One question I often hear is "Which antenna does what on my aircraft". I remember years back, instructors used to go over with the student just what each antenna function were. In years past there were fewer antennas on our certified aircraft than they are today. While we are discussing which antenna does what, let’s also discuss why the antenna is located where it is with relationship to the airframe, what the antenna does and why the antenna is shaped the way it is. Ever wonder why one antenna is only inches long while others are measured in feet? Well, you are soon to find out the reason behind this mystery.

First let’s define just what the purpose of the antenna is. According to Mr. Webster an antenna is a conductor by which electromagnetic waves are sent and received by a device such as our navs and comms. According to Mr. Rogers, an antenna is the eyes and ears of the radios. Let’s define a couple more things at this point. Wavelength is a distance measured between two successive points in the wave that are characterized by the same phase of oscillation. Have a desire to figure see just how big a wavelength can be? Well here’s the formula. Wavelength=Speed of light/Frequency.  W=c/f

From this formula it’s obvious that the larger the frequency the smaller the wavelength. VHF Comm. antennas operate around 126 MHz and the antennas are roughly 2.5 ft long. On the other hand, transponders work at around 1090Mhz and their antennas are approximately 3 inches long. We could go into dipoles and Doppler but that is beyond the scope of this article. The point to all of this is, the Higher the frequency, the smaller the antenna. In a later article we will dive into boring issues such as dipoles, VHF, UHF, SWR, line of sight, ground planes, Doppler radar, coaxial cable and some other fancy words. Regardless of how great your radio is; without a good antenna system it’s capability is limited. I couldn’t begin to tell you the number of times my customers thought they had a radio problem only to find out it was antenna or coax related. Anytime you have new radios installed be sure the antenna system gets a through evaluation. Our Cessna antennas have been out in the elements for years, they may need replacing. I’ve seen several cases where the customer replaced a perfectly good radio with a new one but still had the same problem. The problem turned out to be in the antenna system. Antennas can and do deteriorate in performance due to age and condition. If you’re spending four grand for a new nav/comm., don’t cheap out on a $150.00 antenna. Now let’s take a look at the antennas you would normally expect to see on a Cessna piston aircraft. Other piston aircraft antennas are about the same and serve the same purpose.

The VHF Comm. Antenna: Our aircraft transceivers (Comms) are used for communications between ground, ATC and other aircraft. The transceivers in our aircraft normally operate between the frequencies of 118.00-136.975 Mhz. Unlike many pieces of avionics, the aircraft transceiver does not operate on just one frequency, thus making it difficult to design the ideal antenna. The Comm. antenna on top of the Cessna is often referred to as a "Broad-Band" antenna, meaning it’s designed to work over a broad range of frequencies. In most cases our Comm. antennas sit right on top of the cabin. This is an excellent position for these antennas. The large wing and fuselage area provide a good ground plane plus the antennas aren’t shielded by anything. Being out in the open allows these antennas to receive and transmit well in all directions. The antenna must be bonded well to the airframe for maximum performance. You would be surprised at the amount of corrosion we often find under the antenna base. The Comm. antenna is acceptable to static build-up while flying in the clouds. Notice that the Comm. antennas are coated with a white, shiny coating. This coating is a "P-Stat" coating designed to conduct the static charge that is built up on the antennas while flying in clouds to the airframe and then on out the static wicks (you do have static wicks don’t you?). If the antenna loses it’s P-Stat coating, it will build up a large charge and that charge will do one of two things. One, it will jump to the airframe causing the aircraft radio to pick up the discharge noise. This noise can completely block all incoming calls. I’ve had P-Stat build-up that was so bad that the Comm. sounded like eggs frying in the headset. This lasted for five minutes or more, which is a real bad situation during an IFR flight. If one gets a high static build-up on the Comm. antenna, there’s a chance it will discharge to the center conductor of the antenna and on into the radio, thus damaging the radio. Neither of these situations is desirable.  What’s the cure? Easy, just make sure your antenna has the P-Stat coating as mentioned above. If your Comm. antennas are turning brown and/or fiberglass is starting to show, it’s time for a replacement. In general, the higher the frequency the radio works at, the less of a problem P-Stat is. Lorans and ADFs will die early while flying in P-Stat conditions but GPS will not fail. Some folks pine to mount a Comm. antenna on the bottom of the aircraft. This is OK but chances are it will not perform very well on the ground. Another problem with bottom-mounted antennas is they often pick up dirt, rocks, exhaust and oil, which deteriorates performance. On a Cessna, there’s really no reason to install a Comm. antenna on the bottom. Some Cessna 210’s and R-182’s had the Com antenna buried in the vertical stabilizer. Performance of these poorly placed antennas is horrible. These antennas are very directional orientated and only have a few miles of range to the rear of the aircraft. They didn’t work well 25 years ago and they certainly don’t work any better now. Most of these aircraft have since had the Comm. antennas placed on top of the cabin where they belong. By placing the Comm. antennas inside the vertical stabilizer, did the 210 or R-182 go faster? Absolutely not (well, maybe mentally it does). We’ve placed new antennas on the cabin and you couldn’t tell any difference in airspeed. In fact, I personally think they look sexy up there. The typical Cessna Comm. antenna is rated below 250kts. This is an "indicated" value. The higher speed rated antennas are just beefier and may mount to the airframe differently. No, installing higher speed antennas do not make the Cessna go faster, only lighten your wallet.

The Marker Beacon Antenna: The aircraft marker beacon operates on a frequency of 75Mhz. Marker beacons are used on the ILS approach system. The signal is transmitted up from the ground and as you would guess the marker antenna is always mounted on the bottom of the aircraft. In the early days, Cessna used a huge rod with a piece of coax attached to it. This antenna was huge, caused drag and loved to pick up ice. Later Cessna spent a few more bucks and installed what we call the "boat" antenna. It’s small and works much better than the rod looking device. Some Cessna 210’s and R-182 had an "optional" marker antenna that was flush mounted inside the aircraft. This antenna didn’t work any better, only cost more. Today when one of these flush mounted antennas fail, we just make a plate and install a regular "boat" antenna.  The regular marker antenna costs around $125.00 while the flush mount runs over $3,000. With performance and drag being equal, I know which one I’d purchase. I know what you are thinking. Earlier I mentioned that the lower the frequency the longer the antenna needs to be, why is the marker smaller than the Comm. antenna? OK, remember the marker transmitter is designed to send its signal directly upward. In other words the marker signal is aimed much like a rifle shot verses a shotgun. During an approach the aircraft passes only 200-3,000ft above the marker transmitter. Being the marker signal is aimed up and the aircraft is low, antenna design can get a little unorthodox and still work. It’s not unusual to get a marker lamp and audio at higher altitudes but again, the transmitter beam is being aimed directly up. The normal marker beacon question we get is "I get marker audio but no lamp, what’s wrong"? In most cases the marker antenna is starting to fail. If you have one of Cessna’s stainless steel marker antennas and start having a marker problem, I’d recommend replacing it with a modern type antenna. The newer style work better and cost less. Here’s a question for you. When you fly over the outer marker, you hear a 400hz tone that is modulated with a Morse code, what is the Morse code ident of that tone?

The VHF Navigation Antenna: Most Cessna aircraft employ what we call a "Cat Whisker" VHF navigation antenna. This antenna is normally mounted high on the vertical stabilizer. The so-called Cat Whisker is the two rods sticking out the top of the stabilizer, one located on each side. This antenna may be stainless steel or fiberglass, depending on how new it is. Most replacement VHF navigation antennas are fiberglass with a P-Stat coating. Because VHF navigation (108.00-117.95Mhz) can deteriorate or quit during heavy P-Stat build-up I highly recommend the fiberglass antennas if you must replace them even though the stainless types are still available. During the R-Nav days, some aircraft owners had what we call "Balanced Loop" antennas installed. These antennas have a higher gain and the signal doesn’t seem to scallop around as much as the Cat Whisker type. There are two types of Balanced Loops that I’ve seen on Cessna aircraft. One is what we call the "Towel Bar" style. Basically it looks much like a towel bar in your home’s bathroom, only more expensive. This antenna is installed on the vertical stabilizer near the top. Between the antennas and inside the vertical stabilizer is a device that mirrors the signals on the navigation antenna. Each antenna on the side of the vertical stabilizer is 180 out of phase with the other. This of course produces a "0" output but device mentioned above adds the signal together and sends it on to the navigation receivers. The other "Balanced Loop" type is of a "Blade" design. Basically it’s much like the towel bar except it looks like two blades installed on the tail. This is supposed to be the ultimate VHF navigation antenna but I personally question that. Fact is aircraft owners no longer certify their aircraft up for R-Nav approaches, so I wouldn’t spend the money to upgrade from a fiberglass Cat Whisker type antenna. The blade antennas are exceptionally difficult to install. Often the skin on the vertical tail has to be removed and doubler plates installed. The Blade and Towel Bar VHF navigation antennas are in the $600.00 range plus installation. It’s been my experience that the cost to upgrade to these antennas is cost prohibitive and the performance gain is minimal. But if you have money to burn, give me a callJ Unlike the VHF transceivers, we can run two or more VHF navigation receivers off one antenna. We do this with a device we call a splitter. The same antenna that is used for the VOR is used with localizer and in some cases, the glide slope. The splitter simply accepts one antenna input and has multiple outputs, depending on the model.

Glideslope Antennas: In the early years, Cessna decided it would be best to mount the glideslope antenna inside the cockpit and just behind the windshield. The idea made sense at least in theory. The only time we care about glideslope is when we are on the approach. At this time, the glideslope signal would be aimed directly at the nose of the aircraft, so the thought of mounting the antenna just behind the windshield seemed like a good idea. But in some cases this antenna doesn’t always work as desired. At certain RPM’s of the propeller the glideslope needle will scallop up and down. Change the propeller RPM and the problem goes away. As luck would have it, the needle bounces around at a RPM that you really need to use during the approach! This problem doesn’t show up in every aircraft but when it does, then the issue needs to be addressed. I’ve seen this problem arise more in Cessna Cardinals than any other aircraft. Why you ask? I haven’t a clue. Most Cessna aircraft use a splitter off the already present navigation antenna to receive its signal. Even though glideslope is in the UHF band, it still works fine with the already present VHF navigation antenna. Remember, when using glideslope the aircraft is seldom more than 20 miles out and the glideslope beam is pointed directly at the aircraft nose during an approach. During a new avionics installation we highly recommend removing the windshield mounted glideslope antenna. It’s much cheaper to replace during an avionics panel upgrade.

DME/Transponder Antennas: The frequencies of these two pieces of avionics equipment operate near the 1090Mhz range. For this article, let’s call these fine pieces of avionics "pulse" equipment. Since the operation frequency of DME and transponder is very high, the antenna is small. Normally you will find these antennas on the bottom of the aircraft. We like to have as much separation between the DME and transponder as possible. Often the avionics shop will install suppression between the DME and transponder so only one is on at a time. The switching rate is in microseconds so the pilot would never notice the equipment is being suppressed. Normally the pulse antennas are one of two types. The cheaper pulse antenna is just a steel rod about three inches long. This antenna works OK on non-pressurized aircraft or in non-pressurized areas. The little rod is easily broken during cleaning. It is pretty economical, normally costing around $50.00. A better pulse antenna (in my opinion) is what we call the "shark fin" antenna. It’s white and is secured by two holding devices and a doubler plate and looks like a shark fin sticking out of the water. I recommend these antennas because they seldom get broken off. The shark fin antenna sells for $160.00 on the average and seldom fails. The biggest enemy of the pulse antenna is oil, lead and dirt. Because these antennas are on the bottom of the aircraft, they are subject to getting pretty grimy at times. Picture this if you would. Customer comes into the avionics shop and reports, "My transponder is intermit". Shop sends customer to lunch. While customer is feeding face, avionics manager cleans sludge and grime off antenna. Transponder now checks good with test box. Customer returns from lunch and gets a bill for $95.00, which he/she gladly pays. Transponder now works so customer is happy. Avionics manager is happier than a rooster in a hen house because the fix was easy and fast. Point of this is; many of the transponder/DME problems we see are related to filthy antennas; clean them occasionally. RF doesn’t transmit through grime very well.

TCAD Antennas: With exception of some of the directional TCAD systems, TCAD uses the same antenna type as the DME/transponder. Why? Well, TCAD is looking for a transmitting transponder so it stands to reason that the antennas should be the same. Often a TCAD will have two antennas, one located on the bottom of the aircraft and one on top. Using this arrangement, the TCAD can see what is below and above the aircraft. Some TCADs such as the BFG SkyWatch actually transmit and receive the reply of aircraft with transponders. Later I’ll write a complete article on just how well TCAD works, pros and cons of the system.

ADF Antenna/Antennas: ADF (automatic direction finder) require a sense and loop antenna to determine the direction of the radio beacon. In the older days, these antennas were separate from each other. The sense antenna, better known as the "long wire" normally was positioned from the tail to the top of the cabin or in some cases, along the bottom of the aircraft. The loop antenna can be located either on top of the aircraft or the bottom. Modern ADF antennas are what we call a combined loop/sense antenna. The combined loop sense is smaller and develops less drag, not to mention that they work just as well as the two-piece antenna system. Some of the sense antennas are just steel wire or stainless steel. Others appear to be very thick. These have a "P-Stat" coating to aid in static buildup during IFR flying. The ADF combined loop/sense antenna is another one that must be kept clean for optimal performance. Let this jewel get greasy and the ADF performance will fall on its face. I’d be willing to say that 50% of the ADF problems we see are antenna related. By the way, the length of the sense antenna wire is critical.

The Radar Altimeter is another avionics box that works at high frequency. Most general aviation radar altimeters use a single antenna. This antenna is small and flat. The radar altimeter antenna location is normally on the bottom of the aircraft or on the bottom of the wing. Mounting angle, and location to other antennas is very critical with the radar altimeter. The biggest culprit with the radar altimeter system failure is dirt and grime. Let just a small amount of oil or grease get between the antenna and the airframe and the radar altimeter needle will be all over the scale. If the radar altimeter antenna gets painted, chances are its trash. Most have a label on the antenna that says, "Do not Paint" and that’s just what they mean. Some of the corporate type radar altimeters use two antennas. One antenna transmits and the other receives. Keep them clean and you’ll normally get years of trouble-free use.

The ELT Antenna: All Cessna ELT (Emergency Locator Transmitter) that I’m aware of are located on top of the aircraft. Normally this antenna is located near the baggage compartment or over the tail section. This antenna is very flexible. Many of the newer ELT antennas have a spring base, thus they are hard to break off. The goal is for the ELT antenna not to break off during a crash. Barring the 406Mhz system, the ELT operates in the VHF band (121.5 and 243.00Mhz). Many ELT systems are designed around their own antenna and should not be used with other antenna manufacturers. Make sure your ELT antenna is on the aircraft before each flight; you never know when you may need it.

The Loran Antenna: Most loran antennas are located on top of our Cessna aircraft. They often are nothing more than a VHF COM antenna without the load inside the base of the antenna. Some aircraft owners elected to connect their loran to the ADF sense wire. While this may work, the loran will quit in IFR conditions due to static build-up on the ADF sense wire. A Loran antenna may be located on the bottom of the aircraft as well with what we call a "Bent Whip" antenna. The Loran receiver must be told where the antenna is mounted (top or bottom) for proper operation. It’s been my experience that Loran works best when the antenna is located on top of the aircraft.

The Stormscope/Strikefinder Antenna:  Most weather avoidance antennas are nothing more than ADF loop antennas.  In fact, the weather avoidance equipment such as the Stormscope and Stikefinder are looking for noise.  If the storm activity is strong enough, you will not only cell a strike on the screen of the weather avoidance equipment but will hear it on the ADF also.  Figuring the direction and distance of the strike is some real magic but the antenna is 50 year old technology and it works great.  Normally this Stormscope or Srikefinder antenna can go on the top or bottom of the aircraft.  These antennas must be isolate from interference such as strobes and autopilot servos.  Installation is very critical. 

The GPS Antenna: What an antenna! Due to the frequency, the GPS antenna is quite small, not affected by static build-up (in most cases), causes very little drag and is easy to mount on the aircraft. Normally the GPS antenna will be mounted on top of the cabin, which enables the antenna to view the horizon in all directions. Early GPS receivers needed this excellent view in order to operate properly. But with today’s hot 12-channel GPS receivers one can almost put the antenna anywhere on top of the aircraft and it will work fine. The GPS antenna should never go on the bottom of the airframe or wing. There is a combined VHF Comm./GPS antenna that we’ve used in the past. It works pretty well and doesn’t require punching another hole in the aircraft. The combined COM/GPS antenna is very expensive, so when possible use the GPS antenna supplied by the manufacturer. All factory supplied GPS antennas are rated at the top of the subsonic range. Most general aviation aircraft operate in this range. GPS antennas normally are either passive or active types but the footprints are different with each manufacturer. The question I get often is "Can I install an outside antenna for my portable GPS"? Yes, but the external antenna often runs $350.00 or more plus installation. Never paint the GPS antenna; you’ll hate yourself later.

1.Can my A&P or avionics shop mount an external GPS on the outside of my aircraft for my portable GPS? If your aircraft is not pressurized and AC 43.13 is followed, the A&P or avionics shop can install an external GPS antenna with little trouble. Keep in mind the overhead interior may have to come down and this can take some time. With today’s hot 12-channel GPS receivers one would question if it were really necessary to have that outside GPS antenna.

2. Can I as a pilot determine when an antenna is failing? Yes, to some degree. If the antennas start to look brown, scaled or weather damaged; it’s time to change them out. If you suspect an antenna is failing but it looks OK, have your local avionics shop check it out. Often we find high SWR’s (another fancy/expensive word) and corrosion at the base of the antenna. Most COM/DME/transponder antennas cost less than a GPS database update.

3. Do expensive antennas enhance performance? Seldom. The standard fiberglass antennas that were installed on the aircraft, from the factory are an excellent choice. Antenna location is more important than the antenna price in most cases.

4. Should I install another VHF COM antenna for my handheld transceiver? There’s no need to. There’s a "handheld adapter box" that connects in series between your present COM and it’s antenna. When you plug your handheld radio into the adapter, it opens the panel mounted COM from the system and places the handheld on the external antenna. My test showed using the portable transceiver rubber duck antenna, range was limited to around 7 miles. By plugging the handheld into the adapter, the range increased to 43 miles using the same portable radio! Cost was $55.00 for the adapter and a couple of hours to install. This cost is far less than installing another external antenna.

5. I’m removing my panel mounted GPS and installing another manufacturers panel mounted GPS. Can I use the same antenna? The answer depends on several issues. Who are the manufacturers of the GPS systems? GPS antennas are either active or passive, depending on the manufacturer. Of course the footprint is different on each. Manufacturers often use different types of coaxial cable, be sure the correct one is installed. If the GPS is to be certified for IFR, then the correct antenna that the GPS was manufactured with must be used. They are a few exceptions to the rule but VERY few. I recommend using the antenna supplied by the manufacturer.