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RF Exposure Issues (long)

Subject: RF Exposure Issues (long)
From: dave@egh.com (dave@egh.com)
Date: Thu Aug 8 18:34:31 1996
        I have just read through the document called "Measurements of 
Environmental Electromagnetic Fields at Amateur Radio Stations".  After
reading it, I have a few questions/comments which I think are pertinent 
to the calculation of RF exposure:

        1. I have read the definitions of "Controlled Environment" and
           "Uncontrolled Environment" on page 53.  It is my assumption
           that the "Controlled Environment" may be considered to be
           the property owned/rented by the amateur operator, with
           possible exceptions in the case of multi-family properties.
           I believe that the "transient passage" covers visitors,
           invited or uninvited.  Does this seem reasonable, especially
           to any of you who are familiar with legal nuances?
           
        2. How can we accurately determine the duty cycle?  I saw Brian's
           (K6STI) initial estimate of 25%.  Let's say we have the worst
           case for a CW contest.  This should be the times when we are
           calling CQ without answers.  (When we are making a qso, we can
           assume 50% listening time by virtue of the fact that the 
           received exchange on average takes as long as the sent one.)  
           During a CQ'ing period, we are probably transmitting 6 seconds 
           and listening 4.  The rest of the calculation rests critically 
           on what percentage the duty cycle is for transmit time, i.e.,
           what is the time length of a space versus a 'dit' or 'dah'.
           Does anybody have any idea of this percentage?  Perhaps those
           of you who have written logging programs capable of sending
           CW have already calculated this.

        3. The comment has been made that higher antennas are going to
           cause less exposure than lower ones.  I guess this will make 
           those of us owning 100 foot towers feel pretty good.  However,
           the example stations in the study were measured at various
           locations, and on page 24 I see that station C had a very high
           reading near a section of feedline (nearly 3 times as high as
           those taken in the immediate vincinity of the xcvr).  Now the
           100 foot towers with their 250 foot coax runs may have a 
           problem.  No comment was made as to the quality of coax.  
           (K6NB - Wayne, if you're on this group, do you by any chance 
           remember the coax type?) Offhand, this seems to indicate a 
           problem with the coax.  Is it reasonable to assume that this 
           was perhaps RG-8 or some other type with a shield ratio less
           than that of RG-213? (the xcvr was running 1000 watts)?  If so, 
           will RG-213 give a substantial improvement?  Is this problem 
           perhaps related to the lack of a balun at the antenna, or just 
           a bad feedline?  Perhaps Brian, Dean N6BV, or someone else who 
           has written antenna analysis software could give us some input.  
           (I apologize to whomever I have left off this list - I'm sure 
           there are more of you out there who have written these fine 
           software products.  It also appears that more of us may be 
           purchasing one of these products in the near future.)  

        4. Antenna heights and coax qualities being equal, it would seem 
           that the E field strength limits in the 3.0-30 Mhz frequencies
           (1842/f and 828/f) indicate that we need to concern ourselves
           more with 10 meters than 20 or possibly 40 (for yagis).  Wire
           antennas tend to be closer to the ground for 40,80,160 meters,
           so there is an added uncertainty here.  Fortunately the formula
           allows a higher E field strength due to the lower f value.
  
        Any thoughts on these observations will be appreciated.

73, Dave Clemons K1VUT


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