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.
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.
And here is how to use an SWR meter when making antennas.
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.
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.