VOR Indicator Fluctuation

VOR Indicator Fluctuation

I recently started using my C-172 for instrument training.  Until now I used the VOR indicators only infrequently for cross country flights, and the indicators seemed to work well enough.  Now that I am using them for approaches, it seems when I am a few thousand feet or lower, and within a few miles of the VOR, I get the needle swinging 10 degrees side to side, and inaccurate indications of the radial.

On the Localizer/ILS, it's a different story.  The needle indication is solid all the way to the runway.  This makes me think it may be something other than the equipment.  I would be interested in hearing from anyone who may have had the same problem, and what you did to correct the problem since the indicators are worthless for doing any kind of VOR approaches.

The equipment I have are two TKM MX170B Nav/Coms driving a King KI-214 indicator w/GS receiver and a King KI-208 indicator.  The indicators are old, but how old I don't know because there is no logbook entry as to when they were installed.  The TKM radios are 5 years old, and are slide-in replacements for the King KX-170B radios that were probably installed with the indicators.

I would appreciate any and all advice.

Thanks

Jon Plaxton

jplaxton00
jplaxton00
useravatar
Offline
186 Posts
User info in posts
Only registered users or members can reply or post

Re: VOR Indicator Fluctuation

The phenomena you are seeing may be related to multipath signal anomaly. When the VOR signal is received close to the ground and if there are buildings or mountains on the signal path the signal will take different multiple paths to your aircraft. As the aircraft move accross different paths it will receive different radials emitted from the station. This will cause the needle to swing like a windshield wiper. At altitude (above 5,000 ft) the terrain effect is minimal at close distances (less than 20nm). VOR works best at altitude on relatively flat terrain.

Orv

Guest
Guest
useravatar
Offline
Posts
User info in posts
Only registered users or members can reply or post

Re: VOR Indicator Fluctuation

Hello Orville;

Thanks for your reply.  The King manual for the unit refers to this as "scalloping".  Is there a way to make adjustments to the radios, antenna, etc., to minimize this effect?  My flight instructor owns a Piper Cherokee that he uses for instruction.  He said that his VOR indicators are more solid on VOR tracking and VOR approaches than mine, using the same facilities.  Could this be a "High Wing" problem, since the Nav antenna on my C172 is on the top of the vertical fin, and on a VOR approach the wing structure would be in line between the antenna and the station?

Thanks

Jon

jplaxton00
jplaxton00
useravatar
Offline
186 Posts
User info in posts
Only registered users or members can reply or post

Re: VOR Indicator Fluctuation

Some VOR receivers have different damping techniques to alleviate the problem. The simplest is putting a capacitor across the CDI indicator input terminals. This will slow down the oscillations without affecting the deflection calibration. Depending on the coil resistance the capacitor value would be between 100uF to 400uF

Orv

Guest
Guest
useravatar
Offline
Posts
User info in posts
Only registered users or members can reply or post

Board Info

Board Stats:
 
Total Topics:
6026
Total Polls:
1
Total Posts:
16346
Posts this week:
1
User Info:
 
Total Users:
2404
Newest User:
skyman
Members Online:
0
Guests Online:
2760

Online: 
There are no members online

Forum Legend:

 Topic
 New
 Locked
 Sticky
 Active
 New/Active
 New/Locked
 New Sticky
 Locked/Active
 Active/Sticky
 Sticky/Locked
 Sticky/Active/Locked

ankara escort ankara rus escort ankara escort bayan ankara bayan partner ankara escort kizlar escort ankara ankara escort ankara eskort ankara escort bayan ankara bayan partner ankara escort kizlar escort ankara