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Airworthy Radios and Installations (a few thoughts)!

Hai-Lee

Old and Bold RC PILOT
#1
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.

Left-3-D-radiation-pattern-of-a-dipole-omni-directional-antenna-right-3-D-radiation.png

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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BATTLEAXE

Well-known member
#2
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 wires that protrudes from your radio Rx and 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.

View attachment 136949

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 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 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 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 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 can 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!

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!

I hope I covered most of it even if only a brief touch on the subject!

Have fun!
JBT my friend. I know what to look for in my next communication set up. Thx Bro
 

Merv

Well-known member
#3
One thing to know is, the active element. It is the exposed wire at the end. This what you care about, it should be fairly straight. It needs to be a specific length and away from anything metal, carbon fiber or electronic (ESC & battery). Those items will block the signal. If you have a 90 degree bend in the middle of your active element, you will drastically cut your range. The black part (in this picture), you don't care about, can be any length, tie it in a knot, hide it in carbon fiber. It's just the antenna lead.

If you fray the shielding (the lead) you will allow unwanted rf signal in, which will kill your range. The antenna must be repaired or replaced. To repair, just cut off the frayed coax and solder the remaining coax back onto the Rx board. If you manage to cut off a portion of the active element, just peal back more of the lead. Its best to cut off a bit more of the lead than necessary, then trim the active element to length. Like they are doing in this picture.




I have flown many years with a single antenna Rx (the old Flysky), installed on the inside of my plane. The only time I have had any range issues is when I damaged my antenna lead. A range check is invaluable to discover this damage on the ground and not in the air.

Any time the Rx is stopped by the antenna catching on something, you have the potential to damage the lead.
 
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sprzout

Knower of useless information
Mentor
#4
One thing to know is, the active element. It is the exposed wire at the end. This what you care about, it should be fairly straight. It needs to be a specific length and away from anything metal, carbon fiber or electronic (ESC & battery). Those items will block the signal. If you have a 90 degree bend in the middle of the active element you will cut your range. The black part (in this picture), you don't care about, can be any length, tie it in a knot, hide it in carbon fiber. It's just the antenna lead.

If you fray the shielding (the lead) you will allow unwanted rf signal in, which will kill your range. The antenna must be repaired or replaced. To repair, just cut off the frayed coax and solder the remaining coax back onto the Rx board. If you manage to cut off a portion of the active element, just peal back more of the lead. Its best to cut off a bit more of the lead than necessary, then trim the active element to length. Like they are doing in this picture.




I have flown many years with a single antenna Rx (the old Flysky). The only time I have had any range issues is when I damaged my antenna lead. A range check is invaluable to discover this damage on the ground and not in the air.

Any time the Rx is stopped by the antenna catching on something, you have the potential to damage the lead.
That's one big reason why I liked the older Spektrum receivers. I sheared off an antenna in a quad crash, and all I had to do was pop open the case for my receiver, pull the IPEX connector antenna off, and pop a new one on. $8 for 10 of the antennas, and it works great. :) I can't tell if your antennas on the board are soldered on or using connectors; the replacement antennas I have state that they can be used for FrSky receivers too, so I would imagine there might be some FrSky receivers that utilize the IPEX connections. :)
 

Hai-Lee

Old and Bold RC PILOT
#5
Quads and the like do not put the same demands on the radio system that fixed wing aircraft do! Quad experience and radio performance experienced can be good even on a single antenna radio system because the flight attitudes and the time at attitude vary considerably from that of fixed wing craft.

Sure quads can roll and loop but they cannot knife edge or maintain flight attitudes for the same time periods that fixed planes do. Fixed wing planes are poorly served by the current generation of radios and most used in fixed wing aircraft are not really suitable for the task according to the basic antenna theory!

Ground reflections, structural reflections, combined with high powered ground stations can provide a degree of masking of the radio inadequacies but be aware those inadequacies do exist and will bight when least expected!

Have fun!
 

Hai-Lee

Old and Bold RC PILOT
#7
Just a bit of news! Our local club had another fly away/LOS today!

The user was using a Spektrum DX9, (a good dual antenna radio), on DSMX, (a more capable air protocol), in combination with a 2 antenna Spektrum'Orange Rx, (short wires and buried inside the fuselage). The whole setup passed the usual ground range check.

At the time I was just bringing a motorglider, (a buddy boxed flying lesson using the FlySky iA6 2A), in for a landing in a moderate wind. During the downwind leg I had let the glider go deep to the very beginning of the landing glide slope to show the student the appropriate landmarks and the line of the approach. The other pilot was flying a foamie corsair and he was flying higher but somewhat behind the glider. He followed my path to practice his approach line, (it was a new field for the club).

Anyway I did the turns and was lined up on approach when the other pilot started yelling about control issues. It seems that he could maintain control with the plane flying away but everytime he attempted a turn the plane would start the turn and then start diving and spinning OOC. Each time he managed to recover the plane it was recovered flying away! Eventually he lost all control and watched the plane slowly turn slightly left and disappear behind a massive line of very tall trees on the top of a hill some 800 metres from the flight line.

He had been warned by fellow club members about the quality of his Rx installation for fixed wing usage but he had received alternate, (internet and local retail outlet), advice that the radio would work to maximum range as long as the antennas were at right angles)! Now he knows better! He saved $20 on the Rx but lost a $200 plane because of it!

Spektrum make Rxs with dual long wire antennas and even satellite antennas but they are generally more expensive! When choosing a radio DO YOUR RESEARCH and if the better performing Rxs cost more so be it! It doesn't happen for everyone, every time, but statistically you will eventually pay the price for going for poor design equipment on your fixed wing, (even if you do not recognize it as a LOS)!

Have fun!
 

whackflyer

Active member
#8
This is an excellent thread! I run Spektrum, DX6 G2, and older Spek DSMX receivers, and the “PowerUp” DSMX receivers. I’ve never had a LOS other than once when my transmitter was still running outdated firmware and when the low battery alarm came on, it caused my mini f-22 to meet its end. I get excellent range from the “PowerUp” receivers, but i usually try to fix the two short (maybe inch long) antennas at right angles to each other. I go as far out as I am comfortable, maybe a half mile at the most, and never have had a LOS. Keep in mind I do fly many miles from any city (closest town is 13 miles away) and no transmission towers of any kind are near. For newbs looking to get into the hobby, Spektrum is not the cheapest option, but I will say that they are very user friendly and the customer support is world class. Not saying Spektrum will work for all circumstances, but it works for me and so far I’m happy with it! Keep up the great posts!
 

FDS

Well-known member
#11
I have never had any signal loss on DSMX with my Orange TX6i and Lemon receivers, right out to the limit if my ability to see. I use a satellite receiver on large planes, always have one antennae out the bottom of the fuselage and the other at 90 deg, site the receiver away from the motor and ESC.
 

Piotrsko

Well-known member
#12
@Hai-Lee Based on your example about radiation patterns, might I prevail upon you to create a primer for multi antenna positions and their subsequent relative merit?.

I have only long wire 70 mhz equipment therefore my ramblings are not necessarily relevant.

@whackflyer : there are many users of high powered commercial equipment tight beaming a couple of kilowat through the countryside both sides of our alloted bands about 50-100ft agl.
 
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#13
I've never used a Lemon satellite receiver piggybacked with a Lemon 6 ch. What will that gain operationally and why? I have an extra satellite receiver and might be willing to try it. Thoughts?
 

FDS

Well-known member
#14
I have a large heavy Sportster, I extended the front by 2” to balance the heavy UK foamboard. It runs a big motor and a relatively large battery/esc. The receiver is just under the wing, with one antennae poking out near the front.
I had a spare Lemon satellite, I painted the plane in a metallic paint, the main receiver was in a position where it might get interference from the ESC which is underneath the power pod. It’s also near the metal landing gear.
I put the satellite right back behind the wing with its antennae in the opposing two poles to the main RX, just to maximise coverage.
My thinking was that it’s a big, 800g plane, LOS is the last thing I want.
 

sprzout

Knower of useless information
Mentor
#15
Some of the older OrangeRX receivers had quality issues - primarily with the DSM2 receivers. The newer ones are better, but they're also more expensive than the previous ones. Still doesn't scare me off from using DSMX; I see lots of incidents where people blame "loss of signal" on a crash or flyaway, when it turns out to be mechanical failure. Had an incident this weekend at my field where the guys started blaming his radio for the problem. He walked out to retrieve his downed plane, and discovered that his aileron had separated from the wing, causing his crash into tall grass.

Does this mean that I don't think loss of signal occurs? Heck no. I know it has. I've had it myself. But it's few and far between where it has actually been a signal issue and not a mechanical issue. I've seen far more that have been things like motors getting too hot and seizing, or ESCs burning up, or servos freezing up, or even batteries dying in midair.
 

Hai-Lee

Old and Bold RC PILOT
#17
@Hai-Lee Based on your example about radiation patterns, might I prevail upon you to create a primer for multi antenna positions and their subsequent relative merit?.

I have only long wire 70 mhz equipment therefore my ramblings are not necessarily relevant.

@whackflyer : there are many users of high powered commercial equipment tight beaming a couple of kilowat through the countryside both sides of our alloted bands about 50-100ft agl.
I will definitely supply the "primer" on our current communication issues to further explain the problems as I see them.

FORTUNATELY, I have been distracted somewhat with the upcoming release of the FlySky Paladin series. The first real step forward in 2.4GHz RC radio equipment in a long time! Funnily enough it looks like it covers most, (still not all), radio issues. One issue I had not mentioned in the OP was the vertical antenna position which is effectively pointed at the plane when in the air, (an antenna pointed at the plane provides little or no signal), but the "Paladin" seems to have addressed even this issue! I am sad at the release price and a few functions that are missing but otherwise an exciting radio control milestone/development.

When the 2,4 GHz equipment first appeared using the ISM band the rush was to provide a product and some systems were just the same old radio systems with a different RF module. The properties of radio propagation at 2.4GHz were almost completely ignored. Sure you got all those lovely programmable features, (Bells and Whistles), but the extra features did not make the actual radio work any better.

I believe that in business you do not add features to a product unless the features either make the product better in performance, cheaper, or more desirable. Recently Spektrum and FlySky adopted the dual antenna system on their top end transmitter products. For example you can find a second hand older version Spektrum DX9 and compare it to the latest Spektrum DX9 and the most serious technical change has been the adoption of the dual integrated antennas array. Spektrum would not have adopted it if it was not needed!

OLDER radio systems got away with the use of a single antenna by use of a phenomena called reflection. Longer wave band frequencies are actually reflected by almost everything including the human body but mostly the ground and the reflections can also change the transmitted signal polarity and of course its direction. effectively the earth was acting a a large non-directional reflector. When the RC equipment moved to 2.4GHz the earth reflector effectively was withdrawn except at extremely short range!

Enough rambling for now!

Have fun!
 

Hai-Lee

Old and Bold RC PILOT
#18
I was just looking for a few diagrams to help explain antenna placements and why when I came across someone"s similar work/post on close to the same issue. Whilst the approach and focus are ever so slightly different the placement guide is quite good and well thought out.

I could plagiarizer his work but credit for his work is truly deserving. Here is the link to his antenna positioning advice:- http://open-txu.org/home/continuing-education/introduction-to-antennas/

With a full dual antenna system there are still a few holes in the projected radio coverage but they are easily allowed for. I still hold that a 3 antenna system for the Receiver is required to eliminate coverage holes entirely, (assuming that you get the antennas clear of metal etc and they are mounted on three separate axis's at right angles to each other.

Have fun!
 

Piotrsko

Well-known member
#19
D'Oh, I had forgotten how fussy 2.4 gig was for receiving. Usually not such a big problem with dishes, yagis and a Kilowatt or so to throw at the reception end on top of a huge building.

This begs to question: does the newer stuff suffer these line of sight issues? I have heard of allocation of a new band called 436, not sure it's mhz or ghz
 

Hai-Lee

Old and Bold RC PILOT
#20
D'Oh, I had forgotten how fussy 2.4 gig was for receiving. Usually not such a big problem with dishes, yagis and a Kilowatt or so to throw at the reception end on top of a huge building.

This begs to question: does the newer stuff suffer these line of sight issues? I have heard of allocation of a new band called 436, not sure it's mhz or ghz
Sadly almost any freq over 1 GHz is almost totally Line of Sight. There used to be a 10 GHz ISM Band as well as a 900 MHz band. The 430 odd MHz band is also ISM but it used to be full of garage door openers, (using SAW and ceramic resonators), and is very narrow in bandwidth.

As for the new equipment all I can say is that the FlySky is quoting 3Klm range for fixed wing aircraft using 2.4 GHz with only a moderate increase in Tx power! In addition it is claiming a far faster signal recovery time in the case of a Loss Of Signal! Mind you until it is easily available I will not commit as I what backwards compatibility as well as support for other radio manufacturer radio transmit protocols.

It looks like there is an exciting generation of radios coming in the next few years!

Have fun!