How exactly does an NDB work?

[80/20]

NDB GROUND STATIONS

The NDB transmitter is a very basic transmitter with a simple radio frequency oscillator that produces a carrier wave. The oscillator "filters" out all undesirable frequencies and will only allow one selected frequency to pass through. It produces a carrier wave, which consists of two electromagnetic fields: an electric field called the E-field and a magnetic field called the H-field. The H field, I believe, is named after the American physicist Joseph Henry who discovered the induction of electromagnetic fields.

See the VNugget's recommended link about how the ADF loop antenna works

Modern NDBs use an audio morse code for identification. Audio can not be transmitted directly through space and it is modulated onto the carrier wave in a modulator. After being modulated the signal is sent to an amplifier where it is boosted to the final transmission power. The power of an NDB is based on its intended use. It may vary from 15 watts to several kilowatts. The signal continues to the antenna, which radiates the electromagnetic energy in all directions. NDB antennas may be a single mast or wires strung between masts.

NDBs have an automatic shutdown feature if tone, modulation, power or other essential functions deviate beyond a set level. Modern stations have dual transmitters where a shutdown signal initiates a transfer from the primary transmitter to the secondary transmitter.

Teach all this when introducing ADF?
Too much information can be just as bad as too little. It is difficult to learn NDB tracking. I would keep it short and simple, focus on the practical flying and basics. Use some simple rules about how to track a bearing to and from the station. When the student is ready for more I would focus on the many limitations and factors that can make NDBs inaccurate. I would save this "nice to know story" about NDB stations for the student that wants to know more or for a more advanced program.

 

[wil]

Andy- that was classic!!!!

 

[HerrJeremy]

Sorry for resurecting an old thread, but we spent an inordinate amount of time trying to explain this in CFI ground lab last night, and i'm still a little confused.

I understand that induction lets the loop antenna determine that the NDB is in one of two locations, 180degrees apart. what I dont understand is how the sense antenna diferenciates between them.

 

[F70pilot]

PFM=Pulse

Frequency
and
Modulation

 

[Humty72]

NDB's should be a thing of the past because they DON'T work half of the time. )

yea.. some of you folk may remember this one ...

http://www.myairplane.com/databases/approach/pdfs/05034ND21.PDF

that would take you right for that hill to the N of the approach course..

 

[Mmmmmm Burritos]

The loop antenna will line up in the direction from which the signal is coming from. It's called a loop antenna because they actually was a circular (hula hoop) antenna sticking out of an airplane. Check out pictures of WW2 era aircraft and you will see them. They actually had to tuen the loop by hand until they heard the strongest signal. The loop antenna will tell you that you are lined up with the signal, but you still don't know which way it is coming from. Think of the ADF needle without the needle pointer. It's just a line. Which way is it coming from??

The sense antenna figures out which way the signal is coming from. It places the needle pointer on the needle. On older aircraft, the sense antenna is the long wire which strecthes from nose to tail. On modern ones, the loop and sense are all combined into a big fat lump antenna on the bottom of the aircraft.

Each antenna is pretty useless without the other.

 

[HerrJeremy]

The sense antenna figures out which way the signal is coming from.

Right, that's my question. How does the sense antenna do that?

 

[Thedude]

So you think NDB are dead huh,

Going into SAT about 2 months ago, the guy that I was flying with same the exact same thing and then the ILS went completely down. So guess what was left....NDB only. NDB are great if you know how to use them. I can see a lot of Barbie Dream jet pilots think they are antiquated but there are still are lot a large transport airplanes out there that dont have FMS or glass.
I still say if you can shoot a NDB appraoch you can do the rest of them fairly easily.

 

[350DRIVER]

But I still think the NDB's time has passed!

wrong..... You may not use them but I can tell you that flying into certain airports in Canada you have only one approach and one option, the NDB. Even though this country may be doing away with them you still need to know how to safely execute the approach due to other places that still use them quite often. I will never forget my first trip as captain flying 135 up into Canada going into a small airport that only had 1 approach, yup the NDB approach... You need to know how to fly them unless you want to chance making a fool out of yourself at some point down the line.

NDB's......
If the FMS cant do it, it should not be done.

How many part 135 pilots hauling night freight have the luxury of having a FMS onboard? Kind of thought so.....


3 5 0

 

[taxicabdriver]

Until they put FM radio stations on VORs, they need to keep those NDBs going.

 

[Mmmmmm Burritos]

Right, that's my question. How does the sense antenna do that?

I have no idea, I'm not an avionics technician. Only they need to know that.

All I remember is way back when I was a cadet in the Civil Air Patrol at age 12 or 13, they would hide a fake ELT (that transmitted on something other than 121.5) for practice and we had this contraption with an antenna which looked like an H. It would beep when the signal was in range, then we would hold it upright and spin around until the beeping was loudest. Then we would turn on "sense" mode and turn the unit until it beeped again. It had an arrow on it pointing to one of the sides of the H antenna. When it beeped, that meant the one side was in line with the signal. Then we would go that way and find the ELT. It was pretty fun, but not nearly as fun as riding in a C-130 to go to the Smithsonian.

 

[minus273]

Hey there,


There is a sport called radio orientearing, or ARDF. Someone hides a radio transmitter and you have to go find it using something similiar to an ADF.

http://members.aol.com/homingin/equipment.html

Also I am not sure if I am correct or not, but I always thought that aviation frequencies even in the VHF band were modulated as VHF AM signals. So you would not be able to listen to VHF FM radio even if you wanted to and also the VOR band is put higher than VHF FM so that there should not be any interference.

The NDB always had the ability to pick up AM broadcast, as in days gone bye you could see where towers were on the map and what the name of the station was, this is why they always do name checks so often. Nowadays with companies having so many repeater stations and clear channel communication telling everyone what they should listen to it is not so easy.

You can also if you live in a Marine environment, tune in Marine marker beacons on the coast. They use them up north in Canada, the only problem is that they can be seasonal. And you also have coastal refraction errors.

Anyways I still think that it is a goos idea to teach them, as not everyone gets to jump into new plane with GPS or FMS.

-273

 

[minus273]

Will correct myself before someone else does apparently the VOR is both FM and AM. The Reference phase is FM and the variable phase is AM.

Anyways, here is a web-site that explains some of the fun things in VORs and ILS equipment.

http://www.rohde-schwarz.com/www/appnotes_files.nsf/ANFileByANNoForInternet/49C1F32335CF81B6C1256B2900332CEE/$file/1GPAN09_0E.pdf

ooo the joys of beer and the internet.

-273