Nav Antenna & VOR Theory

[Bob Axsom]

A little history first - When I was in the Air Force the Navigational aids were Low Frequency Radio Ranges and Non Directional Beacons supplimented by Direction Finding equipment (CRD-6) and RADAR (GCA and RAPCON) and I was a radio mechanic (AFSC 30450) for AACS (Airways and Air Communication Service) which was responsible for all USAF Air Traffic Control and Navigational Aids. When I reported to the 2020th AACS Squadron at Shaw Air Force Base upon returning from Korea they had this new navaid made by Wilcox called a VOR that provided 360 radial "beams" instead of 4. I had to maintain it but obviously I never got to study it in my training and knew nothing about it's theory of operation.

Fast forward 52 years and I'm an instrument rated pilot flying in cross country air races. In the past couple of years I have been unscrewing the elements of my Comant nav antenna to reduce drag for races. In a recent configuration I have been flying on instruments vectored for an approach with the VOR or Localizer off to the right and the indicator starts coming off the max left limit and is maybe half way to center and I start turning to lead the intercept when the controlled says you appear to have flown through the final. This happened at Baton Rouge, Louisiana (ILS) and at Taylor, Texas (VOR) in a specific altered configuration before the race at Taylor where I removed them again. I have a ramp tester for checking the system and enroute VOR checks seem OK.

I have heard the antennas referred to as dipole antennas which I would assume is a half wave center feed antenna based on the appearance but I do not know that for a fact. They could be two end feed half wave length antennas set at an angle for a specific function or there configuration may be something I never heard of back in radio school. I know all current pilots are GPS glass panel managers but is there anyone with theoretical VOR knowledge that can describe the effect of incomplete/intermittent/open connection of one element of an aircraft VOR receiver antenna?

Bob Axsom

[Garage Guy]

Originally Posted by Bob Axsom

I have heard the antennas referred to as dipole antennas which I would assume is a half wave center feed antenna based on the appearance

That's right.

They could be two end feed half wave length antennas set at an angle for a specific function or there configuration may be something I never heard of back in radio school.

No, the localizer receiving antenna is just a dipole, not two specialized antennas like an ADF with loop/sense antennas or something like that.

The course deviation information is coded in the radiated signal from the localizer transmitter and its antennas. The receiving antenna just has to sample that radiated signal at a particular point in space (attached to your aircraft ) and feed your localizer receiver. Then your localizer receiver has to decode it, and drive the CDI.

I guess it's possible that your antenna is delivering a weak signal, and your receiver isn't handling that very well. Or maybe your receiver needs aligning. (A VOR check on your receiver won't tell you the whole story since a lot of the LOC decoding circuitry is separate.)

Edit: Reread your post and now I see you clearly said the problem is happening with both LOC and VOR. Well, the receiving antenna is the same in both cases, and it's nothing special. But if it's affecting both, it's probably a problem in something shared by both. Curious that enroute VOR checks are OK, but not OK on the VOR approach to Taylor. I assume you had the OBS set correctly for that approach .

--Paul

[az_gila]

Originally Posted by Bob Axsom

A little history first - When I was in the Air Force the Navigational aids were Low Frequency Radio Ranges and Non Directional Beacons supplimented by Direction Finding equipment (CRD-6) and RADAR (GCA and RAPCON) and I was a radio mechanic (AFSC 30450) for AACS (Airways and Air Communication Service) which was responsible for all USAF Air Traffic Control and Navigational Aids. When I reported to the 2020th AACS Squadron at Shaw Air Force Base upon returning from Korea they had this new navaid made by Wilcox called a VOR that provided 360 radial "beams" instead of 4. I had to maintain it but obviously I never got to study it in my training and knew nothing about it's theory of operation.

Fast forward 52 years and I'm an instrument rated pilot flying in cross country air races. In the past couple of years I have been unscrewing the elements of my Comant nav antenna to reduce drag for races. In a recent configuration I have been flying on instruments vectored for an approach with the VOR or Localizer off to the right and the indicator starts coming off the max left limit and is maybe half way to center and I start turning to lead the intercept when the controlled says you appear to have flown through the final. This happened at Baton Rouge, Louisiana (ILS) and at Taylor, Texas (VOR) in a specific altered configuration before the race at Taylor where I removed them again. I have a ramp tester for checking the system and enroute VOR checks seem OK.

I have heard the antennas referred to as dipole antennas which I would assume is a half wave center feed antenna based on the appearance but I do not know that for a fact. They could be two end feed half wave length antennas set at an angle for a specific function or there configuration may be something I never heard of back in radio school. I know all current pilots are GPS glass panel managers but is there anyone with theoretical VOR knowledge that can describe the effect of incomplete/intermittent/open connection of one element of an aircraft VOR receiver antenna?

Bob Axsom

..does not really affect the results. The AM signal is based on phase to determine which radial you are on.

One bad element on the antenna would reduce the signal and be a mismatch into the receiver, but it should work OK at short ranges.

A detailed technical description of the signal and how the phase modulation is made and detected is on this ham site...

http://www.southgatearc.org/articles/vor.htm

[Garage Guy]

Gil's link there has a good intro to VOR but if you want to get into the theory of LOC and almost every other imaginable thing, you might check out the book Avionics Navigation Systems by Kayton and Fried. 800 pages and the going price is $185 but if you are near a good university library they probably have it. Also http://books.google.com has a major portion of it scanned and available online.

--Paul

[Bob Axsom]

You information has already been helpful. I will go into Gil's link first and see how that works out. My system seems to work fine (when I get within sight of the airport it is right where it is supposed to be) and I know controllers can get hypersensitive even though the plane is /U and the approach is non-precision after working with super precise coupled approach traffic all day. So if the antenna is a straight dipole (well "V" shaped instead of straight) I know a lot of standard receiving antenna rules that apply. The "V" does make a difference of course. I start getting concerned when two controllers in different states give me the same "you appear to have flown through the final" warning. The local avionics guy told me that sometimes the antenna loses one side and the response is something like I appear to have experienced. He advised flying a tight circle on a radial from a distant VOR (like Fort Smith) and recording the response. Since I have been removing and reinstalling the nav antenna elements for racing it is not too hard to imagine that I could have introduced an anomaly. He has offered to check it for free the next time I'm ready to fly - of course when I raced at Taylor last month I removed and reinstalled the elements so any installation related anomaly may have been fixed in the process.

Thanks again and I will check Gil's link.

Bob Axsom

[Garage Guy]

Originally Posted by Bob Axsom

So if the antenna is a straight dipole (well "V" shaped instead of straight) I know a lot of standard receiving antenna rules that apply. The "V" does make a difference of course.

I think the only difference the V makes is that it lowers the radiation resistance from 75 ohms (for a straight dipole) to more like 50 ohms. So a better match for RG-58. And it is I guess a more streamlined shape (but not streamlined enough for you )

The local avionics guy told me that sometimes the antenna loses one side and the response is something like I appear to have experienced.

I just don't see the causal connection there, but I learn something new every day...

Let us know what you find out.

--Paul

[az_gila]

Originally Posted by Garage Guy

I think the only difference the V makes is that it lowers the radiation resistance from 75 ohms (for a straight dipole) to more like 50 ohms. So a better match for RG-58. And it is I guess a more streamlined shape (but not streamlined enough for you )

I just don't see the causal connection there, but I learn something new every day...

Let us know what you find out.

--Paul

...you might be seeing a "shielding" effect from any metal (your vertical stabilizer?) that might be in the way from a signal from the side if you are not heading directly to or from a VOR station....

Otherwise, as Paul says, the two parts of the "V" are only a matching issue to a 50 ohm co-ax line.

[Norman CYYJ]

Sometimes Bob you can get a phase shift in the received signal from a control surface which has a poor ground. This can cause weird reading on the CDI even though the ramp tester shows everything is alright. This usually shows up when traveling in certain directions not all of them. Sometimes it causes the CDI to act like a windshield wiper sweeping back and forth. It is usually a real bugger to trouble shoot.