MURS Dakota Alert IR Sensors and Antennas – Part 2, by Tunnel Rabbit

(Continued from Part 1. This concludes the article.)

a Radio Survey

Performing a radio survey of the area first is a necessary step before purchasing or fabricating the appropriate antennas. You might find that no directional antennas will be necessary, the cost reduced, and the remaining budget used to purchase additional sensors.  The range of any transmitter is in the end limited by or enhanced by the surrounding terrain. Given that very low power transmitters are being used, the 1 watt transmitted by these sensors, versus the 5 watts of a handheld transceiver, the challenge is greater. Having favorable terrain is necessary to extend the range of low-power transmitters. And even if the terrain is not entirely favorable, 1 watt of power is still adequate enough to refract over several hills to a receiving antenna. However, there is a significant loss of signal strength in the signal making that transit.

You must understand how an antenna propagates. Even a high gain omnidirectional antenna, such as a Slim Jim, can be aimed at the crest of a hill in order to send the signal farther.  This antenna does concentrate its propagation lower and toward the horizon as compared to a unity gain 1/4 wave antenna that evenly distributes RF in all directions, and therefore has no gain.  Aim the Slim Jim by placing the crest of the hill 90 degrees, or perpendicular to the length, or axis of the antenna, so that more of the signal is sent toward the top of the hill and will be refracted over and downward on the opposite side.  A higher gain directional antenna is not always needed, but should the hill be high enough, additional power in terms of Effective Radiated Power (ERP) might be used to refract the signal over the hill.  This is where a high gain directional antenna can be used to solve a problem.  But first, we must find out where it would be best to use these sensors, given the low power of the transmitter, and where and what kind of external antenna, if any, might be needed.  Therefore we need to perform a radio survey.

I recommend the free RF Line of Sight (LOS) map and calculator. It is simple to use, and so useful to your radio survey effort, the time saved from its use can greatly offset the time it take to use it.  This makes the job of performing a thorough Radio Survey possible. Identify potential RF dead spots in your AO to test, and develop a ‘workaround’ that solves the problem. This tool is often used for analyzing cell phone propagation.  A tool like this is more important if your transceivers are low-powered and UHF.  As a general rule — not always true — is the lower the frequency, the better a signal can negotiate micro terrain and larger hills.

Radio line of sight is a bit different than actual direct line of sight, and where there can be obstructions discovered with the use of this tool, the presence of obstructions does not necessarily mean that the radio wave will not find a path around it.  In fact, radio waves can have several paths to a receiver. However, if there is a clear Line of Sight (LOS) to a receiving station, then we can rest assured that we would probably not have reception problems to overcome, especially if we are restricting the amount of power that will be used.  When using less power, it is more important  that there not be any obstruction to the LOS.  I’ve spent hours mapping out the entire AO and my region, using screenshots to document the results.  If the Internet is still available when you do a survey, then start with this. By the way, I’ve also used this same tool to examine fields of fire.

Regardless if we use the LOS calculator or not, we’ll still need to go out to see what a transmitter and antenna combination will actually do in the field.  You’ve gotta do it.

Step #1
Install an external antenna on the base station that is at least 15 feet high, or higher than any buildings or obstructions nearby. An omnidirectional, or directional antenna (such as a Moxon or Yagi) can be used on the receiver.  Use a sensitive, good-quality receiver, such as a scanner or transceiver of known high quality.  These will hear the sensors better than a typical Baofeng.

Step #2
Using a handheld transmitting only 1 watt, first with the attached antenna, and then if needed, with an external antenna, visit potential installation sites, and transmit to the base station.  If the successful, that is a possible good location for the sensor.  Or we can use a sensor and an external antenna to perform the survey.  Record the location of any potentially good sites on a map.

Step #3
Install the sensor at the location and determine if it can be heard loud and clear by first the base station,  and then other receivers located within the vicinity of the base station.
If not, then install an external antenna on the sensor, and place the antenna as high as possible. If this is not adequate, then install a high gain directional antenna at the base station, or move the sensor to more RF favorable terrain, or install a directional high gain antenna both on the sensor, and at the base station.<

There is still an advantage to hearing the sensors placed at extended ranges with only the base station receiver that is attached to the best possible antenna, if is advantageous.  It is a good trade-off only if it can be monitored 24/7, or most of the time, and especially starting one hour before dawn, and again starting one hour before dusk as these are the most likely times of an attack. Yet an attack can occur in broad daylight, as well.  If the receiver must be portable, then use the best attached, or external antenna possible on an handheld, such as the much long in length Nagoya 1107, or a similar attached antenna, and use the best and most appropriate antenna on the sensors possible so that a portable receiver can reliably hear it.  We may also use this handheld to communicate on the same frequencies as the sensor to further alert and coordinate a response.

I suggested at least two or more sensors be used in conjunction at extended ranges. If the sensors are at extended ranges, or close in at the perimeter, either or both sensors, set up in this way to be received by a handheld receiver, that directional antennas might be necessary only on those sensors placed at extended ranges.  One might avoid the additional expense by using several external antennas and receivers at the various locations on the property where they can monitored.  If it is to be monitored by a small dedicated security force, the sensors can be monitored at the listening post/observation post (LP/OP), using an appropriate antenna at that location.  As an aside, the team at an LP/OP can also use a scanner to monitor the most commonly used frequencies in your AO.  This can improve situational awareness.

How Directional Antennas Work to Extend the Range.

The use of directional antennas is not only for the purpose of transmitting a strong signal in a way similar to that of a concentrated beam that casts light from a flashlight, but also acts like a device that collects and magnifies an incoming signal.  By using a directional antenna on the transmitter, and receiver, the reception is amplified naturally, and at both points.  This situation is roughly analogous to two flashlights pointed at each other if they are each sending signals.  But what if only one site is sending and the other is only receiving?

A rough estimate of the potential increase in magnification of the combination of these high gain antennas can be estimated by looking at the gain of each antenna.  If a 5-element yagi that has a gain of 10.5 dBi, and is used with the Dakota Alert sensor, or any transmitter that transmits with only 1 watt, the ERP could be as high as 6.8 watts from the sensor.  If the receiving antenna also has a gain of 10.5 dBi, then the signal received is amplified by the antenna to be increased by the same 10.5 dBi of gain in much the same as a radiotelescope magnifies the weak signals from a distant star.  If the 6.8 watts is transmitted in very close proximity to the receiving antenna of the same kind, and otherwise in ideal conditions, it might be received as if it were close to 46 watts ERP.  This would be the hypothetical best case.  At extended distances, a more realistic estimate in actuality might be that the 6.8 watts ERP is transmitted, would arrive at the receiving antenna with a low signal strength of less than S-1, or to better illustrate, we’ll use a numerical representative value of say, 0.5 watts. The receiving antenna that has a gain of 10.5 dBi, would magnify that weak signal, and deliver it to the receiver with a signal strength that would be the equivalent of 3.4 watts. That would be plenty. So that the reader can use their own assumed factors, I suggest using my favorite ERP calculator. Enter the transmitter power in watts into the cable loss calculator and use the result in the ERP calculator.

Even if my math is only representative, the idea that a high gain antenna can magnify an incoming signal is valid.  As we might not be able to afford to install high gain directional antennas on all transmitters, yet it could be justified by installing one such antenna at the base station, and we would use the less expensive high gain omnidirectional antennas, such as a J-pole that has a gain of only 2.1Dbi, on the transmitters.  Or perhaps the manufacturer-provided antenna would prove to be good enough.

How to Maximize the Use of Omni-directional Antennas

A gain of 2.1 dBi of the J-pole, or if using a Slim Jim with an advertised gain of 6 Dbi is useful as more than offsetting the loss in the coaxial cable used.  However, the gain of the antenna is not as important as the length of the radiator and its design, or how it propagates.  Most important of all, is the height of the antenna.  1 watt is propagated efficiently by a 1/2 wave J-pole antenna, is to near its full potential, and better than a 1/4 wave antenna that has no gain.  Antennas shorter than a 1/4-wave antenna are less efficient and arguably have a negative amount of gain.  Gain is a  relative measure to that of 1/4 wave antenna, or ideally an isotropic radiator.  But we do not need to go there in this article.

As a rule, the longer an omnidirectional antenna is, the better that it sends out the available transmitted energy.  A longer in length antenna with an SWR of 2.0:1 is a better antenna, because it sends a signal farther, as compared to a shorter antenna with a perfect SWR of 1.0:1. A shorter antenna with no gain can be a better antenna if is mounted higher than a longer antenna with more gain. The old hame adage is true: “height is might.”  If a person cannot afford a high gain antenna, then install the available antenna as high as possible and the performance will be greatly improved, or possibly be even better than a high gain omnidirectional antenna that is mounted close to the ground.  The main reason that ‘height it might ‘ is that the antenna can avoid propagating into objects that are nearby.  A very high gain antenna will not send a signal through a metal building, yet a no gain antenna mounted higher than the peak of the same building will send the signal far beyond the building, and thus it is a better antenna.

However, if we need to get the signal over a hill, aiming the beam of a high gain directional antenna at its crest, puts as much power as is available where it is needed most to refract the radiation over the top and beyond.  It is more likely to be successful than the best omnidirectional antenna that is mounted as high as possible, yet not as high as the crest of the hill.  We can also bounce a signal around a corner using a directional antenna, if there is a rock face or outcropping, or a large enough metal object available.  Signal strength is lost, but the signal has a chance to get to the receiver in the process.

Antenna Height and Horizontal Polarization

Transmitting 1 watt through a good external antenna mounted at least 15 feet high, or optimally, 50 feet high over the average surrounding terrain, can in itself triple the potential range, not simply because it is not only radiated better, but because the propagation avoids nearby objects that will obstruct, or attenuate the signal, and because the curvature of the earth can cause the signal to run into the ground.  If this same original 1 watt signal is received by a high gain directional antenna that has a gain of 10.5 dBi at extended, or extreme ranges, this antenna would receive any signal as if it were up to 6 times stronger than sent.

Yet if it were not installed high enough, objects on the ground would weaken the signal as it traveled.  If the terrain is heavily forested, a horizontally polarized signal travels further as it avoids vertically standing trees by passing around them. A VHF or UHF antenna, if it cannot be installed high enough to clear treetops and their foliage, can do better if it is horizontally polarized, rather than vertically polarized. A vertically polarized signal must penetrate through the woods, whereas the horizontally polarized signal can avoid enough of the woods to arrive at the distant receiver stronger. VHF is much better at making its way through pine forests than UHF.  Fortunately, the MURS frequencies are VHF.  Many other similar sensors are UHF, and cannot accept an external antenna.

If the terrain permits, investing in a lower-cost directional antenna from Arrow Antennas, or some other vendor that provides this antenna tuned for MURS frequencies, 151 to 155Mhz, such as this source ($76.00) could be advantageous.  Security will be job one, and this investment can provide service for years to come. Rugged J-poles might be purchased.  Learn how to create an air balun, and how to insulate and otherwise install this antenna correctly by consulting KB9VBR’s website.


There is much more to be learned about these sensors that will improve their performance. Extending the range increases the value of the sensor.  These sensors, if used to their potential, can be extremely effective.  If properly deployed and maintained, and their limitations understood and compensated for, this is an electronic security force that does not eat, and does not sleep.  It is very difficult for a threat to avoid detection by a Dakota Alert, day or night, if it is properly installed and maintained. Every additional minute of early warning greatly enhances our readiness to meet a deadly threat. The element of surprise can be removed from a threat, and transferred to the defender.