I suspect that the spurious signal is more likely to come from a VOR or COM frequency. Occasionally, it can result from harmonics of signal leaking thru the front of a Bendix King VOR/COM unit.
I did think of that, and examined the frequencies I knew about, but still could not see an obvious harmonic relationship.
Also switching off the DME (KN63) stopped the issue, so it wasn't just the act of tuning a VHF radio to the well known 11th or 13th sub-harmonic of 1575MHz.
I must get hold of that DME frequency!!
Backpacker - I had a go at replacing your URLs with links, using the Link button above the text entry box. The result is still wrong
Peter, David, thanks for fixing the URLs.
The VOR/LOC/GS/DME frequency pairing is a bitch to find. I remember having searched the internet previously, and I think I finally ended up in the official ICAO Annex something which specifies it.
The other location where I found it is the preamble of the old Bottlang I have here. It's on page Communications 2-1. You may also find it in the preamble section of a Jeppesen IFR flight guide or something like that. And that page only lists the VOR/LOC/GS pairing and the DME channel. It does not specify the DME frequencies.
One of the stupid pointless useless JAA IR exam questions was whether the GS frequency is 3x the LOC frequency.
It does actually sound plausible, and is certainly in the right ballpark, but I think it is wrong.
I haven't looked it up
But I have the schematic of a KN75 right here, to hopefully settle the argument definitively
Remember that some poor bugger had to implement all this stuff in the 1960s, using TTL logic... "Respect" is the right word.
Actually I think it was the other way around at this time.
Instead of a designer saying to an engineer: "Implement this", I think the engineers actually got a rather free description. "Here's our larger-world problem, here's approximately what we need, what can you build?"
VORs, for instance, are very hard to understand for anybody growing up with an iPad nanny. But if you know a bit about analog logic, frequency modulation and such, you'll find that it's actually a very clever idea which could be implemented with relatively few circuits. But you first need to understand the larger problem that needed to be solved, and the limitations that the engineers had to work with.
In other words: In those days the engineers were listened to and respected. Wheras today somebody without a clue writes a set of requirements, throws it over the wall and expects the engineers to build it.
Yes; I think VORs are like TV technology: specially designed to make the receiver very simple.
Also the whole VOR concept is specifically for flying long legs VOR-VOR-VOR, using the CDI presentation. Presenting the data in any other way is torturous. An RMI presents it as a bearing, but it doesn't make sense until you have a slaved compass system as well.
DMEs and transponders were IMHO clearly designed for a minimal hardware implementation at the aircraft end. But still not trivial in the early days.
In the early 1970's, I owned a Piper Arrow 200. It had a Narco DME with an analog indicator. The indicator was marked in NM. When it was searching for a lock on, the needle would sweep back and forth across the meter movement like a wind shield wiper. Finally after 10 seconds or so, it would usually lock on while the needle was sweeping and stop at the DME distance.
Later on, in 1980, we installed a KNS80 that was a Rho Theta area navigation unit with four waypoints. For each waypoint, it had a VOR frequency, a radial, and a distance. With this unit, one could proceed direct from point to point. When we installed it, having a DME in a general aviation aircraft was considered top of the line and not too many aircraft were equipped.
Then in 1990, we installed a KLN88 LORAN and could go anywhere direct to an airport, VOR, or waypoint in the database. It was approved for enroute IFR. This was replaced with a KLN90B GPS in 1996 which added IFR approval for terminal and GPS approach. In 2000, the GPS was updated to a GNS430, no new capabilities, but we could see where we were and I became a child of the magenta line. As I was selling my avionics shop, the last thing I did was replace the GPS with its larger brother the GNS530. In early 2007, the GNS530 was upgraded to WAAS. It added LPV, LNAV/VNAV, and LNAV+V procedures and I haven't used a VOR or ILS in anger since, practice only. In fact we no longer have to perform the 30 day VOR checks, although I still do. In the summer of 2009, the software in the GNS530W was upgraded to include the LP approach procedure type, but it took Jeppesen until this last database cycle (February 7, 2013) to include LP procedures in the database. Next is ADSB and maybe the ability to fly RF leg types.
Arthur C Clarke wrote a fictional book called 'glidepath', based on his experiences working on early radar assisted landing systems. Not about VORs which weren't invented then, but still well worth a read.
You would be amazed how many KNS80s are still around.
I get asked for pre-buy views on various TB20s (because I wrote a fair bit of stuff on the topic) and I have seen several with a KNS80 in them.
It is now a useless piece of kit (other than as a DME) because, on Eurocontrol IFR flight, ATC dish out waypoints anything up to 200nm away, and there is no way to be in range of a physical VOR/DME at that distance. One can hack such a long leg with several nearby VOR/DMEs but it's a lot of work.
The funny thing is that a KNS80 can be BRNAV approved which makes it legal for exactly that
But then, here in Europe, the emperor has been stark naked for best part of 20 years...
And at low levels, UK Class G, there is often no reception at low down.
The Germans were baffled by the accuracy (< 30m)
Surprising as the Germans had a more accurate system called Knickebein which was used on the Coventry raid. The German Consol system designed for submarines, was also very accurate and could be used in the air, it remained in use well into the 1970s outliving both Oboe and Gee. I certainly used it many times in the RAF as all you needed was a chart a pencil and a long wave radio.