Originally Posted by
India Four Two
However the navigator (in the right seat) was very experienced and he showed how you could check for the ELT signal on a frequency 50 kHz offset from the ELT's frequency - 123.15 in this case. When the signal showed up on this frequency, we were always within half a mile of the "crash site" so it was easy to pick it up visually.
For any radio experts out there, how does this work? My understanding is that the bandwidth for a VHF channel is only a few kilohertz either side of the centre frequency, so we shouldn't be receiving it 50 kHz off the tuned frequency.
Interesting information, thanks.
The phenomenon you experienced is due to the passband characteristics of the receiver, and the effective 'width'. of the transmitter signal.
While nominal channel spacing in the aviation band used to be 25kHz (and for many is now 8.33kHz) this doesn't mean that you'll hear nothing if you're say 5kHz away from a particular frequency - particularly as you get closer to a transmitter.
The reason for this is that receivers and transmitters aren't perfect. The width of the transmitting signal, and the receiver passband filter (ie. how 'wide' it will receive) is measured on a scale (in dB) and is declared to be
xHz wide typically when it reaches the -3dB point, however below this point - and progressively further away from the centre point of the filter - it will continue to pass signals, albeit at a lesser level or 'signal', until the 0dB point is reached.
This diagram, courtesy analogictips.com, shows pretty much everything:
As you'll see in this case the bandwidth is declared at the -3dB level, but the filter is actually wider than this and signals will pass further from the centre frequency, albeit at reduced levels. Different filters will have different characteristics and therefore their slope can be steeper, or more shallow. I'd suggest that as you go back in age of equipment the passband filter slope will become shallower (wider) and so you could possibly receive signals further off frequency than a more modern receiver. In addition to this both transmitters and receivers, especially older units, may not be working perfectly on frequency and could be some way from where they're meant to be.
So, given that the transmitter will actually be transmitting wider in frequency, but at reduced signal, than its
declared bandwidth, and the receiver will receive signals wider than its declared passband you can in fact be quite a few Hz off-frequency and still hear something. Obviously this will be down one side or other of the filter slope and so the signal is effectively attenuated, which means you need to be correspondingly closer to the transmitting source in order to hear it. Bear in mind that both the transmitter and receiver have similar characteristics (ie. the transmitter will transmit further from its centre frequency at reduced signal, and the receiver will receive further from its centre frequency at reduced sensitivity) and so the two will interact to give you what you'll hear.
Coming back to your experience - in other words going off-frequency on the receiver 'weakens' the signal as if you were quite a distance from it, meaning you can effectively get quite a lot closer to the transmitting source before it registers on your receiver, thus reducing the limits of the search area. Quite a useful technique, and a good thing for people to know - top marks to your navigator.
FP.