|Aug 23, 2011, 05:00 PM|
Joined Jan 2008
2.4GHz Antenna System Fundamentals
An “antenna system” for a radio link needs to include both the transmitter and receiver antennas to be able to analyze the link performance. For RC this isn’t as easy as analyzing a simple point-to-point fixed link since there is a continually changing geometric relationship between the transmitter and receiver. This can account for a lot of the momentary control glitches we sometime see with typical antennas. Some other factors that also affect the link integrity are interference from other users of the spectrum as well as interference we generate ourselves by reflections of our own signal from surrounding objects. The choice of antennas used on both the transmitter and receiver can have a significant bearing on these issues and link range. The antennas supplied by OEMs with their radio systems are usually far from optimum for the task at hand and are made to a price – meaning cheap.
Two antenna characteristics, among others, that impact link performance to a large degree are directivity and polarization. Some people tend to confuse the two but it’s important to sort it out. Another factor that’s often quoted (mostly to sell after-market antennas) is “gain”. Antenna gain can be a useful number if understood in the right context. Gain is a product of antenna directivity and to be meaningful, requires a reference antenna for comparison, which is most commonly an isotropic radiator. This is a hypothetical antenna that radiates (or receives) equally well in all directions i.e., it is an antenna where the radiation pattern is a sphere with the antenna located at the center. Units for gain using this reference are dbi, meaning decibels improvement (or loss -dbi) compared to the isotropic reference.
A simple dipole antenna has a gain of 2.15 dbi and has a directivity pattern, in free space, shaped like a torus or donut. Since the radiation is much less in the “hole” of the donut, it holds that it has to be greater in some other directions. Thus, the dipole has gain in its favored directions compared to isotropic. This can be an advantage in some circumstances but in RC systems, that “ hole in the middle” can be a real problem. The usual transmitter antenna is a dipole or monopole and If pointed straight up, the donut pattern is horizontal and minimum radiation is from the tip. This is why we are told to not point the antenna at the plane. This is counter-intuitive. Do we always do this while concentrating on flying? I doubt it. It’s too bad that we even have to be concerned about it.
Linear antennas with higher gain (>2dbi) need to be used with caution. The directivity pattern often has multiple narrow lobes with nulls between. This results in more signal fading as the plane/transmitter orientation changes and monitoring the transmitter position in your hands becomes necessary. We don’t need more things to think about while flying. If more antenna gain is needed to extend range, there are antenna types with increased gain without breaking up the pattern into narrow lobes but you’re not going to find them at your favorite RC supplier.
Antenna polarization is the orientation of the electric field component of the transmitted radiation and for simple linear antennas, is the same as the orientation of the antenna conductor (wire). If the wire is vertical the polarization is also vertical, if horizontal the polarization is horizontal and everywhere in between it follows the wire orientation. The main point is that the polarization is linear and is independent of directivity. The same linear polarization is present in all directions of the directivity pattern. Transmitting and receiving antennas behave the same in this regard.
One characteristic of linear receiving antennas is that they respond best to an incoming polarization that matches their own. At any other linear polarization the signal is reduced and at 90 degrees it can reach –25db or more. This is a huge penalty in signal strength and glitches are probable, particularly near maximum range.
As the plane orientation changes – banks, climbs, dives and inverts, the signal level delivered to the receiver by the antenna(s) is all over the place. This signal fading is due to two primary factors - the mentioned polarization difference between the transmitter and receiver antennas and changes in gain of both antennas because of the directivity pattern response. Taken together, signal level changes of more than 50db are possible. Fortunately, they’re usually momentary but still represent a degradation of link integrity and can occur at a bad time, like when close to the ground.
Many of today’s systems use a form of receiver antenna diversity, which can help with the fading issues, both polarization and directivity. That’s why they usually have multiple receive antennas. If these antennas are oriented and spaced properly, they can definitely help.
There is another type of polarization that solves some of these problems. Unfortunately, it is not used very often in RC due to greater complexity and cost. However, if you’re willing to DIY it can be a big help and not cost very much. Circular polarization results when the electric field vector continually rotates (at a very high rate) and because of this, will respond near equally to any linear polarization. There is a slight downside in that a circular antenna’s response to linear polarization is down –3db compared to its own polarization response – a small price when compared to –20db plus fades.
Circular polarization comes in two flavors (sense), left hand (LHCP) and right hand (RHCP), which refers to the direction of rotation of the wavefront. Like two opposite linear polarizations, the two circular types don’t talk to each other well. RHCP response will be –20db or so to LHCP and vice versa. Actually, this turns out to be a significant advantage to using circular for RC.
One issue of using 2.4GHz compared to the lower VHF bands is that reflection from surrounding objects is more prevalent at the higher frequency because of the shorter wavelength, which can result in signal interference at the receiver. Circular polarization has the property that upon reflection, the polarization sense reverses. RHCP becomes LHCP and vice versa. This is not true for linear polarization – the polarization remains the same. So, if we are transmitting and receiving circular of the same sense, any reflections, having been reversed, would be reduced considerably. A BIG advantage!
Basically, antennas are reciprocal devices – they behave the same whether transmitting or receiving. However, there are some parameters that apply more significantly to one mode or the other. The feed point of an antenna, where a transmitter is connected, has a certain impedance (RF resistance), which ideally should be the same as the transmitter output impedance for maximum power transfer. If there is a transmission line (coaxial cable) connecting the transmitter to the antenna, the line also has a characteristic impedance (most commonly 50 ohms) and again, for best performance, all three impedances (transmitter, feed line and antenna) should match. If the antenna impedance is mismatched to the transmitter or feed line, a standing wave results, which can represent a power loss and the transmitter isn’t very happy with a mismatched load.
You may see a SWR or VSWR (Voltage Standing Wave Ratio) specification stated for a given antenna. A perfectly matched antenna (referenced to 50 ohms) would have a SWR of 1.0, but generally a number less than 2.0 is acceptable. The lower the better. Depending on receiver design, antenna SWR may not be as critical for receiving, however some of the newer telemetry radios both transmit and receive using the same antenna, so it’s worth considering.
Another parameter of importance is the antenna bandwidth (don’t confuse with beamwidth). An antenna designed for a specific frequency has certain characteristics at that frequency only. As the frequency is changed, plus or minus, from this center frequency, those characteristics will change to some degree. The degree of change vs. frequency determines the range of frequency where the antenna performs in an acceptable manner. This is the bandwidth, which is highly dependent on the antenna type and design.
For radio systems that operate on a single frequency, bandwidth isn’t much of an issue. However, a number (if not most) of 2.4 GHz systems use spread spectrum or frequency hopping technology, which require operation over a range of frequencies. For these systems, bandwidth is significant.
Two of the more significant parameters connected to bandwidth are impedance and directivity (pattern or gain). An example is that an antenna may have an SWR of say 1.2 at its center frequency and 2.0 at some plus and minus frequency, which would represent the bandwidth if 2.0 is our specification limit. We would also like the directivity to be reasonably consistent over the bandwidth for predictable results.
Antennas offer an area for many RC’ers to improve the performance of their radio systems by DIY without much cost. It does require a bit of education to be effective but isn’t learning part of the fun of the hobby? The following are some of my thoughts on the subject.
For me, using circular polarization is a no-brainer. The advantages of reduced reflection interference and polarization fading is well worth the extra trouble. Of course, to realize the reflection advantage, both transmit and receive antennas need to be circular. However, changing just the transmitting antenna will give you good immunity to polarization fading at the expense of -3db less overall signal. This could be made up with additional transmitter antenna gain.
If you couple receive diversity with matched circular polarization, the improvement can be significantly greater than either alone. Also, circular can offer some rejection to other interfering signals. If they are linearly polarized the rejection is only -3db but if they happen to be using the opposite sense circular, the rejection is –20db or so. For the FPV folks who have problems with interference between the radio system and the video system, using opposite sense circular for the two systems can often cure the problem while providing other benefits.
Antenna directivity is very important to a reliable system, with the goal being to reduce signal amplitude fading as the plane maneuvers. This applies to both the transmitter and receiver, but the requirements for the two are quite different. It seems that the ideal transmitter antenna radiation pattern would be a quarter sphere directed up and in front of us since we don’t need to direct signal toward the ground and we usually don’t fly the plane in back of us. The typical system-supplied antenna doesn’t even come close to this. Now, no antenna type has exactly this pattern, but we can come a lot closer and by limiting the pattern to the useful directions, we can pick up some additional gain.
For receiving antennas the need is completely different. Since the antenna(s) in the plane are constantly changing their orientation relative to the transmitter, what would seem to be ideal would be an isotropic circularly polarized antenna. Again, in reality, this doesn’t exist. Receiver diversity helps by effectively superimposing the patterns of the individual antennas to simulate something closer to isotropic with the receiver selecting the strongest signal at the moment. Simple monopole or dipole antennas don’t do to badly at this if installed and oriented correctly, but can still be improved upon. Of course, if we want the advantages of circular, a different type is needed.
Antenna gain should be considered on the system basis. Gain, of course, relates to the range we can obtain from the system. For a given system antenna gain (transmitter plus receiver), it makes no difference in range whether we apply gain at the transmitter or the receiver. However, gain comes with narrower beamwidth (directivity), which is just the opposite of what we want at the receiver. The place for additional gain is at the transmitter (up to a point). Remember, this is a “system”.
To complete a set of desirable specifications, I’ve selected antenna bandwidth as 2400 to 2500 MHz with a SWR of 2.0 or less and a nominal impedance of 50 Ohms. The choice of circular polarization sense is a toss up between RHCP and LHCP unless someone can suggest to me a prevalent circular polarization used by other services in this band. I would then choose the opposite.
There are hundreds of antenna types. Choosing one can be a rather daunting task and some means of evaluating them must be found to make a reasonable choice. An online free program that can help considerably is 4NEC2. It takes a bit of learning but it can answer a lot of antenna questions quickly with reasonable accuracy. Numerous examples are provided and you can roll your own. After antenna types are selected, they must be built and tested.
Antenna testing is a difficult chore and doing it professionally takes substantial facilities. For most RC’ers, simpler methods must be found. The easiest is to simply fly it! This doesn’t produce quantitative results, but with enough flights it can give us a ballpark feel or guesstimate as to effectiveness.
The method I’ve decided to use utilizes a FrSky telemetry system, which allows monitoring of receive signal quality (RSSI) at the transmitter, along with a homebrew pan-tilt platform to mount the receiver on a tripod and is controlled by the transmitter. Using the low-power range check mode, range can be determined for various receiver antenna attitudes. This doesn’t test everything but comparing to the stock antenna system should provide some useful answers.
I looked at many antenna types that produce circular polarization and a number had to be rejected (turnstile, phased arrays) because although they did produce circular, they also produced both senses simultaneously, albeit in different directions. This isn’t good because the opposite sense can be reflected from objects that then produce the desired sense and interference. Others had too narrow a beamwidth or were too ungainly to build and mount.
One that caught my eye for the transmitter is the quadrifilar helical antenna (QFH or QHA). This has a number of good attributes: it produces single sense circular in all directions of its pattern, small maximum dimension (40mm), reasonable gain (~5dbi), a wide (130 degrees) single-lobe pattern in the right direction, reasonable bandwidth, SWR, circularity and cheap to make (cheap, not easy). It consists of two rectangular loop elements mounted at 90 degrees to each other and twisted 180 degrees over their height, with the twist direction determining the sense. Seems to fit the bill pretty well and no longer needing to be concerned about not pointing the antenna at the plane is a benefit.
This investigation let to the QHA’s little brother – the bifilar helical antenna (BHA). This has most of the attributes of the QHA but is a single loop and has a donut pattern similar to a dipole but with wider lobes. Its gain (~2.3dbi) is less than the QHA but a tad better than a dipole. They look good for the receive side circular pair. The overall dimensions (18mm x 40mm) are about the same as the QHA but it’s still okay for most aircraft and is a bit easier to build than the QHA.
The next installment of this thread will get into the construction and testing of these antenna types to see if the theoretical evaluation advantage is borne out in practice.
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|Feb 02, 2012, 06:38 AM|
Hungary, Borsod-Abaúj-Zemplén, Miskolc
Joined Mar 2011
Whoah, that's quite a bit of information SamoaFlyer, you cleared up a lot about the new radio systems.
I'm eagerly awaiting the next part.
|Oct 23, 2012, 04:46 PM|
Thanks for the exciting and learning reading.
Is not Weatronics 2.4 GHz system and the upcoming new Multiplex 2.4 GHz* system good enough according to your opinion? Can it get much better?
It will really exciting to follow this thread.
*The PROFI TX M-LINK sets new standards in its class, with 2.4 GHz transmission technology and numerous innovative and pioneering features:
• Integral aerial technology (IOAT) (Integrated Optimized Antenna Technology): integral aerial with enhanced radiation pattern, optimised for model control: double the signal density transmitted to the model, offering a significant enhancement in security.
|Oct 24, 2012, 09:04 AM|
Joined Mar 2011
This might be a little off topic but I noticed you mentioned "cross polarization" as being bad for the link; It made me think about cross polarization when used to interfere with radars in electronic warfare, the targeted plane "listens" to the radar and then transmits a signal at the same frequency but with crossed polarization... I think the idea is to use the "crossed" signal as a skin over the echoe or something like that. Don't know if only specialty planes like the EA-6 and the E/F-18G are the only types to use cross-polarization or if most planes with an AESA radar can use it...
|Dec 07, 2013, 01:58 PM|
Joined Jan 2013
Understand the basics but want to make sure I'm not crazy here. If I buy two of the same left handed polarization antennas... one for multicopter and other for receiver.... I should be good to go? My brain is telling me I need opposites but...... Most all of my flying is above me very high or close in around me so the radiation patterns look perfect for me. I tried making a cloverleaf and planar and had very bad signal directly overhead.
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