In some cases, satellite antennas might be strategically installed without having to drill through the airframe, but make sure they have an unobstructed view of the sky.
As they should, builders tend to spend lots of time planning the avionics interface but often shortchange the antenna portion, which is the most important subsystem of the suite. There is far more than just buying the antennas. The install includes a variety of essential tasks, including strategically routing signal cable through the airframe in hopes of eliminating, or at least minimizing, RF interference.
The incorrect placement of ADS-B antennas will kill performance and is often the cause of tail-chasing troubleshooting once the aircraft is assembled. The key is to get the antenna install right before closing the structure. In this article, we’ll offer our tips for doing all of that, with a few words on modern antenna design and maintaining them as the aircraft ages.
Antennas are only half the system. In addition to main runs of coax routed through the airframe, there might be short critical pigtails that connect the cable to the radios.
Know Thy Frequencies—and the Manual
You don’t need to be a ham radio whiz to build your own aircraft antenna farm, but a basic knowledge of the frequency band will help with the big picture, especially with placement on the airframe. For aircraft antennas, there aren’t a lot of frequencies to be concerned with, but most are critical to performance when it comes to potential RF interference and signal jamming.
This is a Garmin SXM antenna. Like most others, be sure it doesn’t get painted.
Consider a combination GPS/VHF antenna. It can do the job of two antennas in one, saving installation effort. That’s a Comant CI 2580 combo pictured here.
The current avionics install data should be the final say as to where you mount antennas and how you terminate connectors and route the coaxial cabling, but you’ll likely make some compromises. Most avionics manufacturers do a good job of spelling this out in their installation manuals while also offering valuable guidance on building a ground plane on non-metal surfaces. Garmin is one that gets it right, specifically offering detailed guidance in its equipment installation manuals for installing antennas. So does Avidyne.
For instance, the install manual for the Garmin GTR200 VHF com radio is pretty specific about locating the com antenna. According to the manual, the antenna should be well removed from all projections, engines, and propellers, of course. But that may be easier said than done. Just because there is space that accommodates the mounting doesn’t mean it’s the right space. Think in terms of performance. The ground plane surface directly below the antenna should be a flat plane over as large an area as possible (Garmin echoes what many pro installers would recommend, which is 18 inches square—at a minimum).
Make absolutely certain there is sufficient support structure for any antenna, which means fabricating a doubler plate. If not, skin repairs could be in your future.
But the other challenge is the lack of real estate for co-locating various antennas, especially on smaller airframes. For the com radio, its antenna should be mounted a minimum of six feet from any DME or other VHF com antennas and should be mounted as far as practical from the ELT antenna. We’ve witnessed some ELTs exhibiting reradiation problems that cause interference with other gear—including WAAS GPS receivers, perhaps the most critical antenna in the farm.
For fabric and composite airframes, be sure there is sufficient ground plane for best antenna performance. The larger, the better.
Worth mentioning is properly planning the antenna placement for modern audio panels, especially if you are installing antennas before you get into the audio panel wiring. Both Garmin and PS Engineering panels have a split-com mode. This is where the pilot can transmit on one radio and the copilot on the other simultaneously. While most com radios should be wired for transmit interlock (another reason to carefully follow the wiring schematics), for the split mode to work, one antenna should be mounted on top of the aircraft and the other on the bottom. That should be enough separation for closely tuned frequencies on both radios.
Ground Planes and Doubler Plates
We talked with Don Jeckell, a veteran antenna pro at antenna manufacturer Comant Industries, for tips on getting the antenna portion of the project right the first time. For years Comant has been one of the most popular choices for aircraft antennas and accessories, and popular suppliers (including Aircraft Spruce and Chief Aircraft, to name just two) stock a wide variety of Comant antennas—everything from L-band transponder/ADS-B blade-style antennas to combination VHF/GPS stick models. But Jeckell cautioned that selecting the right antenna for the airframe and the avionics system is only part of the equation.
“If you shortchange any antenna install, it might still work, but not optimally, with varied performance at different airports,” Jeckell wisely told us. He’s right, and botching an antenna install is an easy way to rack up a serious invoice (and shop teardown time) for troubleshooting. This is especially important when building a ground plane on fabric and fiberglass airframes. Poor continuity generally means poor performance. Perhaps you’re fabricating a mesh ground plane for an antenna on a composite structure. You’ll need to be certain there is proper contact between the antenna’s mounting screws (oftentimes where the grounding takes place) and the mesh. While this is a very basic concept, it’s one of the easiest tasks to get wrong. The FAA’s Advisory Circular AC 43.13-2B is a good place to learn a bit about acceptable methods, and we suggest some reading before drilling.
We asked Jeckell if there’s a general spec for designing the size of a ground plane, and he reiterated what we already know from years of antenna installation experience: Bigger is better. At a minimum, a circular ground plane’s diameter can’t be any shorter than the antenna’s base. Understand that not every install is always identical, which is also the case for the end performance result.
The bead of sealant around this antenna still looks good, but you’ll eventually see cracks develop, which means stripping and reapplying.
A ground plane may not be required for all antennas, but it’s still recommended. That’s what Garmin says in the installation manual for the WAAS antenna used with its G3X Touch integrated avionics system, and they suggest the conductive ground plane be a minimum diameter of 8 inches. Like many antennas, the WAAS antenna is grounded through the mounting hardware and the coaxial antenna connection. The Garmin manual suggests that the mounting hardware (washers and nuts) and doubler plate should make contact with an unpainted grounded surface to ensure proper antenna grounding. It is important to have good conductivity between the coaxial shield and the ground plane. You’d think it would, but the bottom of the antenna does not need to make contact with the ground plane because the antenna will capacitively couple to the ground plane beneath the paint or covering.
For fabric aircraft, technicians often fabricate a ground plane using heavy foil tape or other metallic surfaces for a solid bonding of the antenna. Since there might be limited structural space available for mounting antennas on fabric aircraft, the installation of some systems just might not be possible. Again, AC43.13 provides guidance.
Maintain antennas by inspecting and replacing the silicone sealant around the base. If you don’t, water intrusion invites corrosion.
Another important part of installing the antenna is maintaining the structural integrity of the skin below it. That almost always includes installing a doubler plate or stiffener to keep the antenna from damaging the skin. In general, antenna manufacturers don’t provide a backing plate or doubler, which means you’ll be fabricating one from sheet metal and attaching it to the structure. This is most important for higher profile antennas, including VHF com antennas. Antennas in general take a beating. They sustain vibration and radically changing temperatures, they collect ice, and they do flex enough to tear the skin.
Metal fatigue can still occur even with a hefty backing plate if it isn’t properly attached to the stringers. Think in terms of oil canning—with the antenna (and the skin below it) buckling under stress. Some builders get so caught up in saving weight, the last thing they want to do is add more metal to the airframe, but it’s necessary and really doesn’t add that much weight. We’ve dealt with enough expensive skin repairs to vouch for it.
VHF nav dipole antennas live hard lives on the vertical tail. Inspect them regularly.
Even low-profile GPS WAAS antennas need reinforcement. Looking again at Garmin’s WAAS antenna installation instructions shows the installation must provide adequate support for the antenna considering a maximum drag load of 5 pounds (at subsonic speed). The install manual goes on to prescribe that when penetrating the skin with a large hole (for accommodating the coaxial connector), a doubler plate is required to reinstate the integrity of the aircraft skin. Before you bore a hole through the structure, consider if the antenna installation will weaken it, and always make use of any available reinforcements to beef it up.
Consider the speed of the aircraft when selecting antennas. After all, many if not most antennas carry a speed rating. Even when properly installed using doubler plates (this is a stiffener fabricated from sheet metal that attaches between the antenna’s base and the surface of the aircraft’s skin), an antenna that isn’t properly rated can come off the aircraft, or worse damage the skin, requiring a big repair. And as you’d expect, antennas with a higher speed rating come at a higher price—sometimes double the price of one with a lower speed rating.
Interference-free placement of WAAS GPS antennas like this Garmin GA35 is critical for reliable precision GPS approach performance.
How Many Antennas?
This is where you might want to lobby some advice from your avionics shop. You might have a few options for spec’ing out an antenna combination based on the avionics you have—or plan to install down the road.
Typically, there are two com antennas (one for each radio), one VHF nav antenna that connects to both radios via a splitter, one GPS antenna for each navigator (combined com/nav/GPS systems like a GNS/GTN or Avidyne IFD navigator still require separate com and nav antennas), a transponder antenna, an ADS-B antenna (or two for some systems), a marker beacon antenna if still installed, an ELT antenna, and a TAS/TCAS antenna if installed. Got a Stormscope for lightning detection? That requires an antenna, too.
While it is possible for two VHF com radios to share a single antenna through a splitter, this is a rare and expensive interface. If you’ve lost count, that’s a total of 11 antennas all fighting for an interference-free spot on the airframe. It’s a wonder some systems even work at all with all of these antennas fighting for interference-free performance.
Spec It Out, First
There is a lot that goes into planning an antenna setup. In addition to the technical guidelines we lightly touch upon here, you might select antennas based on aesthetics and also performance. We can’t come close to listing them all, so we won’t. But there are some common ones, especially for VHF com.
Com antennas come in different varieties, from an inexpensive metal rod whip design to pricey, high-speed fiberglass models appropriate for turboprops and jets. For the underbelly mount, one common antenna is the Comant CI122, typically sold for around $220. This is a bent-whip antenna with a fiberglass base and metal rod element. Models like the ITT DM C70-1A are priced around $400 each, and slightly more for the flavor that mounts on the belly.
You might be shocked to find that navigation antennas won’t be cheap, either. Luckily, you might only have to replace one. That’s because it’s common for a single navigational antenna to feed both nav radios in the aircraft through a coaxial splitter.
ADS-B installations require antennas. That’s a blade-style L-band antenna shown here, which is compatible with a variety of transponder and UAT-based systems.
And don’t forget ADS-B. Complying with the FAA’s ADS-B mandate shouldn’t require specialty antennas because ADS-B systems use typical L-band transponder antennas. For ADS-B Out via a 1090ES extended squitter transponder, a common blade-style antenna will work just fine. But systems with diversity (which helps eliminate shadowing) require a top and bottom L-band antenna. UAT systems (which operate on 978 MHz) also use a typical L-band antenna.
If you have an existing kit with some hours on it and you’re having the shop install ADS-B (especially a system that’s 1090ES transponder based), don’t be surprised if it suggests changing the existing transponder antenna for better ADS-B performance. Since the L-band antenna lives on the belly, it might be contaminated with oil and grease. We’ve seen some fiberglass blades melt because they were installed too close to the exhaust stack. Remember that when installing one on a new kit. Exhaust blow-by grime quickly coats the surface of the antenna with performance-killing grime. You’ll work hard to keep it clean.
L-band antennas also come in a rod/ball design, which is just that—a metal rod with a ball at the end. These are generally low performance and aren’t nearly as durable as their fiberglass counterparts. Since they’re low profile and difficult to see, they’re easy to damage when washing the aircraft with a brush. If you leave the chore to them, remind the FBO staff to use caution when washing, regardless of which antenna you have.
Going in for paint work? It’s a good time to replace antennas and not the time to paint over existing ones. (Photo: Courtesy of Judy and Sam Wayman)
Old Kit? Consider an Antenna Upgrade
If you have a vintage kit, and especially if you are battling performance issues, an antenna system overhaul could be in order. It will take some teardown, perhaps better saved for inspections and other major maintenance. Dig deep.
In many cases, the cabling that attaches the antenna to the radio could be as old as the aircraft and is subject to deterioration and wear from years of vibration, heat, and cold. Consider too that old coaxial cable is likely non-shielded RG58 (or older) and could be the source of interfering noise that’s worked its way into your radios and audio system.
As for the antennas themselves, you won’t have to look closely to see if they need replacement. Is the fiberglass wearing off? Is it cracking? Cracks invite water intrusion, especially around the base and it’s what corrosion is made of, especially if the base of the antenna and the skin are dissimilar metals. Some, but not all, antennas come with mounting gaskets, and Don Jeckell at Comant told us not to install one if the manual doesn’t call for it. Instead, spread a thin layer of RTV around the base, and let it dry completely before moving the aircraft. Make the antennas a regular part of inspections and add new RTV when the old sealing bead cracks—and it will.
Last, if you’re considering new paint work, consider changing the antennas, too. It’s the perfect time to do so because they should be removed anyway. No, don’t paint over them; that’s likely a sure way to kill performance.
