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Tom Rogers, Ph.D. |
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| About the Author ... |
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Tom Rogers is Avionics Editor for AVweb.
Tom operates
Avionics West, Inc. at Santa
Maria, California, one of the finest radio shops on the West Coast. Tom is an
instrument pilot, an FAA Designated Engineering Representative (DER) for
avionics, and has a Ph.D. in nuclear physics. (We're not sure why he got the
doctorate, but we call him "Dr. Tom," and he seems to like that.)
You can send Tom your avionics questions at
avionics@avweb.com.
Tom's company, Avionics West,
is one of the worlds largest discounters of handheld and
panel-mount avionics for general aviation. They offer all leading brands of
aviation electronics at deep-discount prices. Every item sold by AWI is
covered by a 30-day no-questions-asked return privilege.
Avionics West features handheld GPS receivers from Garmin and Lowrance;
headsets from David Clark,
LightSPEED and Telex; and panel-mount GPS navigators from II Morrow. AWI
stocks these units and generally sells them at prices substantially below the
manufacturers' "Minimum Advertised Price" (MAP). Consequently, AWI is not
permitted to advertise these prices on its web site, but you can obtain them
by sending an email to the AWI auto-responder at
avionics-specials@avweb.com, or
by telephoning AWI at 1-805-928-3601 (M-F, 8-5 Pacific Time). |
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| Ask Tom an Avionics Question |
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VHF Comm
Many 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.
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