Hi everybody, I'd like to throw my two-cents worth into this discussion, since it's an area where I'm knowledgeable. First, aircraft comm antennas are what are known as quarterwave groundplane antennas. That simply means that the physical length of the wire (radiating element) is an electrical quarter wavelength at the operating frequency; that is the distance a given point on one RF cycle travels before the corresponding point on the next RF cycle leaves the antenna. The wavelength at these frequencies is about 2 1/4 meters long, so a quarter wave length is about 20-21 inches. Antennas are inherently balanced devices, meaning that what is on one conductor of the cable from the radio must be duplicated on the other conductor of the cable. If you look at the "rabbit ears" nav antenna, that is a balanced dipole antenna. Many years ago, RF engineers found that one side of the antenna could be replaced by the ground or a metalic surface perpendicular to the antenna, or a "ground plane (as in surface, not to be confused with aircraft)". The skin of the aircraft serves the function of this ground plane. As far as RF radiation is concerned, picture a doughnut laying on the ground (or a metalic ground plane) with the quarter wave antenna setting in the middle of the doughnut perpendicular to the ground, or ground plane. On a much larger scale, this is the radiation pattern of a quarterwave ground plane antenna. You will notice, there is essentially no radiation below ground level. This in not precisely true, but a good approximation. Also, notice that most of the radiation is between 30 and 60 degrees above horizontal. Extrapolating this to an aircraft antenna mounted on top of the cockpit or on top of the tailcone, you can see that most of the radiation is upward. By turning the whole model upside down (without the doughnut falling off), you can see that most of the radiation will be downward. This would be the case of a comm antenna mounted on the belly or bottom of the tailcone. With the aircraft at altitude, this would be a desirable condition but would have very reduced range on the ground. However, with the aircraft on the ground, we're generally not trying to communicate over long distances, usually less than a mile. A bottom mounted antenna would have better range at altitude and reduced, but acceptable, range on the ground. Why don't we see many bottom mounted antennas? First, they are much more vulnerable to damage from objects on the ground such as mechanics, chocks, taxiway lights, etc. Second, there is probably a lot of "We've always put them on top, so that's where they're supposed to be." mentality. If you'll notice, transponder antennas (also quarterwave ground plane antennas) are bottom mounted, but since they're much smaller (due to the much higher frequency) they aren't nearly as prone to damage. Also, if you will notice, most airliners have comm antennas (the larger blade type - also a quarterwave ground plane in a streamlined fairing) mounted on top and bottom. Again, with the airliners being much larger, the antennas sit much higher and are not nearly so prone to damage; nor do they intentionally make off-airport landings. BTW, I'm also an RF communications technician, so this information is not just theoretical; I've seen it proven in practice. I just thought a little more in-depth explanation would clear up some of the mystery. Hope it helps rather than confuses. Regards, Dave in Alamogodo, NM (Ercouper wannabe)
Artie Langston wrote: > Great info, Claude! How does the bottom mounting perform on the > ground? Also, do you feel the broadband claims for the fiberglass > antennas are true, and do they really offer better SWR over the whole > aviation band? Where do you have the transponder spike on your > plane? Artie N2666H __________________________________________________________________________ ______ To unsubscribe from this list please send mailto:[EMAIL PROTECTED] _____________________________________________________________ Who will win the Oscars? Spout off on our Entertainment list! http://www.topica.com/lists/showbiztalk
<<attachment: winmail.dat>>
