Yup, One of the local guys had a 2m antenna that melted the springy-strap. I suspected the same cause you stated. 73 Mike Perryman www.k5jmp.us
-----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] Behalf Of skipp025 Sent: Monday, March 06, 2006 12:14 PM To: [email protected] Subject: [Repeater-Builder] 42-50 MHz antennas up to 6 meters (Fc 52.525) Hello Sailors, I rained so hard.... (..how hard did it rain?) that I was only able to play with a silly six meter antenna project over the weekend. I can't imagine how some of you deal with all the snow and rain you get in other parts of the country/world. **** I've got a snot full of low band ~42MHz antennas removed from service. The CHP in California is on low band (and it works well for them). Their old pulled antennas end up at surplus sales and flea markets for very low prices. My goal is to find an easy mod to relocate the 42 MHz antennas up to about 52.525 or slightly higher, which I hope will cover the upper end of the Six-Meter FM Band. I've got a small amount of Antennex, Comtelco, Larsen and Maxrad low band antennas to use as test jigs. To be honest the mostly plastic body Larsen stuff doesn't look like very hardy or easy to convert. Most of the commercial antennas use the same base/jig materials for construction through a mostly mechanical assembly. I've found the Maxrad antenna coils/loads unscrew if you know how to fix a lathe or wrench onto them. Using a wrench will often mark the metal so access to a lathe makes a much cleaner dissassembly. The Antennex unit coil covers are a very firm press fit over a dual direction seration, which can be removed by a strong - firm lathe pull. I'm working with the Antennex brand loads now... this weekend I started by shorting the upper loading coil turns. When the smoke cleared I had 4 upper load coil turns shorted and had moved the impedance tap up 1/4 inch on the coil for 50 ohms. The rod length required about 1.5 inches to be removed. The results are great! The antenna gives a near 1.1 match at 52.600 (my actual target frequency) and retains a < 1.5 swr/match for well over 1MHz each side of the band. The final modification was to remove the excess turns vs shorting, which didtn't make a big change in the antenna tuning. I'm going to move the tap point back to the original position to see how far/where the impedance changes. The goal here would be to write up an easy to reproduce antenna modification for surplus commercial antennas. **** One last part... I can easily see why and where these base loaded coil antennas go bad. The Antennex brand of antennas use a very weak thin brass foil strape press-fit contact at the ground end of the loading coil. The coil high current causes this location to fail over relatively short time lengths from serious arcing. Every one of the low-band Antennex base loaded coil antennas I've pulled appart have this problem (serious black arc and pitting marks). A wire brass or stainless steel brush cleans it back up, but I'm not really one to trust this type of dorky connection for the long term. Since 90% of the surplus antennas I've found at the flea market (from the ex chp application) are the Antennex brand, I''m going to assume the ground foil arcing issue is more than a small problem as yet unresolved by the mfgr. This week will produce the final resultant antenna modified, cleaned-up and ready to use on 6 meters. More information to follow as this project works out. 73's skipp Yahoo! Groups Links Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
