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Old 22nd Sep 2010, 08:22   #21 (permalink)
Join Date: May 2001
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Likewise, the same can be done airborne with an ELT and an ADF, or simply the aircraft radio tuned to 121.5.
I think this was the statement of Mr Guppy's that everyone was reacting to.
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Old 22nd Sep 2010, 08:34   #22 (permalink)
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Several contributers have taken the view that it is impossible to receive Amplitude Modulation transmissions on 121.5 MHz on an ADF receiving on a few hundred Kilohertz and I have the same opinion. The frequencies are separated by a huge margin which is most definitely outside the frequency range of receivers designed for ADF.

That said, Amplitude Mod as a transmit-receive system is susceptible to being able to detect signals outside the normal operating range provided that they are in close physical proximity, such that the first stages of signal filtering in the receiver are “overpowered” as has been described by the reception of strong local signals from Police stations (and local taxis etc).

Another factor is how pure the transmitted signal is – a “dirty”, mal-adjusted one will radiate on several frequencies simultaneously. Indeed, it may be that historically, 243.0 MHz came into use to be able to capture the second harmonic of 121.5 MHz.

So, using a cheap transistor radio on the “AM” waveband in close range on the ramp would probably work and tuning across the range wouldn’t have much effect on how well the ELT is received. The VA offering to use ADF to locate the ELT is very commendable but completely misguided for not appreciating how frequency is measured, equipment capability and for my reasons above.
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Old 22nd Sep 2010, 08:40   #23 (permalink)

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Here's the only practical way of homing in on a VHF transmission (121.5 perhaps) with commercial aircraft equipment. Copybook effort.

Qantas Crew Help Lost Pilot in the 1950/60s
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Old 22nd Sep 2010, 08:47   #24 (permalink)
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That said, Amplitude Mod as a transmit-receive system is susceptible to being able to detect signals outside the normal operating range provided that they are in close physical proximity,

I agree (as an electronics engineer who at the age of 13 used to build pirate radio transmitters comprising of a pair of 807s, followed by a less than exhaustive PI-tank ) but I think if this worked habitually, it would soon be realised that the NDB at any airport which also has a VOR there would be pretty useless.

Very interesting about 243 being 121.5. Never spotted that It must be that they simply generated a dirty 121.5 and then filtered everything above 243, to achieve a simple design. 406 is much more complicated in comparison.
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Old 22nd Sep 2010, 15:41   #25 (permalink)
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Airborne DF has existed for over 50 years. We had the facility on the old HS Argosy to home in on a buddy using a UHF frequency. He transmitted (a sprung loaded 1000 kcs tone switch was also supplied for this purpose) and the homing aircraft followed an instrument just like a zero reader which gave a fly left/right and a fly up/down indication.

The equipment was known as "Violet Picture".

Needless to say, we could find an ELT (we called them SARBE) with great accuracy. I can remember doing an exercise at Thorney Island. The ELT was inside a hangar and we were even able to tell them which corner of the hangar it was in!
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