Vor Antenna On Aircraft - In the beginning, there was dark water. Marconi bridged the murky waters between North America and Europe with a 300-foot (100-meter) high antenna with a balloon and a kite, so the antenna concept was the key to communication. And it remains to this day.
I work in a field where a master's degree is an entry-level requirement, but my colleagues see my antenna specialization as using fresh eyes and frog toes in my designs. Nothing can be further from the truth, and if you want to follow me, I will do my best to guide you to the right path.
Vor Antenna On Aircraft
An artist's conception of how waves propagate. Vertical polarization is shown. Rotate the image 90 degrees for horizontal polarization.
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First, let's talk about "polarization"; there are three polarizations. The first two are easy. If the active ("radiating") element is vertical to the earth's surface, then it is vertically polarized. If the active element(s) is horizontal to ground, the polarization is horizontal. The last one is a circle, and there are two shapes of circles, right circle and left circle.
Old narrowband com antenna. It had to be mounted on the belly of the aircraft and bent back to allow ground clearance. It's a pretty bad antenna for com because it radiates horizontal polarization as well as vertical.
Vertical polarization is mainly used if we are not interested in where the signal comes from, but only concerned with receiving it. In most cases, our comm antennas are vertically polarized with so-called "whip" antennas placed above a horizontal conducting surface that mimics the earth's surface. The second vertical antenna is a small transponder antenna that is generally mounted vertically on the lower surface of the aircraft. The signal from and to the vertical antenna vibrates up and down relative to the earth's surface (or to the metal that simulates the earth). Remember this (even though it doesn't make much sense now): the impedance at the bottom end of a quarter-wave vertical antenna is about 32Ω (ohms). I will explain later.
A pretty clever dual comm and nav antenna from the 1970s. The vertical pole is the com antenna and the horizontal wing is the nav antenna. He suffered from a rather serious nav signal overload disease when the comm antenna transmitted a few watts to a microwatt sensitive nav antenna very close.
Cessna 182t Skylane N726sa
Horizontal polarization is mainly used if it is very important to us where the signal comes from. Here's the deal: When horizontal radio waves hit a large reflective surface (like a building or mountain), they generally reflect as a vertically polarized signal. The mountain is taller than it is wide, like the building, and the reflected waves are taken vertically. Think about this when you're punching a hole in the sky navigating a VOR or LOC station or a glide path. The station transmits a horizontally polarized signal, and you are very careful to receive a direct signal from the transmitter. But what happens when a VOR signal bounces off a mountain 20 miles to the right, or a LOC/GS signal bounces off a building near a landslide. Fortunately, no. Horizontally polarized navigation antennas reject all such vertically polarized signals. Like vertical antennas, horizontal antennas emit and receive signals that vibrate left and right relative to the earth's surface. Again, wait a few minutes, but the impedance at the center of the horizontal dipole is about 72Ω.
The standard cat-whisker navigation antenna is used on most everything out of Wichita from the 1950s to the present.
But what happens if the transmitting station can be horizontal or vertical at any time? What about satellites in space that aren't sure how to rotate while orbiting the Earth? GPS signals are easy to figure out. Those GPS satellites can do crazy maneuvers, and we can't rely on orientation. We then move to circular polarization where the wave actually rotates as it travels. Let me use a baseball analogy. If the pitcher throws the ball with a clockwise (right-handed) spin, it will break to the left when it crosses the plate. Also, if a pitcher throws the ball in a clockwise (left-handed) direction, it will break to the right. If the batter mishits and swings right to break left, miss the ball (and the same for swing left to break right). But if he hits it right, he hits the ball. Thus, the GPS signal is polarized with right circular polarization. This means that the antenna on the plane must use right circular polarization (RHCP) to receive the correct GPS signal. Fortunately for us, planning for RHCP (and LHCP for that matter) is a relatively simple process, and any antenna you buy for GPS is covered by RHCP. For starters, GPS antennas can be made to any impedance you want, but most are designed to match 50Ω cable.
1950s communications antenna for (at most) 90-channel radio stations. We will talk about why this is so in a later column.
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Stay with me here. Most vertical and horizontal antennas are made with "quarter-wave" conducting elements. What does that mean? Radio waves travel at the speed of light - 186,000 miles per second (or 300,000 kilometers per second if you prefer). But it also vibrates back and forth at several frequencies... for example, the only air "chatter" frequency allowed is 122.75 MHz. This means that the signal vibrates "start-down-down-down" 122.75 million times per second. The question arises, how many times did he cross one complete wave? We get that by dividing the speed of light by the frequency. Without going into a lot of algebraic conversions, I'll just say that if you take 11.785 and divide the frequency in MHz, you get the wavelength in inches, which in this case is about 96 inches. It's a wavelength. A quarter wavelength is a quarter, or about 24 inches. So, to make a quarter-wave antenna element, we need to cut a 24-inch length of wire.
This "BB" transponder antenna is a short whip antenna with a ball on the end. The ball has no electrical purpose; it is there to prevent it from putting a hole in the skull when it suddenly comes from clearing the stomach and hitting the head on the antenna.
Almost. I hate it when someone moves the correction factor, so I just say that after a few thousand trial and error measurements, we should consider the factor called "end effect" and reduce this length by 5%, or the practical length. of 22¾ inches. You can go through the same iteration for the metric system and find that the practical quarter wavelength for this frequency is 60 cm.
At this point you have two options...horizontal or vertical polarization. You have two other options, a single element ground plane or a dual element construction ("dipole". Four combinations are possible and it is only a matter of installation that you will do. For example, if you are building a plastic plane, you may want to run a vertical dipole on the vertical fins. If you are building a metal boat, you may want to use a metal ground skin with a single vertical rod for the radiating element.
Aircraft N8700a (1949 Beech A35 Bonanza C/n D 2110) Photo By Doug Robertson (photo Id: Ac133853)
With the same calculations for a simple VOR / LOC antenna for the Buffalo (NI) VOR (116.4 MHz), we find that the antenna elements must be 24 inches long (each), but because the VOR and LOC signals are horizontally polarized. , you probably want to use a horizontal "rabbit ear" dipole with these two elements that will measure 48 inches from tip to tip.
ADF circular antenna. In my opinion, ADF is only used for one purpose in airplanes today: to listen to ball games on long flights.
Or maybe a little less. We'll get into the finer points of antenna placement, setup, what type of coax to use, and more in the next column. Just remember that I went to the wedding of two antennas on your plane, and the ceremony was not big, but the reception was great. Until then… Stay tuned…
Shark fin transponder / DME antenna. This includes transponder and DME frequencies, while the "BB" antenna is only for transponders.
Atr Aircraft Hi Res Stock Photography And Images
Marker ship antenna. This is one of the few places that wants cross polarization (vertical-horizontal) so that the marker lights and tones only come on when we are directly over the marker transmitter.
Vintage 1960s "sled" antenna sign. Good during the day, but a bit tight compared to boat antennas and not resistant to "belly" contamination.
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