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Path: utzoo!watmath!clyde!cbosgd!ihnp4!mgnetp!mgweed!rjr
From: rjr@mgweed.UUCP (Bob Roehrig)
Newsgroups: net.ham-radio
Subject: Dipole antenna theory observation/question
Message-ID: <20780@mgweed.UUCP>
Date: Fri, 4-Oct-85 13:44:27 EDT
Article-I.D.: mgweed.20780
Posted: Fri Oct  4 13:44:27 1985
Date-Received: Sat, 5-Oct-85 06:55:21 EDT
Organization: AT&T Information Systems - Montgomery Illinois
Lines: 27


The question concerns the formula given in  the  handbooks  for
determining the length of a half wave dipole.

The  handbooks  say  that  dipole  length  in  free  space   is
determined  by the formula: 492/MHz = feet. End effects on wire
antennas reduce this figure by 5% so  that  the  common  figure
used is 468/MHz = feet.

Several friends and I have erected various antennas  this  year
and  have  all independently arrived at the conclusion that 492
should be multiplied by

This was  observed  during  the  installation  of  at  least  3
antennas  (  80,  40  and  12  meter  dipoles) all at different
locations, and all at various heights. In every case, it  meant
shortening  an antenna that was cut to length with the handbook
formula. It made no difference if a  balun  was  used  or  not,
except  that  higher  frequency antennas (say 20 meters and up)
often have to be slightly shorter if  a  balun  is  used  since
inductance adds to the electrical length.

If what we are seeing is real, where was the .95 factor derived
from and why has it not been corrected?

Bob, K9EUI


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