Tuesday, May 1, 2012

Simple Antennas for Amateur Radio Operators--a continuing series


In my last post, I described a simple inverted vee that could be built and erected by one person at a modest cost.  In my case, I had enough wire and and pvc mast sections to build this 40-10 meter antenna without going to the nearest hardware store.  Fortunately, I also had a hundred or so feet of 450-ohm twinlead, a spare 4:1 balun, and a good ATU (Drake MN-4) to finish the job.  With each leg of the "v" 33-feet long, you have an antenna that can work from 40 to 10 meters.  If you want to explore 80 meters, make both legs of the "v" 65-feet long.  These measurements are approximate--you may have to trim each leg a few inches to make a better match.

If you don't have twin lead, you can also use approximately 40 to 50 feet of coax to feed the antenna.  Of course, the coax will feel happiest on one band and its third harmonic.  That characteristic makes it possible to use a 40-meter inverted "v" on 15 meters.  The match will be a little off for 15 meters and may put the best match a little near the upper limit of 15 meters.  You can either let your ATU smooth out the mismatch or you can add "outrigger" wires with clip leads to produce a better match on 15 meters.  Some amateur radio opertors have also attached capacity hats made from stiff wire to the 40 meter wire to help lower the SWR.  Most antenna books published by the ARRL explain the process.  If you want to use 20 meters on your coax fed 40 meter inverted "v", it might be best to add another, separate 20 meter inverted "v" to the antenna.  This additional length (approximately 16-feet, 6 inches for each element) should be run 90 degrees offset from the 40 meter wire elements.  You could also modify a "fan dipole" for this purpose, with separate wires running beneath the 40 meter wire elements.  There will probably be some interaction between the elements, so be prepared to trim antenna elements as needed.

As for the coax connection to the apex of the inverted "v", I've found the Budwig coax connector ideally suitable for this application.  Fair Radio Sales in Lima, Ohio markets this type of connector.  When it comes to coax, any type from RG-58 to RG -213 would be suitable.  I've found RG-6 (the type cable installers use) can be used without much difficulty.  Although this cable is designed for 75 ohms impedance, my Drake MN-4 has no trouble tuning out the slight mismatch.  RG-8X could also be used.  However, with the smaller diameter coax cables, such as RG-58, one should probably keep power below 100 watts.  RG-58 also has more loss than RG-8 or RG-213.  In most cases, I avoid coax altogether, since I prefer 300 and 450-ohm twin lead for most of my hf antenna work.

The important thing is to build your own antenna with the resources you can find locally.  I've seen some homebrew antennas that rival the commercial products in both construction and effectiveness.  While none of my simple antennas can compare with those available in the commercial sector, they do work and get me on the air with a minimul of fuss and cost.

That's about all for this segment.  Good luck on designing and building your next antenna.

Aloha es 73 de KH6JRM--BK29jx15.