NDB frequencies

[Squawk7777]

I don't know if this is the correct forum or "Aviation History" :D

Anyway, ICAO has assigned the frequency range 200 - 1750kHz to our lovely and soon-to-be-doomed NDBs. Most pilots love their ADF since they can listen to AM. :rolleyes:

I have never seen an ADF whose frequency was above 500kHz. Anybody out there who has seen higher frequency NDBs? The lowest frequencies I have seen was TUK 194 over Nantucket (now OTS) and DIW 198 NE of ILM VOR (North Carolina).


7 7 7 7

[fireflybob]

I believe Lichfield in the UK (LIC) is on 545 kHz!

[OzExpat]

The lower frequencies in the NDB LF and MF band are best suited to locations where high terrain isn't closely adjacent. Come to Papua New Guinea and you'll see a lot of them in the 1700s. They have quite a short range and are best suited to use in broad valleys, where they normally provide a more stable signal.

To put this in perspective, if you were to use a frequency in, say, the 200 mHz range, you would be most unlikely to get a stable needle (and indeed an ident) more than about 20 miles away. Using a (much) higher frequency, in the MF band, you might only get another 10 miles or so, but you'll have a much more stable signal and, therefore, more solid needle indication.

We have some high frequency NDBs here that consistently deliver a range of 50 miles. That might not seem like much but its really pretty good when you consider the terrain that surrounds them. Of course (and this is probably a bit off the topic), range can always be improved but increasing transmitter power and/or adding "radials" to the earth mat.

The latter is especially useful if the NDB must be sited in soggy ground. I doubt that the UK really needs NDB frequencies much above the 500-600 mark. Indeed, they're probably only that high to avoid co-channel interference.

[HEATHROW DIRECTOR]

Oz said: "To put this in perspective, if you were to use a frequency in, say, the 200 mHz range, you would be most unlikely to get a stable needle (and indeed an ident) more than about 20 miles away. Using a (much) higher frequency, in the MF band, "

I assume you've got your MHz and KHz confused? A frequency in the MF band would be very considerably lower, not higher, than 200Mhz. 200MHz would only be line of sight BUT would not be affected by time of day. MF and LF frequencies can be severely affected by QRM around dark and after.

Extra radial on a groundplane antenna can be effective - says he having "worked" Australia and New Zealand on amateur radio using 10 watts on 1.8MHz with a shortened vertical and one or two radials.... but aren't some NDBs dipoles, which obviate the need for a serious earth mat?

[Stan Evil]

Most UK NDBs are in the 300-400 KHz range but one, at Cranfield, is 850 KHz. There's nothing special about it with a DOC of 20 miles which is typical of UK NDBs.

Odd:confused:

[Tinstaafl]

Oz tends to always use a dipole antenna. Can't think of any there that aren't.

UK seems to veer the other way. Don't know if earth mats are used. There isn't a mat at my base, anyway.

[Brookmans Park]

Could someone please explain" extra radials for me "thanks

[Squawk7777]

Thank you all for your replies!

They have quite a short range and are best suited to use in broad valleys, where they normally provide a more stable signal.

It seems that the higher frequency might be associated with the class A accuracy requirement (+/- 2 degrees). Is the one at Cranfield class A?

Another thing that does not make sense to me is that some NDB frequencies are .5 kHz e.g. 344.5 Any reason behind it? Most GA nav equipment does not have 1/2 kHz capability.

[HEATHROW DIRECTOR]

Brookmans Park... There are all manner of different types of radio antennae and the subject is complex. However, two popular types are dipole and groundplane. A dipole is usually a horizontal wire, fed in the middle and equal on both sides. If this is erected at a particular height relative to the frequency it serves it will function quite normally in its own right but will usually be directional, with strongest signals at right angles to the line of wire (but this function will be affected by the height and whether the wires are horizontal or sloping downwards). A groundplane antenna, in simple terms, is like half a dipole but erected vertically, usually on the ground. It radiates equally in all directions. To provide for the "other half" of the dipole an "earth" system is required under the groundplane and this often takes the form of "radials" - lengths of wire extending away from the antenna like the spokes of a wheel. The more you use, the better the antenna functions. The radials do not have to be buried, or even on the surface of the ground if the antenna system is elevated. Incidentally the "earth" effect can also be created, for example, by the body of an aircraft or a vehicle. This is a very simplified explanation but hope it helps.

[OzExpat]

Well spotted HD ... I was just checking to see who was awake! :D Discovered that it was everyone except me! :( We have more than a few single mast NDBs here, so good earth mats are vital.

However, because we depend so much on our NDBs, their range needs to be the best we can possibly provide. Thus, again, there is no substitute for a good earth mat - and heaps of transmission power, of course! :D

Stan Evil ... the range of a NDB can be reduced by reducing the power output levels. It can even be done by removing some of the earth mat radials. Some countries reduce NDB range for strategic reasons and others for airspace reasons or to confine usage to a Locator function, perhaps in association with an ILS.

Brookmans Park ... putting it at its simplest, an earth mat is nothing more than a series of copper wires that emanate radially from the centre of the NDB or, in the case of a dipole unit, from the midpoint between the two masts. The radial arrangement is the thing I was referring to as radials so don't confuse this with a VOR.

Squawk7777 ... the only place I've ever seen the half kilohertz used is in America. And that was a while ago. Might also have been used in Europe at one stage too, but I wouldn't swear to that. I suspect that the only reason for it was because the responsible authority was running out of whole-number frequencies. There might be another reason for it, but I haven't yet heard of it.

[Soddit]

The half kilohertz is not uncommon in Europe. And the higher frequencies ( up to 900 KhZ) are still common as en route navigational aids in many of the countries which were formerly part of the Soviet Union ( as it then was).

[fireflybob]

Sqwuak 7777,

<Another thing that does not make sense to me is that some NDB frequencies are .5 kHz e.g. 344.5 Any reason behind it? Most GA nav equipment does not have 1/2 kHz capability.<

Well thereby lies a tale! Some of us are old enough to remember manual tune ADF equipment ("Coffee Grinders"). An advantage of this system (!) is that you could tune for max signal (usually assisted by a signal meter) so a frequency ending in .5 was not an issue - assuming you could locate it between all the other stuff on the ether.

Enter the digital tune ADFs in GA aircraft without 1/2 khz capability. I think that for these systems the bandwidth has to be increased to cater for those NDBs which end in 0.5. The result of this is an effective reduction in range as you are not spot on frequency. Convenient but less efficient.

ADFs in most large transport a/c are able to select the 0.5 kHz and sometimes have variable bandwidths as well.

[knobbygb]

I was using a 1/2khz spaced NDB in the UK only the other day - EME 353.5 As I couldn't tune to the 1/2khz, I just used 353 and it seemed OK, although I got nothing on 354. Anyway, didn't rely on it - I was just passing nearby and knew exactly where I was anyway.

It's right under the extended centre line of rwy 09 at East Midlands - so I assume it forms some part of a precision approach procedure of some sort, but they don't teach us lowly PPL's about that sort of thing so I wouldn't really know. What's the story? Are they for 'general' use or not?