From philabs!prls!mips!wyse!vsi1!apple!bloom-beacon!tut.cis.ohio-state.edu!ucbvax!ibma.nerc-wallingford.ac.UK!PJML Fri Apr 7 14:40:14 EDT 1989 Article 10954 of rec.ham-radio: Path: philabs!prls!mips!wyse!vsi1!apple!bloom-beacon!tut.cis.ohio-state.edu!ucbvax!ibma.nerc-wallingford.ac.UK!PJML >From: PJML@ibma.nerc-wallingford.ac.UK ("Peter Morgan Lucas, Network Support, Swindon") Newsgroups: rec.ham-radio Subject: (none) Message-ID: <06.Apr.89.17:06:42.BST.#2219@UK.AC.NWL.IA> Date: 6 Apr 89 16:04:27 GMT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 107 Subject: More of G6WBJ's Budget Antennas. 'Slim Jim' Antennas for VHF applications ========================================= The following diagram shows a practical 'slim jim' antenna (derivative of the 'J-pole') with cutting details for popular frequencies (i.e. 'Lo-band', aircraft and marine frequencies). The antennas do not require radials, and provide a low angle of radiation, whilst at the same time being relatively tolerant to surrounding metalwork. Best construction materials are either copper or aluminium tube. I have made the 70MHz one using copper water pipe and a range of solder-in cappilary plumbing fittings! (a word of warning - DO NOT, repeat NOT, attempt to bolt together aluminium tubes with BRASS nuts/bolts - they form an electrochemical couple in the presence of moisture - the aluminium will corrode seriously) Use a short length of plastic rod (Nylon, Tufnol) machined to the required size, to bridge the gap 'E'. ------> +====+ <----! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! 'B' 'A' ! ! ! ! ! ! ! ! ! ! <---! ! ! E 'E' is a gap between element ends ! ! ! <---! ! ! ! ! ! ! ! ! X...X Coax feedpoint ! ! ! ! (50 ohms - connect inner to ! X.. .X 'C' short leg, outer to long, and ! ! ! ! slide up/down elements for lowest ! ! ! ! SWR or best received signal) ! ! ! ! !--> +====+ <---! <'D'> ============================================================================ + Centre frequency + 70MHz + 158MHz + 125MHz + ============================================================================ + Length 'A' + 2.989M + 1.325M + 1.706M + ---------------------------------------------------------------------------- + length 'B' + 1.999M + 0.833M + 1.137M + ---------------------------------------------------------------------------- + Length 'C' + 0.980M + 0.432M + 0.549M + ---------------------------------------------------------------------------- + Spacing 'D' + 0.075M + 0.040M + 0.060M + ---------------------------------------------------------------------------- + Gap 'E' + 0.020M + 0.010M + 0.020M + ---------------------------------------------------------------------------- + Tube thickness + 0.025M + 0.012M + 0.025M + ============================================================================ Construction - i made mine out of copper tube (see your local plumber or airconditioning specialist - they often have offcuts available cheaply), with the ends beaten flat and joined by a length of threaded rod inserted into another length of tube bolted through the flattened end. The important thing is to obtain good electrical contact - soldering the joint after bolting it all up is recommended (the bolts provide mechanical continuity, solder provides the electrical continuity). Attach the coax feeder initially using alligator clips and slide them up/down the elements to obtain the best SWR or strongest received signal. To mount the antennas, either hang them from a hook by a length of nylon cord (i use the aircraft-band one for portable expeditions hanging from a branch of a tree!), or make up an INSULATED mount to clamp to a pole. The important thing is not to short together the elements, or to accidentally ground one or both in the mounting arrangement. The whole thing can be scaled for other frequencies - i am working on a version for 430-460MHz at the moment - looks like it will come out about 22 inches high! Happy Experimenting! G6WBJ Please use the following addresses for reply: + \/Natural + \/\Environment JANET: PJML@UK.AC.NERC-WALLINGFORD.IBMA + \/\/Research ARPA : PJML%IA.NWL.AC.UK@NSS.CS.UCL.AC.UK + \/\/\Council EARN : PJML%UK.AC.NWL.IA@UKACRL + AX25 : G6WBJ {144.650MHz} + Holbrook House SPAN : STAR::"PJML%IA.NWL.AC.UK@NSS.CS.UCL.AC.UK" + Station Road PHONE: +44 793 411665 + SWINDON SN1 1DE +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ + 'Freedom is being able to say that two plus two is three, or four, or five' + + + + George Orwell 'Nineteen Eighty Four' + +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++