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Understanding basic antenna working principle :

An antenna produces both a magnetic and an electric high frequency field which, combined, form an electro-magnetic wave.

In order that this wave gets formed properly, ideally the magnetic and an electric high frequency fields should not be disturbed, particularly close to the antenna.

Once the wave is formed, it propagates like a light beam: anything being an obstacle to its propagation creates a shadow zone.


Basic installation principles:

In order to define where the antenna will be best installed, one has to carefully consider the surrounding in which it will be placed, and follow as much as possible the following principles:


1°) move away the antenna as much as possible from anything that will disturb the magnetic and an electric high frequency fields: any kind of metal, conductive material (carbon fibre for example), magnetic elements (compass for example) . Ideally, one should look for an area free of perturbations for about one wave length around the antenna, i.e. 35 cm….


2°) consider that any metal or conductive object (even if considered in 1°) as far enough to disturb marginally) will form an obstacle for waves, both transmitted and received. If such object gets in front of the antenna, the antenna, similarly to a light beam source, will not send wave behind the object, nor receive any wave from this area. This will create a shadow zone.


In our crowded gliders, shadow zones cannot be completely avoided. But in most cases, one can find a location for the antenna such that shadow zones created by surrounding obstacles be located in directions which are the less critical (often considered for Flarm device as the rear direction).

A bit more optimism: the metal or conductive surrounding of the antenna will often reflect and refract waves, which is likely to reduce shadow zones effects. It is extremely difficult to predict such behaviours in complex environments; therefore it is recommended to not relying on these effects in advance.




Mandatory:

-install the dipole in vertical position (the dipole is the two half bars of the T shape formed by the antenna) . It can be glued with tape (no aluminium tape) or epoxy, neoprene…etc.

-maintain the feeding coaxial cable (the one linking the Flarm to the T center) perpendicular to the dipole for at least 15 cm long from the dipole centre.

-maintain the two half dipoles aligned as much as possible (misalignment angle no more than 10° to 15°). Usual fuselage curve in the cockpit is an acceptable misalignment if it is being considered to glue the antenna against it (not too close to the glider nose of course)


Possible :

-installation against a glass fibre wall

-installation close to the pilot, pretty transparent to rf waves at such frequency. No risk for pilot, Flarm emitting power is extremely low.

-make the feeding coaxial cable « run » and attached against metal parts (except in its terminal length, close to the antenna).



Do not :

-install the antenna against a carbon fibre wall, even if it is not electrically connected to the glider ground.

-install the antenna against metal or conductive parts (beware of conductive paintings)

-make the feeding coaxial cable « run » against the VHF radio or transponder antenna cable

-install the antenna too close from VHF radio, or transponder antenna

-roll the feeding coaxial cable around metal parts, and / or in small whorls ; if rolling the cable is necessary , roll diameter should be at least 10 cm



Testing the installation:


Ground test: place the glider under test on its dolly tail wheel. Take another glider with a Flarm unit in it, and switch on both units. Place the glider under test at one end of the runway, and pull the other glider facing the glider under test with a tow bar toward the other end of the runway until the green led RX, or “receiving” of any Flarm unit stops blinking. Usually on the ground the air range is below 1 km, this should not be worrying, it will be much better in the air. Then reduce the distance between the two gliders by roughly half. The RX led blinks again. Then start rotating the glider under test on its landing gear wheel touch point, and record positions in which the "RX" led stops blinking: these are the shadow zones that has to be recorded on paper (use on board compass for angle measurements). This provides very useful information on the shadow areas located on the horizontal plan, and allows to test several antenna locations in order to select the one which seems the most appropriate.

Alternatively, if at the end of the runway the RX led's are still blinking, rotate the pulled glider until the RX led stops blinking, and then rotate backward until the led just starts blinking again. Note the glider heading with the on board compass, reduce the distance between gliders by half and align the pulled glider with the same heading as previously noted. Then rotate the glider under test as described above.



In flight test: Flarm Technology company offers a very valuable service for getting the inflight air range pattern of a Flarm installation. It requires a flight record file with as many as possible other Flarms being encountered in it. See Flarm Support site, 3.09 sw release details.





This way you will most probably be able to get the best performance out of your Flarm installation. But remember, whatever this performance is, your eyes are your best anti collision systemLook outside, and land safely !