Hai-Lee
Old and Bold RC PILOT
There has been a lot of to and fro about what radio is best and whether a model crash was caused by a LOS, (Loss Of Signal), issue, interference, or poor piloting. Sadly most is based upon conjecture, misinformation, or wrong assumptions. The biggest thing to come out of the discussions here on the forum is the real lack of understanding as to what really constitutes a radio that is suitable for airborne usage in models. So herein I will try to point out a few physical properties of radio antennas as used in RC models and the design limitations and performance limitations on some radio systems. This post will not be discussing brands of radios as all brands have the same issues for the uninitiated. It has been dumbed down a little so I encourage you to research for yourself if you wish to find any scientific explanations!
ANTENNAS.
The wire/s that protrude/s from your radio Rx and that is/are found on the top of your Tx are the antennas and their function is to radiate the transmitter power, (and the included information), in the case of the Tx antenna, into the air as well as receive the required signal, (in the case of the Rx antenna). On a Rx the antenna has the function of collecting the radiated/transmitted signal and forwarding it to the Rx where it is decoded and the information used. Sounds ideal! Well like all things there are limitations and factors that are less than helpful to our need for solid communication. A basic Omni-directional, (All Directions), antenna actually radiates in a donut fashion with the antenna at its centre or core this means that most of the signal is radiated at right angles to the antenna. On a Rx antenna the same donut applies and so where the 2 antennas are parallel to each other, and the Rx is close enough to the Tx, the Rx will get the required signal. See illustration for the radiated donut pattern.
As you can see there is NO power coming out of the top or bottom of the antenna. This means it is possible for our Tx and Rx antennas to be close to each other and have no communication. THIS IS NOT AIRWORTHY! Sure we could place another antenna at right angles to the first and then the "null" signal area would disappear! Well yes and no! Firstly consider that if the Rx has 2 antennas, at right angles, and the plane flies through the Transmitter NULL then both Rx antennas receive no signal at all! So we would need 2 transmit antennas,(at right angles), to give us the two radiated donuts! Would a single Rx antenna then suffice? NO! there is another matter or property of the antenna radiation to consider!
TRANSMIT ANTENNA POLARIZATION
When the Tx and Rx antennas are not orientated the same there is an additional loss to be considered. This loss it at its maximum when the antennas are orientated at right angles to each other and the loss can be around 20db, (a factor of 1/100th of the signal received), compared to when the antennas were parallel.
With a plane that has a single Rx antenna not only must if stay out of the "NULLs", (the Tx and Rx antennas have nulls), but it must not fly into an attitude where the antenna is at right angles to the Transmit antenna.
Obviously a single Tx antenna is not really an airworthy system, and a single Rx antenna is dodgy even when used on a system with dual Tx antennas. (The single antenna system can still have the plane in the null of one Tx antenna and cross polarity with the other! A dual antenna Rx and a dual antenna Tx are the best we currently have to offer or select, BUT even the dual Rx antenna can find itself without signal is some special attitudes!
SIGNAL LOSS RECOVERY
As we live in an imperfect world, and we do very rarely get interference caused LOS, the Radio system should have a fast recovery function. In the days of analog when the radio signal was restored the control functions also immediately returned but modern systems, (well some and most with some form of flight controller), seem to have a rather long delay whilst the radio reinitializes and restores control. This alone makes the current radio systems fail the airworthiness test!
RANGE TESTS
I cannot give my honest, (full of expletives), opinion on the range test feature as to me it is a HOODWINKING tool included by some/most manufacturers. Whilst it does give a very rough guide to the performance of the Rx or Tx it is effectively a go/no-go test at best. Passing a range test on the ground where polarities match proves nothing of real value when airborne! Even with systems that boast having a BER , (Bit Error Rate), or an RSSI, (Receive Signal Strength Indicator), still fail to give the full picture when it comes to airborne communication quality though if I had to select one I would chose BER as it can reflect the presence of possible interference!
IDEAL RADIO SYSTEM
The ideal radio system based upon current technology and not on current trends is to have a 3 antenna Rx with all antennas set at right angles to each other combined with a dual antenna transmitter, (3 antenna transmitter would be even better)! The chipsets for 3 or more antennas are easily available and in use in modern WiFi applications and WIFI devices are almost dirt cheap!
If there is an outage or loss of control for any reason there should be a fast recovery mode to allow the radio to regain ANY control possible ASAP for the crafts safety.
A BER indication rather than a simple Tx power reduction feature as all range tests really are!.
The inclusion of a Micro SD card slot to allow the recording of flight features and radio signal performance. The slot could be disabled on boot up if there is no card present. This would assist in determining if you had interference, a LOS or the crash had another non-radio cause!
A possible flight data recording function.
If the RC model aircraft fraternity and national associations were to unite behind the insurance companies and not only seek an airworthiness rating for radio systems but also push for the design of a proper radio system then LOS crashes would quickly become so very infrequent that our insurance costs could actually drop!
GENERAL WARNING!
Water and 2.4GHz do not co-exist happily! When flying avoid flying with the plane dripping in water as you would in rain or on a morning with heavy dew. 2.4GHz is used in microwave ovens because water absorbs the radio waves and converts them to heat. Using 2.4GHz in your plane with the antennas behind a sheet of water does cause a loss of signal or at the very least a decrease in effective radio range! You have been warned!
ANTENNA POSITIONS FOR MODEL AIRCRAFT
We should all have heard by now that the antenna/s in your model should be clear of all wires and other metal objects. This is because the metal objects in your plane can interfere with the Rxs ability to receive a clean signal or even a signal at all. With 2 antenna Rxs the 2 antennas should be mounted at right angles to each other as well as being clear of all wires etc.
If you only have a single antenna then mount it out of the top or bottom of the fuselage and fly the plane up and down the field without too much banking in the turns because if you bank it too hard or even loop/roll the plane a LOS will almost definitely occur!
With dual antenna Rxs I mount them on the outside of the fuselage away from as much metal as possible with one 45 degrees forward and the other 45 degrees rearward. They can also be fitted to the top/bottom of the wings with both leaning 45 degrees backwards. No matter which position you find works for you you should ensure that the 2 antennas are still at right angles to each other!
BALLET
You can of course put the ground station through matching maneuvers to attempt to maintain the same polarization and thereby maintain communications but sadly I am getting too old for ballet and I go to the flying field to fly and not dance!
I hope I covered most of it even if only a brief touch on the subject!
Have fun!
ANTENNAS.
The wire/s that protrude/s from your radio Rx and that is/are found on the top of your Tx are the antennas and their function is to radiate the transmitter power, (and the included information), in the case of the Tx antenna, into the air as well as receive the required signal, (in the case of the Rx antenna). On a Rx the antenna has the function of collecting the radiated/transmitted signal and forwarding it to the Rx where it is decoded and the information used. Sounds ideal! Well like all things there are limitations and factors that are less than helpful to our need for solid communication. A basic Omni-directional, (All Directions), antenna actually radiates in a donut fashion with the antenna at its centre or core this means that most of the signal is radiated at right angles to the antenna. On a Rx antenna the same donut applies and so where the 2 antennas are parallel to each other, and the Rx is close enough to the Tx, the Rx will get the required signal. See illustration for the radiated donut pattern.
As you can see there is NO power coming out of the top or bottom of the antenna. This means it is possible for our Tx and Rx antennas to be close to each other and have no communication. THIS IS NOT AIRWORTHY! Sure we could place another antenna at right angles to the first and then the "null" signal area would disappear! Well yes and no! Firstly consider that if the Rx has 2 antennas, at right angles, and the plane flies through the Transmitter NULL then both Rx antennas receive no signal at all! So we would need 2 transmit antennas,(at right angles), to give us the two radiated donuts! Would a single Rx antenna then suffice? NO! there is another matter or property of the antenna radiation to consider!
TRANSMIT ANTENNA POLARIZATION
When the Tx and Rx antennas are not orientated the same there is an additional loss to be considered. This loss it at its maximum when the antennas are orientated at right angles to each other and the loss can be around 20db, (a factor of 1/100th of the signal received), compared to when the antennas were parallel.
With a plane that has a single Rx antenna not only must if stay out of the "NULLs", (the Tx and Rx antennas have nulls), but it must not fly into an attitude where the antenna is at right angles to the Transmit antenna.
Obviously a single Tx antenna is not really an airworthy system, and a single Rx antenna is dodgy even when used on a system with dual Tx antennas. (The single antenna system can still have the plane in the null of one Tx antenna and cross polarity with the other! A dual antenna Rx and a dual antenna Tx are the best we currently have to offer or select, BUT even the dual Rx antenna can find itself without signal is some special attitudes!
SIGNAL LOSS RECOVERY
As we live in an imperfect world, and we do very rarely get interference caused LOS, the Radio system should have a fast recovery function. In the days of analog when the radio signal was restored the control functions also immediately returned but modern systems, (well some and most with some form of flight controller), seem to have a rather long delay whilst the radio reinitializes and restores control. This alone makes the current radio systems fail the airworthiness test!
RANGE TESTS
I cannot give my honest, (full of expletives), opinion on the range test feature as to me it is a HOODWINKING tool included by some/most manufacturers. Whilst it does give a very rough guide to the performance of the Rx or Tx it is effectively a go/no-go test at best. Passing a range test on the ground where polarities match proves nothing of real value when airborne! Even with systems that boast having a BER , (Bit Error Rate), or an RSSI, (Receive Signal Strength Indicator), still fail to give the full picture when it comes to airborne communication quality though if I had to select one I would chose BER as it can reflect the presence of possible interference!
IDEAL RADIO SYSTEM
The ideal radio system based upon current technology and not on current trends is to have a 3 antenna Rx with all antennas set at right angles to each other combined with a dual antenna transmitter, (3 antenna transmitter would be even better)! The chipsets for 3 or more antennas are easily available and in use in modern WiFi applications and WIFI devices are almost dirt cheap!
If there is an outage or loss of control for any reason there should be a fast recovery mode to allow the radio to regain ANY control possible ASAP for the crafts safety.
A BER indication rather than a simple Tx power reduction feature as all range tests really are!.
The inclusion of a Micro SD card slot to allow the recording of flight features and radio signal performance. The slot could be disabled on boot up if there is no card present. This would assist in determining if you had interference, a LOS or the crash had another non-radio cause!
A possible flight data recording function.
If the RC model aircraft fraternity and national associations were to unite behind the insurance companies and not only seek an airworthiness rating for radio systems but also push for the design of a proper radio system then LOS crashes would quickly become so very infrequent that our insurance costs could actually drop!
GENERAL WARNING!
Water and 2.4GHz do not co-exist happily! When flying avoid flying with the plane dripping in water as you would in rain or on a morning with heavy dew. 2.4GHz is used in microwave ovens because water absorbs the radio waves and converts them to heat. Using 2.4GHz in your plane with the antennas behind a sheet of water does cause a loss of signal or at the very least a decrease in effective radio range! You have been warned!
ANTENNA POSITIONS FOR MODEL AIRCRAFT
We should all have heard by now that the antenna/s in your model should be clear of all wires and other metal objects. This is because the metal objects in your plane can interfere with the Rxs ability to receive a clean signal or even a signal at all. With 2 antenna Rxs the 2 antennas should be mounted at right angles to each other as well as being clear of all wires etc.
If you only have a single antenna then mount it out of the top or bottom of the fuselage and fly the plane up and down the field without too much banking in the turns because if you bank it too hard or even loop/roll the plane a LOS will almost definitely occur!
With dual antenna Rxs I mount them on the outside of the fuselage away from as much metal as possible with one 45 degrees forward and the other 45 degrees rearward. They can also be fitted to the top/bottom of the wings with both leaning 45 degrees backwards. No matter which position you find works for you you should ensure that the 2 antennas are still at right angles to each other!
BALLET
You can of course put the ground station through matching maneuvers to attempt to maintain the same polarization and thereby maintain communications but sadly I am getting too old for ballet and I go to the flying field to fly and not dance!
I hope I covered most of it even if only a brief touch on the subject!
Have fun!
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