TopBand: Re: Rohn 25

Earl W Cunningham k6se@juno.com
Thu, 14 Aug 1997 02:50:38 EDT


Hi, Top-banders,

John, W0UN has informed me that each section of Rohn 25 when installed is
a net 9' 8-1/2", so here's the corrected info for Phil, K5PC, who wanted
to know how many sections with a 10' adjustable mast at the top would
resonate at 1830 kHz.

13 sections of Rohn 25 (126' 2-1/2") with 3' 7" of 2" diameter mast
sticking out of the top (total height 129' 9-1/2") is 1/4-wave resonant
at 1830 kHz.  Feedpoint impedance at resonance is 36 + j0 ohms, or
.389:1 SWR on 50-ohm coax.  The 2:1 SWR bandwidth on a 50-ohm line is
219 kHz (1748 to 1967 kHz).  The lowest SWR (1.363:1) will be between
1845 & 1850 kHz, where the input Z is about 37.5 + j4 ohms (unless
perfectly matched at resonance, the frequency of lowest SWR is NOT the
frequency of resonance!).

Because my table of values previously posted was based on 10' increments,
I recalculated for various numbers of Rohn 25 sections.  Still using #8
gauge for the gamma wire spaced 24" from the tower.  Values were computed
to match 50-ohm coax.  Voltage calculations are for 1 kw into the
antenna.  These are only towers, with no masts, no top loading (beams) or
other wire hanging from the towers.  Also using non-conductive guy cable
or wire guys insulated from the towers and broken up with insulators.

   # of           Tower         Gamma    Gamma     Capacitor  
Sections      Height          Height     Capacity      Voltage
------------   -----------------     ------------   -------------    
--------------
      9          87'  4-1/2"       86' 9"       76 pF          5084
    10          97' 1"              76'         103 pF          3755
    11         106' 9-1/2"        61'         153 pF          2524
    12         116' 6"            42' 9"      259 pF          1488
    13         126' 2-1/2"        31'         449 pF           858
    14         135' 11"            40'         402 pF           958
    15         145' 7-1/2"        56'         275 pF         1403

Sorry about the 3-1/2" per section error -- I've never used Rohn 25.

73, de Earl, K6SE


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