grounding and counterpoise
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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. _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
RE: grounding and counterpoise
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"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: [hidden email] You must be a subscriber to post to the list. 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Re: grounding and counterpoise
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Hi Ron,
You gave a nice summary of the antennas used on boats and aircraft in response to the question asked by "mc". I especially enjoyed your description of the Zepp, an excellent but often misunderstood antenna. There are a few other antennas that have been and are used on airplanes that I have learned about over the years. In the late 1970s I was working at Bell Labs in NJ and got to work with AT&T's "air force" regarding flight operations manuals and such. They flew a number airplanes including some Gulfstream 4s and several smaller corporate-type jets. In discussions with the pilots, some of whom had been pilots for the commercial airlines, these additional designs came to light. 1. The leading edge of the vertical stabilizer is typically an insulating strip that either contains a wire or metal bar that is used as a short vertical fed by a wide-range ATU that covers HF and/or VHF frequencies. 2. The insulated leading edge can instead be used to form a tunable slot antenna. The gap so formed is effectively embedded is a large, although not infinite, metal surface. The length of the slot and the position at which the slot is fed can be varied to adjust the resonant frequency and matching condition. 3. Just yesterday I was having lunch with a corporate pilot and a few other ham friends. He said another technique is to run a wire from near the front of the plane to the vertical stabilizer tip to form a loop consisting of the body of the plane and the wire. I presume the loop is fed by an ATU, probably installed inside the vertical stab or near the nose with one side tied to the plane's metal shell and the other to the wire. I'm sure there are many other ways to put an antenna on a boat or plane that we haven't mentioned yet, and others yet to be dreamed up. Cheers, Gus Hansen / KB0YH _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
RE: grounding and counterpoise
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Tnx Augie!
Yes, my 'large' aircraft work involved mostly piston-engined stuff like B-50 bombers (B-29's on steroids) and RC-121s (Super-G Constellations) in the 1950's. I remember reading about the leading edge radiators, but I never worked with them. Jets that I worked on, like the F-86, F-101, etc., were a bit small for "HF" antennas <G>. Some aircraft I worked with did use the "loop" you mentioned with the wire to the top of the stab. That eliminated the tendency for the fuselage to shield the antenna from the ground too! I have always been impressed with the simple elegance of the original Zepp, once I learned how the feeders worked even with one side of the open wire feedline terminated in "nothing", Hi! 73, Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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Hello Ron!
If you look at pictures of the old 707 or 720, you'll often see a "stinger" pointing forward from the top of the vertical stabilizer. This is an HF antenna. There was often a stinger pointing aft on a wingtip of a 747 (before the 400 series aircraft). This, too, was an HF antenna. The 707-style system worked pretty well; the 747-style had a *lot* of static due to bleed-off from the aircraft. The antenna became an extension of the airframe static dissipation system. This was (and is) a poor location... Lyle KK7P _______________________________________________ Elecraft mailing list Post to: [hidden email] 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 |
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