Posted from Usenet by W3VS...

To rec.ham-radio and sci.electronics:

Recent postings have asked about modifying scanners to receive the 
aircraft band. I use the following cheap/dirty (but educational) 
method whenever I go plane-watching or must kill time at an airport.  
This article was originally posted about 2 years ago on rec.ham-radio:


             RECEIVING AIRCRAFT FREQUENCIES ON TWO METERS

Most 2-meter receivers and VHF scanners will detect strong signals 
from a portion of the VHF aircraft-communication band, by making 
constructive use of Image Response, a normally-undesirable attribute 
of superheterodyne receivers:

1.   Consult the owner's manual to find the first intermediate 
     frequency (i-f) of your receiver.  It's usually near 10.7 MHz.

2.   Multiply the i-f by 2 and add to the desired aircraft frequency.  
     Set your receiver to the sum.

Example:  To listen to 124.15 MHz with an Icom IC-2AT handheld 
          transceiver:

     The IC-2AT's i-f is 10.695 MHz.

     2 x 10.695 MHz = 21.39 MHz.

     124.15 MHz + 21.39 MHz = 145.54 MHz.

     Set receiver to 145.54.

Aircraft radio is AM but most transmitters contain enough FM or 
incidental phase modulation so that they are easily readable on 
narrow-band FM receivers, depending upon the type of FM detector in 
the receiver.  It's usually necessary to increase the volume-control 
setting.  Of course, multimode receivers work better for this 
application than do FM-only types.

The catch-- Sensitivity is very low.  You must be less than a mile 
from the transmitter (2-3 miles with a beam antenna).  Receiver 
designers do their best to eliminate image response; the poorer its 
image rejection, the BETTER a receiver works for intentionally 
receiving image frequencies.

The tuned circuits in the front end of the receiver reject image 
frequencies.  A sufficiently strong signal can overwhelm the filters 
or couple directly into the mixer. To deliberately degrade image 
rejection, try installing a temporary jumper to bypass the input 
filters, or couple the antenna directly to the receiver's mixer input.

Even the low sensitivity can be turned to advantage:  A friend who 
services aircraft radios uses his handheld 2-m ham transceiver tuned 
to an image frequency to make preliminary tests of transmitters in 
parked planes.  If he hears nothing, the aircraft radio is faulty.

The aircraft band (118-136 MHz) is much larger than the 2-meter ham 
band.  The image response of a ham-band-only receiver covers only from 
about 122.6 to 126.6 MHz, a well-populated and interesting segment.  
VHF scanners can receive most or all of the aircraft band by the image 
technique, as can certain ham rigs which are modifiable for wider 
frequency range.

Two-meter repeater owners are often dismayed and mystified by aircraft 
interference on "their" frequency.  The only solution is to replace 
the receiver with one having a different intermediate frequency.  
Before establishing a repeater it is wise to calculate the receiver 
image frequency and check for activity there.

FAA regulations prohibit operation of electronic devices by passengers 
aboard commercial aircraft, without permission from the operator.  
"Operator" means the airline management, not the pilot.  Pacemakers 
and a few other devices are excepted.  One reason for the regulation 
is that local-oscillator radiation from radio receivers can interfere 
with the plane's navigation or communications equipment.

--

Frank Reid     W9MKV
reid@gold.bacs.indiana.edu