Optimizing MURS Dakota Alert Sensors

by Tunnel Rabbit
SurvivalBlog Contributor October 12, 2019

Editor’s Introductory Note: This essay describes one approach to optimizing the performance, extending the range, and securing the signal of MURS band Dakota Alert intrusion detection sensors–and other low power transmitters.

—

To begin, here’s some ground thruth on perimeter security: Security will be Job One. Everything else supports that objective. Manpower for most tasks will be greatly lacking. Every trick, hack, or tactic should be considered. If we don’t see’em, hear’em or smell’em coming, then it is over before it starts. You lose.

Organizing with your community is the best defense for those without their own manpower. Defend at a distance, not at the mail box. Don’t let them into the area in the first place. Thus, potential intruders will be reduced to a few potentially lawless neighbors.

One of the best plug-and-play force multipliers–often recommended by JWR–is the MURS band Dakota Alert system. We all should know about those by now. If not, then please see a video on it.

Although only a 1/2-watt transmitter with a limited range, external antennas can greatly improve the distance at which Dakota Alert sensors can be deployed. I make my own antennas, but I cannot beat the least expensive and most effective external antenna on the market.

When ordering these antennas, request the longest continuous cable length of at least 15 feet or more, if possible, specify the cable end, and the frequency it should be tuned for. In this case, a good center frequency should be 153.500 Mhz. A less expensive DYI antenna is a dipole made from RG58 coaxial cable, but you’ll need a Standing Wave Ratio (SWR) meter. The least expensive one on the market is the Workman 104.

I can make one of these antennas inexpensively by cutting a 20′ cable with two PL259 ends, and make two antennas for about 10 bucks each or less. Here is how. https://www.youtube.com/watch?v=N-rUeIMYXbg

And here is how to use an SWR meter when making antennas. https://www.youtube.com/watch?v=qSea5FjcTDEt

Milking Extreme Range

To squeeze every bit of range out these, put it on a yagi, or moxon antenna up high as possible, and install it parallel with the ground, in ‘horizontally polarized’ configuration, and point it at the crest of the hill that is in the way. This can be done to an equal effect using a vertically polarized yagi, or even using a slim jim that has a narrow take off angle or compressed pattern–hence additional gain. The signal will diffract at the ridge line, and may or may not bend enough to get the signal to the receiver. The signal will not be heard on the opposite side at the base of the hill or obstacle, but can be heard further away from the diffraction point.

A five element yagi has at least 7dBi of gain at practical heights, and will boost the 0.5 watt signal to an estimated radiated power (ERP) of almost 1.5 watts, if the cable is shorter, and if connections are not too ‘lossy’. If the transmitting antenna is horizontally polarized to maximize the range through thick forests, and to diffract over obstacles in it’s radio line of sight, and to reduce detection, then the receiving antenna must also be horizontally polarized.

To go to extreme ranges, the receiving antenna can also be a directional antenna! To save money, convert an old VHF TV antenna to be used on your receiver. Directional antennas are very hard to direction find (DF). The larger the front to back (F/B) ratio, and narrower the radiating beam, the better. Using lower power transmitters on a yagi, and signal received by another yagi, is stealthy way to secure radio transmissions. The ERP of a 1 watt hand held would be about 3 watts out of a 5 element yagi. If transmitting signal is horizontally polarized, the signal would be attenuated by 30db, if intercepted by a vertically-polarized antenna. This is whisper quiet in radio terms. If heard, the signal strength will like be below S-1 and more difficult to DF.

Home Brew Antennas

I make my own OWA (optimized wide banded antenna) 6 element yagis that are wide banded and direct connect, so no matching device needed. A moxon is much easier to make, however it has less gain and casts a broad 180 degree RF pattern, much like a 2 element yagi. However it has a fantastic F/B ratio. It is more wide-banded than than any yagi, and is also direct connect. Face the back side of a moxon toward the intercept station, polarized horizontally and you will be very, very quiet when using low power.

One can transmit directly at a intercept station with lower power, and not be heard if you know what you are doing. But please do not try this at home, or in a war zone, unless you are mobile and have no other way. If ‘shooting this signal’ by their antennas, then you must move asap at least 500 yards. Special order a yagi antenna from Arrow Antennas. This is a multi-purpose hand-held version.

Hoist any antenna as high as possible and the range could be improved as much as three times–as the terrain permits–or more if the antenna is above and is ‘clear’ of a previous obstacle that once blocked the signal. Using a high gain 5/8-wave folded dipole omnidirectional antenna, a slim jim, I have had reliable results out to 6 miles in ideal terrain. Place them along the road as far away as possible in pairs separated by 50 to 100 hundred yards. Used in pairs we now have ‘gates’ that allow the user to determine, direction of travel, number of vehicles, and speed of their approach. This also provides redundancy.

Redundancy!

Do not rely entirely on any electronic sensor. Rain causes the sensor to become somewhat unreliable, and can produce false alarms, or no alarm. The best way to deploy sensors is in pairs. Higher voltages improves the range and reliability in the cold and wet, yet that sort of alteration is not available to all. There is a number of simple ways to improve the power supply to increase range, reduce false alarms, and avoid frequent battery change, making the sensor generally more reliable.

Situation: The unit functions well, with no ‘false alarms’, for awhile, then begins to ‘alert’ frequently, and repeatedly issuing ‘false alarms’. According to the manual this the unit signaling that the batteries need to be replaced. Lithium batteries are usually worth the expense in this application. But if not, install fresh alkaline batteries, and see if it stops putting out ‘false alarms’. If it does not, then reinstall the ‘old’ batteries, and see what happens. If I recall correctly, when the 6 batteries only provide 7.2 or less volts, the unit will signal with a low battery alert. Eneloop batteries when fully charged will test at about 1.45 colts, versus fresh alkaline batteries that will test at 1.5 to 1.6. Rechargeable batteries do not last as long. If using the unit in the cold climates, battery life is also greatly diminished. So I would use the lithium type, if necessary.

Power Supply Options

There are several way to power these sensors. If using rechargeable batteries, one way is buy an aftermarket AA battery holder that has place for more than 6 AA batteries. To extend the time that rechargeable will power the unit, add one more rechargeable. For spots for any extra batteries that the holder can hold, you should have dead batteries wrapped in aluminum foil to serve as ‘place holders’. These place holders provide no power. The unit will tolerate more than 9 volts–perhaps even briefly 12 volts, but 12 volts may reduce the life of the unit. If we have a rechargeable battery with voltages of 1.4 volts x 7 = 9.8 volts, as compared to 6 fresh alkaline with 1.6 volt x 6 = 9.6 volts. Thus, by adding one more rechargeable battery we are near the same starting point as the set of alkaline batteries.

Another trick is to use a small 12 VDC SLA (sealed lead acid) battery, or car battery and a universal voltage transformer set to 9 VDC. The battery can be charged with a 10-30 watt solar panel using an inexpensive charge controller.



This 30 watt panel is the most cost effective and good for cloudy conditions, although ‘overkill’ for this application The panel has other uses.

Perhaps select a less expensive 10-watt panel that could be connected directly, and spare the expensive and complication of a charge controller, but only if it is used to charge a larger and standard 12 VDC automotive or marine battery. A marine battery, BTW is not a deep cycle battery!

This could be done with units that are set up at longer distances. Use an old and failing car battery that has a ‘bad’ cell or two, and tests between 8 to 10 VDC and power the unit directly. If charged by a PV panel, a universal voltage step down transformer should be use to limit the voltage. PV systems will produce 14.1 to 14.6 volts at the battery. There is often plenty of capacity at that voltage, even in cold temperatures to run these sensors that sip power, and an old failing car battery would have an extended life if supported by small PV panel that does not need to be regulated and can be connected directly.

—

MURS band Dakota Alert systems are very useful, but they often frustrate the user. Read up. It would be a huge loss if one could not operate their sensors correctly. And just like anything else, take one out of the Faraday cage and use it for awhile to gain experience with it. In a time of stress, getting a ‘false alarm’, could be nerve racking, when all hat it needs is batteries. In a pinch, but only in a pinch, would I connect directly to 12 VDC. With higher voltages they are more reliable in the rain, and cold. My first choice would be to add an ‘extra’ rechargeable AA battery, or two sets of 7-battery packs, wired in parallel to double the ‘run time’ when using rechargeable, or 6 regular alkalines, if the lithium type were not available. Just keep the voltage below 10 VDC, to be safe. The most stable source of power would be the 12 volt car battery with the universal voltage adapter set to 9 VDC. So as not to ‘waste’ a good car battery, use one with a bad cell that no longer can start a car. Once again, here is an inexpensive 10 watt PV panel that can be connected directly to that battery.

Especially if a PV panel is used, be sure to use a universal voltage adapter. And always confirm polarity with a volt meter before connecting.

Of course, placing these along public road may not appropriate during normal times, they might be stolen if not properly camouflaged. Yet they might be deployed on a temporary basis for testing. Find out now useful is this method. For instance if one has a dirt road 1 mile long and the maximum safe speed is say 35 mph, then the vehicle may need 6 minutes to arrive at your location. 6 minutes is a lot of time when you are in a hurry. If one can hear the sensor from 2 miles away, then the likely time of arrival could be 12 minutes. Set this up now if possible and learn to listen and interpret the traffic coming and going. It is surprising how effective this can be. If on a spur off the main dirt road, placing a sensor at the point of intersection also makes sense.

Stealthy Sensor Deployment

A stealthy way to deploy them on trail is to set them on the ground and bury them in debris. An external antenna can assist, as the unit can then be placed into places that where it would be otherwise impossible to have an attached antenna. The external antenna can also be mounted higher for better range. “Height is might” as they say. Don’t worry about cable loss if you do not have enough height. Enough height to make reliable communications easily offsets the loss in the ‘excess’ cable used. It might be nice to have a roll of LMR 400, or RG8, but quit dreaming. It is on UHF that the loss can be noticeable. Feeding a high gain yagi with lossy cable, and the loss is offset by the high gain of the antenna. It is about the ERP.

For the best results, this roll up type of Slim Jim from N9TAX is easy to hang with a light weight made of a non metallic material, or use cordage to create light tension, so that it is strung to it’s design length. Attempt to space the antenna way from the tree or pole at least 19″, and on the side facing the receiving antenna. It is not difficult to do.

This antenna has a broad bandwidth of at least 8Mhz on the VHF side and should cover the Business Band, Public Service Band, and Marine Band as well. Ask the manufacturer. The UHF is good for GMRS, and has the narrow band with of only 4 MHz. Ask if it will cover the UHF business band. If not in use on a sensor, this antenna can also be used elsewhere.

Connector Confusion

Verify what type of connector is on your sensor. Older models use the BNC type, so select the BNC Male option. The recent production Dakota Alerts use SMA connectors. I cannot at the moment verify which, so talk to the vendor, or research it. These take either the SMA Male, or the SMA Female. These antennas also work well with standard radios, mobiles, or handhelds. Adapters are available on the site. The Baofeng UV5R takes a SMA Female. Mobiles typically use PL-259 cable ends. An SO-239 (aka. UHF) to SMA Female adapter for the Baofeng UV5R, allows this and other antennas to be attached to the Baofeng UV-5R. Transmitting on a Baofeng on a homemade copper Slim Jim on 70cm, I could hit the Blue Mountain repeater 80 miles away with ‘full quieting’ with only 4 watts. The Slim Jim is a good antenna for many applications, but not all. And it is great for scanners.

For those wishing to mount an antenna on a roof top there is far better antenna. These are what I make and have tested over the years: A Slim Jim antenna suitable for the MURS band.

Cable into the house should have PL259 ends, that are sold separately. Use a UHF-to-SMA Female adapter for your Baofengs.

Of course the less expensive option is to make a dipole and hang it high as possible and at least 19 inches from any parallel surfaces that when wet can couple with the antenna, hence de-tuning it.

I almost forgot: To increase the range of the sensor, the first and least expensive thing to do is to install an external antenna on the receiver side. I would get the copper pipe version for this purpose. Of course a 1/4 wave antenna will work too, not as well, but it is cheap to make a receiving antenna that is not tuned and good for transmitting. This can be done use common 75ohm cable television coaxial cable. Just separate the inner wire from the braided shield, and have 18″ of length for the inner wire, and leave 18″ of the braided wire hang and tie the inner wire up. Connect the inner wire to the receiver in several ways. The total length 36 inches. I would not transmit on this antenna because it is not tuned, or any antenna, even if tuned, because it would greatly increase the range at which I could heard, unless there was no other way to establish reliable communications.

Commo and Sensor Goals

We want to limit the range, by using the lowest power or ‘smallest’ antenna we have to avoid detection, yet effect solid comms. However, we want to hear as well as we possibly can, and that means using an external antenna. We could therefore hear each other better, and yet limit the range we broadcast, if we use an external antenna to ‘hear’, and the antennas on the Baofeng, to talk. This set up on all stations, improves reliable comms, and reduces the chance of detection. We can also better listen for threats that are using low powered radios, and any radio at distance.

Well to get back on topic, first install an external antenna to listen to Dakota Alert sensors. And if that does not provide the range needed, then install an external antenna on those sensors that have difficulty reaching your receiving antenna. This is the sensible way to get it done, rather than installing external antenna on all sensors. The one advantage of installing antennas on all sensors, is that receiving station that is a hand held, would have a better chance of hearing all the sensor should they beyond the reception range of a tiny Baofeng antenna. Test the reception of both the fixed receiving station, and the mobile hand-helds at all locations within the retreat, and install antennas as needed.

Other Uses

The techniques discussed herein can also be applied to a community radio network. Listen to your neighbors low-powered radios with external antennas, yet transmit on your low powered radio to their external antenna. And then we can get horizontally polarized antennas involved! The cost of making a good receiving antenna is next to nothing, and a lower RF (radio frequency) footprint will increase COMSEC, and greatly lower the chances of the being located with direction finding methods.

Using the Baofeng dual receiver function, means one can use split frequencies, where one talks on one frequency and receives on another, causing the eavesdropper to potentially miss one side of the conversation. But this topic is beyond the scope of this article. However note that if one can master this technique, the number of frequencies combinations that are used for TX/RX, compound and forces the intercept station to use lots of high speed scanners to monitor huge chunks of bandwidth to monitor both sides of the conversation. For every high dollar, high tech threat, there is usually a 25 cent low-tech countermeasure. But more on this later…

Here is a video showing how to make a cheap and easy transmitting antenna if tuned: How To Make Coax Antennas (FM receiving) Folded Dipole. These can also be used as receiving antennas that can be mounted above your roofline using a plastic pipe or pole, using RG 58 or better 50-ohm for transmit/receive, or using free-to-me 75 ohm, RG59 or RG6, cable television coaxial cable. This cable is good for all receiving antenna projects, and there are hundreds feet of it laying around. I even use this stuff for transmitting antennas for VHF, and HF, even though that is against the rules. It works just fine if you know how to cut to 1/2 wave lengths to avoid the impedance problem, or speed bump. A bit tricky, but it works great. Standard F-Type cable television connectors can also be used. As a radio guy, using this repurposed television cable, one can “live off the land”, so to speak. And with little or no cost I can afford to mount hidden stationary TX/RX antennas along patrol routes. There is so much more that could be discussed on this topic. The best cable to have around is RG8X for most applications. Definitely use the heavy stuff for UHF if you need to run beyond 20 feet.