On Friday, June 1, 2012 3:25:27 PM UTC-7, Matt Herron Jr. wrote:
For a product specifically designed for gliders, the designers don't
seem to have paid much attention to the form factor of either the
brick or the antenna as relates to gliders. Maybe a shorter antenna
with less gain would be better than a long antenna with no place to
install properly. Also (Darryl may correct me on this) a dual antenna
configuration on the brick (not the dipole) will be very directional,
with most of the signal radiating fore an aft of the plane of the
antennas. So be careful how you orient them. I will eventually get
flarm, but this is just another reason to wait until the bugs are
shaken out. Incorrect polarization of the butterfly display is
another one.

Hi Matt

I suspect the Flarm developers have a good understanding of antenna gain and radiation pasterns (remember the more on-axis gain the less off axis gain and Flarm has to optimize all this to detect threats that may be on the horizon or significantly angled below or above the glider) to optimize their product. The lower gain stubby little antenna of a Zaon MRX by comparison only has to receive relatively powerful transponder signals (compared to the flarm-flarm signals) and how PCAS operates means its only really interesting to work at relatively short range whereas the 1090ES data-in capability of the PowerFLARM means you probalby want higher antenna gain PCAS/1090ES antenna than just for a PCAS only device).

I am not sure what you mean by the "dual antenna configuration on the brick". But maybe I can try to see if I can cover that.

Both the PowerFLARM portable and brick units have connectors for Flarm A and Flarm B antennas. The Flarm A antenna transmits an receives flarm messages, the Flarm B antenna only receives flarm messages. The Flarm B antenna is really intended for mounting in a location to give improved reception in directions that would be largely shielded today when using a single Flarm A antenna (ideally located on top of the glareshield). That shielded direction is mostly rearward (and also below the glider), with areas shielded by the pilot's body and carbon fiber fuselage etc. I see a Flarm B antenna mounted in future under or on top of my glider tail boom (on top to potentially avoid being too close to the transponder antenna). In a configuration like this overall signal sensitivity may or may not be that directional. It might be much more isotropic than a single antenna blocked to the rear by a pilots body and carbon fuselage.

The 1090ES/PCAS antenna next to the Flarm A antenna on the PF Portable will result in some directionality of both antennas (weaker signal towards the otehr antenna -- I'd wild guess several 10% signal reduction) but I am sure that is all well understood as a part of many tradeoffs the developers have to live with.

Or am I missing what you meant?

Darryl