Isn't that what I said without modelling ?
I think it is dangerous to base all our knowledge exclusively on the
results of modelling! The technical guys should know the answer to such
question without having to resourt to modelling. Modelling is a super tool,
but it can be a dangerous tool as well...
73
John
>To: jon.zaimes@dol.net, topband@contesting.com
>Subject: Re: TopBand: sloping parasitic array
>X-Juno-Line-Breaks: 0,4-5,7-8,10-13,15-18,20-21,24-25,29-30,32-35
>From: k6se@juno.com (Earl W Cunningham)
To: <topband@contesting.com>
>Date: Sun, 29 Mar 1998 16:51:38 EST
>Sender: owner-topband@contesting.com
>X-List-Info: http://www.contesting.com/topband.html
>X-Sponsor: W4AN, KM3T, N5KO & AD1C
>
>Jon wrote:
>"I'm planning on erecting 128 feet of Rohn 25 as a vertical radiator,
>with ropes supporting vertical parasitic elements with sloping top-hats
>as described in ON4UN's book. (3 elements with Reflector, Driven and
>Director).
>
>"What difference will it make whether I series-feed the tower or
>shunt-feed it, if any?
>===============
>I modeled 3 configurations of the 128' vertical without any parasitic
>elements in the field:
>
> 1) Series fed
> 2) Shunt fed (gamma match)
> 3) Folded unipole with 3-wire cage of equal size as tower, spaced 2
>feet from the tower.
>
>All three models had virtually the same gain and TOA.
>
>The series-fed 128' tower is very close to resonance at 1830 kHz, with a
>feedpoint resistance of about 35 ohms.
>
>Adding the cage to make it into a folded unipole shifted the resonant
>frequency down to 1800 kHz, with a feedpoint resistance of about 140
>ohms.
>
>The shunt-fed 128' tower, at 1830 kHz, required about 600pF of gamma
>capacity. Voltage across the capacitor was less than 1000 at 1kw. Shunt
>wire was .25" diameter, spaced 24" from the tower, tied to the tower at
>32' high.
>
>All three configurations had a 2:1 SWR bandwidth of better than +/- 100
>kHz.
>
>73, de Earl, K6SE
>
>
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>
***************************************************************
* John Devoldere, ON4UN, AA4OI, OT8T *
* e-mail: john.devoldere@innet.be *
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