Monday, October 21, 2013

Laser Collimation of a Newtonian Telescope

There are endless articles on Newtonian telescope collimation all over the Internet. A recurring theme is that plain laser collimation is not the best option since Laser Collimators should be collimated first and most have poor collimation out of the box to begin with. This is probably true and I am not arguing with although my Baader Planetarium Colli SC actually was near perfect out of the box.
Here are a few thoughts on the subject while I was toying with collimation methods on my 8" Meade Schmidt-Newtonian LXD75 SN-8. This articles is intended to explain why the SCA Laser collimators (Self-Centering type) are not the answer and IMHO are over-hyped.
Collimation is the process of aligning the optical axis of each optical component in a common optical axis for the entire instrument. In a Newtonian telescope this means that the Primary Mirror optical axis should align with the eyepiece optical axis perfectly. The secondary mirror which is on the light's path has to be angled properly to match both axises. The tilt angle of the secondary serves to position the eyepiece's optical axis in the physical center of the primary. Then the primary is angled to align the direction of the optical axis so it will match the one of the eyepiece The eyepiece is the reference point since the position and angle of  its optical axis is not adjustable, but the other two components - both mirrors can be angled  and moved in the case of the secondary. Each component in the system - human eye, lenses in eyepiece, secondary, primary mirror and the light from an  object should be treated as vectors - they have XYZ position and direction!

For a laser collimator, collimation means that laser beam should be perfectly centered and parallel with the collimator's mating barrel axis. When inserted in the focuser, it mimics an eyepiece, and its the reference for the entire collimation process - again, a vector connecting and starting from laser LED, located in the XY center of the barrel and exiting the barrel at zero degrees from both X and Y. If there is a slight angular error, it will manifest as much larger linear value after it travels thru the entire OTA system.
An eyepiece is secured in the focuser by either a thumb screw(s) or a compression ring. In both cases, the eyepiece's barrel OD is a tad smaller than the focuser's insertion tube ID to facilitate easy insertion. When secured, the thumb screws push onto the barrel, pressing it against the opposite wall of the focuser, thus offsetting the optical center of the eyepiece inside the fouser tube. (same goes true for the compression ring btw, contrary to the believe that the compression ring grabs the barrel with the same force all around).
Here is the main problem with Self-Centering Collimators - they do work as stated - rubber rings around the barrel are compressed and expand evenly, centering the barrel inside the focuser tube which is fine, except for the fact that your eyepieces does not employ the same self-centering technique. When you collimate the telescope with a SCA collimator, you reference the entire optical system to a perfectly centered eyepiece, but when you use an actual eyepiece, the compression ring or thumb screws will push it to the side and create miss-alignment. Here is an article by Hotech USA describing the issues addressed by the SCA collimator. In fact, while they do solve the centering problem, they create another one by creating an optical axis which, while perfectly centered in the focuser will never match the one of the normal eyepiece when placed and secured by normal means.
What is the answer to the problem with securing and using a laser colimator, besides having the laser collimator collimated - make sure that the OTA is horizontal, the focuser is pointing straight up (90 degrees from the level OTA), place the collimator in the focuser tube and let it rest with its shoulder flush with the top surface of the focuser, tighten carefully but firmly the thumb screws (if two thumb screws are used by the focuser, tighten the first screw until it makes contact with the eyepiece / collimator but its not too tight, tighten the second screw at 100% force and go back and re-tighten the first screw again - this will re-position the eyepiece properly) and lastly, dont touch or wiggle the collimator or move the OTA during collimation.

 Collimator should emulate an eyepiece, including the way it is positioned and secured in the focuser, not introduce a completely new method for centering, which can not be ever achieved with an actual eyepiece. Who cares about repeatability between collimator insertions when it can not be repeated with an eyepiece?
The best is to incorporate the self-centering mechanics in the focuser itself - hear that Moonlite? Or instead of using thumb screws / brass compression ring - use a deep/long wedge with 120 degrees of eyepiece barrel curvature and two angled radial thumb screws near the edges of the wedge.

Friday, July 19, 2013

Using smartphone as a tire toe angle gauge - E-Revo VXL

Here is one quick tip for the RC Cars crowd.
I don't like "eyeballing" things that can be easily measured. For example, the tire's toe angle on many RC cars is often adjusted by drivers just by looking at the tires from the top and estimating the angle - hardly a precise method.
Nowadays, almost everyone is sporting some type of a smartphone and every smartphone has built-in an accelerometer and/or gyroscope sensors. There is a galore of apps using these sensors to turn the smartphones into "spirit levels"and as added bonus, they can be calibrated for relative measurements as well (automatically subtracting the angle measured during the calibration process from the current reading). Just to name a few such apps (all Android OS btw) - Smart Tools, Spirit Level Plus, Precision Bubble Level, Gravitometer, etc...

Step 1 - Place the E-Revo VXL on the corner of the table, laying the car body only on the battery compartment door (bottom tires hanging free in the air). Calibration of the software is done on the flat work surface or even better - placing the phone onto the top battery compartment door - exactly parallel with the length of the car. This is the axis that needs to be calibrated - the other (perpendicular) axis is not important for toe angle adjustment and it is OK to be angled (Revo's battery door is angled in such way). The back of the phone should be laying flat over the surface of the door and the phone should be pressed and held firmly when calibrating it. The picture shows almost zeroed X axis - this is the axis running with the car's length.

Step 2 - Place the phone on top of the wheel (even better if you have only a rim with no tire installed), managing phone's orientation strictly parallel to the length of the car and read / adjust the tire toe angle. (in this case - axis X reads 3.1 degrees toe-in a little too much).

Step 3 - After adjusting the desired toe angle, take off the toe push rod on the adjusted side and use it to adjust the length of the other toe push rod. 

That's all - no need for an expensive specialized gauge.

Tuesday, July 16, 2013

Spektrum Telemetry on E-Revo VXL

My 3 year old son tends to over-steer and over-throttle his E-Revo (he has somewhat fuzzy understanding of the word "gentle" :) so to make the vehicle more kid-friendly I needed an "Exponential control" (Expo) feature and Dual-Rates - something  not available on the cheap stock Traxxas remote control. Naturally, I turned to Spektrum and their DX3S surface radio in particular. The kit came with telemetry capable SR3300T receiver and sensors - such nice functionality should not go to waste.
The SR3300T has inputs for a temperature sensor, a RPM sensor and a LAP sensor. The voltage sensor is internal to the receiver (more on this later...)

 Installing the temperature sensor is a very straight-forward process. I just attached the sensor to the under side of the motor housing using self-adhesive Kapton tape (high-temperature silicone based adhesive).
The RPM sensor install turned out to be a bit more involved. The stock spur gear cover has a mount for RPM sensor but Traxxas telemetry is using magnetic sensor - a small magnet is placed in the spur gear and the sensor on the cover picks up the magnet's rotation. Spektrum, on the other hand is using optical (infra-red) sensor. IMHO this is a more universal solution - a reflective or non-reflective sticker (depending on the situation) can be placed on almost every spinning part and it will not cause off-balance issue like the heavy magnet will.
This picture shows the optical sensor mounted on the spur-gear cover. I drilled a small hole for the opto-couple and secured the little sensor PCB inside the magnetic sensor bed, using the screws provided by Spektrum. A word of caution - the clearance between the inside wall of the spur-gear cover and the spur gear is very small - I had to shorten the screws so they are now flush with the inside wall and don't catch on the spur gear.

This is the outside of the spur-gear cover. The PCB is mounted with two screws inside the "bed" designed for the Traxxas magnetic sensor. I sealed the Spektrum RPM sensor board with black UV resistant silicone sealant for water/dust-proofing and mechanical protection. In addition, by keeping the sensor in the dark, I eliminate the chance for "confusion" in bright sun light. There is very little clearance on the outside of the spur-gear cover too. One should be careful to place the PCB close to the cover and not put too much sealant on the back so when the cover is installed in place, it doesn't get pressed on by the radio receiver box.

The RPM sensor uses either reflective stickers (placed on non-reflective surface) or non-reflective stickers if the rotating object is reflective (bare metal). The spur-gear surface is not smooth and I wasn't sure how well the sticker will adhere. Instead of using the provided stickers, I made my own reflectors.
I used a thin aluminum sheet (roof flashing), which I polished with polishing compound to a mirror surface and covered the polished side with transparent Scotch(tm) tape for weather protection. Then I cut a few "pizza slice" pieces - different sizes to fit on the 3 different spur gears I might use. I epoxy glued the metal slice to the spur gear as shown on the picture. To minimize any potential balancing issues caused by the added eccentric weight, I selected a spot opposite of the magnet's cavity to glue the reflector and filled the cavity with a mixture of epoxy and metal shavings - approximately the same weight of my mirror + glue combo used (I used very precise micro-gram scale). This is probably not needed at all and it is just me, obsessing with accuracy. The spur gear is small enough and light enough that balancing is not a concern - Traxxas certainly didn't provide a way to balance the much heavier magnet in their setup.

The Spektrum DX3S has a nice calibration feature which allows for the RPM reading to be converted and displayed on-screen as an actual speed (either Km/h or MPH are options besides just the raw spur gear RPM count). Maybe I am nitpicking here, but why you have to input the Roll Radius calibration in inches even when using km/h readout - centimeters is the logical unit to be used.
Speaking of Spektrum shortcomings - why on Earth there is no external voltage sensor input? The voltage reported by the Telemetry module is the internal receiver / servo voltage and not the actual battery pack voltage. On electric models, the ESC supplies power to the receiver and it is usually 6v regulated. Only explanation is that it probably never occurred to the designer that the receiver might be used with electric models too.
Too bad that Spektrum never thought of this! I'd take battery pack telemetry reading on any day instead of the stupid LAP timer. What good is lap time for if you don't know that you should be preserving power just to finish?
A possible work-around is to power only the servos from the regulated ESC and have the receiver powered directly by the battery pack. The price to pay is when the setup is used with 3S or 4S LiPo packs or dual NiMH packs connected in series  -  such solution needs a proper preset voltage drop circuit at the input as the receiver can not handle more than 9.6V. Going that route, one needs to mentally add the amount of the voltage drop to the displayed value to get an actual reading.
I might try to open up the receiver and see if I can hack into the internal voltage sense input. Then I can re-purpose the useless LAP sensor connector as a external voltage sensor. Not sure if it is possible - wish I could find the schematics. If successful, I'll eliminate extra wiring and connectors to separate receiver and servo power and can have just a couple of voltage sensor pigtails for different power sources.

All of the Telemetry sensor wires were tucked in the receiver box and in the space under the motor, making for a pretty clean look.

Monday, July 15, 2013

Installing lights on Traxxas E-Revo VXL

My son really wanted his RC car to have lights "just like on the real cars". The Spektrum SR3300T receiver is a 3 channel receiver and the AUX channel can be used to control lights - so why not? Turning the head and tail lights by flipping the AUX switch on the remote control was an extra feature, I thought would be pretty cool. 
Our design goal was : 2 high-intensity white head lights, 2 red (light red / deep orange) position (tail) lights, 1 red/blue flashing ("dash mounted, "police strobe mode" light) - all switched remotely. 2 dual-step intensity, combined break lights / position lights (deep red color, bright - break, dim - tail) - switched on/off at the model, automatic break signal detection and intensity control.
Parts used: kits - RCL5004E, RCL5090 and miscellaneous connectors, wires, resistors

Front View - head lights - high-intensity cool-white LEDs in chrome "reflector" fixtures providing great level of illumination. The flashing blue/red  strobe light is seen just behind the windshield. This light improves the model's visibility during the twilight hours.

 I made two elongated holes in the Lexan body (using Unibit drill bit) in order to properly align the head lights - parallel to the body, almost level (very slightly tilted forward for maximum illumination of the road ahead of the vehicle). Adjustments were made based on the current suspension settings.

 The head lights wiring harness. The chrome reflector fixtures were glued at the specific angle / position in the elongated openings using hot melt glue. It was a bit tricky to keep them properly oriented while the glue cools down.

 Rear light bar - the black plastic plate with the 6 holes came off the Spektrum DX3S packaging (used to secure the remote control to the cardboard retail box with wire ties. It is the perfect size for the tail light bar. All I need to do is to enlarge 4 of the holes  to accommodate the black plastic bezels and to drill two more mounting holes. The bar is secured to the Revo's wing mount with 3 zip-ties for easy installation/removal.

The outside two LEDs are the dual-intensity, deep red color, tail / brake lights. The inside two LEDs are a second set of tail lights - slightly different red color (deep orange). The LEDs are wired in parallel (on the back side) in two separate pairs and all connections are water-proofed using black UV resistant silicone sealant.

This picture shows the water-proof receiver box with the Spektrum SR3300T receiver mounted, the Y-split wire harness for the dual-servo steering, EBS (electronic break-detection switch) connected in-line with the THR (Throttle) signal going to the ESC (EBS is the clear heat-shrink covered  unit at the top). Pictured also is the RPS (Remote Power Switch) connected between the LED controller and the AUX output of the receiver. It got really tight in the small receiver box but I was able to fit everything inside.

This is my DIY "LED Controller". The RCL5004E comes with a "LED Controller" but it is too bulky and the connector placement didn't work for me. It is not water-resistant too. What they actually call a "LED Controller" in this case is nothing more than a combo of two "power strips" with built-in current limiting resistors.
 I made my own "LED Controller" except I used a single connector, soldered the current-limiting resistors on one side and covered it with silicone and heat-shrink tubing for water-proofing. The wire lead from EBS+RPS plugs in the center of the power strip - left side is RPS controlled for head lights, tail lights and emergency light, the right side is EBS controlled for the dual-intensity brake/tail lights. It could have been even smaller but I left a couple of positions for future expansion and for extra configuration flexibility provided direct power outputs (in case the current limiting takes place at (or near) the LEDs and it is part of the wire harness. In addition, I incorporated two different sets of current limiting resistors with different values per channel - 82 Ohms and 150 Ohms as options for different current (brightness) levels.

The "LED Controller" is positioned between the receiver box and the heatsink of the motor and it is held in place with Velcro. The wire harness for the rear light bar is place in a braided polyamide sleeve. Visible, just in front of the motor between the two servos  is the "emergency light" LED  - it is a special high-intensity dual color (red-blue) fast flashing LED and it is directly plugged in one of the current limited ports of the LED power distributor (on the RPS controlled side).
My goal was to have the wiring look neat - probably this is as good as it will get.

One reason I was trying to avoid the "wire-salad" look is because sometimes, we drive the car with a clear Lexan body that shows off the beautiful vehicle internals (no headlights on the unpainted body).

This is how the complete model looks like. We went with the dark blue/bright yellow color scheme for an improved visibility. The windows were masked as well as the most of the car body, leaving exposed only the areas to be painted blue. Then the yellow paint was applied. All of the painting is done on the under side to protect it from scratches during roll-overs and crashes.

Monday, June 24, 2013

Dual steering servos for E-Revo VXL

Installing a second steering servo on the Traxxas Mini Revo (1/16) VXL is an easy and fairly inexpensive upgrade (about $30 in parts). The E-Revo VXL has already second servo mounting position on the chassis - it is just closed off with a blank cover.
Parts needed - Traxxas 2080 Mini Servo, Traxxas #7043 steering arm kit, link ends and some screws.
There are a few advantages to dual servo steering - the steering is more precise and it holds on better at high speeds, the extra torque provided by the second servo allows you to run much bigger tires, which could otherwise overload  and possibly damage a single servo. Additionally, this setup decreases the wear-and-tear of the servos as the steering load is evenly distributed across both servos.
The main challenge with mounting a second servo is to make sure that both servos are mechanically linked to the bell crank steering arms in such a way that their center (zero) positions (radio: 0 trim, 0 sub-trim) will match perfectly - otherwise they will work against each other!
If their center position is not exactly the same, when linked with the standard, fixed, non-adjustable links, they will "fight" for the center position "ownership", each pulling its side of the bell crank to its own center (zero) and this will cause a premature failure from the constant load (not to mention the increased electrical current).
Some drivers, who use aftermarket 3-channel radios (like the Spektrum DX3S) suggest the use of the AUX channel for the second servo control and then enable the steering (STR) channel mix to the AUX. Doing so brings some serious limitations due to the poorly designed firmware in DX3S - the steering trim on DX3S does not affect the mixed AUX channel - same thing goes for the sub-trim adjustment! If trim is added to the steering channel, AUX will still stay fixed on its own position causing servo binding. Mixing works only during steering wheel commands but not with any of the trims adjustments. AUX channel has its own servo trim function but using it means that one should adjust the steering trim first until the car drives straight with only main servo (STR) linked to the bell crank arm, then adjust AUX trim until spacing between the steering arm and the second servo horn is exactly as the fixed linked and then install the link between that servo and steering arm. This is a severe limitation - no steering trim will be possible on-the-fly without removing a mechanical linkage from one of the servos and going through this procedure again. It turns trimming of the steering into a rather slow and painful process - forget about quick trims when suspension or wheel alignment is adjusted. In addition, the "fighting servos" condition will still exist if the car (ESC) is powered on with no radio turned on to apply trims.

The other way (better IMHO) to do it is to drive both servos from the same steering channel - the original Traxxas receiver has two ports for CH1 or if using a Spektrum receiver one can use a Y-split for the steering channel. Then, the difference between servo's centers is compensated by using an adjustable length mechanical link - turnbuckle or regular screw type adjustable link will do the job. Things are never simple tho - the spacing between the steering arm and the servo horn is very small and a custom adjustable link must be fabricated to fit.

I just used two plastic toe link ends (GPM toe links), trimmed them short enough so their combined length when put back-to-back is just a tiny bit less than the original Traxxas steering link and used an M3 screw as a push rod between them (the screw head was cut off with a Dremel tool)
This approach worked perfectly. I connected both servos to the STR channel on my Spektrum receiver and set the radio to 0 trim, 0 sub trim. Using the original Traxxas fixed link, I linked the servo with the best 90-degrees-servo-horn-to-body position to the steering arm and then connected the other servo with the adjustable link so there was no binding (no buzzing or humming from any of the servos). Using steering sub-trim, I made the bell-crank arm exactly perpendicular to chassis.
I also set the radio for servo travel adjustment of 116% and it seems to give me a little more steering range.
That's all there is to it - steering trim is now available as usual. Adjustable link is not needed if you are really lucky to have both servos matching their center positions, but this is very unlikely due to the way the splines are made in the servo horn - normally, the servo will take the horn exactly at 90 deg. on one side, but it will be off when placed 180 degrees from that position (which is actually required for dual servo setup).
I installed metal servo horns, but they have exactly the same splines as the plastic ones.
Another mod I made to the steering system is to replace the plastic bushings in the bell-crank with real bearings - Traxxas #5114 - 5mm x 8mm x 2.5mm / sealed which made the movement absolutely buttery smooth.

Thursday, June 6, 2013

Align 450 DFC - Blade grip control link mod

One disadvantage of the DFC (Direct Flight Control) Rotor Head is that it puts the blade grip control links under a lot of strain and they eventually can fail causing a crash. Align tried to solve the issue by changing the design of these links and supposedly making them stronger .
The new links are called "Main Rotor Grip Arm Integrated Control Links". These new links are made from solid aluminum and there is a pair of ball bearings at the pivot point where the link is attached to the blade grip.
The blade control links transfer the axial movement of the swashplate (when the servos move it up/down to change blade pitch angle or tilt it for aileron / elevator control) up to the blade grips while transferring down the rotational force of the blade grips to the  top (rotational) part of the swashplate.
The main issue with this design as part of a DFC rotor head is that it relies on a very stiff head dampening in order to minimize the axial movement of the feathering shaft. Any significant movement of the feathering shaft puts serious strain on the linkage, trying to "pry" it off the swashplate's linkage ball due to the created lateral force on the link arm.
By coming up with the metal "integrated arm" design, Align attempts to reinforce the linkage strength but they have focused on the top portion of the linkage. By keeping the standard for the 450 series plastic link ends on the arm, what they did is just to move the failure point down - a chain is as strong as its weakest link.
DFC pilots often see control link failures in that area, especially when flying 3D - the plastic ends sometimes break off or are being pulled off the link arm.

Point and case is this facebook post.
It is a shame when failure of a $1 plastic part causes the destruction of a few hundred dollar heli due to poor design.

To improve on the reliability of these links, what I did is to replace the lower portion of the Integrated Control Link Arm with a beefier hardware designed for Align T-Rex 550.
The part number needed for this mod is "Align T-Rex 550 Stainless Steel Linkage Rod A - HN6065A"

This my 450 PRO FBL DFC rotor head with the modified Blade Grip Control Links already fitted on.

I used a 2 mm drill bit to carefully enlarge the threaded hole on the bottom of each Integrated Arm link. The 550's Stainless steel  Linkage Rod A has a slightly larger diameter - 2 mm (actual 2.2mm) vs. 1.6 mm on the original rod. I didn't use the motor of my electric drill - I just placed the drill bit in the chuck and rotate it manually, very slowly, while making sure I drill straight along the axis, until the bit bottoms out. There was very little material that was actually removed.
To thread the new 550 linkage rods that came with the kit, I had to cut a new thread in the aluminum arm. The main problem is that the steel Linkage rod A has a square thread (it is designed to normally go into plastic link ends on both sides), not the regular screw type of thread, so using a standard 2 mm tap is of very little help (it did next to nothing).  What I ended up doing is to use one of the extra linkage rods from the kit as a tap by holding it firmly with pliers and working it in - rocking it back and forth. The rods are made from much harder stainless steel and the arm is soft aluminum so it is not that difficult to make the new thread.
Because I had to hold the rod for one of the threaded portions with my pliers I just marked it for future use as "tap" and didn't use it for actual linkage.
I took a new linkage rod, applied some RED (high-strength) thread locker and threaded the rod inside the arm until it bottomed out. All of the threaded portion of the rod should sink in.
It is very important to clean all metal shavings and debris from the hole beforehand.  If the rod is too loose because of sloppy drilling -  CA glue or even better JB Weld metal epoxy can be used instead of thread locker to permanently fix the rod - the link arm side of the rod does not need to be adjusted so permanent fix is actually a better solution.
After a few hours for the thread lock to cure, I just screwed onto the rod ends the heavy duty 550 plastic link ends that came in the package - they have exactly the same ID and fit as the standard ones for the 450 series but are much stronger with more plastic material on them. Furthermore, the rod screws much deeper into the plastic and it will require way more force to be pulled off across the thread.
The plastic link ends screw almost all the way down against the aluminum arm when setting up for 0 degree blade pitch. (on both - my 450 PRO DFC and Sport V2 DFC I had aprox. 1.5 mm gap exposing the smooth center portion of the rod - seen on the picture)
This mod is not a must if you are not a 3D pilot but at least should put some peace in your mind by taking care of a known weak point in the DFC setup.

Spektrum AR7200BX antenna repair / antenna extension

Here is a tip for the RC heli crowd on how to repair or extend the antennas of the popular AR7200BX flybarless controller.
AR7200BX is pretty much a Microbeast BeastX flybarless controller bundled with a 7 channels 2.4 GHz Spektrum DSM2/DSMX receiver. This setup is really nice because it saves a lot of wiring and cable management for those flying with Spektrum / JR radios, not to mention it saves space and weight too.
The receiver has two antennas in order to address possible polarization issues. In the GHz range proper polarization is very important and on a RC heli it is a challenge to maintain consistent Tx-Rx antenna polarization. Often, the only instance when the Heli has assumed a normal orientation (blades up, skids down)  is just before take off ( 3D pilots know exactly what i am talking about). The two antennas must be oriented in a way to cover at least two different spatial planes (X and Y and ideally, the third plane Z as well via a satellite receiver). The two AR7200BX antennas are normally placed at 90 degrees to each other. An important condition is that the antennas have to clear the carbon fiber frame (which is conductive and "lossy" when it comes to radio-waves) and at the same time stick out sufficiently so the antennas are not "radio-shadowed" by the Heli's fuselage and other parts.

I made this antenna mount for my 450 PRO using a plastic straw (salvaged from a compressed air can), a cable tie and a cable tie mount (the double-sided self-adhesive foam was removed from the mount and replaced with a single-side adhesive foam so the mount does not adhere to the FBL controller - it is held in place by the Velcro strap). It works great to maintain a good 90 degrees polarization difference. The thin, grey, 1.13 mm antenna coax can be seen, exiting the black grommet of the FBL controller.
The two antennas are made out of a miniature coaxial cable (1.13 mm OD) and are long 110 mm and 40 mm respectively. The actual antenna element is the very end portion of the coax, where the outer jacket and the coax shield braid have been removed, leaving exposed approximately 31 mm (1/4 wavelength @ 2.4 GHz) of the insulated, center conductor only.
As per Murphy's law: one needs just a few more centimeters extra coax to clear the Heli's frame or other parts.
 Another potential problem is that these antennas can get damaged fairly easy in a crash or just by being scraped during flight by the sharp carbon fiber frame edges - such a thin coax is very fragile.

Here is my Align Trex 450 Sport V2 DFC. The AR7200BX is mounted inside the frame on the gyro plate. I wanted to have the short antenna pointing vertically downwards but alongside the plastic skid frame. This way, the frame will act as a mechanical shield protecting the antenna whip. Unfortunately, I need about 10 mm more coax on the short antenna to really clear the heli's CF frame. Another good antenna location is behind the anti-rotation bracket of the swashplate but the coax is way too short to reach there too.

After some investigation, here is what I found out: the AR7200BX is using standard IPX connector in the cable assembly for both antennas

This image is courtesy of Helifreak member sup77095. It shows both miniature IPEX / IPX  coax connectors on the AR7200BX receiver board.

As it turns out, IPEX / IPX cable assemblies are "dirt cheap" - just search on eBay for "IPX cable". They are used as interconnects for many WiFi devices, inside laptops, cell phones, etc and always come in the form of a "pigtail" (ready-made cable assembly with connectors installed).
I got two cables, completed with the connectors for just under $4 (free shipping). 
To make an antenna, just measure and cut the overall length needed for an extended antenna or repair (including the proper connector on one side and add an extra 1/4" of cable. Very carefully (!), using a sharp blade, strip only and remove the outer insulation of coax shield about 31 mm from the end. Carefully, using a needle, un-braid the exposed coax shield and trim it down to where the outer insulation begins.
(!) Be very careful not to damage the center conductor and the Teflon insulator around it - it is very easy to nick the Teflon insulation and then when bent, it will break off.
Re-measure and if needed trim down the center conductor - the goal is to have exactly 31 mm of insulated center conductor with no coax shield around it.
(Probably not need, but I'll mention anyway that such modification will void the warranty on the FBL unit and it is mostly for the brave ones)
The coaxial loss is ~3.1dB/m @ 2.4GHz or 0.031 dB per centimeter. One needs to optimize the length to the absolutely minimum needed to avoid signal strength issues in the receiver but generally up to 10 cm extension for the short antenna should be OK. 
If you fly your heli as a "dot in the sky", installing a satellite receiver is recommended anyway. 
Remember to perform Radio RANGE CHECK after doing any antenna work on your heli.