BLEVEN’S TECH TIPS

Tech Tips April 2004

RADIO CIRCUITS???



Sorry, this one is a bit long, so hang in there and read it over twice!


Airplane radios consist of several circuits to allow them to operate properly. As with electrical system circuits, we can break radio circuits down into secondary circuits to more easily understand their interconnections.

The first circuit to consider when we do this is the power circuit. Nav/Com radios are usually fed with one power feed to supply both the Nav and the Com on/off switches. If you elect to incorporate a radio master switch, the power supply should begin at the fuse panel with a breaker that is large enough to handle all of your radios. From this radio master breaker, you would go through a radio master switch and then back to the breaker panel in order to power each individual radio circuit breaker. In the event that you desire so much radio/electronics that you cannot find a switch with sufficient current rating, you can elect to utilize a solenoid switch to do the job. There are a couple of points to remember here. Firstly, each breaker must be of the amperage called for by the radio it will be powering, and secondly, the sum of these amperages will be the determining factor in selecting the size of the radio master breaker and switch.

The communication radio has a switching relay inside. This relay is similar to a solenoid relay and its job is to transfer the communication radio from receive mode to send mode by grounding one lead. This relay also opens the feed line from your mike to the radio (your push to talk circuit) to enable you to hear your voice transmission. You would not hear it otherwise as the radio would no longer be receiving a signal to send to the speaker or to your headset. Also, your transmission over the audio speakers would be confusing. Two separate audio outputs are usually supplied. As with any circuit, these outlets send the sound signal, an electrical pulse, to the speaker. Once received at the speaker, the pulse is converted into English, hopefully. From there, the pulse goes to the ground to complete the circuit.

Another single inlet is provided for the mike connection. When the mike is connected directly to your radio, this push to talk relay allows voice transmission. When connected to an intercom system, your voice will activate the intercom speaker system. The push to talk button will still need to be pressed for radio transmissions. To make all of this easier to understand, a schematic drawing is supplied with your radio and with your intercom. Outlet pins are numbered on the back of the radio and identified on the schematics. Just connect the dots! It really is that simple. Radios have a side tone within them that allows you to hear yourself as you speak over the radio when wearing a head set. Without a headset, you have to yell to hear yourself. (at least in my plane that is)


Now, what are some of the issues to be aware of regarding the subject of R.F., or Radio Frequency interference? It is important to remember that there is electricity surging through the wiring and electrical components of your airplane. Also, static electricity can produce a couple of problems in this area. Firstly, it can make electrical frequency waves close to the actual range of your radio.(not good) Secondly, it can cause interference on the power lead feeds to the radio. For these reasons, it is important that we provide protection for the radio. This protection is accomplished in several ways:


  1. Shielding the antenna wire. A coaxial cable is used for this purpose.

  2. Also, we should keep this cable as far away as practical from other wiring and electrical components.

  3. Both ends of the outer jacket of this cable should be grounded.

  4. All wiring in your airplane should be of aircraft quality and shielded.

  5. One end of the wire shielding should be grounded

  6. Using Teflon coated wiring will allow a shorted wire to burn inside its cover without too much effect to the surrounding components.

  7. Bond all metal components to eliminate static transfers

  8. Attach noise suppression filters to each mag ground lead, to the output line from the alternator, and the feed line to the strobe power supply if needed.

  9. If your plane has an unshielded high voltage system as might be the case if you are using automotive ignition wiring or strobe lights, the antennas for the radio should be protected from line of sight to these systems with a metal shielding.

  10. And remember, these same protective measures are important for the navigation radio even though it is not something we “hear”.


Finally, just bear in mind that wiring the nav/com and connecting the C.D.I., is only as difficult as you make it. Look over the schematics and then connect the wires. Make a neat harness and good connections. With all of the different radios available out there today, even professional avionics personnel would be lost without following those schematics.


Dick i.d.k.s. about this either


Next topic---Bolts and Nuts, a no brainer, right?