FLARM dipole mount

The major problem with installing FLARM in a Libelle is working out how and where to fit the transceiver antenna. I put the FLARM electronics on the instrument tray, and assume most people will do the same, but simply fitting the transceiver antenna there will cause the upper row of instruments to obscure any gliders that might be lurking behind you. I know of two solutions:

• Use the standard LX FLARM dipole antenna but mount it as high as possible inside the nose, placed roughly half way between the front of the instrument tray and the rudder pedals on a non conductive support.
• Fitting a dual antenna is another possibility: one being fitted in front of the panel and the other in the rear fuselage. I contacted Glasfaser to find out how other people had solved this problem. Hansjörg Streifeneder told me about using a dual antenna system. He knows a German engineer who can supply them.

I'm using the first of these ideas, which came from Thorsten Mauritsen, a Danish Libelle pilot. My version is shown below. The rest of this page should give enough detail for anybody with good DIY skills to design and make a similar antenna mount.

Click to enlarge

Construction and setup notes

• Make sure you buy a FLARM unit with some means of downloading log files. You'll need to analyse them to ensure that your FLARM installation has adequate detection range in all directions. My LX Red Box has the vanilla SD card reader option, which it uses to save non-certified log files and also provides a convenient way to configure the FLARM or upgrade its firmware. The IGC logfile option is an additional feature which may be implemented by a subsequent firmware upgrade. It is not needed unless you intend to use your FLARM as an IGC-certified logger.
• It is essential that the antenna mount has no metal or carbon parts near to the dipole, which is why my antenna mount is made from glass epoxy board, a section of a glassfibre fishing rod, nylon bolts and a wooden block.
• The fitting that attaches the whole assembly to the Libelle instrument tray was cut from 1.5mm epoxy board and glued together with slow setting Standard Araldite. It uses a metal bolt as a pivot for the rod, since its far enough from the dipole for this to be OK. It is attached to the instrument tray by the same steel bolts that were holding the perspex pneumatic line support when I bought the glider. You can just see a black M5 nylon screw under the front of the fitting: this allows the angle of the rod to be adjusted so the dipole is as high as possible without touching the inside of the glider's nose shell
• The green tube was cut from one of the butt sections of a fibreglass fishing rod after I'd measured the distance from the front of the instrument tray to my usual pedal position. I drilled a hole through its base end and epoxied in a section of brass tube that slides over the mounting bolt. I filed the tube ends flush with the rod's outer surface when the epoxy was set. This protects the thin walls of the rod section from wear by the threads and also prevents the rod from being squashed and the mount fitting being damaged if the pivot retaining nut is tightened too enthusiastically: fishing rod sections have very little resistance to being squeezed.
• The block needs to be a fine-grained hardwood, such as beech, because you'll will be cutting threads into it. Start by drilling the block to fit on the rod and hold the dipole as shown above. You want sloppy fits so the rod section and dipole slide easily into the block. Then drill two further holes, which will be tapped to take nylon bolts. Any size is fine, but M4 or M5 are probably best: drill these holes to the appropriate tapping diameter for the bolts you've selected. After tapping the holes, harden the threads by wetting them with cyanoacrylate glue[1] and then clear the threads by running the tap through again when you're quite sure the cyano has set: you really, really don't want to find you've glued the tap into the block. The tapped holes must not be drilled along the grain or the threads will strip. The photo shows the rod locking bolt in the underside of the block and the dipole retention bolt in the front face where it is almost hidden behind the rod.
• Roughly position the dipole halfway between the instrument tray and the rudder pedals, reinstall the panel and go flying. Half an hour or so of local soaring would be a good idea because that will let the FLARM register more contacts than if you wander off cross-country. Do this at least two or three times before downloading the logs and submitting them to the FLARM range analysis tool to check your installation's performance.
• My installation initially showed excellent range to either side, poor range in front and barely acceptable to the rear. So, I moved the dipole about 20mm further forward and tried again. This time the side range was reduced a bit but the rearward range was considerably improved and the forward range was also better. The overall performance is now acceptable, but I'm still collecting data and fiddling to see if I can get even better coverage.
• In other words, if your range in some directions is poor, move the dipole forward or aft a small distance and try again. If that gives an improvement, try moving it a bit further in the same direction: if it doesn't help, try in the other direction.
• Don't forget to record exactly what changes you made and keep screenshots of the range analysis for each setting. You'll need this data so you can place the dipole at the optimum position when you've finished the tests.

[1] Cyanoacrylate is the generic name for a class of almost instant setting medical glues which set in the presence of moisture. The best known consumer brand is Superglue, though there are very much better brands available through your local RC model shop. These come in different formulations for different uses, e.g. thin cyano is very runny. It is intended for sticking close fitting parts and sets fast enough to stick your fingers together if you aren't careful, while thick cyano sets a lot slower (10-20 seconds) and is a good gap filler. I recommend thin Zap or HotStuff for general use as well as for hardening threads tapped in soft materials.

Coverage after adjusting the antenna

Click to enlarge