and ammo that is buried in the spot near the house that the imaginary relatives know about.  This might lead to them digging holes around everywhere the soil looks disturbed.  If they are an obviously hostile force, their digging might distract them from security.  Since this is your honey pot, you will know all the best ways to approach it if you decide that attacking is your best option.  Personally, I think attacking is only the best option if it is the only option.
In some situations, you might want to configure the radio for two-way voice communications.  This would allow you to converse with the intruders if you are trying to decide whether they could become contributing members of your MAG.  To do this, you would run remote wiring from the radio for both the microphone and speaker so they would not be able to simply follow the sounds to the RSU.  You would enable the VOX function on the RSU.
Blue Sky Stuff:
These are pretty basic options, but the sky is really the limit.  You could add an external Dual Tone Multiple Frequency (DTMF) decoder to enable some remote control functions.  One good thing about having an attached DTMF decoder would be the ability to “poll” the device for status of sensors and radio rather than have it check in on a schedule.  There are a variety of events that can be initiated using a DTMF remote control, most of which I would not recommend or discuss.
If you have read my earlier posts on the subject of securing your stronghold, you will recall I mention that you should figure out where someone would set up an observation post to watch your operation and either develop a plan of attack, or perhaps to decide how to approach your group for membership or a handout.  If you have identified this location and are able to conceal it, it would be nice to have a concealed RSU with a remote microphone very close to the spot you expect them to set up to watch you with VOX enabled.  It sure would be nice to know for sure what to expect from strangers.
Connecting a computer to an RSU is as simple as cross-connecting microphone and earphone ports on the computer to the microphone and speaker ports on the radio.  Digital modes of transmission open up whole new areas for the experimenter to tinker with. Video and still photos can be transmitted.  Maps and diagrams can be sent via the radio version of fax messaging. The sky is the limit.