Capital Avionics Tech Time Article

Applications in Aircraft: Logarithms, Decibels – March 1999

October 31, 2012

The previous two months we have covered basic power equations, this month we look at applications in aircraft.

I recently attended an AEA regional meeting where GPS interference was discussed. The presenter explained that the 12th or 13th harmonic of certain VHF communications transmissions falls into the GPS receiver band of 1575.42 Mhz. It was suggested that a minimum of 100dB of isolation be maintained between the offending transmitters and the GPS antenna. Also, an inline filter could be installed to minimize interference but the insertion loss should not exceed 2 dB.

Let us start with the 100dB figure. A VHF communications radio transmitting at 10 Watts is producing a 40dBm signal.

dBm = 10 log 10000mW / 1 mW = 10 log 10 4 = 10 x 4 = 40.

 

The GPS signal from the satellite reaching the antenna on an aircraft is approximately -138 dBm (!).  Because this level is so low there usually is an amplifier in the antenna which adds 26 dB of gain. This results in a signal of about -112 dBm reaching the receiver. When you transmit and produce interference that equals -60 dBm ( after providing our hypothetical  100 dB of isolation )  you have swamped the GPS receiver with a signal one hundred and fifty thousand  times greater than that from the satellite.

Difference in levels = -60 dBm  and -112 dBm = 52 dB = 10 log ~ 1.5×101.5

 

This situation would be unacceptable. It was stated that you need greater than 100 dB of isolation but how much is enough? Let us give ourselves all the help we can get and look for a solution in the form of filters. They typically provide about 30 dB of rejection which we can add to the isolation number which is itself dependent upon antenna location, cable placement, etc. To reduce the interference to a level equal to the satellite signal we need 122 dB of antenna / cable isolation .

+40dBm  VHF transmitter

– 122 dB     Isolation derived from antenna location, cable placement

– 30 db      In- line low-pass filter

-112 dBm   Signal level at GPS receiver

 

Providing 122 dB  of isolation requires a careful evaluation of the aircraft before the installation is performed. We typically must relocate one VHF antenna to the belly and move the remaining top mounted VHF antenna to the rearmost available position and install the filter.

 

Let us now talk about the insertion loss of the filter that we have installed. Be alert to the fact that nothing comes for free and that the addition of such a filter will have unintended effects on your comm transceiver. The example given at the regional meeting was that the maximum acceptable insertion loss of such a filter is 2 dB. This means that this filter could cause your transmitter power to drop by 37% ! It could also raise the squelch threshold by the same amount.

-2 dB = 10 log ~ 0.63

 

This again is not an acceptable figure. Fortunately for us the filters that are available have a much lower figure. Comant’s new CI508 GPS/VHF Isolation Coupler lists its insertion loss for the VHF and GPS ports as being 0.1 dB.This amounts to a loss of only 2.3%.

-0.1 dB = 10 log ~ 0.977

 

To sum up: Beware what the numbers say on technical specifications and the unintended consequences they can have. Logarithms ( and by extension the decible dB ) are powerful numbers that explain how our world operates and ultimately why we must settle for less than perfection. Which leads us to audio noise in aircraft.

 

Going back to last month’s article, the human ear has a range extending to 10-16 W/cm 2. How low is this? In a healthy person at mid-range frequencies this is the amount of energy needed to move the eardrum a distance roughly equal to the diameter of a hydrogen atom!

Because of our ear’s sensitivity we will never entirely rid an aircraft of audio noises. Inductively and capacitively coupled signals are sources but my experience has been that the most common installation error is not grounding audio systems at one point. The aircraft alternator / generator  provides a pulsating DC current which is damped or filtered by the aircraft battery. Imagine all of the aircraft loads which use the airframe for the return conductor back to the battery. Examples such as strobes, landing lights, gear, flaps, avionics ( Who has ever run a ground strap to the battery for avionics?). The airframe therefore has substantial current coursing through it and because electrons are like-charged they like to keep their distance from one another and what you have is an equilibrium set up in the return paths of these circuits. Airframes have some resistance so you have a voltage drop throughout the structure. When any type of audio device is installed which uses the airframe for ground i.e. mic or phone jacks mounted in instrument panel, or rear mounted jacks etc. the audio panel or intercom ground is probably not going to be at the same potential as the device ground. This difference in potential is nothing more than a pulsating DC or AC signal being produced by the common mode currents seeking battery ground. You know how sensitive your ears are and you or your customer are going to hear it. If its too loud the customer is going to object. The solution? Do what the OEMs say- float loads above airframe ground ( by using insulating washers in the case of mic and phone jacks) and give each load its own return wire back to the audio system ground. A good example is the AlliedSignal KMA-24 Audio Panel with the ground strap above the mating connector on the tray. All grounds and load returns are terminated there preventing common mode currents from wreaking havoc. Make your own strip from copper stock or even aluminum and put all audio commons/grounds there. It will make a significant difference.

Next Month: On to the bench