Vor Antenna On Aircraft - A vertical ground plane mounted next to the vertical stabilizer. Note that the side of the fuselage (the ground plane) immediately tilts to a vertical position. It provides good coverage of the stations here.
Last month we had a brief general chat about antennas. Let's get a little more specific about what we need to do to get a decent antenna.
Vor Antenna On Aircraft
Except for GPS and ADF, all aircraft antennas are terrestrial or dipole. A ground plane antenna has one element 90 degrees above the transmission surface which is infinite in all directions. A dipole antenna has two collinear elements (straight pencil). This is the theory.
Comant Radio Telephone Antenna P/n Ci275 Nos (1022 421)
The ground plane or dipole antenna elements can be made of any conductor, usually metal. The best metals are (in order) silver, copper, gold, aluminum, zinc, nickel, copper, and bronze. Honestly, the difference between a silver antenna and a bronze antenna is so small that sensitive instruments are needed to tell the difference. You really couldn't tell the difference on the plane. Use whatever metal works best for you.
A writer with a dipole copper tape antenna on a wooden dowel. Note that the antenna is fed in the center. It should also be noted that two coaxial power lines are connected to the center. It's the coax balun that we'll talk about next month.
Now for the practical side. A principal plane in a plane cannot appear on an infinite surface. We are often limited to antennas built on the ground (the original "ground" of Marconi's "ground plane" and still used for radio broadcasting and the like). An aluminum plane is also not "infinite", but works as a first approximation. This often works in our favor.
A true "ground plane" antenna has zero radiation below the ground plane; all the radiation is out and up, away from the ground. Imagine a donut being tossed onto a vertical element from above. This makes it very difficult to talk to a ground station from an airplane, because all the transmitted signal goes into space (except directly - the "hole in the cone" effect). However, the plane's ground plane is not perfect, and if you look at the shape of the fuselage, you'll see that it sticks out a little and points down. So an antenna that uses the body as a base plane will have a large amount of radiation to the sides and bottom, which is what we want.
Aviation Cyber Security Testing
The author's personal editor holds a copper tape dipole inside a weatherproof housing (plastic water pipe). This will be a vertical com-top antenna with a right angle bracket on the left hand side of the editor.
And the dipole does not want anything on the metal surface. Its signal is as wide as a dipole, and a vertical dipole produces a maximum signal at the horizon. Imagine a cone passing through one of the elements and moving down to cover the center of the dipole; there would be maximum radiation at right angles to the dipole, but there would be a large hole at the end of each element.
Dipoles are also somewhat "insensitive" to metal in the center. The tips are really sensitive to wires, but in the middle you can get away with mounting them on or over a metal surface. It is also true that the "hole in the cone" effect can be avoided by bending each element slightly horizontally (say up to an angle of 30 degrees) rather than aligning the elements directly. This makes them a very shallow V, with the best reception at the open end of the V. That's why the "rabbit ears" on the horizontal VOR/LOC antenna on the vertical stabilizer of many metal airplanes point forward for best results. receiving from where you're going, not where you're going.
A horizontal "cat's whisker" VOR/LOC antenna is mounted on top of the vertical stabilizer. This particular antenna will have the best reception behind an airplane.
Boeing 737 Flight Instruments
Both ground planes and dipoles have fairly loose requirements for being "straight". This means that you can bend the elements to fit the shape of the airframe, especially in a plastic aircraft, where the elements generally remain vertical or horizontal - especially in the last 25% of the element length.
One thing that requires a dipole not to be a ground plane is a balun (pronounced BAAH-luhn; more on that in a future column).
Let's make a dipole antenna and see what we can detect (the same general principles apply to the ground plane).
What do we make it from? Copper tops the list because it is cheap and available. What kind of copper? From me: "Would #40 copper wire go through my glass roof?" The answer is yes, but not well. Why?
Vor (vhf Omnidirectional Range) Navigation System Overview
Basic planar radiation pattern. Note the "cone hole" at the top of the main plane.
The answer is bandwidth. The copper wire is cut at the exact frequency, which works well. It doesn't work well on either side of the frequency and the answer lies in what we call the volt standing wave ratio (VSWR - pronounced VIZZ-war). In short, the transmitter tries to pump energy into the antenna and radiate it. VSWR says, "No, I'm reflecting some of that power back because the antenna isn't perfectly aligned with the conductor and coax." How can this be?
If you remember last month, we said that the impedance at the center of the resonant dipole is about 70 ohms, and the center of the resonant ground plane is half that value, or about 35 ohms. However, we design our conductors and coax cables to be 50Ω. Why? Because we either feed the ground plane or the dipole, and we have to decide to split the difference, so the transmitter can drive with little loss. Calculating VSWR without much trouble? Divide the larger number by the smaller number. 70/50 = VSWR 1.4 or 50/35 = VSWR 1.4. See how it works? Again, without much math, at a 1.4:1 VSWR, about 3% of the transmitter's power is reflected back into the line. For a 10-watt transmitter, this would be about 300 milliwatts. Nothing big.
Broadband transponder antenna. Almost all of the radiated energy goes directly to the ground station.
Reducing Radio Interference On Aircraft
At the exact resonant frequency, it is. But look what happens when you get to the edge of the ball. I do the numbers for the ground plane; dipole will be the same. At a center frequency of 127 MHz, transmitter #40 has a VSWR of about 1.4:1, but almost 3:1 at the low end of the band and almost 4:1 at the high end of 137 MHz. These are 25% and 35% power reflections - reflecting a quarter to a third of your power and hitting your weak conductor very badly.
(left) Figure 1: VSWR from 100 to 150 MHz with a center frequency of 125 MHz (near the center of the aircraft's VHF communication band). Wire size #40 (0.003 inch diameter). Note how narrow the V graph is (narrow bar). (Center) Figure 2: The same wire as Figure 1 is 1/8 inch (0.125 inch) in diameter. (Right) Figure 3: Same as Figure 1 except the wire size is 1/2 inch copper tape.
Now let's take a half inch copper tape antenna. The average 1.4:1 VSWR is about the same, but at both ends of the band (118 and 137 MHz) the VSWR is lower than 2:1, which is less than 10% reflected. This will make your transmission a much happier motor.
The VOR/LOC horizontal dipole antenna is made of copper tape and mounted inside the plastic aircraft fuselage. The best reception of this antenna is V .
Microsoft Flight Simulator
So we've talked about polarization, we've talked about ground planes and dipoles, and we've talked about baluns and coaxial cables. I think one more session on "floobydust" (balls, coax and other bits and bobs) and we can talk about building realistic antennas for our planes. Until then… Stay tuned…
Horizontal sliding antenna made of copper tape. The best reception of this antenna is V .
Paul Bertorelli reviews Garmins highly anticipated GTN 650/750 navigators to find out how they stack up against the successful GNS 430/530 series.
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How A Vor Navigation System Works
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