Antenna placement and interference

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autoreply

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I'm thinking about antenna placement, interference and all that stuff.

I don't want any "stuff", hanging out in the breeze. Yes, aerodynamic efficiency is an issue ;)

I've a design that's pretty similar to a glider. Downwards fin and pusherprop, mid-engine. Construction is composite, so the lowly-loaded parts can be replaced with glass or kevlar (those are transparant for radiation).

I have 5 places "available" for antennas. On top of the instrument panel, behind the headrest for the pilot, in the wingtips (preferably not, detachable wings so that would require connectors), on the lower side of the nose (the whole canopy and the lower part of the fuselage front cover slide forward) and in the fin (would required kevlar/glass spar).

Just thinking about all possible antennas that would require placing. I'm especially curious about interference.

Here's a good source for all frequencies:
Frequency Allocations


GPS: 1200-1600 MHz
*On top (instrument pane or behind headrest). Small antenna (3X5X1")

COM, 100+ MHz
*Conventional ones are pretty tall (19", vertically polarized) Possible mounting in the fin. There are also "tape-like" antennas, but they have to be mounted in a non-carbon structure. Possibly in the nose cone or fin.

NAV + ILS: 100+ and 330 MHz
*Same problems as above, 19", horizontally polarized. Is typically the NAV/COM antenna used? How about the 330 MHz antanna?

ELT: 400 MHz?
*On top? Crash-resistant location (below headrest?) Far from COM

Transponder: 1000 MHz
*"All-round" visibility, front and slightly downwards have priority. Lower part of the nose? How about interference with NAV/COM?

ADF: 0.1-2MHz
*Obscure, don't need it I guess?

Magnetometer (EFIS).
*Quite sensitive for antennas. Necessary to keep away from direct current, being landing lights and generators-alternators. Batteries might also be a problem.


Any comments on placement? How about interference between the above antennas?
 
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Toobuilder

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I'm far from an authority, but my avionics guy said to keep the comm antenna a good distance from your head (rather, headsets) to prevent feedback.
 

djschwartz

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GPS is at 1500 MHz, not 240. That's why the antennas can be so small.

Your VHF antennas are COM (118-136 MHz), vertically polarized; and, NAV/ILS (108 -118 MHz and 330 MHz) horizontally polarized. VOR and Localizer are at 108 - 118 MHz and will always use the same antenna. Glide slope is at 330 MHz. You can get antennas that are dual band (VOR/LOC and GS) if you want to use the same antenna for both or you can use a separate antenna for GS. You should not use a NAV/ILS antenna for COM. Most NAV antennas have a small balun transformer in them that will burn out if you attempt to transmit through them.

Forget ADF, it's a dead technology.

If by "magnetometer" you mean the flux gate used to sense magnetic heading this will not be affected by radio signals at all. It will be very sensitive to DC currents such as those that are drawn by a landing or nav light or other electronic device. Keep it away from DC wiring.
 

Dan Thomas

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Comm antenna is usually about 19" long. Quarter-wave. Nav antenna is a dipole with two legs each about the same length as the Com. I could measure both from the aircraft we have here if you need something more exact.

ELT antenna should be at least 30" away from the Com antenna, Artex says. Hard to do sometimes.

ILS uses the NAV dipole. DME has a horizontal rod under the belly. Transponder is tiny and needs to be on the bottom. And it nees to be kept clean and clear of obstructions; any dirt or oil or other stuff can detune the transponder and ATC loses you. I wouldn't bury it inside the airplane.

Best ADF antennae are the belly comes about 8" x 12" or so, inch and a half thick. Big, obsolete. Older ADFs had the dome for the loops and a long sense wire from the cabin roof to the top of the fin.

Dan
 

KC135DELTA

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GPS is at 1500 MHz, not 240. That's why the antennas can be so small.

Your VHF antennas are COM (118-136 MHz), vertically polarized; and, NAV/ILS (108 -118 MHz and 330 MHz) horizontally polarized. VOR and Localizer are at 108 - 118 MHz and will always use the same antenna. Glide slope is at 330 MHz. You can get antennas that are dual band (VOR/LOC and GS) if you want to use the same antenna for both or you can use a separate antenna for GS. You should not use a NAV/ILS antenna for COM. Most NAV antennas have a small balun transformer in them that will burn out if you attempt to transmit through them.

Forget ADF, it's a dead technology.

If by "magnetometer" you mean the flux gate used to sense magnetic heading this will not be affected by radio signals at all. It will be very sensitive to DC currents such as those that are drawn by a landing or nav light or other electronic device. Keep it away from DC wiring.
I use it twice a week.

Get stuck up under some thick IMC, lose your GPS signal and the only thing dead will be you. :roll:

Seriously, don't write it off. It's simple, everybody knows how to use it and it works.

LORAN on the other hand. Is dead.
 

autoreply

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Well, that's a lot of good comments, thanks a lot. Since we're not allowed to go IFR in Europe (unless you N-register it), I propably don't need all those antennas and will only have a COM/ELT/Xponder. Nevertheless, I want to be sure it's possible to mount everything in the aircraft once it's necessary. A contributing factor is that - by the time my design flies - VOR is probably something that's almost obsolete too.


Can NAV and COM (plus glideslope) antennas be mounted next to each other? The JS-1 glider uses a kevlar spar in the fin and kevlar or glass skin. I like that solution (mount both antennas in the torsion nose of the stabilizer) but is it realistic to put them 1" or so apart? Or should I put the COM in the fin and the NAV in the stabilizer?

Dan comments that burying the transponder in the aircraft isn't a good idea. Would a typical sandwich (2X2 layers of glass, bit of Divinycell at around 1-1.5" distance) also be a problem?
 

djschwartz

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The COM antenna should be vertical. The NAV/ILS antenna(s) should be horizontal. You will get a very noticeable reduction in range if they are mounted incorrectly. To put this in technical terms a 45 degree cross polarization will result it about 3 dB of loss of signal strength which means the range will be approximately 70% that of the properly oriented antenna. Not too bad, that's why we don't lose the signal in a bank or other normal maneuver. Theoretically the signal loss at 90 degrees from the correct polarization is infinte; but, fortunately, that requires a mathematically perfect antenna that cannot exist in real life. For a typical aircraft installation the loss at 90 degrees (i.e. horizontal instead of vertical) will be between 10 and 20 dB. This will result in the range being reduced to something between 1/3 and 1/10 of the range with the proper orientation.
 

Dan Thomas

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Well, that's a lot of good comments, thanks a lot. Since we're not allowed to go IFR in Europe (unless you N-register it), I propably don't need all those antennas and will only have a COM/ELT/Xponder. Nevertheless, I want to be sure it's possible to mount everything in the aircraft once it's necessary. A contributing factor is that - by the time my design flies - VOR is probably something that's almost obsolete too.


Can NAV and COM (plus glideslope) antennas be mounted next to each other? The JS-1 glider uses a kevlar spar in the fin and kevlar or glass skin. I like that solution (mount both antennas in the torsion nose of the stabilizer) but is it realistic to put them 1" or so apart? Or should I put the COM in the fin and the NAV in the stabilizer?

Dan comments that burying the transponder in the aircraft isn't a good idea. Would a typical sandwich (2X2 layers of glass, bit of Divinycell at around 1-1.5" distance) also be a problem?
I wouldn't put the Nav and Com antennae too close. The transmitter of the Com creates considerable energy around its antenna and would force RF into an adjacent Nav antenna and could damage the Nav's amplifiers.

The guy to ask about this stuff is Jim Weir, the avionics kit guy.

Dan
 

Joe Fisher

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Don't forget about ground planes. It is not an issue in metal airplanes and not a big deal with steel tube but with composite your antennas will not work at all with out a some big conductor.
 

Dan Thomas

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Don't forget about ground planes. It is not an issue in metal airplanes and not a big deal with steel tube but with composite your antennas will not work at all with out a some big conductor.
Exactly. That can be tough to do in a composite or wooden airplane if you're trying to hide everything. Some guys use aluminum foil tape in an X shape under the skin, grounding that foil to the antenna's cable shield, and some have used aluminum window screen.

The ground plane acts, as I understand it, as one of the plates of a capacitor (the antenna is the other) to generate an electrostatic field between the antenna and ground plane. That field generates the electromagnetic radio wave. Without the ground plane the wave will be weak. With an offset plane (large on one side, small on the other) the wave will radiate more strongly from the large side. Some guys have had Com problems when facing certain directions, caused by an offset plane. Remember the old CB antenna mounted on one end of the rear bumper? The best transmission direction would be across the car's roof toward the opposite corner.

Dan
 

JamesAero

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What about dipole antennas that claim to not need a ground plane? Also, almost all com antennas come pre-molded at an angle other than vertical. I'm pretty sure that is for drag reduction, but is it really at a loss of 3db or more?
 

djschwartz

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What about dipole antennas that claim to not need a ground plane? Also, almost all com antennas come pre-molded at an angle other than vertical. I'm pretty sure that is for drag reduction, but is it really at a loss of 3db or more?
Dipole antennas (VOR/ILS) do not need a ground plane and they should have no metal close by in the plane of the antenna elements or parallel to them. Obviously this can't be a total exclusion as the wings and horizontal tail will be parallel (more or less) to the antenna. But that's life, it will work OK. The sweep angle of the COM antennas actually just for appearance (unless you're talking about a transonic aircraft) and is typically only about 30 degrees or less, which results in minimal loss. And since aircraft antennas are rarely mounted over a "perfect" ground plane the effect is hard to measure. The radiation pattern of a typically mounted antenna isn't the perfect circle the text books show, so the main effect of the sweep back or the bent over end on some is to slightly further distort the already somewhat distorted radiation pattern.

FWIW, there's no "magic" distance from the antenna to other metal or other antennas that is good or bad. In general, the farther the better. But just do the best you can within the practical constraints of the aircraft configuration. With todays good radios you'll get excellent performance as long as you buy or build a good antenna and are reasonably good on the installation.
 

wsimpso1

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Check out RST Engineering (RST Engineering). This is Jim Weir's company, and he puts out a booklet and materials kit on how to build dipole antennas inside plastic airplanes. He even gives rules on how close to other conductors you can be and still expect good antenna behaviour. His antenna have flown in many thousands of airplanes, are reputed to work better than the ones we use on our spam cans, and even hang in the Smithsonion after circling the globe non-stop (Voyager). Good enough for Burt Rutan is good enough for me.

The COM antenna needs to be roughly vertical and each arm needs to be 20.3" long. They can be bent around, but yeah, no carbon fiber or other large conductors (more that 1/8th wavelength) within a 1/4 wavelength. To ground you on this, these are 1/4 wave dipoles - yeah, a quarter wave is slightly longer than the 20.3" cited above.

The GPS and transponder antenna are talking to higher frequency stuff, so the wavelength is correspondingly smaller, so the antenna are really small and need a correspondingly smaller area free of conductors.

Remember also that conductors between your antenna and ATC's can block the signal, so putting a vertically oriented antenna near the firewall is likely to go dumb on some bearings relative to other antennas. Jim recommends things like vertical tails, and fuselage walls far from the engine for COM antenna. He also publishes a scheme for hanging antennas in fiberglass wingtips, splitters, what cable to use (and what cable to avoid), his balun designs, and the news that antenna are reciprocal - if they listen, they can talk too.

Worth the money for the reading, and his materials kit is terrific too.

Oh, and dipoles don't have one leg acting as a ground plane, ground planes are acting as the second leg of a dipole
Semantics.

Billski
 

Dan Thomas

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Oh, and dipoles don't have one leg acting as a ground plane, ground planes are acting as the second leg of a dipole
Semantics.

Billski
Semantics, maybe, but I appreciate the correction. Hope it's right:)

Dan
 

autoreply

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Ok, took me a couple of days to read up on my (rusty) antenna theory.

Thanks a lot for the references, especially the Weir and Archer references are very interesting. Hadn't thought about the ILS/VOR being blocked by the engine/fuselage (if in the H-stab) because you're flying directly towards it.

As for the glass permeability for an antenna, I couldn't find anything useful. Dinivycell though claims a Dielectric coefficient of 1.1 which is very low.
 

djschwartz

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The dielectric constant isn't necessarily the most important parameter. Surrounding the antenna with a material whose dielectric constant is greater than one will make the antenna seem slightly longer than its physical length and lower its characteristic impedance; however, there would need to be a substantial volume of material for that effect to be noticeable in operation. Putting the antenna in the middle of a solid foam core structure would not be a good idea. But putting it inside a relatively thin sandwich shell should be OK. If need be the antenna can be tuned (trimmed in length) to compensate for the effects of dielectric constant. This does require some test equipment. The more important characteristic is dielectric absorption, also known as loss tangent or loss coefficient. At the VOR/LOC frequencies the loss of these materials is quite low and again, as long as you don't surround them with a LOT of material you're fine. By the time you get to the transponder and GPS frequencies the loss can get much worse and the shorter wavelength means a smaller amount of material is needed to have an effect. Unfortunately, unless the materials are ones commonly used for radomes or similar structures it may be very difficult to find that information.
 

Topaz

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The dielectric constant isn't necessarily the most important parameter. Surrounding the antenna with a material whose dielectric constant is greater than one will make the antenna seem slightly longer than its physical length and lower its characteristic impedance; however, there would need to be a substantial volume of material for that effect to be noticeable in operation. Putting the antenna in the middle of a solid foam core structure would not be a good idea. ...
Eeesh. Glad I've been lurking this thread, since that's exactly what I was planning to do on my own ship. Assuming the vertical tail is full-depth-foam and glass, would running the COM antenna up the leading edge, between the glass and foam, result in a large loss of the type you're discussing here? Or, if there is any significant loss, would it be confined to the rearward "shadow" of the vertical tail?
 

djschwartz

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Eeesh. Glad I've been lurking this thread, since that's exactly what I was planning to do on my own ship. Assuming the vertical tail is full-depth-foam and glass, would running the COM antenna up the leading edge, between the glass and foam, result in a large loss of the type you're discussing here? Or, if there is any significant loss, would it be confined to the rearward "shadow" of the vertical tail?
I would certainly expect it to "detune" the antenna, meaning that the length would need to be adjusted for the effect of the material of the fin. Unfortunately, there's no simple way to predict how much of an adjustment is needed. You'll need to sweep the antenna with a network analyzer or make a series of measurements with a VSWR bridge to figure that out. The fin will distort the radiation pattern and may cause some loss. Again, without using a very complex finite element field solver there's no way to predict exactly how much.

My advice would be to go ahead and build the antenna into the fin and give it a try; but, have a plan for an alternate more conventional antenna installation if it doesn't work. You can probably get close enough on the antenna tuning by making a mockup of the fin using a block of foam similar in shape and size to the fin. The mockup won't need to be carefully airfoil shaped, as long as it is approximately the same size it will be OK. Once you've got the antenna tuned on the mockup you can build one of the same dimensions into the real fin.
 
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