VHF Nav/Comm Basics

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Ever wonder how a VOR receiver knows what radial you're on? How the localizer needle knows whether you're left or right of runway centerline? Or how much effect power output has on the range of your comm transmitter? AVweb's avionics editor takes you through the basics of the most taken-for-granted radios on your panel, your VHF comm and nav.

VHF Comm

AvionicsMany years ago it was decided that civil aircraft communications radios would use the 118-137 MHz band, and would use amplitude modulation ("AM"). Like many other things in aviation, this has remained unchanged for many decades. It's a pity, because our air-ground communications would have much better audio quality if we could switch to frequency modulation ("FM") as the majority of commercial broadcast stations have. But it looks like we're stuck with AM for the foreseeable future.

In the old days our aircraft communication radios were limited to ninety channels spaced 200 Khz apart. As more channels were needed, the channel spacing was reduced. Modern aircraft comm radios have 760 channels spaced 25 Khz apart. These comm frequencies lie in the portion of the radio spectrum known as "Very High Frequency" or VHF, defined as 30-300 Mhz. Military aircraft use a different band in the "Ultra High Frequency" or UHF spectrum (300-3000 Mhz).

Power, range, and modulation

Frequently I hear a pilot say "I wish I had more power so I could talk to Center while on the ramp." General aviation comm radios transmit at a power output of 2 to 25 watts. In most cases, more power wouldn't help. VHF radios operate strictly line-of-sight. If Center can't hear your 5-watt radio because there's a hill in the way, 100 watts wouldn't do any better. Think about the ELT for a moment. It can send a signal to a satellite thousands of miles away on just one watt of power, because there's a clear line-of-sight.

I figure anything over ten watts is a waste and is added load on the radio. Another thing to look for is the way the manufacturer measures power output. Some use phrases like PEP, RMS, average, etc. If you're attempting to compare the power output rating of two radios, make sure you're comparing apples with apples (e.g., PEP with PEP).

The best way to improve the range of an aircraft comm radio is by installing a good antenna system. As with all radios, the antenna is the heart of the system and a poor one will do a poor job regardless of how good a radio you have. I recommend that if you're installing a new comm radio, you have the antenna system checked out also. Nothing worse than paying $4,000.00 for a new radio, only to find out that it preforms no better than the old clunker you pulled out because the antenna is no good.

King makes a little gizmo that is installed between the antenna and the aircraft radio and permits you to plug in a handheld transceiver. When plugged in, the handheld uses the aircraft antenna for its antenna, providing greatly increased range. Normally, at seven thousand feet, you should be able to receive and transmit a range of around fifty miles. This is of course is true only if it is line of sight and no big rocks like the Rockies or Sierras in the way.

The receiver

Another important thing that must be set properly is the modulation level of the transceiver. Most radio manufacturers call for 90% modulation of the carrier by voice. If modulation is too low, your voice will sound weak; if too high, it will be badly distorted. This adjustment must be done by a shop with the proper testing equipment. Newer radios have build-in protection against overmodulation, but most older radios do not incorporate this feature. If you overmodulate the radio, your transmissions will be garbled, and may also interfere with adjacent channels.

The receiver portion of a comm radio is every bit as important as the transmitter. Receivers incorporate a "squelch" circuit to eliminate background noise when nobody is transmitting on the frequency. On most of the older comms, you manually adjusted a squelch knob until you heard the back ground noise, then backed it off slightly until the noise went away. Most of the newer comms have "auto-squelch" in which the squelch level is pre-set on the bench and the pilot simply has a switch to turn the squelch on or off. Most auto-squelch circuits are set to open at a signal strength of three microvolts (just in case you wanted to know). If the received signal is less than three microvolts, then you hear nothing. If the signal is greater than three microvolts, then you hear whatever is there.

Reliability

Older comm transceivers (particularly those from the vacuum tube era) have a high failure rate. The newer solid-state units like the King KX-155 and Narco MK-12D seldom fail. The older radios had crystals to determine the frequency in use. These get out of tolerance often and are expensive to replace. The newer radios use a synthesizer to select the desired frequency and are very reliable.

Many of us use "nav-comm" units that combine a VOR/LOC navigation receiver with a communications transceiver in a single chassis. Even though they share the same box, very few components are shared between the nav and comm sides. So if the nav receiver fails, the comm is still likely to be working...and vice-versa.


VHF Nav

The most used piece of navigation equipment in the world today is the VOR or "very-high-frequency omnidirectional range". They are around 800 VOR stations in use today in the U.S. The VOR operates from 108.00 to 117.950 Mhz which is in the VHF band like the comm is. This is good because VHF frequencies are relatively immune to static and interference, making them excellent for navigation. All VOR stations have a three letter identifier and some have voice weather.

The VOR station produces a radial pattern by transmitting a 30-Hz reference and a 30-Hz variable-phase signal. The nav receiver in the aircraft compares the phase of these two signals and figures out what radial from the station it is on. It then compares the computed radial to the radial that the pilot selected with the "omni bearing selector" (OBS) and deflects the "course deviation indicator" (CDI) needle to indicate any deviation between the desired radial and the actual one.

How it works

To understand how the receiver can tell what radial it is on, let me give you an analogy. Suppose you have a lighthouse that sends out a powerful light beam which rotates one full revolution each minute. Suppose also that the lighthouse has a strobe light on top which flashes precisely when the beam passes magnetic north. Now if you were flying in visual contact with the lighthouse, you could start a timer when you saw the strobe flash and stop it when you saw the searchlight beam. The time difference between the two would always tell you what radial you were on relative to the lighthouse. If you saw the beam 15 seconds after the strobe, you'd know you were on the 090 radial from the lighthouse; if 30 seconds, you'd be on the 180 radial, and so forth.

The VOR station and receiver work exactly the same way, except that both the "beam" (variable signal) and "strobe" (reference signal) are replaced by radio signals, and the "beam" rotates 30 times a second. If the reference and variable signals are the same phase, then the nav receiver knows it's on the 360 degree radial. If the variable signal is 90 degrees out of phase with the reference signal, then the nav receiver knows it's on the 90 degree radial.

As with comms, the older nav receivers use crystals for selecting the correct frequency and these fail with age and are expensive to repair. The newer radios use a synthesizer for tuning and are very reliable.

Localizers and glideslopes

Almost all VHF nav receivers handle localizers as well as VORs. The localizer is in the same band as the VOR, and uses certain channels in the lower portion of the nav bad that are dedicated for that use. Localizers have four letter identifiers starting with "I" (for ILS).

The localizer beam is produced by two transmitters operating on the same frequency but modulated with different audio signals. The transmitter on the left has 90 Hz signal on it and the right one has a 150 Hz on it. The two signals are carefully aligned so that they are of equal strenth precisely on the extended runway centerline. If the aircraft is left of course, the 90 Hz signal is stronger than the 150 Hz signal, and the nav receiver deflects the CDI to show a "fly-right" indication. Conversely, if the aircraft is right of course, the 150 Hz signal is stronger than the 90 Hz signal, and the receiver produces a "fly-left" indication. Basically, that's all there is to it.

Interestingly enough, although we're required to check and log the accuracy of our VOR receivers every 30 days for IFR operation, there's absolutely no regulation that requires the localizer or glideslope receivers to be checked...EVER! But I'd strongly suggest checking them at least once a year if you do IFR approaches. Can you imagine shooting an ILS in low weather and discovering that the indicator is three dots off? Most radio shops have a portable battery-powered checker that allows checking the VOR, LOC, and GS calibration right on the ramp in just a few minutes.