April 1999 - American Bonanza Society

Before we leave the audio panel, mostof ours also contain a separate receiverto receive marker beacons. This receivercan only receive on one frequency: 75Megahert z. herea fter abbreviated asMHz. It outputs both one of three blinkinglights for OM. MM , or 1M and amodulated tone, which is different fo reach (i.e., outer, middle, or inner marker).Here, Dr. Edmonds' radio interferenceletter rears its ugly head. Seems 75 MHz isright in the midd le of one of the mostheavily used frequency bands in radio communications.Between 30 MHz and 108MHz is everything from military field radios,amateur six-meter band transceivers,pelice band radios. commercial VHF television,Marine band VHF radios. commercialFM radios, etc., etc. lance had a garagedoor opener (since superseded on themarket) that would activate the marker beaconlights on my airplane!Moving on down our Bonanza stack,we normally next fi nd one or twoAVCOM VHF radios. Since Air TrafficControl is always throwing numbersat us, we know that our VHF transmitter/receiverworks on all frequencies betweenIIS .OOO and 137.97 MHz. On theNAV side, we can receive navaids on allfrequencies between 10S.OO and 117.97.What some of may 1101 know is that whenwe select a AV frequency for a localizer,the receiver also automatically tunes anadditional receiver for the gtideslope in theUHF frequency band.I could spend the next six pages of themagazine relating things that get intoandin some cases, come out of-VHFCOM radios. Most of you have heard mylamentations about Mark Twelve radiostran smi tt ing on one frequency, say122.80, and not realizing they are alsotransmitting on 122. 77 alld 122.825.That's going to get even worse soon, alreadyin Europe, and soon in NorthAmerican Continent, radios with SKCspacing, instead of 25KC are going toenter the system, first going into air carriers.At lhat point, someone transmittingon hi s Mark Twel ve may be blockingeiglil other frequencies, in addition to theone he thinks he is using !)U j J nzas are a tlyrng 1'1 Ie 0, mU'(]c ing Interierino 11C p )t(-ntial problem dec r Jnl(In addition to the radio spurious signalsand some of their causes outlined inDr. Edmonds' letter. there are some newones that have jumped up to plague ussince I threw away my physics book.Display technology- such as LCD TFT,AVTIVE MATR IX, LED, BACKLI T,SUPERTWIST, RASTER, and so onhavecome into bei ng to display data.Each sy tem ha been used first in thelaptop computer industry, then adapted toother uses. All avionics manufacturers aresearching for that Holy Grail of displays,which is sunlight readable, dims at night,runs cool and takes very little voltage.In order for the display not to eat batteriesor burn up the airplane, most of thedisplays are not "continuous duty." Thatis, they rapidly cycle on and off, too rapidlyfor the human eye to be aware of.The mechanism to control this "flicker"is an oscillator set to a frequency higherthan that of human eye perception. Thebad news is that the display oscillatorsfrequency is often very close to, or is aharmonic of your radio receiver's oscillator.The result: Radio lIoise!Radio engineers are smart enough toknow this, and can add noise filters , tunedto the frequency of the installed display.However, what happens when the pilotinstalls allollier avionics device with adifferelll display in the stack? The seconddevice has also been engineered totune out ITS oscillator noise, but not necessarily the noise of the device nearby.Add to this. the unsuspecting pi lot wholoads his laptop with JeppData, plugs hisyoke-mounted GPS into its serial port,and plans to use his laptop to fly hardIFR all the way down to Preci sion Approachminimums!Assuming he survi ves, and we pull hisbusted laptop out of the wreckage, he willbelatedly discover that the laptop manufacturerdid not ever envision its use inairplanes in flight. Now do you see whythose nasty airline cabin attendants areso mean about your use of laptops andsmall FM radios when the airplane isbelow 10,000 feet ?Actually. when you are sitting in thecabin of an airliner using a laptop or SonyWalkman, there is I'er), lillie chance thatyour device cou Id affect the radios ornavigation systems of the airliner. Theoscillator would probably have to rad i­ate strongly enough to get into theairli ner's antennas, which are mountedbelow 10 feet of packed baggage compartmentaway from you.However, the airlines and the FAA arenot willing to take that chance. Are youwilling to take that chance that yourlaptop, palmtop, FM radio, your child'sCD player, which cannot be more than10 feet away from your radio stack in aBonanza or Baron, cannot radiate enoughunwanted RF energy to screw up your radiosand/or AVs? Personally, I've seentoo many strange things in airplane panelsto wam to bet on it in JFR conditions.ABS April 1999Page 5660