REFLECTOR: Transponder Signal Strength Anomaly - Recommendations for Replacement

Dave Black dvblack at comcast.net
Tue Mar 15 03:29:17 CST 2005


Gary,

> I have an epoxy circuit board dipole transponder antenna that is
> mounted along the right-side of the fuselage, in the nose compartment,
> about twelve inches foward of the canard bulkhead. The anomaly that I've
> encountered is when flying directly away from the radar station and
> somewhere around 40-50NM from the station, the contollers lose my
> transponder signal. 

Does this problem occur when you are flying directly toward the station? If it
only happens when flying away, it certainly suggests signal shadowing by
battery, people, or engine. How close is the nearest metal? 

Is the antenna vertically polarized? If horizontally polarized, it would be
weak in all directions, but virtually nonexistent when flying to or from the
radar. 

> Question:
> Is this a generally known problem with pushers? 

I've never heard of it before. 

> Is an antenna such as the AV-22 a suitable substitute?

Certainly, but be sure to install a ground plane. Ground plane for a
transponder antenna is only 9" diameter or so, so it's not too big a deal to
find a place for it.
 
> I was planning to locate the AV-22 somewhere along the bottom of the
> fuselage, perhaps between the nose-wheel strut and speedbrake. Is this
> satisfactory? 

It should be an excellent location. Cable length is critical for the
transponder, so you can't go very far from the panel. That's why the strake
end transponder antennas don't work well. 

Dave Black
Shortwing RG


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