(Continued from Part 1.)
Broad-banded antennas are very useful. However, if forced to fabricate an expedient antenna, and we only had an antenna capable of 462 to 463 Mhz, we would be in business as the channels that come in the commercial radios are 1-7, and 15 to 22, are GMRS and are within the 462 Mhz to 463 Mhz range. FRS channels are 8 thru 14 and are between 467 and 468 Mhz.
Material requirements are much less for a J-pole, and these antennas can be made to be nearly indestructible. A larger-in-diameter radiator will typically have broader bandwidths. 3/4 inch copper pipe is better than the 1/2 inch copper pipe for this purpose, yet the larger pipe is more expensive and relatively hard to locate. FRS radios use 467 to 468 Mhz. FRS is transmitted using only 500mw (1/2 watt, mw = milli watt) or less Effective Radiated Power (ERP). Usually, the antenna on the handheld FRS transceiver is so poor that the ERP could be much less than the maximum allowed by the FCC. The actual ERP might be less than 1/4 watt (250mw).
Explain to neighbors not to use FRS frequencies to communicate if they wish to communicate at further distances. FRS channel 8 thru 14 should be avoided for this reason. Set aside these FRS very low power channels for in house use to increase security/privacy. It is also illegal to transmit with more than 500mw ERP on FRS frequencies. Fortunately the FCC recently updated its rules. Take note that current production Midland and other GMRS/FRS radios, can now transmit up to a maximum of 2 watts on GMRS frequencies without a license. Those who are licensed GMRS operators can use 5 watts in handhelds, and a maximum of 50 watts with mobile transceivers. The no-test license fee has recently been reduced to only $35 for a 10-year period.
Antenna Designs and Materials for Austere Settings
Soon — or eventually — we will all face a problem that will, to use radio terminology, ‘attenuate’ our habit. The guerilla antenna builder, could face shortages of materials, and higher prices for available materials that eventually become unobtainable due to either price or availability. And then, magically, these materials will at some point become transformed into ‘unobtainium’. Just like silver is poised to rocket in price, the supply of silver solder could become ‘unobtainium’, even if it is only .01 percent silver, and the rest antimony or tin. He is then forced to hit the trash piles and use lead-based solder. Or no solder at all. Or he might resort to smelting bullets into thin sticks to be used as solder. As the pile of preferred materials dwindles, there is the hard choice between building an affordable antenna and buying more bananas. So the Guerilla must become creative. A wise Guerilla is always looking for more bananas, and if he is a radio nut, he is always looking for innovative ways to feed his other habit.
In the following links are some detailed instructions that show making dirt-cheap antennas. This kind of thing works, and allows one to learn by trial and error, and learn about antennas on the cheap. The difficulty of these ‘projects’ is moderare.
Video example for a 2 Meter dipole:
Directional Antennas That Improved COMSEC
For the do-it-yourself (DIY) types, or non-radio geeks, I recommend the Moxon antenna that can be made or purchased in completed form from Sal Electronics. Or we can purchase or make a yagi from one of the links listed. For geeks like myself, I recommend making a 6 element OWA yagi that can be designed to be used on 2 Meter, and the first 3 MURS frequencies with an acceptable VSWR for low power applications.
Here is an excellent demonstration video of how a yagi antennas works: Directional Antennas.
You can use 1/2 inch metal tubing to construct a Moxon or the 6-element OWA yagi. Careful tuning can result in an SWR of under 2:1 from 144 Mhz to 155 Mhz. Design frequency should be 149.500, or thereabouts. Unless strict production standards are possible, each antenna is unique. Or, it could be tuned to operate best from 150.500 to 160 Mhz, a range that includes the Marine Band.
Of course, any yagi can be used to good effect, but it could be limited in its bandwidth and therefore its utility. Look up the detailed discussion and design by L.B. Cebik for the OWA yagi. Begin by reading Cebik’s PDF. This design is as broad-banded as the Moxon, and has a higher yet modest gain of only 9.2 to 10.3 Dbi at its peak. It has a narrower footprint that could be essential for a point-to-point communications circuit. It is a compromise antenna that sacrifices some gain for bandwidth.
Both designs have exceptional broad bandwidths and have useful high front-to-back ratios, and are 50-Ohm direct connection that makes them easier to build. Because we are using an unbalanced coaxial cable to feed a balanced dipole, we will need a balun, or air choke to prevent common-mode currents that alter the resonance of the antenna. This is one aspect of building Moxons that can be frustrating, since an air choke or balun must be used. If your antenna build behaves badly, and no discernible cause can be identified in the physical construction,mthen improve the choke or balun. Here is an excellent introduction to the Moxon by L.B. Celbik himself.
Choice of Polarization
As I’ve mentioned in previous SurvivalBlog articles, horizontally polarizing an antenna, be it a J-pole (See: J-pole calculator), or a directional antenna decreases the likelihood of being heard to the front and the rear as the signal from a horizontally polarized antenna is attenuated by around 20 dBds when it is received by vertically polarized antenna. A 1/2 inch copper pipe Moxon is outstanding in performance and ruggedness. (See: Moxon calculator.) Again, additional attenuation occurs when horizontally polarized and is realized for both front and reward propagation.
Tune a Moxon for a ‘center’, or a ‘design’ frequency of 152Mhz, and the harmonic is close enough that it will likely also transmit on 462-465nMhz that include GMRS frequencies. However, the RF pattern would not be the same as if it were used with the design frequency. This means you can talk on MURS, and GMRS on the same Moxon. Same with the 2 meter variety, 70cm, above 430Mhz can be used. And there is 10 megs (megahertz) of usable bandwidth. A Moxon is ideal in terms of being easy to build, exceptional bandwidth, broad area coverage to the front, 100 degrees plus at closer ranges. And it has a very high front to back ratio (F/B ratio) that blocks the transmission to the rear and blocks incoming signals from the rear. For my purpose, its high F/B ratio is more important than its gain of only 5.75Dbi. Its modest gain is actually an advantage in my book if its purpose is to contain the signal within an AO.
As an additional method for controlling and containing a signal to the front and the rear, I would point the directional antenna toward a nearby mountain to block its propagation or tilt it downward so that most of the RF goes into the ground, and where it is needed most, and not where it is not needed. Conversely, when orienting the front toward the ground, the rear of the antenna pointing more to the sky, sending RF upward and away from the horizon. This is a double plus good effect, that works when a Moxon is used vertically polarized, but not as well when horizontally polarized.
Yagis also work better this way when horizontally polarized. In this way, we are limiting its forward propagation to hopefully be mostly within the AO (Area of Operation) only. Moxons are desirable because the F/B ratio is an extraordinary 18Dbd at the edges, and up to 40Dbd at the null (center of the rearward pattern), if the transmission or incoming signal to be received is on or nearest to the design frequency. An average F/B might be 30Dbd, but I’ll use the lowest measurement, 18Dbd. If we rotate the antenna so that it is horizontally polarized, and the attenuation is increased about 20Dbd. 20Dbd + 18Dbd (F/B) = 38Dbd total possible attenuation. It is likely less, yet the numbers are good for comparative purposes. Nearby objects can reflect forward propagation to the rear, negating some of the advantages of a high Front to back ratio. Operated above the average terrain and certainly above nearby and potential objects that might reflect a signal are necessary considerations to optimize the advantages of using a directional antenna. Use the sharp 40dBd null in the rear to obtain a rough bearing.
Gain Versus Stealth
It is important to understand directional antennas. Gain is usually the primary motivation for using a directional antenna, yet if our goal is to limit communication to a select few as possible, then no gain to the front is actually preferred, if it were possible. A 5-watt transmitter would have an ERP, given a gain of 5.75Dbi using RG8x. would be about 9.16 watts after coaxial cable line loss is deducted. The signal could travel more than 20 miles. If the signal that is transmitted to the rear is attenuated by 38Dbd, then the ERP of the signal out the back would be around 0.001 watts. Again, in reality, the signal can bounce off reflective objects and change polarization, so the math is good for comparative purposes only. We must properly install, or understand the limits and problems that the terrain presents. If not mounted high enough, more signal finds its way to the rear than expected. If one used only 1 watt through a Moxon, the ERP out the front would by only around 2 watts after line loss is deducted. That is just enough to be heard out the front, but with nearly zero signal out the back, and there is less signal strength that can be reflected, and scattered to the front and rear.
Lower power is our friend. I say again: low power is our friend. Used in conjunction with a brevity code, this, and other low tech means and methods are formidable means to avoid interception. If there is less chance the signal is intelligible, there is also less need for sophisticated codes and encryption. If we can avoid detection, the potential interceptor does not know, what he does not know. This situation, fully appreciated, might be the needed motivation to work harder, and to master the use of directional antennas.
Reception and Crude DF
There is more that can be done with directional antennas. Receiving: If we are scanning, we could also use up to 4 Moxons, or the similar broad patterned 2 element yagi to listen in the 4 cardinal directions, or along the azimuths of likely avenues of approach that typically are roadways. In a remote location, this could mean we only need 2 directional antennas, one that is pointed opposite of the other, and along the major county road. Because the Moxon has a much superior and useful front-to-back ratio that rejects signals from its rear, it is a good choice for this application. A 3 element Yagi design for fox hunts and direction finding, would be a good alternative.
These are easier to build, yet sturdier construction is necessary.
Or we could get by with just one Moxon, and one omnidirectional antenna. If the receiver connected to the omnidirectional antenna hears a transmission, but the Moxon does not hear it as well, or not at all, then by deduction we conclude that the signal is not coming from the direction of the main avenue of approach. In this way we have instantly determined a rough bearing without the use of more expensive equipment and minimal training for the attendant. This is in my book, a good investment of time and money, as we can shift our defenses given a known direction of approach and transfer the element of surprise from the attacker to the defenders.
If we might have $1,000 invested in transceivers, we might be able to justify an equal amount in useful antennas, antennas for all occasions, and the many radio services, and have spares on the shelf. In the event that I am left with only an old and dusty transceiver of any kind, I can build a suitable antenna for the job, even if I only have scrap metal to work with, discarded cable TV coaxial cable, and a tape measure. In a worst-case scenario we would need to adapt and overcome, and the more we know, the less we need. It is almost “show time”. So have at least one directional antenna.
(To be concluded tomorrow, in Part 3.)