VOR/NAV sensitivity questions

Matthew

Matthew

Touchdown! Greaser!
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Matthew
I'm not quite sure of the exact inner workings of a VOR transmitter - I've read up on them and had them explained on a pretty detailed level, but it's been a while and I have a curiosity-type question.

If I am, say, at the maximum range for receiving a VOR signal at that altitude, would the NAV radio have a better chance of receiving first the FROM radial or the reciprocal TO radial? Or would it matter?

Just wondering if the directional information from the VOR results in any difference in when you start to pick up certain radials.

I'm thinking that the receiver decodes the timing signals and determines the radial you are on, so it won't matter.
 
Your last sentence is the key. The VOR receiver doesn't understand TO or FROM, just what radial you're on, and transmits that to the CDI. The converter in the CDI head compares the selected course on the OBS with the radial from the VOR receiver, and positions the needle and T/F flag appropriately.
 
Matthew said:
I'm not quite sure of the exact inner workings of a VOR transmitter - I've read up on them and had them explained on a pretty detailed level, but it's been a while and I have a curiosity-type question.

If I am, say, at the maximum range for receiving a VOR signal at that altitude, would the NAV radio have a better chance of receiving first the FROM radial or the reciprocal TO radial? Or would it matter?

Just wondering if the directional information from the VOR results in any difference in when you start to pick up certain radials.

I'm thinking that the receiver decodes the timing signals and determines the radial you are on, so it won't matter.
Click to expand...

We've all been taught that VOR's have "Radials" as if each degree has some discrete coding in the signals transmitted from the ground but this is not the case. All VOR ground stations modulate their carrier with two 30Hz sinewaves, one that's the same phase (peaks occur at the same time) in all directions and one who's phase varies continuously with the angle of the line to the receiver. The two phases are equal along the line from the station in the direction of the station's reference magnetic north and 180 degrees out of phase along the line in the opposite direction.
 
I don't know why a sensitivity check isn't part of the monthly Nav radio check. Center the needle, then crank the OBI 10 degrees and check for 10 degrees deflection. I had one that passed the required checks but only had about half the expected deflection.
 
Graueradler said:
I don't know why a sensitivity check isn't part of the monthly Nav radio check. Center the needle, then crank the OBI 10 degrees and check for 10 degrees deflection. I had one that passed the required checks but only had about half the expected deflection.
Click to expand...

There's a lot that's not tested on the required VOR check. Some you'd probably notice even though the test could be considered "successful" some that you'd never detect. To be completely thorough you'd need to perform a test with a signal just barely strong enough to pull the flag, test at least every ten degrees, preferably every degree of OBS phase with several different relative bearings to the station (looking for antenna shadowing and multipath effects), and as you pointed out verify the amount of needle deflection with course offset. And that's just to test the VOR side. There are no mandated tests for LOC or GS indication and those use different circuitry and in the case of the GS, different everything.
 
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