How does the ADF work?

[FlyingForFun]

Hi!

Could someone please explain to me how the ADF works, or point me to a good reference (preferably on the www)?

I understand how the loop antenna works. I understand the ambiguity. And I understand that the sense antenna is there to resolve the ambiguity. What I don't undestand is how the sense antenna resolves the ambiguity.

The Bristol groundschool notes contain a diagram of the electrical fields around the antennas. I'm pretty sure that understanding this diagram is the key to understanding how the sense antenna works, but I can't make sense of the diagram.

The diagram itself is pretty much identical to one on page 4 of this CAA document (http://www.eurocontrol.int/sma/SPG%20Positions%20documents/spg13p20-NDB.pdf). The document describes the diagram and how the sense antenna works, but it makes even less sense than my Bristol notes. And the only other thing I can find is an old AvWeb article (http://www.avweb.com/articles/adf.html) which makes less sense still!

Thanks for any pointers you can give me.

FFF
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[woderick]

Do your Bristol notes contain the diagram (s) similar to the one in the CAA description but with + and - signs added ?
I hope so and that there are two of them otherwise this could be difficult !

[FlyingForFun]

Yes, woderick, I think they do (will check when I get home tonight though). But that hasn't helped me understand it!

FFF
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[mono]

It's all to do with Phase Angle.

The loop produces 2 nulls (it is easier to tune to a null than a peak) and 2 peaks with signal amplitude and phase angle varying according to the relative position of the station. The sense ae phase angle is the same irrespective of the relative station position. The two signals are 'combined' within the ADF Rx such that the gonio is driven so its' null is 90 degs in advance of the sense ae signal. the ADF pointer then points TO the station. If it were driven to 90 degs phase lag it would point FROM the station.

It is a little like VOR except the signal is split on the a/c not on the ground.

Hope that helps (though reading it now I'm not too sure :D :D )

[FlyingForFun]

woderick,

Yes, there are 2 + signs, and 1 - sign on the Bristol diagram.

mono

No, that doesn't help at all - sorry! You say: "The sense ae phase angle is the same irrespective of the relative station position". In which case, what difference does it make whether the NDB is in front of or behind the aerial? In both cases, the phase angle at the sense aerial is the same :confused: I must be missing something obvious!

Also, I understand the relationship between the VOR and the ADF - but I don't understand the how the amplitude modulation "old" VOR works. I'm guessing that once I understand the ADF, the VOR will suddenly become clear too, since they're so similar. (Although, ironically, I have no problem understanding the "new" VORs which use doppler shift to frequency-modulate depending on the radial.)

Thanks!

FFF
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[forget]

http://www.avweb.com/articles/adf.html

[FlyingForFun]

forget,

Thanks for that. But it's actually the same link which I included in my original post, saying that it made even less sense than anything else! Actually, I don't think it doesn't make sense, as such, it just doesn't go into any more detail than what I understand already. And unfortunately, it doesn't help any more when you post the link than it did when I posted it! :D

FFF
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[mono]

Oh dear,

I suppose I've got myself into this mess I should try to get out of it.

FFF

It seems to me from your answer that what you have difficulty grasping is infact the loop ae modus operandi and not the sense.

The sense ae signal, being of the same phase irrespective of station relative position means it acts as a reference signal against which to compare the phase varied loop ae input.

If we assume that at a given moment in time the sense ae i/p to the ADF Rx is at maximum with a positive voltage. Then depending on the position of the station relative to the a/c the loop input to the Rx will be anything from maximum positive to null to maximum negative (i.e. same amplitude but 180 degs phase difference) to null and back towards maximum positive again. In other words the loop ae i/p to the Rx varies sinusoidally. With its amplitude, phase angle and sense (i.e. positive or negative) varying as the sine of its' angle relative to the a/c nose.

So. assuming the same moment in time, lets say the station is at 90 degs to the left of the a/c nose then the loop signal into the Rx would be maximum but with a negative voltage (sine 270 degs = -1) compared to the sense signal. If the station were at 90 degs to the right of the a/c nose then the signal would be maximum but with a positive voltage (sine 90 = +1)

So you can see (hopefully) that it is by comparing within the ADF Rx the constant (relative) phase angle of the sense signal with the varying (according to station position) loop signal that the ambiguity is resolved.

hope this is a bit clearer. If not a longer explanation may be required but I will have to sort out my typing skils first.:D :D

Cheers.

[Don D Cake]

Take a loop antenna and make it a square one because it's easier to explain. Let's call one vertical section "A" and the other vertical bit "B".

When the antenna is perpendicular to a signal source, voltages are induced in both vertical sections and as both sections are in a loop the voltages cancel. No current flows.

When A is pointed at the signal source, the wave will reach A before B, the voltages in the vertical sections will not equal zero and current will flow. The same happens of you point B at the signal.

I believe you're happy with this. For the next bit get your paper and pencil ready.

We point A at the signal. Current flows in the antenna and on the oscilloscope (which is triggered by the signal source) we have attached to our receiver we see a sine wave, starting at zero and going positive. We then point B at the signal, again current flows but in the reverse direction around the loop so we see on our ‘scope a sine wave that starts at zero and goes negative. If we couple in an omnidirectional sense antenna to the receiver circuit such that the voltages induced in it are in phase with the A induced voltages they will of course be in anti phase with B induced voltages. This will give max signal when A is pointed at the signal source and min when B is (as shown by the cardioid diagram).

Thus when we obtain our two bearings with the loop, we can determine which one is the "real" bearing by pointing the A side of the loop antenna with the sense antenna coupled in and searching for maximum signal.

If this still doesn’t make sense, I’ll try and find my marine navigation notes that are buried in my attic and fax them to you.

[I. M. Esperto]

One thing to keep in mind. The ADF needle will point in the direction of lightning discharges.

[FlyingForFun]

Don - thanks! I think your explaination makes sense!

So, if I've understood correctly, the position of the sense aerial, relative to the loop, is critical to the operation. I think this was the piece of information which I was missing!

Mono, I think I'll need to re-read your explaination a couple more times before I understand it. But, with a bit of help from Don's explaination, it might make sense, and fill in the few gaps which Don has left!

Thanks very much guys - I knew I could rely on you! :)

FFF
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[411A]

FlyingForFun,

Just be glad the ADF you have is A (for automatic). Manual DF could be a real challange in the "good 'ole days"....not to mention four-course radio range orientation, oh what a delight...NOT.

[mono]

FFF,

The position of the sense ae with respect to the loop matters not a jot.

It is the relative phase relationship twixt the two which is important. The sense can be above, below, forward, aft or as is the case with most modern ADFs integral within the loop.

The loop produces positive, negative and null, the sense only positive. If the two signals cancel each other out it's in front (for example) if they combine then it's behind.

That's it. I think you are trying to make it more complicated than it really is.

Cheers.

[FlyingForFun]

Ok, in that case, I need to think about it again.....

*Smell of burning sawdust as FFF tries to get his brain around this one*

mono, you said:

It seems to me from your answer that what you have difficulty grasping is infact the loop ae modus operandi and not the sense
Suddenly it makes sense, and I think you're right!

For some reason, despite the fact that everyone and everything was telling me that this was the case, it never occured to me that the phase of the signal from the loop aerial would vary as the bearing changes. I realised that the amplitude would vary, and that by turning the aerial until you get a null signal you knew the station was to the side of the aerial. But it's only now that I realise that the phase changes as well - and suddenly it all makes sense!

Thanks to everyone who's responded, especially mono for your detailed answers and for persisting with my stupidity! The next time you're at a bash and I'm there too, I owe you a beer mate!

FFF
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[Ignition Override]

Many, if not most ADFs are impossible to identify or don't have enough strength for the RMI needle to point to them, at the many US airports which my company flies to. My company can not afford the newer digital ADFs which many turboprops had in the early 80s. If I know during preflight that only an NDB approach is available, we board extra fuel for an alternate. And I sure don't give a d**n what the Chief Dispatcher or brand new (or previous) VP of Flight Ops thinks about the cost of the extra fuel weight. They can send the new release and a few revised enroute burn figures on ACARS.

No matter how 'hot' some PPrune pilots appear to think they are, an NDB approach is a joke to many of the folks I fly with, and would be the case even if most US airport and navaid NOTAMS for what looks like a jillion approaches, were not listed and crammed in one large non-stop block of almost gibberish, regarding minima and approach lights/taxiway signs and construction. What century are we in? Kind of like a very crappy statistics testbook I once had just before dropping business as a major (what a relief) at a certain "trade school" univ.

If you want to put yourself in a tight, dark corner off of a long one-way street in a bad neighbor'hood', depend on an NDB approach in the US in limited VMC or worse conditions. Without VASI, PAPI or Navy/Marine NAS "meatball" lights for vertical guidance, you could be endangering your career. Some PLASI lights don't even function correctly, and were designed for helicopters. An ASR, if available, is probably much safer. If we declare an emergency, we can fly a PAR.

[FlyingForFun]

IO,

Yes, I noticed a definite dis-like for NDBs when I was flying in the USA. Even though I was only flying VFR, the instructors at the school I was renting from laughed at me when I considered using an NDB as a nav-aid.

I've not come across that attitude in the UK at all. Of course a VOR is a far better nav-aid - less prone to interference, and easier to track. But even so, during my PPL training, my instructor had me track to an NDB about 5 miles from the airfield, several times, when coming back from the local area. Totally unnecessary, of course, because I know the area well enough to find my way back visually, but he wanted to stress the usefullness of the NDB in finding my way back if I ever needed it. Tracking towards an NDB seems to be considered a fairly normal thing to do over here, whereas it's not in the US.

FFF
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[Tinstaafl]

In Oz for your initial instrument rating & renewal (it's a yearly renewal) you must do an NDB approach for the rating to be issued. All the other other types of approaches are add-ons to this.

But then NDBs tend to outnumber VORs by a factor of ten or so. Ditto VORs outnumbering ILS' by a similar amount.

For a Night VFR rating the candidate must demonstrate the ability to track using either NDB &/or VOR. Whatever aid(s) is demonstrated within the required tolerance is then the aid(s) the N.VFR holder may use. Given the preponderance of NDBs it's unusual for someone not to get both NDB & VOR

[big pistons forever]

Ah yes the delights of the NDB approach. Watching the altimeter unwind as you sink further into the clag knowing you are surrounded by higher terrain in all quadrants and you life is in the hands of that wobbly ADF needle. Life sure got better when you could cross check your track with a GPS bearing or better still just fly the overlay. I love old airplanes but the day they decommision the last NDB approach will be happy days indeed.:)