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

[TexasRF--- via TowerTalk]

Hi Pete and Dick, modeling closely spaced conductors has always been  
problematic.
 
My experience with shunt fed verticals has shown that a reasonably sized  
shunt feed device ends up with an impedance of much less than R=50 and 
includes  a series L of a few hundred ohms at the feed point.
 
The included series inductance provides the key to matching without having  
to add an extra inductor. One basically adds a series C  to allow  
cancellation of just enough L to arrive at a parallel circuit with R=50 and a a  
shunt value of inductance that is then canceled with a parallel C. Also known 
as  series to parallel conversion.
 
Sounds a lot like an Omega match doesn't it? Once adjusted, one can run up  
and down the band by small adjustments to series C. The shunt part doesn't  
change very much so usually it can be left alone.
 
One can use small fixed and/or variable capacitors to perfect the design  
and then replace with the bigger high power parts.
 
73,
Gerald K5GW
 
 
 
 
 
 
In a message dated 6/23/2015 1:03:34 P.M. Central Daylight Time,  
n4zr at contesting.com writes:

>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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>
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