The use of Directional Antennas are the most basic way to improve communications security even if all you’ve got is a set of inexpensive radios. In Part 1 we talked about the theory of use and why they’re important to have for the dedicated RTO of a small unit. Sending your signal in one direction versus all directions does a couple of things for us- creates security through only sending a signal along the necessary path and second it greatly improves the range along that path. In this section we’re going to discuss the antennas themselves and how they work.
Remember just a few years back when every house had those funny antennas on its roof? The ones you had a rotor (or if you were poor you had an set of big channel locks on the pole) to turn to get a better signal on the TV stations you wanted to watch? They’re mostly gone these days but that was a type of Yagi antenna.
Named after two electrical engineers in Imperial Japan during the interwar period, Yagi and Uda, the antenna was a solution to two problems. First, it sent a signal in one direction much further with a lot less power. Second, it listened much stronger in that same direction. It enabled directional communications links with even the very crude radio gear of the day, and found itself in service with the primitive radar systems just coming into use.
Yagis work through having a basic dipole (called the driven element) connected to the radio, with one dipole slightly longer just behind it (called a reflector) and one or more dipoles out front (called directors) that have no electrical connection to the driven element. They work on magnetic harmony- in other words, the reflector reflects the energy from the driven dipole forward, and the directors further pull that energy in the forward direction. The more directors you have, the tighter the beam. In addition, the more directors you have, the higher the decibels of gain you have in that direction- keep in mind that with each 3db of gain, you’ve effectively doubled your radiated power output in that direction. Since gain is in orders of magnitude, for each additional 3db you’re doubling your gain again.
Wait, what? Think about it like this. If you have a three element Yagi antenna, like the one picture above in from the Advanced RTO Course, which has 7.5 decibels of gain, you’ve taken the 4 or 5 watts from a handheld and increased its radiated power to over 20 watts just based on the gain of that antenna, in the direction that antenna is pointed. You’ve taken a very basic tool (the humble handheld) and made a potent communications device while using the same amount of battery power. Not bad. Not bad at all.
But the Yagi also has gain on reception, meaning it hears better in that given direction over a single omnidirectional antenna. That’s why going back to our old over the air TV antenna example above, you’d rotate the antenna towards the direction of the TV station, and why hams use the same antennas on towers to listen in a given direction. Its a heck of a lot of capability in a relatively small package for line of sight (VHF/UHF) use. And since they’re small, they should be part of every patrol loadout.
Similar to the Yagi, a Moxon is a directional antenna that’s wildly simple to build. The best way to describe it is a Yagi with the director removed- a driven element and a reflector. So rather than a tight beam LED flashlight radiation pattern, these are more like a broad maglite. The antenna direction itself is pretty broad compared to the Yagi, but where these really shine is through their simplicity. With just a few pieces of wire and some material for the frame, you can build one in less than an hour and have a directional antenna ready to roll for a team. Check Moxgen for a downloadable program that gives all of the measurements you’ll need for cutting the wires.
Longwire and Resistor
The last directional antenna is known as the Longwire. The Longwire antenna was best known to the Vietnam generation as LRP team members used them for directional communications back to their bases of operations while evading the Signals Intelligence assets of the NVA. As the name would imply, it’s one really long wire that runs to a resistor placed in series with the wire and then driven into the ground. The long wire itself radiates with the ground end running along the earth ground. The resistor works to pull all of the current in its direction and with it most of the radiated energy.
Of each of the antennas discussed, the long wire is by far the simplest and most clandestine when used. And it also presents a very low Standing Wave Ratio (SWR) due to the residual energy being pulled to the resistor at the end of the line. Very little gets reflected back to the radio itself, But most of all, it packs up the most compact and is the easiest to build or repair in the field. All you really need is a long run of wire and a high Ohm carbon resistor- just make sure you have a lot of them.
Summing It Up
These three antennas are each fairly easy to build on your own and present a huge advantage over omnidirectional antennas. The security offered through using directional communications is not to be overlooked. For a team of guerrillas communicating critical information, its the only option. But that said it takes a bit of work and training to get right. In part three we’ll be discussing exactly how to do that, covering the basic planning requirements and how to incorporate them into your patrolling equipment.
6 thoughts on “Directional Antennas For The Small Unit, Part 2”
When you say to have a lot of high ohm carbon resistors for the long wire antenna, does that mean they have to be replaced periodically?
If so, how do you tell when it is time for a replacement?
Only if you burn them out.
I made a three el yagi recently. Two pieces of wire coathanger and a section of 450ohm ladderline.
Reflector 13 inches
Driven element 10inches (ladder line-soldered at each end split in the middle on one side for soldeing on the coax )
Director 12 inches
Distance from reflector to driven element is 8.85 inches
Distance from driven element to director is 3.5inches.
Vswr is 1.3/1 at 446mhz
Took me 20mins. Duct tape the elements to a pice of wood and you have a yagi antenna in no time. I used a piece of plumbing tubing.
My experience of small walkie talkie type radios is that they can handle -bad- swr levels as they are designed to use the operator as half the antenna system.
Keep up the good work.
Quick- I got my measurements the wrong way round ( just went and looked at the actual antenna ) reflector to DE distance is 3.5 inches and distance from DE to director is 8.85 inches. My apologies. I posted whilst only one cup of coffee in….
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