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Article: 2.4GHz Rx Installation Primer
- Join Date
- Jan 2006
2.4GHz Rx Installation Primer
Here is a basic RF Knowledge primer which should come in handy in understanding how to install 2.4GHz radio gear in your model.
The article will only cover RF issues rather than power supply requirements for receivers. RF tends to be more of a mystery for most.
I’ll try and keep the theory to a minimum but I need to explain some of the basics up front in detail so that it will impart some knowledge on to those who want some understanding of the concepts. For those that cannot read, you can scroll down to the pics!
In many instances I find that people still ignore the basics when it comes to correct installations. Even though in the majority of instances the equipment still seems to work ok ie 'you can get away with it 99% of the time', there are many instances where one loses a model due to a suspected loss of their radio link and then they are all to quick to point the finger at interference or other unseen cause when the the answer usually lies in poor installation practices.
With the advent of 2.4Ghz RC systems it is even more imperative that modelers are aware of the limitations and advantages of operating at these frequencies as it has certain implications for the installation of our radio gear.
One of the first things that the non RF inclined modeler notes on 2.4Ghz systems is the lack of antenna - well, the lack of length actually. This has all to do with basic physics.
If any of you did some physics in high-school, one of the most common formulas is "Velocity = Frequency x Wavelength".
Where Velocity is the velocity of the signal [usually the speed of light in a vacuum or free space], Frequency is the actual frequency of the signal [expressed in Hz] and finally, Wavelength - the actual length of the waveform of the signal expressed in Metres.
Antennas are designed to operate at certain fractions [eg ¼, ½] of their full wavelength which is related [inversely proportional] to their operating frequency.
You will note that as frequencies increase antenna sizes get smaller. So if we look at the old 36MHz gear, a Rx or Tx will have a relatively long antenna as compared to a 2.4Ghz Rx or Tx. Note that the overall length of the antenna is very important so don’t ever lengthen or cut antennas unless you know what you are doing.
The most common antenna utilised for our RC gear is the most simple – the ¼ wavelength antenna. Just about all the 2.4Ghz receivers that I have seen use one form of this antenna in one way or another. If you have ever accurately measured the length of the active element of your 2.4GHz Rx antenna you will find that it should be just under 31mm [30.7mm to be slightly more precise].
So how do we achieve this 31mm magic figure. Well all we need to do is use the basic formula mentioned earlier.
Ie. Velocity = Frequency x Wavelength
The velocity is basically the speed of light as RF waves travel at the same speed ie. 2.998 E8 Metres/Sec
The frequency is the exact frequency we are interested in. Our RC gear operates in a range from 2.40 to 2.485 GHz so lets pick the middle at 2.44 GHz for this example [2.44 E9 Hz]
Wavelength is the full wavelength of the signal expressed in metres.
Rearranging the formula gives us:
Wavelength = V/F therefore 2.998 E8 divided by 2.44 E9
This gives us 122.869 E-3 or 122.869mm for a FULL wavelength.
Note that our common RC antenna for 2.4GHz is actually a ¼ wavelength type so we need to divide by 4 for the final answer which gives us 30.7mm – Simple!
Last edited by Costas; 29-06-2010 at 09:45 AM.
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Total Comments 8
21-06-2010, 02:22 PM #2
Ok – So now that we have some concept of why the antennas are a certain length, lets see what affects the propagation of our RF signal [apart from the absolute distance between Rx and Tx].
The most important propagation mechanism for our short-range communication on the 2.4Ghz band is that which occurs in an open field, where the received signal is a vector sum of a direct line-of-sight signal and signals from the Tx that are reflected off conductive objects and also the earth/ground.
There are three distinct phenomenon which apply to transmitted signals and they are – reflection, diffraction and scattering of the signal.
The path lengths of these signals differ from that of the line-of-sight signal, so the receiver sees a combined signal with components having different amplitudes and phases.
A reflection causes a phase reversal.
A reflected signal having a path length exceeding the line-of-sight distance by exactly the signal wavelength or a multiple of it will almost cancel completely the desired signal ("almost" because its amplitude will be slightly less than the direct signal amplitude). On the other hand, if the path length of the reflected signal differs exactly by an odd multiple of half the wavelength, the total signal will be strengthened by "almost" two times the free space direct signal.
Thus, the total signal strength at the receiver is the vector sum of not just two signals, but of many signals travelling over multiple paths. This is termed ‘Multipathing’.
So in more simplistic terms: RF signals will reflect, diffract, and scatter off of various obstructions they encounter [predominately conductive objects] – eg, metal, carbon etc.
A Rx may receive both the primary signal and one or more reflections of that same signal ie. multi-pathing. Strong reflected signals cause interference by adding or subtracting to primary signal amplitude. Reflections can also leave gaps in RF coverage by cancelling out (nulling) the primary signal.
One major way to defeat multipath interference is to utilise antenna/receiver diversity.
In this case, a receiver with multiple antennas [or similarly using multiple receivers] can continuously sample incoming signals, and choose the input source [antenna and/or receiver] with the best signal for that instant in time. To increase the probability of getting a higher average output you could use three or more receivers.
Now you can see the actual reasons behind why the big name manufacturers utilise more than one antenna or receiver for their larger receivers designed for models bigger than park flyers.
Polarization Diversity is another method in defeating multipath issues.
Fading characteristics are dependent on polarization.
A signal can be transmitted and received separately on horizontal and vertical antennas to create two diversity receive paths.
Reflections can cause changes in the direction of polarization of a radio wave, so this characteristic of a signal can be used to create two separate receive paths.
Polarization diversity can be particularly advantageous in our models, since the orientation of our receiver antennas will not be rigidly defined.
Space Diversity is a third method that can be easily implemented by the modeller to keep multipathing issues from affecting the receivers.
A signal that is transmitted over slightly different distances to a receiver may be received at very different signal strengths.
The distance is related to the wavelength of the frequency in use.
A good tip is to space out antennas or receivers at a minimum of ½ a wavelength in order to have reasonably independent, or decorrelated, antenna/receiver paths.
For 2.4Ghz that’s about 60mm separation for optimum results. Obviously handier to use this method on larger sized models but even on smaller models there is an advantage in keeping the receivers or antennas separated as much as physically possible.
In a true multipath environment, the receiver, and therefore the reflections are usually constantly in motion, so the nulls and the peaks would occur at different times on each antenna/receiver.
Last edited by Costas; 26-06-2010 at 11:03 PM.
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21-06-2010, 07:41 PM #3
Just summing up for those that prefer diagrams.
A few pointers to remember with RF:
Radio waves decrease in amplitude as they pass through obstacles.
As the radio frequency increases, the rate of attenuation increases - that is, the radio strength dies off faster, and the attenuation effect of passing through obstacles is much greater.
Therefore a 36MHz signal will penetrate obstacles much better than a 2.4GHz signal will.
Bending around Obstacles:
Radio waves travel in a straight line; however, a radio “beam” can diffract or bend when it hits an edge in the same way as light can.
The angle of diffraction is higher as frequency decreases – in other words, radio waves tend to bend around obstacles more as frequency decreases.
A lower frequency radio signal is “blocked” by an obstacle to a lesser extent as it is able to bend around the obstacle. So a 36MHz signal will be able to 'bend' around objects much better than a 2.4GHz can.
Think of this next time you mount a 2.4GHz Rx or antenna on an opposing side of your carbon frame with next to no visibility of the Tx from the opposing side.
Radio waves also reflect from dense surfaces such as the ground, heli frame work etc. Very often the radio signal has been reflected several times before it reaches the receiver unit.
When a radio signal is reflected, some of the RF power is absorbed by the obstacle, reducing, or attenuating, the strength of the reflected signal.
This attenuation increases with frequency. That is, the reflected signal is weaker for higher frequencies. If the path is very congested, with a lot of
consecutive reflections, a 2.4 GHz signal can fade out quickly - Refer to Multipathing issues in posts above.
Not an issue for 36MHz but certainly needs to be considered for 2.4Ghz radio signals.
Last edited by Costas; 30-09-2010 at 10:23 PM.
23-06-2010, 02:51 PM #4
Ok - So now lets move onto where it matters - Placement of your satellite Rx's or coaxial antennas on the model.
One thing I would like to mention at this point is that you should always mount your receivers [or any electronics for that matter] using quite THICK double sided tape or ZEAL tape etc.
Even though these things are quite tolerant of vibrations due to the utilisation of surface mount components, they are in no way impervious to damage caused by long term exposure to vibration and mounting them to a heli with say, just one layer of thin double sided tape is asking for trouble.
OK - Now lets have a look at the most common antenna/receiver setups that we have available to us nowadays.
First of all is the simple coaxial antenna. this is used by many manufacturers. It consists of a standard coaxial cable which has the inner conductor exposed [shielding removed] at the very end of the coaxial cable.
You will note that the length of the exposed coax is around 31mm which means that this forms our standard ¼ wavelength antenna. The active section of the antenna is highlighted in red. The active section is where the RF signal is received [the actual antenna] so this is the most important part of the cable.
Next up is a Spektrum satellite receiver assembly - this contains not only the antenna but a full blown receiver directly connected to the antenna elements. You will note that the antenna has two elements. These two elements form an antenna called a dipole antenna. One element is connected to the RF input amplifiers [ACTIVE element] and the other is simply connected to the ground plane [GROUND element].
The important thing here is to note that the ACTIVE element is always positioned on the LHS of the satellite Rx. The ACTIVE element is of main importance as we will see later.
Last edited by Costas; 26-06-2010 at 11:05 PM.
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26-06-2010, 10:29 PM #5
You will note that I have made some effort to highlight the active element as far as these coaxial antennas and satellite receivers are concerned.
It is important to realise that the antenna’s or satellite Rx’s ACTIVE element must always be mounted so that it is in clear air and not shielded by carbon framework or any other large conductive object such as an engine or battery pack etc.
Mounting an antenna adjacent to carbon framework will actually detune the antenna and while the Rx will still seem to work, it will have much reduced range due to the detuning effect and you will also have the added problem of having the antenna shielded from the opposite side of the model.
Another important point is that you should endeavour to keep the antenna wires as straight as possible. If you allow them to fold over or squash them etc it will also cause them to be detuned and the resultant effect is that the receiver will have much reduced sensitivity which will result in reduced performance.
Here are a couple of photos showing poor installation techniques:
Coaxial antenna mounted adjacent carbon framework - This will result in poor performance due to detuning of the antenna as well as having the antenna shielded from the opposite side of the heli.
Another similar poor installation – In this case the antenna is also mounted adjacent some servo wiring. You really want to keep any antennas away from any other electrical wiring – This is most important near ESC power wiring where high currents can induce noise directly into receiver circuitry.
Poor installation with a Spektrum satellite Rx – Same issues encountered as per the two previous examples.
Below is an example where the owner has not realised that the ACTIVE element of the satellite Rx is the one shielded by the carbon framework.
Ideally you would want to rotate or flip the satellite around so that the ACTIVE element is the one poking up above the framework instead of the GND element as shown in the pic. Remember that the ACTIVE element on a Spektrum Rx is the antenna located on the LHS of the unit when viewed from the front.
Just paying attention to the orientation wrt the ACTIVE and GND elements on a Spektrum satellite Rx will result in improved overall performance for that receiver.
In the example below – if we rotate or flip the satellite 180 degrees then issues such as antenna detuning and shielding will not be a problem than if we left it as shown in the pic below.
So how many of you now are just grabbing your heli to re-arrange your receiver orientations...
Last edited by Costas; 27-06-2010 at 09:32 PM.
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27-06-2010, 09:42 PM #6
Here is an example of how to do it correctly - I currently rearranged my heli to run 2x Spektrum satellites directly into my VBAR when I converted it to FBL a couple of weeks ago.
I have one satellite mounted at the lower front clear of any conductive obstructions. NOTE that I have inverted the receiver so that the ACTIVE element faces horizontally forward to keep it as far away from any RF shielding obstructions as much as possible.
Here is my second receiver which is mounted at the top rear of my heli. Note the vertical orientation which is at 90 degrees to the other Rx which gives me receive polarisation diversity.
Again note the receiver is orientated so that the ACTIVE element is not up against the carbon frame but in clear space
Many pilots install both receivers up in the front but I try an maximise my reception so that the Tx will ALWAYS have line of site visibility to at least any one Rx at any instant in time. Mounting one at the lower front of the heli and one at the top rear helps to achieve this.
Hopefully this article is useful to many who do not have any experience with RF and installing 2.4GHz receivers/antennas for optimum performance.
Last edited by Costas; 27-06-2010 at 09:48 PM.
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30-06-2010, 08:34 PM #7
Thanks Costas. Can I ask 2 questions about aerial placement. An Rx that has the aerials at right angles to each other, I think some JR are like this. Is the one on the left, regardless oif which way it faces still the active one? Second question really doesn't relate to placement but is about stiffening the aerials so they do stand out straight. Is it OK to put heat shrink tube over them and hot glue the heat shrink to the Rx body?A geriatric flier - pushing the brain cells and reaction times.RC junkie, lots of heli's, some planks and cars.
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30-06-2010, 09:59 PM #8
These 9ch Rx's have 2 separate receivers mounted in the one housing, so for these BOTH antennas are ACTIVE elements. Their ground plane is formed by their internal PCBs.
Ideally you would want to position these so that both antennas are clear as possible of conductive objects.
The heatshrink provides extra vibration support/protection and it also keeps the antenna straight which is exactly what we want. BTW I would not bother with the hot melt glue unless you have the very early Rx's which don't have the small shoulders moulded as part of the case.
Last edited by Costas; 30-06-2010 at 10:19 PM.