what on Earth is vdf let's find [Music] out hi I'm Grant and welcome to the third class in the radio navigation Series today we're going to be having a look at very high frequency Direction finding which is a method of finding a direction using VHF signals unsurprisingly but the difference with this is we use communication signals to find out the direction we're going in so we'll start off this class with a little breakdown of how radio communication Works aircraft communication requires the use of antennas as we saw in the previous class the ideal length of one being half of the wavelength of the radio wave be being transmitted or wanting to be received but a quarter length can also be used although it's not going to be as efficient this means that in some of the frequency bands on the low end of the scale with very long waves uh wavelengths aren't suitable for example a low frequency signal of 300 khz has a 1 kilometer wavelength meaning that we would need a aerial poking out the top of our aircraft 500 meter tall which isn't very practical so for communications on aircraft we use the high frequency band as well as the very high frequency band of radio waves for communication this way we can have antennas in the aircraft of a reasonable size to receive those signals HF communication radios high frequency communication radios are ideally suited to long range communication ations this is because of the propagation characteristics that we looked at in class one basically the surface waves bend around the curvature of the earth and returning Sky waves allow for signals to re be received from very far away in aviation we use frequency from about 2 to 22 mahz which yes I know is sort of outside of that uh high frequency band that we defined and that means that we have wavelengths of about 150 m to 13.6 M if we take half the wavelength for use in our antenna then on the high end of the scale we're looking at an ideal antenna length of let's say 7 m rounded up what we can do with this 7 m tall antenna is put it in the tail of the aircraft just up the back there that way it's semi-exposed it's not got another aircraft in the way that it's not going the rest of the aircraft in the way sorry and it can receive the signal nice and clear the issue with high frequency is that the antenna still isn't quite big enough if we had a signal with a shorter wavelength for example it's okay because we are close to the ideal size for that signal but when we get to the lower frequency AKA higher wavelength the uh antenna is no longer in that correct range it's no longer half wavelength it might not be quarter it might be an eighth it might be a 16th that kind of thing so what we want to do for our signal is ideally use the highest frequency aka the shortest wavelength possible to get the clearest signal but if you remember in wave propagation the higher frequencies won't travel as far as they will refract around the earth less and they won't Bend as much back for these returning Sky waves so it's a fine Bal bance between range and Clarity of signal in the human uh in the uh high frequency communication world and often controllers will have a range of frequency they use and they will change over the course of the day as the characteristics of the ionosphere and the returning Sky waves change as well VHF Communications very high frequency Communications are a lot more straightforward these waves are in the range of about 118 mahz to 137 MH meaning wavelengths of about 2.5 M to 2 m the ideal length for uh antenna is therefore going to be about 1.25 M which is very easy to fit into the tail of the aircraft where it's nice and exposed to get a clear aircraft they propagate in space waves only those line of sight waves only meaning that the range is dictated by the max theoretical range formula that we saw in the previous class this is fine if we're not traveling over vast stretches of water and there's plenty of ground transmitters that we can hop between as we make our journey vdf very high frequency Direction finding is a bit of Kit some gr stations have that will basically let Air Traffic Control see on the ground which direction a signal is coming from this is done by using a circular setup of dipole antennas called an Adcock aerial the circular nature means that each individual anten antenna on this array will receive this radio wave coming in at a very slightly different time and therefore phase of the radio wave we're talking tiny little differences here and then using some clever Computing software we can tell where the radio wave is coming from based on those small differences in Phase between each of these antennas what happens next is then the air traffic control can then give that information to the pilots over the radio um via voice Communications and it's important to note that because these waves are electromagnetic the direction provided will be a magnetic Direction unless the variation on the ground station is applied if you don't know what I'm talking about here then go watch my videos on Direction in the gnav series where we Define all the different types of direction that there are so the controller can either provide a QDR or a qdm code which are one of many Q codes that are used in aviation that stand for various things in this case the QDR is the magnetic bearing from the station to the aircraft for example say we have the vdf over here and the aircraft over here in this example the QDR is the bearing from the station to the aircraft so in this example our QDR would be about 045 if you think that's North this is about yeah 045 something like that the QD m is the reverse of this it's the magnetic bearing to the station from the aircraft and it should always be 180° off um the QDR so in this case if we took north of the aircraft here our qdm is this angle in here and it's going to be the reciprocal of 045 which is 225° and again this is going to be magnetic and these bearings can be classed um either A B C or D and Class A is accurate to within 2° it's plus or minus 2° Class B is five C is 10 and D is accurate to less than plus or minus 10° most bearings are in that class B range and class D isn't really that useful if it's 10° out you could be miles off course um when you follow it due to the high my workload of vdf procedures for the air traffic controller using a vdf isn't that common as a way of navigating but there are a few places normally quite quiet airports that have vdf approach procedures but more commonly this sort of thing you would use this is the sort of thing you would use if you were lost you're in clouds you thought you were near a certain airport so you tune up the frequency to speak to them and ask if they can give you a QDR or a qdm towards uh the air that airport you can then look at your map draw a line and go right I'm somewhere along this line I thought I was to the north I'm actually to the West right I can make a plan of how to get back that kind of thing so vdf is okay at providing magnetic bearings to and from Air Traffic Control ground stations but it does have a few errors which can result in the bearing class degrading so it could normally be a Class B plus orus 5° but the errors could downgrade it to a Class D which as I said before isn't really that useful for navigation the most common errors are like a multiple signal error if there's multiple people broadcasting on the same frequency then the ground antennas can't differentiate between the two and the qdm or QDR signal will be hard to pinpoint I've been up to the tower before uh where they had a bit of they had the vdf kit and there was people going around the circuit pattern and every time there's a communication it's going bang bang bang bang bang it's flopping back and forth so it could be be quite hard to Define who's actually asking for the signal without asking for complete radio silence and that leads to other problems if there's radio silence then uh how do you know where people are how are people going to tell you where they are as a controller and you're not going to hear what's going on around you as a pilot so multiple signals cause a bit of error and a bit of confusion in the vdf system another thing that can happen is terrain VHF signals on that line of sight so if there's going to be mountains in the way then the signal might not be able to get through them um simple as that another one is refraction you can get refraction the bending of signals around these mountains and also it can happen around water there's a bit more resistance over water than over land and that slows down the waves and as they slow they change direction so the signal coming into the ground station may have bent on its way and when the controller gives back the information to the pilot it might not be accurate so for instance in this little example here you may be 10° off where the vdf signal where the controller thinks you are um so those just a few errors that are involved in vdf systems