With the small GA cockpit turning into a sort of video Disney World, serious IFR pilots worry more about what to do if all that stuff goes dark.Not that we expect any reliability problems, mind you, but none of the new certified mapcoms have more than a few months of operational field experience. Who knows whats lurking in those circuits.
The standard back-up scheme these days seems to be a portable GPS of some kind, plus a handheld VHF comm. Thats good as far as it goes, but the comm wont be nearly as reliable as you think and you can substantially improve the performance of both GPS and VHF comm with external antennas.
If youre using portable GPS routinely, not just as back-up, an external antenna will make it work as we’ll as any panel mount does and you’ll save yourself the hassle of fooling with antenna suction cups, tangled wire leads and other paraphernalia when you fly. Lets deal with the comm first.
Comm Antennas
Try this experiment: Next time youre flying, haul out your Icom or Sportys handheld VHF comm and give Center a call. we’ll bet you even money no one will hear you. Why? The truth is that even a 5-watt transmitter radiating with a rubber ducky type antenna from inside the cabin isn’t at all efficient. The antennas length is compromised to make the unit compact and as a result, the reliable range isn’t more than 5 miles. It will be less if there’s aircraft structure-wings, tail, doors-between you and the ground station.
The solution is an external comm antenna designed specifically for connection to a handheld transceiver. Auxiliary comm antennas can be mounted on the top or bottom of the fuselage but the bottom mount is usually less obtrusive and often less time consuming to install.
The antenna doesnt have to be of super performance as long as its tuned for the proper frequency and, of course, is electrically functional, with the correct coax and no corroded connectors. The stainless bent whip antennas are the most common and cost under $150 for the hardware. Depending on the airplane, installation will cost another $300. There are a couple of ways to handle the wiring. The easiest is to route the coax into the cabin with enough slack to stuff it into a map pocket. When you need the radio-even for routine ground clearance ops-remove the ducky and connect the external antenna.
Option two is to route the cable to a female BNC connector somewhere on the panel then make up a short jumper from the radio to the connector. This costs a few more bucks but eliminates the nuisance of loose wiring hanging around the cockpit. (The BNC jack should have a placard identifying it.)
This set-up will give similar performance as your panel mounted comms but it will match neither the range nor the audio quality of the panel gear.
Portable ransceivers will accept push-to-talk switches and most headsets seem compatible with the audio output of the radio. Caution here: We said most headsets. In our headset tests, weve noted that some headsets arent electronically compatible with portable comms. Try yours to see how it sounds.
No Splitters
Another method of getting the RF outside the airplane is to use an antenna splitter or junction box that allows a portable transceiver to share a primary comm antenna. When the portable transceiver is connected, the panel mounted comm radio is disabled, prohibiting keying of two transmitters into one antenna.
We frown on this type of installation because the splitter can malfunction and induce a primary comm system failure. Moreover, depending on where the failure is, it could also silence your back-up. In general, comm reliability is reduced by adding connectors of any kind to a coaxial antenna lead. These can come loose, accumulate corrosion or just otherwise result in a poor connection or ground. The splitter approach is certainly cheaper-about $100-but you get what you pay for. If you want true back-up reliability, the external antenna should be a standalone system.
As for the antenna itself, a basic wire whip on the belly is fine. The belly mount makes sense because there’s usually more room there and there’s less structure to shadow the radiation pattern. Comant, Dorne and Margolin and Antenna Specialists all make these antennas and prices are competitive. Weve used all three suppliers with good results.
If aesthetics are a concern, go for a conventional blade-type comm antenna, complete with fiberglass housing and anti-precipitation coating. These look a little nicer than wire whips but cost more, too. Performance wont be any better, however. Also, they should be mounted topside and if youve already got two blades up there, your airplane may develop that porcupine look.
Nav Antennas
Your portable VHF comm probably has a VOR receiver and in a pinch, you might even navigate with it. But if you also have a portable GPS receiver, why would you bother? The GPS can reliably put you on a runway in almost any circumstances; its doubtful if the portable VOR could do as well.
Portable GPS receivers rely on built-in onboard antennas or remote antennas that mount somewhere inside the cockpit, either on the glareshield or via suction cup to a window. Besides the nuisance of dealing with this less-than-elegant hardware, portable GPS antennas occasionally have marginal-or even zero-performance.
When the satellites are mostly overhead or perhaps behind the airplane and out of the antennas sight, the nav solution may fade or drop in and out. Sometimes moving the antenna helps, sometimes not. Since GPS antennas are always mounted on the roof of the cabin, they can see any available satellite, usually right down to the horizon.
If youve experienced intermittent performance with your portable GPS, an external antenna will almost certainly improve it. Older vintage GPS engines were of the six or eight-channel variety-some were even single channel- that simply will not perform we’ll inside the cabin without a remote antenna, especially in aircraft with high wing design.
Amplified
GPS antennas arent cheap for several reasons. Most portable GPS systems will require an active type antenna to amplify the signal, otherwise known as preamplification. The theory here is that the incoming satellite signal-which is so weak as to be below the ambient electronic noise level-breaks down further with longer runs of cable. If the signal is amplified before its sent down to the receiver, performance is greatly improved.
In some instances, a less expensive passive or non-amplified antenna can be used if low-loss type coaxial cable connects it to the receiver. Considering the cost of low-loss coaxial cable, however, it might be more cost effective to buy the active antenna in the first place.
The GPS manufacturer will advise of the type of antenna required for any particular GPS engine. Some receivers can use active only, some use active or passive. Comant is a major supplier of GPS antennas and its Classic GPS antenna line includes a model for virtually every handheld navigator made.
The CI405 series, for example, is a low-profile teardrop design complete with wiring kit. The passive versions retail for about $295, the active version for about $400. If you have a Garmin 195 or GPS III Pilot, youd want the Comant CI405-7KC, which sells for about $400 complete. Garmin is one of the few manufacturers to offer its own antennas. The GA56 retails for $382 and will work with most of the companys recent model portable GPS navigators.
Another possibility-and one that cuts down on installation complexity-is to consider a dual-purpose antenna such as the Comant CI4510 GPS/VHF combo. At $625 retail, its pricier than either a standalone VHF or GPS antenna, but will be cheaper than installing both. The antenna has dual outputs and thus at the cable end inside the airplane, it can be dealt with just as any other aux antenna. The installation of the aux GPS antenna is more time consuming mainly because of limited mounting locations and the requirement to remove the aircraft headliner to install the various fittings. Figure on half a day or about $300 to $400 to install a GPS antenna.
While youre at it, consider having the shop hardwire your GPS into the avionics bus, so you wont have to fuss with wires and balky lighter adapter sockets. The GPS can have its own breaker and both the power cable and antenna lead can be neatly routed to be less obtrusive without losing the receivers portability.
-by Larry Anglianso
Larry Anglianso is an avionics technician at Exxel Avionics in Hartford, Connecticut. Hes a regular Aviation Consumer contributor.
