Introduction to Antennas

Introduction to Antennas

Your Host:  Justin Pool

Introduction to Antennas

SCOPE:

Antenna placement is a subject of much conversation in the RC arena. Various methods and philosophies are used by many with success, so this is not intended to be the one-and-only way to do this, just a guide that may be used to initially setup an RC receiver in an airframe and expect a reasonable level of success. Differences in airframes, RC equipment, flying area, radio frequency environment, etc. may dictate different approaches, but by following these suggestions you can expect good initial results. Some RC gear has an RSSI function (Received Signal Strength) that can be used to fine tune the system to suit your needs.

There is much information available that covers all of the highly technical aspects of how antennas interact with each other, so no attempt will be made here to explain these mysteries, only guidelines that the average modeler needs to know to get the best performance from his/her gear, and hopefully, avoid unplanned trips to the workshop. There are some links at the end of this article for those that wish more in depth information.

THE RF LINK:

The ability to relay information from the transmitter to the receiver is called the RF link. Most modern RC systems use the 2.4 GHz band to carry the signal from the TX to the RX, and is generally considered to be superior to the older systems that used bands from 27Mhz to 72Mhz. One drawback to 2.4 GHz however is that the signal is easily absorbed by almost any material that can conduct electricity. The transmitted signal can be affected by local features such as hills, trees, buildings, the presence of broadcast antennas, even weather (think wet grass). This is one reason why you will hear the term “line of sight” or “LOS”, because the signal will be lost if there is not a clear path between the TX antenna and the RX antenna. In other words, if you cannot see your craft, then the receiver likely cannot “see” the TX signal either. The RF environment, usually consisting of other 2.4Ghz signals in the area, can play a part as well, although some systems seem to tolerate this better than others. The RF link will be strongest when the TX and RX antennas are parallel to each other, and weakest when they are perpendicular, and non-existent if either the TX or RX antenna is pointed directly at the other antenna.

I couldn’t find a picture or a drawing of a pair of parallel TX and RX antennae. However, if you can imagine that you are on a football field while you’re flying, and looking downfield at your aircraft, then the TX antenna and the RX antenna should both line up with a Yardage line, like this:

AntennaPic1

This, however, is how NOT you keep your TX antenna parallel:

 AntennaPic2

A FEW ANTENNA FACTS:

Without going into antenna theory here (trust me, you wouldn’t enjoy it) let’s just discuss some of the information that is relevant to understanding how to get the most out of our RC systems.

A typical antenna assembly will consist of a length of coaxial wire (coax) with a connector at one end, and a portion of the center conductor exposed (unshielded) at the other end. The figure below shows three antenna assemblies of varying lengths:

AntennaPic3There are other types of 2.4Ghz antennas out there*, but for the sake of brevity (and to keep this short) 😉 we will use this style as an example. The coax wire that the Antenna assembly is made from is constructed like this:

AntennaPic4The center core, which is usually a copper wire, carries the signal from one end to the other.   The metallic shield is there to protect the inner wire from transmitting or receiving unwanted signals. The dielectric insulator has special properties, but it will suffice to know that it keeps the core and the shield separated. The plastic jacket provides insulation from the rest of the world.

As you can see from the above images the overall length of the antenna assemblies can vary, but the length of the active part of the antenna (the unshielded portion of the coax) is always the same. For the rest of this document the term “antenna” will refer to only the unshielded portion of the coax. The remaining portion of the wire (the antenna lead) is to allow the user some leeway in positioning the antenna assembly in the airframe. The length of the antenna lead can degrade the performance of the antenna, so using the shortest practical length for your application is recommended. No need to get all O.C.D. here, just be aware that if your antenna lead is arraigned in a loop inside the airframe or TX case that you may see better RF performance with a shorter lead.

Tx antennas are typically mounted inside of a plastic enclosure that is mounted to the top of the TX case, with a lead long enough to reach the electronics. The lead may be attached with a connector, or be soldered directly to the RF circuit board.

RX antennas simply exit the receiver case, allowing the user to route it to suit the airframe.

There is much that we can do to maximize our chances of maintaining a robust RF link between our transmitters and receivers, and that is the goal of this article. First we must understand the variables that are under our control, and then what we can do to stack the odds in our favor.

 

THE TRANSMITTING ANTENNA:

The ground based antenna (TX) must send the highest quality signal possible to the airborne (RX) antenna using the power allowed by regulations in that area. The main options here are the location and orientation of the TX antenna. Because 2.4Ghz is line of sight, we want to fly in a location with as much free space around us as is practical. A perfectly flat area with no buildings, trees, or commercial antennas, is preferred, but this is the exception, so use your judgment to select the best position for your area. Orientation is defined by the direction in which the TX antenna is pointing. When the antenna is pointing up or down it is called vertical orientation. When the antenna is pointing to the left or to the right it is called horizontal orientation. The orientation of the TX antenna will change with any movement of the TX as we fly, so be aware of your body language.

It is important to understand that the signal from the TX antenna radiates away from the antenna in a predetermined pattern. Of course you can’t see this pattern, but it is easy to imagine what it would look like if you think of a donut (mmm, donut), and imagine placing the donut over the antenna with the antenna sticking through the hole in the donut. Now imagine that the donut is big, really big. Now you have a pretty good idea how the pattern looks.

AntennaPick5

 

If the antenna is pointing straight up, as in the figure above, then the power will radiate out in all horizontal directions, forward, left, right, and to the rear, with less signal going up or down, as in the image above. If the antenna is sticking out to the left or to the right, then the power will radiate in a vertical plane, forward, up, down, and to the rear, with less signal going to the left or right sides. Remember that whatever direction the antenna is pointing, there is no signal being transmitted in that direction, sometimes called the null area, so you should avoid pointing the TX antenna directly at the aircraft.

If your radio system has a fixed antenna that points straight out from the top of the TX be extra careful to avoid pointing the antenna directly at the aircraft, as this will resulting in poor signal strength and possibly loss of control.

 

THE RECEIVING ANTENNA:

The airborne antenna (RX) receives the signal being sent by the transmitting antenna, and because the 2.4 Ghz signal is line of sight we need to be aware of any part of the aircraft that can obstruct the signal coming from the TX. Proper installation of the RX antenna is essential to maintaining the RF link. The airborne antenna has more variables to consider when mounting in an airframe, as it is generally surrounded by things that can affect its ability to receive the transmitted signals. Try to avoid placing the antenna where a piece of equipment (battery, RX, ESC), or any conductive materials (wire, carbon fiber, metal, fuel, etc.), may block the transmitted signal from reaching the RX antenna. Remember that your aircraft will change it’s orientation relative to the TX during flight, and as it does various parts of the airframe will come between the TX and RX antennas in various flight attitudes.

 

CHOOSING THE PRIMARY AXIS:

We discussed that the TX and RX antennas should be oriented parallel to each other for the best possible RF link, so our first task is to determine which orientation is best for our particular flight conditions. Imagine a typical flight, and notice the attitude of the airframe during the majority of the flight. Most aircraft, be they standard airframes, flying wings, or rotorcraft, will fly mostly in the “upright” position similar to when it is sitting on a table, so let’s start with this as an example.

 

VERTICAL ORIENTATION:

If we choose the vertical orientation for our receive and transmit antennas, then the “donut” pattern of the TX antenna would cover an area straight in front of the pilot, and also to both sides. The “null” area, where there is no signal, will be directly over the pilots head. The receive antenna in this example would also be oriented vertically, and be parallel with the TX antenna throughout most of the flight.

Picture the aircraft rotating about the vertical axis (yaw) and imagine what things may get in between the antennas. As an example, when the craft is on a heading directly toward, the pilot, the motor, and probably the battery pack, will be in the way of the signal, for fuelies this would be the engine and fuel tank. This is unavoidable, but can be minimized by keeping as much distance between the antenna and these sources of interference. Notice that as the aircraft antenna rotates it will still be parallel to the TX antenna, the optimum orientation.

Next picture the aircraft rotating about the horizontal axis (roll) and imagine what things may get in between the antennas. As the aircraft rolls it is likely that the only things that will get in the way of the RF signal is the servos in the wings, and any carbon fiber that is there. Notice that as the aircraft rolls, the antenna rotates in such a way that there are two positions where it will be pointing directly at the TX antenna and probably unable to pick up any signal whatsoever.

Now picture the aircraft rotating about the pitch axis as it would in a loop. If the loop is performed such that it appears as an “O” to the pilot the RX antenna will likely not encounter any additional interference, however it will at two points in the loop be oriented 90 degrees to the TX antenna, and may for a short time be unable to receive signal.

 

HORIZONTAL ORIENTATION:

Many aircraft simply do not have enough room to accommodate the vertical antenna placement, so let’s look at using a horizontal arrangement. If we choose the horizontal orientation for our receive and transmit antennas, then the “donut” pattern of the TX antenna would cover an area straight in front of the pilot, and also above the pilots position. The “null” area, where there is no signal, will be both to the left and to the right of the pilot. The receive antenna in this example would also be oriented horizontally, and be parallel with the TX antenna throughout most of the flight.

Picture the aircraft rotating about the horizontal axis (roll) and imagine what things may get in between the antennas. As the aircraft rolls it is likely that the only things that will get in the way of the RF signal are any wires that must pass along the length of the fuselage, or any control linkages. Notice that as the aircraft antenna rotates it will still be parallel to the TX antenna, the optimum orientation.

Next picture the aircraft rotating about the vertical axis (yaw) and you will see that the path for the RF signal will be blocked by various parts of the aircraft as it rotates, and also will point directly at the TX at two points in the rotation.

Now picture the aircraft rotating about the pitch axis as it would in a loop. If the loop is performed such that it appears as an “O” to the pilot the RX antenna will likely not encounter any additional interference, however it will at two points in the loop be oriented 90 degrees to the TX antenna, and may for a short time be unable to receive signal.

 

ELIMINATING NULLS:

Nulls are the periods when the either the TX or RX antenna are pointed at the other antenna, or are aligned at 90 degrees to the other antenna, such that no usable signal is received. As we saw earlier, no matter which way you decide to orient the RX antenna there will always be aircraft attitudes that can cause nulls, and loss of signal. With the exception of short range “park flyer” rated RX’s, most modern 2.4 GHz receivers have two antennas. The Receiver constantly monitors the signal quality of both receive antennas, and then uses the signal from the one with the best signal. There are two advantages to this arraignment, 1. It allows for the second antenna to be mounted at a different angle in the air frame relative to the first, so that there will be a better chance that at least one RX antenna to be relatively parallel to the TX antenna. 2. It allows the two antennas to be located in different areas of the airframe, so if one RX antenna is blocked from receiving the TX signal the other RX antenna may be in a better position to receive a clear signal. So how do we use this second antenna to its best advantage?

 

CHOOSING THE SECONDARY POSITION:

Now that we are accustomed to imagining the aircraft rotating through the three axis (roll, yaw, and pitch), and we are familiar with the positions that cause loss of signal, we can picture the aircraft with the first antenna in a bad position and choose the best position for the second antenna.

When we were looking at the vertical antenna orientation (above) when we imagined the aircraft rotating about the roll axis. Remember that the RX antenna will be pointing directly at the TX antenna when the aircraft has either the top or the bottom of the fuselage facing the TX antenna. To solve this problem we can orient the second antenna such that it will be parallel to the TX antenna when the first antenna is in a poor orientation. One example of this would be to mount the second antenna pointing at one of the wingtips. Installing the second antenna this way will compensate for when the vertical antenna is in a null zone.

Let’s review the horizontal orientation (above) where we imagined the aircraft rotating about the pitch axis and having a null when either the nose or the tail was pointing at the TX. Here we need an antenna placement that will be optimal when the plane is pointing directly towards or away from the pilot. This can also be accomplished with an antenna mounted so that it is pointing at a wingtip.

 

These are just two examples, and the real world mat demand a more creative approach, possibly with the antennas mounted diagonally in the fuselage or the wing. The only thing that matters is that you establish a RF link that suits your style of flying, and is robust for the environment in which you fly.

 

ORIENTATION SELECTION OVERVIEW:

Decide what your primary axis will be based on your flying style and needs. Do you want the TX antenna vertical or horizontal? Remember the Donut example above to help you decide.

Vertical orientation of the TX antenna will give the widest signal for flying at low altitudes, tilting the TX up will be helpful when flying overhead to avoid the null zone where the TX antenna is pointing directly at the aircraft.

Horizontal orientation of the TX antenna will give a signal that is more useful when flying overhead, but when flying off to the left or right, as in a landing or takeoff, will be aided by turning to face the aircraft to keep the TX antenna parallel to the aircraft.

 

AXIS SELECTION GOALS:

In selecting the primary axis you want to orient one of the receiver antennae in a location that will be as parallel to the TX antenna as practical in the most common flight attitudes. The secondary axis should be selected so that the second antenna will be in a favorable position when the primary antenna is in a null position, or at 90 degrees to the TX antenna, or when temporarily shielded by equipment or materials in the airframe.

When possible, orient the two RX antennas such that there is approximately a 90 degree angle between them and separation of the two antennas is maximized.

 

SELECTING A LOCATION IN THE AIRPLANE:

Vertical antennae can be located either attached to one side of the fuselage, or if necessary, in a small RF neutral tube either above or below the fuselage. If the receiver is located towards the rear of the fuselage, and the antenna length is sufficient, then running the antenna on or in the vertical stab is OK.

Horizontal antennae are easier to locate, as the fuselage offers many options for antenna placement. The top and bottom of the fuselage are easy to mount antennae in the front-to-rear axis, and if wide enough, can support left-to-right orientation as well. Wings are also useful for horizontal placement. Antennae can be simply taped (with non-conductive tape) to the wing surface. One can use a piece of RF neutral tubing inserted into the wing as a guide to facilitate easy antenna mounting if the wing is regularly removed from the fuselage.

Multi-rotor craft generally present a more complex environment for antenna mounting, and the variations between these types of airframes are many. Most pilots of multi-rotors have found the best compromise to be mounting the antennae underneath the airframe to avoid the metal and/or carbon fiber chassis. Because vertical mounting is a challenge, and horizontal mounting space is restricted the favored configuration is the inverted “V”, where both antennas point downwards at approximately a 45 degree angle.

 

RANGE TEST:

PSA: Any time I hear the word TEST, I think of the word PROP. Here’s why: testing and propellers do not belong together! By definition one can either pass or fail a test, and with powered aircraft fail can mean an out of control spinning prop. Think “ninja throwing star”. If you have never suffered a blade strike, keep it that way. If you have, then you probably don’t want to do that again. Multi-rotor craft pose a unique challenge in that it may be impossible to determine if it is responding to the TX without the props installed. In any case, if the circumstances dictate that the testing be done with the propellers mounted then secure the airframe to a solid structure that can’t be moved by the force of the model at full throttle. Tethers work well here.

Now, back to our regular programming …

Once the best mounting locations for our antennas have been selected we are ready to verify that the installation doesn’t have any bugs that we did not foresee. Range test will help with this. The minimum range test consists of putting the transmitter in range test mode and walking away from the model until the controls no longer react reliably to stick movements. This test can be done by having a helper hold the aircraft and move it through various attitudes to simulate actual flight while communicating the control movements to the pilot. Another method is to place the model on a non – metallic table or stool and walk around the model to simulate various flight attitudes. When doing this solo it may be difficult to see the control movements at a distance, so some pilots tape colored pieces of paper to the control surfaces to enhance visibility (business cards work well for this). Range checks performed with the aircraft on the ground are less accurate than the methods mentioned above because the aircraft is at ground level (hopefully your flights are mainly above this) and because the earth itself can affect the results, especially damp earth.

Any new antenna installation should be range tested prior to the first flight. Any change to the configuration of an existing setup, antennae, radio gear, control rods, wiring, etc. should be range checked as well.

 

TIPS FOR AN EXCELLENT RF LINK:

  • Match the orientation of the transmitting and receiving antennas for best performance. If the TX antenna is positioned horizontally make sure the RX antennas are horizontal too.
  • Position the second RX antenna to provide coverage when the first RX antenna is at the least optimal position, like pointing directly at the TX.
  • RX antennas will work best if oriented 90 degrees to each other, and as far apart as is practical.
  • When positioning the antennas in the airframe keep them away from any conductive or reflective materials.
  • The shorter the antenna lead is, the better the antenna will work, so only use extended length antenna assemblies if a shorter one won’t fit your application.
  • Unless you are a qualified radio technician, do not attempt to modify your antennas. If your application requires a different length antenna then buy one that is the length you need.
  • Avoid sharp bends to any part of an antenna assembly. RF is weird stuff, and will do funny things if asked to hang a Uey half way down the antenna lead wire.
  • DO NOT cram the RX antennas into the fuse saying “I haven’t crashed yet”.
  • Do a range check before every maiden.
  • Do a range check after any mods, even if they weren’t radio related, as wires can get bumped or connectors pulled loose.
  • Do a range check after every crash.
  • If it’s too windy to fly … do a range check.
  • RANGE CHECK! This cannot be overstated. Think of what a beautiful world this would be if your flying buddies asked “Did you do a range check after that last mod?” … before the crash.

 

MORE INFORMATION:

http://www.rcgroups.com/forums/showthread.php?t=1159968

https://vimeo.com/8826952

 

REFERENCES:

*Google 2.4ghz coax antenna

 

Intro to Antennas                                                                       April – 2015


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