From owner-qrp-l@Lehigh.EDU Fri Jan 16 17:52:55 1998 Received: from fidoii.CC.lehigh.EDU (fidoii.CC.lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.8.5/8.8.5) with ESMTP id RAA13261 for ; Fri, 16 Jan 1998 17:52:54 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <12768-62042>; Fri, 16 Jan 1998 17:52:27 -0500 Received: from nss4.cc.Lehigh.EDU ([128.180.1.13]) by fidoii.cc.Lehigh.EDU with ESMTP id <12736-43352>; Fri, 16 Jan 1998 17:52:00 -0500 Received: from relay.bt.net (relay.bt.net [194.72.6.52]) by nss4.cc.Lehigh.EDU (8.8.8/8.8.5) with SMTP id RAA117384 for ; Fri, 16 Jan 1998 17:51:55 -0500 Received: from mail.infotrade.co.uk by relay.bt.net with SMTP (PP); Fri, 16 Jan 1998 22:03:29 +0000 Received: from mail.infotrade.co.uk ;16 Jan 1998 22:04:46 Received: from 1.0.0.0 by webserver;Fri, 16 Jan 1998 22:03:24 Received: by default with Microsoft Mail id <01BD22CA.4E8DDCA0@default>; Fri, 16 Jan 1998 22:01:33 -0000 Message-Id: <01BD22CA.4E8DDCA0@default> Date: Fri, 16 Jan 1998 21:43:50 -0000 Reply-To: D2250077@infotrade.co.uk Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: John Anthony Reynolds To: "Low Power Amateur Radio Discussion" Subject: Bazooka Antenna X-To: "'gqrp-l@blacksheep.org.'" X-Cc: "'qrp-l@lehigh.edu'" X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO Hi Guys, Thought this may be of interest to members, sorry for the cross-posting, taken from Ham-ant Digest V98 # 32 73 de John G3PTO GQRP 595 ................................................................................................................. From: k9euv@juno.COM (Dan G CAESAR) Subject: K9EUV bazooka DOUBLE BAZOOKA/COAXIAL DIPOLE ANTENNA ------------------------------------- A typical, dipole antenna has bandwidth of about 50 to 100 KHz. The bazooka is resonant in the center of the band and had an SWR of 2:1 or less across the entire 80 meter band? The "Bazooka" antenna was developed by the staff of M.I.T. for radar use. The original "Bazooka" used coaxial cable for the entire radiating elements. The adaptation used in amateur radio uses coax only for the broadbanding portion of the antenna, while the remaining portion of the elements are constructed of twinlead or ladder line (see attached sketch). Ladder line is preferable for its inherent strength. This is a single band antenna. It will not radiate harmonics of your operating frequency. In addition, there is very little feedline radiation, which is great for those who have problems with TVI. Its broadband characteristic makes it ideal for 80 meters and 10 meters. The Bazooka antenna consists of a half- wavelength of coaxial line with the outer conductor opened at the center and the feedline connected to the open ends. The outside of the coax and the ladder line operate as a half-wave dipole. The inside of the coax elements, which do not radiate, are quarter-wave shorted stubs which present a high resistive impedance to the feed point at resonance. Off resonance, the stub reactances change in such a way as to cancel the antenna reactance, thus increasing the bandwidth of the antenna. At the very center of the coax carefully cut away about one inch of the outer vinyl jacket. Then cut the exposed shield all the way around at the center of the exposed area. Be careful that you do not cut the dielectric material or the center conductor in the process. Twist the two pieces of exposed shield into small pig-tails. These are the feed-point terminals for the antenna. The center conductor of the feedline is soldered to one and the shield of the feedline to the other. Now solder the center conductor and shield together at each end of the antenna element. Solder the two ladder line wires to the end of the antenna element. At the other end of the ladder line, solder the two wires together. Use a square piece of plastic at the antenna center, drilling a small hole on each side of the coax, wrapping a small wire around the coax and through the holes and twisting the wire together on the other side. A small amount of quick setting epoxy secures the coax to the plastic support and prevents the wire from untwisting. A coating of silicone rubber or epoxy seals and protects the feed-point from the weather. Do the same where the ladder line is soldered to the shorted end of the coax. Short the center to shield at each end of the coax. Short the twin lead at each end and solder the coax/shield juction to the coax. 50 Ohm Coax feedline at least 66 feet long. shield side center connection side ___________ ________________| |________________ ___________ |___________|- [|__________________________________]- [___________]| Twin-Lead coax Twin Lead <----------------L=325/F-------------> coax length <--------------------------------------------L=460/F-------------------------------------------> total length example: 3.888 kc total length 118.3 feet coax length 83.6 feet twin lead length 34.7 feet Dan Caesar, K9EUV ex-ni9y 55966 FIR RD MISHAWAKA IN 46545-4202 219/259-9445 home/fax 219/255-4455 office in home