TopBand: Re: Rohn Tower on 160m

[Jack Harper]

At 04:15 8/13/97 EDT, you wrote:
>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".
>
<snip>

>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).
>
<snip>

>
>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
>
<snip>

>73, GL, de Earl, K6SE
>
<snip>

Earl -- I found your analysis quite interesting.

Am curious about several points:

  * What is the ELNEC program? Availability?

  * Also -- I have a HyGain HyTower vertical that is, of course, supported
on insulators -- assuming that a kw is run into the thing, how would I
calculate the maximum voltages at the base? I am concerned about the danger
of such an antenna in a neighborhood with kids etc running about and have
absolutely no idea of what sort of voltages are present. I have a little
white picket fence up around the base of the tower but still worry about
zapping someone that climbs over the fence... I have not yet loaded the
thing up with the kw as I have not yet completed the resurrection of a
Hammarlund HXL-ONE linear amplifier but that is emminent...

  * Also -- What in the world do you use to support a 100'+ tower so as to
be insulated from the ground. I assume that there is a source somewhere for
commercial broadcast stations (surplus?) or perhaps something is available
from Rohm?? The HyGain HyTower is, of course, considerably lighter than
100' of the Rohm -- HyGain supplies three large heavy molded plastic
insulators for the base that are about 4" tall or so.

Regards to all...

Jack, KC0LR (Friend to all things Hammarlund) 


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Jack Harper                                   Bank Systems 2000, Inc.
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