"mc" asked: what do people do in airplanes and boats where there is no ground or counterpoise possible,,,
In metal planes and boats does the metal frame shield the signal from going anywhere or does the antenna have to be on the out side of the vehicle, on the 40 and 30 and 20 meter bands that is. ---------------------------------------------------- Boats can have an excellent "ground" with a metallic connection to the water. Also, the water surrounding the boat (especially salt water) is the lowest-loss ground you can ever find! That minimizes ground losses in the near field, which is the biggest drawback to using a vertical antenna (remember, that's why most standard broadcast antennas use 120 radials surrounding their antennas. It's also why you see so many broadcast towers near salt water on coastal cities!). Thanks to the excellent "ground" properties of water, boats tend to 'get out' extremely well with only a basic, simple antenna. Also, notice the DX-peditions that like to set up on the shore of an island! Using simple vertical antennas they can work the world with excellent signals. Airplanes with medium or high frequency radios work the antenna against the metal aircraft fuselage. In some cases a "trailing wire" antenna is used. This is a wire that reels out from the rear of the aircraft and sails along behind the plane in the air. A 1/2 wave of wire provides a very efficient "ground independent" antenna since it takes little current: it is a voltage-fed antenna. Of course, it's important to wind in the wire before landing. Compromise MH/HF antennas were also constructed by running a wire from the radio operator's position up near the cockpit, out through the fuselage at the top, back to an insulator at the top of the vertical fin, and then down an insulator near one wing tip. It worked quite well, not as well as most "trailing wires", but didn't require reeling in to land. Perhaps the most famous "aircraft" antenna is the Zeppelin antenna, which Hams now abbreviate to the "Zepp". Most heavier-than-air aircraft are very small, so the radio was very close to the skin of the airplane and only a short run of wire was needed to get "outside'. But a Zeppelin is BIG! They needed a way to run a trailing wire antenna, and connect the END of the trailing wire to the radio inside the airframe with a minimum of radiation between the transmitter and the end of the trailing wire. The solution was to run balanced feeders from the transmitter to the END of a 1/2 wave antenna. At the antenna, one feeder connected to the end of the radiator and the other feeder simply stopped at an insulator. At first glance one would think this would produce a very unbalanced situation and so promote a lot of feeder radiation. Not so. First, the antenna is 1/2 wave long, so it's voltage fed. Very little current flows at the point where the feeder connects to the antenna. Only a high RF voltage is present at that point. So the currents in the feeders are well-balanced with one side of the open wire line going to the antenna and the other side going only to an insulator. There is very little current on either side of the feeders at the end (only a small leakage through the insulator for the side not connected to the antenna). Normally the feeders were made 1/4 wavelength long, so the very high impedance at the end of the antenna was transformed into a low impedance at the transmitter, ensuring easy RF feed and without making the rig "hot" with RF in spite of there being no real "ground" connection. Most Ham "Zepps" today are not like the original Zeppelin antenna. Both the radiator and the feeder are usually whatever fits the space available. In that case it acts like a simply "random wire" with the feeder radiating about as much as the antenna, except on the frequency where the antenna is exactly 1/2 wave long. A common Ham variation is the "double Zepp" antenna, which is simply a wire with open wire fed at the center. Originally each side was 1/2 wave long, making the antenna a full wavelength overall. The feeder was made 1/4 wave long which ensured easy-to-handle low-impedance feed at the rig. That nicety in the design has been mostly lost today. Most hams will call any center fed wire using any length of open wire line a "double Zepp" antenna. Virtually all aircraft communications these days is by VHF/UHF where the body of the airplane is an excellent "counterpoise" or ground for a simple 1/4 wave long "whip". On aircraft these take the form of either small aerodynamically-shaped blades that protrude from the plane. In some cases a dipole is used. These are usually mounted behind a fiberglass cover flush with the body of the aircraft. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
