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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