[TowerTalk] Need help modeling an 80-meter Shunt Fed tower

[Pete Smith N4ZR]

 From my own experience, this is not an easy thing to model with 
precision.  I've been doing NEC-2 modeling for a long time, and had a 
comprehensive model of my complete towerand all antennas when I set out 
to model a 160-meter shunt feed.  Even so, and despite a number of 
climbs to try to find what the software suggested might be a 50-ohm tap 
point, no joy.  I finally wound up using a 45-foot or so tap point and 
an Omega match. No inductor required.

73, Pete N4ZR
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On 6/22/2015 2:43 PM, RLVZ--- via TowerTalk wrote:
>   
> Hi Guys,
>   
> I'm looking for someone to model a simple Shunt-Fed tower for use  on
> 80-meters.  I have no experience using modeling software and don't have  time to
> learn until after I retire, so I'm hoping someone is willing to  model this
> for me.
>   
> I'd like to model this tower to help determine the aprx. location of  the
> 50 ohm resistive feed point tap at a frequency of 3.550  mhz.   I realize
> that I could feed the tower at almost any height and  make it work using a LC
> Network, but I'd like to keep it simple as W8JI once  wrote, by using: "a
> simple gamma capacitor and shunt feed conductor with the tap  point adjusted to
> find 50 ohms resistive".   (no inductor  required with this simple feed
> method)  Since the tower is short at  46', it will be relatively easy to climb
> it in order to find the exact 50  ohm resistive tap location.  However, with
> my bad back, modeling it should  get me in the general area of the 50 ohm
> tap location and save me  several trips up and down the tower which would be
> much  appreciated.
>   
> Here's the tower specs:
>   
> 1) Rohn 45 tower with 18" face width, tower height  is 46'.
>   
> 2) Non-Conductive guy wires.  (no steel guy wires)
>   
> 3) Top mounted Bencher Skyhawk Tribander.  (24' boom)
>   
> 4) 10' aluminum DC grounded VHF antenna top mounted, extends the  overall
> tower/antenna height to 56'.
>   
> 5) Proposed Shunt Feed system: 10 gauge copper wire run parallel to the
> tower, from the base of the tower upward to the 50 ohm resistive tap  point.
> I'm flexible with the spacing distance of the wire from the  tower, but am
> leaning towards an 18" separation unless modeling  recommends otherwise.
>   
> In addition to determining the estimated 50 ohm resistive tap height,  it
> would also be very helpful if the modeling software would estimate  the value
> of the needed capacitance.  (such as 150pf)
>   
> Thanks & 73!
>   
> Dick- K9OM
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