DragonLink Instruction Manual: Operation

Support related stuff, for various products
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Daniel Wee
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DragonLink Instruction Manual: Operation

Post by Daniel Wee » Fri 31 Jul 31 2009 11:42 pm

Assuming that you have everything connected correctly and have double-checked your connections, you are now ready to power up the system. Let us start with the transmitter unit. Before you do anything, ensure that your antenna is attached and securely connected to the transmitter output SMA connector. Turning on the transmitter without an antenna will result in very high SWR which could damage the transmitter's output amplifier or degrade the output power over time.

When you first turn it on, you should see a flash of red on the status LED, followed by a steady green light. If this is what you are getting, you are in very good shape. If you see no light at all, this could mean that the transmitter module is not detecting any PPM signal, or an inadequate signal level. The LED will not light up until it gets a proper PPM signal.

If you are not getting any light - immediately power down the radio and inspect your connections. If you have a meter, check the continuity of the connections and see if there are shorted wires. You should also check that sufficient voltage is present on the power lines (red/orange and shield). The DragonLink transmitter will require at least 6V and no more than 18V for proper operation.

*Note that the output power is unaffected by the voltage as it is internally regulated. Neither is the output power affected by the particular model of radio that you are using.

Each transmitter unit has its' own ID number and the receiver will need to be bound to this ID number before it will respond to the corresponding transmitter. At this point, before you performed the binding operation, the receiver will not respond to your transmitter - this is normal behaviour. (It is possible that John has pre-binded the transmitter to the receiver. Even so, it is better to re-bind.) So the first thing you will need to do is to bind the receiver to the transmitter. The binding process not only associates the receiver to the transmitter in question, it also tells the receiver the number of channels to honour. So if you change the number of channels used in your radio (such as is possible on the Multiplex Royal radios), you need to re-bind for the receiver to recognize the additional channels.

Daniel Wee
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Re: DragonLink Instruction Manual: Operation

Post by Daniel Wee » Mon 10 Aug 10 2009 6:25 pm

Binding the receiver to the transmitter

This is a necessary step in most cases because the transmitters come with a unique internal ID. The receivers may or may not already have an ID that corresponds to the transmitter ID. In either case, it is good to explicitly bind the receivers to the transmitters or the receivers may not obey the transmitted commands. The procedure for binding is as follows:-

1. Turn off the receiver and transmitter, ensure that the antenna is connected to the transmitter
2. Turn on the transmitter while pressing the red button
3. Release button after about 2 or 3 seconds - the LED should be flashing green (binding mode)
4. Turn on the receiver

This should complete the bind operation.

5. Turn off both the receiver and the transmitter
6. Turn on the transmitter, LED should be a steady green
7. Turn on the receiver - test to see that the channels respond

Programming the failsafe

It is recommended that the failsafe is programmed before using the system. In particular, we want to ensure that the throttle channel doesn't go ballistic should anything go wrong. The receiver is programmed not to output any servo signals until valid packets are received from the transmitter (that it has been bound to). Once a proper link is established, pressing the red button on the transmitter will command the receiver to save the current servo positions as the failsafe position. So, simply ensure that all channels are as you would like it in failsafe, then press the button - the LED should go red - and then release the button.

You can test the failsafe by simply turning off the transmitter. Turning the transmitter back on should restore the link control.

Hooking up the receiver

There is not much to be said about this except that the negative pin is the one along the edge of the board. So if you're connecting anything, just make sure that the black wire of the servo cable is towards the edge of the receiver board. Channel 1 is the channel closest to the top edge of the board, where the yellow tantalum capacitor is. Channel 9 is closest to the MCU.

When you power up the receiver, it will not output any servo control signals until it receives consecutive valid PPM packets from the transmitter. Once it receives this, it will initialize and the servos will become active. After this, if it loses the signal for any reason, it will kick into failsafe. Please please please set your failsafe before you fly. It only takes 2-seconds to set so please do it. After the receiver starts to receive control packets from the transmitter, it will start making automatic tuning adjustments if any are required. This is transparent to the user but please check to see that all the controls are smooth and there are no jerky movements.

If this is the first time you are testing out a system, please perform an extensive ground range test before flying.

Using the Dragon Link

It is strongly advised to ensure that the antenna is connected before the transmitter is powered up. A steady green LED indicates that PPM is being received. If the LED turns red, that indicates that PPM is lost. However, if no PPM is received at power up, the LED will not be lit. So if you have applied power but the LED remains dark, check the PPM signal. Ensure that the radio is set to PPM mode (as opposed to PCM). In some cases you may need to put the radio into some kind of trainer mode. Different radios have different quirks with regards to getting a PPM signal out.

With the receiver, the best antenna should have some kind of radial attached to the ground side (as shown in my home-made antennas.) The angle should be around 50 degrees down or maybe a bit more. The length of the radial should be about 164mm.

The first thing you want to do is to make sure the link is operation and failsafe has been set. For your initial attempts, ALWAYS ALWAYS ALWAYS do an extensive ground range check, preferably in the same flight setup you intend to fly with. I cannot over-emphasize this. The way to do this is to remove the antenna from the receiver and see if you get decent range out of it (you should see quite a bit of range.) Check that all channels are working and that failsafe is also working. Make sure that your transmitter has sufficient voltage - the transmitter will run from about 5.5V up to about 14V and will draw only about 190mA at 8.4V so it should give you considerable run time. If your radio has a regular transmitter module (for 72MHz for example), remove it to conserve power.

Test the unit thoroughly on the ground until you feel fairly confident with it before attempting to fly with it. The transmitter antenna is articulating and you should try to have that antenna stand vertically (pointing upwards or slightly towards you the operator) as much as possible as this will give you better performance. When you do a test, try to have everything running - video, GPS, camera, and so on. And do some motor runs (with the propeller off if necessary) to see if you are glitching or losing packets. Using this system, you should be able to feel a progressive loss of smoothness when the link degrades. It helps that you recognize this degradation behaviour as it will inform you of link issues.

LED indications

The following describes the status indicated by the LED:-
Dark - no PPM signal received or no power to the module (pre 1.7 firmware)
Red+Green mix - RF module error (only in firmware 1.7 onwards)
Red steady - Power to the module but no PPM at boot-up (only in firmware 1.7 onwards)
Green steady - Normal operation, all okay
Green flashing - Binding mode, no control possible
Red flashing - PPM signal lost, no control possible
Green and Red alternating - Calibration mode, no control possible

If you get other status - such as irregular blinking lights, or solid RED, or any other undefined status, please remove power and contact us immediately.

Antenna orientation

As we are operating in fairly high frequencies, it is advisable that the antenna is oriented vertically (if the receiver antenna is also vertically oriented that is). The monopole whip that is used has a radiation plot in the shape of a donut. This means that if you point your antenna towards the plane, there will be a blind spot there and you are liable to lose signal. Always be mindful of how the antenna is oriented during your flights.

Remember, better safe than sorry and patience is a great virtue. Using the Dragon Link correctly will entail a certain learning curve and understanding the factors that affect the performance and it is best to become familiar with these factors before trying to fly with the system.

Daniel Wee
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Re: DragonLink Instruction Manual: Operation

Post by Daniel Wee » Sun 16 Aug 16 2009 8:38 pm


Okay, this next tutorial is about antennas and how to make the most out of them. Note that RF signal can have different characteristics at different frequencies and knowing what these characteristics are will help us maximize the performance of the system. Higher RF frequencies tend to be more susceptible to multi-pathing and polarization issues whereas lower RF signals tend to convolute more easily through the atmosphere and do not suffer as much from polarization issues. At 434MHz, we're somewhere in between where we are not as susceptible to polarization issues as, say a 2.4GHz RF signal. This means that cross polarized diversity antennas aren't going to be as effective at 434MHz than at 2.4GHz. I believe there would still be some advantage but nowhere near what you would see at 2.4GHz. Having said that, for ideal performance, it is necessary to maintain the transmitting and receiving antenna in the same orientation. The best orientation in our case is the vertical orientation since it remains the same no matter which way you turn. This is why the DragonLink is supplied with an articulating antenna for the transmitter. Even so, it is my opinion that a proper dipole (such as the one provided for the receiver) will work much better than the monopole whip provided for the transmitter.
Avoid this if possible
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Ideal arrangement
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The standard antenna provided with the DragonLink is a quarter-wave whip (for the transmitter) and such antennas work best at least one wavelength (70cm) above ground. The height above ground affects the signal's launch angle and at 1-wavelength, the angle is probably around 15-degrees or so upwards. See the following link for more discussion on launch angles (albeit on a different frequency but just use the wavelength as a guide):-



Another important aspect of a good antenna system is actually the ground. Many people do not realize that the ground is part of the antenna system and the quarter-wave whips often do not come with a counterpoise. This means that it will rely on your radio system and your body to act as the ground/counterpoise. As a result, your holding of the radio could impact the performance of the antenna system - usually holding the radio will let it perform better unless you have a specific ground arrangement. This is typically the case for unbalanced antennas such as the monopole whip. A proper balanced dipole, on the other hand, doesn't suffer as much from this problem. This is why there is a ground wire on the plane antenna. The counterpoise length is important and is cut to specifications in order to perform correctly so not any wire length will work.

To improve your range further, one of the easiest way would be to use a better antenna - such as a small yagi (number of elements depends on the gain and beam width that you want).
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Because of the way the receiver is mounted, many users will want to use an extension cable for the antenna. In my next tutorial I will explain a little bit about what is needed for a good extension cable. Hint: It's not just about adding a piece of wire.

Daniel Wee
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Re: DragonLink Instruction Manual: Operation

Post by Daniel Wee » Wed 19 Aug 19 2009 10:07 pm

Extension cables

I imagine that many users would want to use an extension cable on the receiving antenna feed. While this is totally possible, it is important to be aware of the things that can trip you up. Of these the most important is the impact on the feed length (which includes the extension cable if any) on antenna performance. Most people do not realize that when feeding a balanced antenna system from/to an unbalanced source (DragonLink TX/RX) - the feed will automatically become part of the antenna system, potentially de-tuning the antenna (severely). The reason this is so is because the reflected RF signal will start making its' way down the shield of the extension cable so that the length of the cable is added to the antenna length. This is, in most cases, undesirable. The way to resolve this is to make sure that you have a current choke on the extension feed/cable right at the antenna end, thus preventing the RF signal from making its' way past the choke.

Such current chokes (often called baluns as well) are fairly easy to make. It can be as simple as having some ferrite collars on the cable at the antenna end, or several loops of the cable, preferably through a ferrite clip or similar. For a more in-depth explanation of baluns, see the attached PDF file.
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A second potential problem with extension cables is the loss incurred over a length of cable. Depending on the type of cable, the loss can be quite significant if the cable is long. Usually though, for short runs of less than a meter, the loss is insignificant. If you are planning a long cable run, you should be concerned about how much loss you are incurring.

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Daniel Wee
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Re: DragonLink Instruction Manual: Operation

Post by Daniel Wee » Mon 31 Aug 31 2009 10:45 am

Operating together with other 433MHz FHSS systems

As these LRS units gain popularity, you may find yourself operating together with other similar FHSS systems. This tutorial goes through some of the considerations you need to take when in that situation.

FHSS means that the transmitted signal is hopping over a set of frequencies in a unique pattern. Each transmitter should have a unique UnitID, which other than being used to identify the correctly designated packets of data, is also used as the basis for generating what is known as a hopping sequence. What this means is that for every different UnitID, a different and unique hopping sequence is generated. This does not, however, mean that totally different frequencies are being used. It only means that the sequence in which it uses those frequencies would be different. It may well be that the frequencies are completely different but they all come from the same pool of channels that are available so there may be some shared frequencies even in different hopping sequences. So, for example:-

UnitID 123
Sequence A B J H K Y

UnitID 47
Sequence D E J B U

Notice that channel J and B are used by both sequences but other frequencies are not shared. Also note that the sequence length is not necessarily the same. In the DragonLink, the sequence is typically around 19- to 24-channels long, out of a pool of about 30 frequencies.

The hopping rate is locked to the PPM frame-rate of the radio. This is why it is important that the PPM stream uses a constant frame-rate. Most radios do but some Graupner radios, for example, don't, and this would break the FHSS system. Synchronizing the hopping-rate to the PPM frame-rate gives us the lowest possible latency (best response time).

With all these in mind, there are several implications that we can already see:-

1. Even if two radios with the same UnitID (and thus the same hopping sequence and frequencies are used), full collision will only take place if both radios have the exact same frame-rate, and are started in the same time slot (within about 20ms of each other). If the radios are started more than 20ms apart, the frequencies would never collide since they would not occupy the same channels at the same time. This also means that if there is a collision, simply re-starting one of the radios could completely resolve the problem. Normally, though, it is not recommended that two radios with the same UnitID be used simultaneously because if the receiver loses lock and attempts to re-acquire, it could acquire the wrong signal.

2. Even with completely different UnitID's, and therefore, hopping sequences, there may be the occasional collision. These should be pretty minimal and probably unnoticeable to the user. The different sequence length, channels, and rate - work together to ensure that such collisions are kept to a minimum.

Thomas's UHF system (now called the Scherrer's LRS) uses the same sequence generator algorithm as that of the DragonLink (since I wrote the firmware for both of them.) However, since the frequency pool is different, this should not create too many problems. Of greater concern is the fact that the Scherrer LRS uses wide-band channels and tends to occupy more bandwidth than the DragonLink. As a result, it doesn't share the band as well as DragonLink does. Nevertheless, the FHSS implementation should be robust enough to deal with minor collisions.

*The impact of using wider band signals is that the same power is distributed more widely, as opposed to narrow band. As such, for the same power levels, the narrow band signal will have better penetration. Think about a focused torchlight beam compared to a diffused one.

Daniel Wee
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Re: DragonLink Instruction Manual: Operation

Post by Daniel Wee » Mon 31 Aug 31 2009 4:59 pm

Documenting the un-documented features

There are some features on both the TX and RX board that is not documented. These are mostly diagnostic features though and some may require special equipment to use. Most of the features discussed in this post are those that can be jumper activated. Typically, these would be jumpers between the signal pins 5 to 9.

TX module
5/6 Calibration
6/7 Low-power mode
7/8 Set UnitID
8/9 Alternate UnitID

RX module
7/8 Force PPM output (only jumper during binding)

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