Re: Ultralight DX on an Airplane?


Superhet radio receivers are those that have a local oscillator and can emit a radio signal.  

A quick course in basic superhet theory.

There are two main frequencies at play in a superhet - the tuned frequency and the local oscillator frequency.   The local oscillator is normally set to a frequency above the tuned frequency and the two frequencies "track" each other as the radio is tuned to maintain a constant "difference" frequency.   This difference frequency is called the Intermediate Frequency (IF).   As it is a constant frequency, simple circuit techniques can be used to provide the massive amplification needed.

The local oscillator signal is small and does not contain any modulation and so will not carry any sounds from inside the aircraft.  At worst it will generate a fixed tone in the pilots' headsets.  And then only if a specific (and highly unlikely) arithmetic relationship exists between the aircraft receive frequency and the superhet's receive frequency.  This arithmetic relationship is equal to the IF of the superhet.

About the only time a superhet will generate a tone is when the superhet is tuned to a specific frequency some hundreds of KHz away from the aircraft receive frequency.  If the superhet is tuned to the same frequency as the aircraft, no tones will be generated. 

If the local oscillator is operating outside the aircraft radio's tuning selectivity,  the aircraft tuning circuitry will reject it and so any interfering radio would, not only need to be tuned within a small band of specific frequencies, but be in a location where its miniscule power could enter the aircraft radio circuitry.   

In the days of valve (tube) radios, the local oscillator could make power in the tens to hundreds of milliwatts and be detected over distances of twenty metres (sometimes more).   My first (and highly illegal) transmitter was a modified valve local oscillator circuit connected to an antenna.  It transmitted from the barracks to the car park.  These days, transistor local oscillators make very little power.

When flying in commercial aircraft, you can be assured that the aircraft is totally secure from locally generated interference.  

Here is an experiment if anyone is interested;

Tune one of your sensitive receivers to a vacant frequency above the middle of the AM band.  Turn the volume high.

Get a superhet radio and position it close to the sensitive receiver.  It must be a superhet - look for a group of little square cans on the circuit board.  A cheap one is best because it won't have any screening.

Tune it about 455KHz below the frequency of the sensitive radio. 

Slowly tune the superhet up and down and listen for a whistle or a damping of the static in the sensitive receiver. 

Once (if) you establish a whistle or static damping, move the domestic radio away.  You will find that it is not long before any signs of the local oscillator vanish.   Now you know how far that superhet's local oscillator will transmit - it won't be far.




From: ultralightdx@... on behalf of andresperezf@... [ultralightdx]
Sent: Tuesday, 18 September 2018 12:14 AM
To: ultralightdx@...
Subject: [ultralightdx] Re: Ultralight DX on an Airplane?

Thanks, guys.

One last question... Does listening to the pilots with a receiver, which is not able to transmit, cause any interruption in their communication? If so, I am the first person to stop using listening to them because my life is in danger.

I would understand if the radio was able to transmit, I would not want the pilots to hear noises from the cabin rather than the actual instructions from the tower.

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