On Tue, 12 Aug 1997 12:38:35 -0500 philk5pc@connect.net (Phil Clements)
writes:
>I am planning the installation of a Rohn 25 tower as a 1/4 wave 160m
>vertical. There will be an adjustable 10' aluminum mast on top to
resonate the
>tower on 1830 khz.
>
>How tall must the Rohn 25 be to be resonant?
>
===========
Hi, Phil,
I couldn't resist using a NEC-based program (ELNEC) on this one. Rohn 25
is 12.5" on a side. To simplify things, I substituted a 12.5" diameter
conductor for the tower and assumed the 10' mast diameter is 2".
Because Rohn 25 comes in 10' lengths, this posed somewhat of a dilemma --
using 120' of tower requires 13' of 2" mast above it (total height =
133') to resonate at 1830 kHz.
Using 130' of Rohn 25 without a mast resonates too low in frequency --
128' of the tower will resonate at 1830 kHz, and I'm sure you don't want
to cut 2' of tower off.
You didn't specify whether you planned to insulate the tower from ground,
which you would do if you planned to simply direct-feed it at the base
with 50-ohm coax (a slight mismatch to the 36-ohm feedpoint resistance of
the antenna at resonance).
If you were planning on base insulators, I can offer a less-expensive
solution where exact 1/4-wave resonance is not necessary -- ground the
tower (don't use insulators) and shunt-feed it!
I came up with the following shunt-feed values (to match 50-ohm coax) for
Rohn 25 tower heights from 90' to 140' (no top-loading & without cutting
any tower sections). In all cases, the shunt-feed wire is #8 gauge,
spaced 24" from the tower. This wire is attached to the tower at the
specified height using a 36" horizontal section of 1" diameter aluminum
tubing.
The gamma capacity values are the "tuned" values at 1830 kHz. "Volts" is
the voltage across the gamma capacitor when 1 kw is applied to the
antenna.
Tower Attach height Gamma cap Volts
--------- -------------------- ------------------- -------
90' 85' 79 pF 4905
100' 72' 114 pF 3411
110' 55' 180 pF 2165
120' 36.5' 326 pF 1193
130' 33' 461 pF 844
140' 48' 333 pF 1167
Note that the 130' tower has the lowest attachment point, largest
capacity value, and least voltage across the capacitor. (This is the
size closest to 1/4-wave resonance at 1830 kHz.)
With a good ground radial system, any of the above towers will be an
effective DX antenna on 160m.
If you use a different size gamma wire or spacing, the above values will
change, especially the point of attachment.
Seems a shame to have all that tower up & without an HF beam atop
(top-loader for 160m), like most do.
73, GL, de Earl, K6SE
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