Avionics News April 2013 - 64

WHAT RADIAL ARE WE ON?
Continued from page 63

associated VOR antennas (photo 5) into the circle. The GNS 530 now
showed us on the 097 radial, and after adjusting the OBS knobs, the No.
1 and No. 2 CDIs indicated 096 and 100, respectively – within the plus
or minus 4 degree range of each other… though the No. 2 instrument was
still out of range.
With the benefit of what I’ve learned researching this article, I did
something unusual. I taxied through the checkpoint, turned and came
back – this time lining myself up with the arrow in the center of the
circle, pointed directly at the VOR. Now the GNS 530 showed 094, the
No. 1 CDI 093 and the No. 2 CDI 097… all within limits.

What’s going on here?
The VOR antenna on our Skylane is typical of piston singles – a simple V-shaped dipole. Most pilots don’t realize that’s a highly directional
antenna sensitive mainly to signals directly ahead and behind. Pulled
in at an angle, as I had originally, one side of the dipole was shadowed.
I strongly suspect both receivers were picking up a side-lobe from the
VOR (probably reflected by nearby buildings), which gave a false radial.
The fact that moving a few feet changed the reported radials significantly
tends to back this up. When I taxied and put the Skylane’s nose on the
VOR, we were in the range where the antenna has maximum sensitivity,
and there was line of sight (confirmed by my eyeballs) from the VOR to
the airplane. Now the errors were much reduced – to 1 degree or less on
the Garmin GNS 530, and to 4 degrees or less on the King KX-155.
A partial solution is shown in Photo 6 – a balanced loop antenna, as
installed on a Piper Arrow I owned a few years ago. This airplane had a
King KNS-80 RNAV in it, which was expected to work properly with
VORs at a wide range of azimuths around the airplane. I’ve recently
spent time flying a Pilatus PC-12 that has blade antennas for the VOR.
But inside those blades are balanced loops, just as on my Arrow. Fair
warning: While the balanced loop isn’t as susceptible to off-axis errors as
a simple dipole, it still has zones of greater or lesser sensitivity.

The culprit: Directional “cat’s whisker” VOR antennas

Consider the System

6
Less directional balanced-loop VOR antenna
(blade-type antennas work similarly)

This brings us to the most significant point for avionics technicians and
shops. Where the VOR is concerned (and its close cousins, the localizer
and ILS), you must consider the system as a whole, not just the receiver.
Bench testing the GNS 530 would probably show it as perfectly functioning instrument, yet it gave an error well in excess of IFR limits when I
initially ran my test because of antenna issues. That’s not a huge surprise.
While the GNS 530 is new in our Skylane, the antenna and cables probably
date to when the airplane was built, more than 30 years ago… and that’s
not unique to us. The PC-12 I’ve been flying was certificated with Bendix/
King avionics when it was built in 2007. It has since been converted to
Garmin avionics, but the antennas look like what it was built with originally, and I seriously doubt the cables have been updated.
Continued on page 66

avionics news

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april

2013

Avionics News April 2013

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